�Wn6ura!): 100, PRINCES STREET Eonlroit: H. K. LEWIS, ise, GOWER STREET, W.C.nbsp;IStilin: A. ASHER AND CO.

F. A. BROCKHAUS ifteto Sotfe: G. P. PUTNAM�S SONSnbsp;BombsH ano �alrutla; MACMILLAN AND CO., Ltd.

Part of a transverse section of a Permian Osmundaceous Fern stem, Thamnopteris Schlechtendalii (Eichwald). a, outer xylem; b, inner xylem. For description,nbsp;see page 329. (After Kidston and Gwynne-Vaughan. Very slightly reduced.)

A TEXT-BOOK FOB STUDENTS OF BOTANY AND GEOLOGY

A. C. SEWABD, M.A., F.B.S.

PROFESSOR OF BOTANY IN THE UNIVERSITY ; FEULOW OP ST JOHn�S COLLEGE AND HONORARY FELLOW OF EMMANUEL COLLEGE, CAMBRIDGE

llllllllllllllll

CambriBge:

PEBFACE

T REGRET that pressure of other work has prevented the completion of this Volume within a reasonable time sincenbsp;the publication of Volume I. Had Volume II been writtennbsp;ten years ago, the discoveries made in the course of the lastnbsp;decade would have given an out-of-date character to muchnbsp;of the subject-matter. It is more especially in regard to thenbsp;Ferns and the extinct members of the Gymnosperms that ournbsp;outlook has been materially altered by recent contributions tonbsp;Palaeobotany. It is, howe^�er, some satisfaction to be able tonbsp;add that recent progress has been relatively slight in that partnbsp;of the subject dealt with in the first volume.

The original intention was to complete the whole work in two volumes. Soon after the second volume was begun, itnbsp;became evident that the remaining divisions of the plant-kingdom could not be included within the compass of a singlenbsp;volume. I decided, therefore, to take the consequences ofnbsp;having embarked on too ambitious a plan of treatment, andnbsp;to preserve uniformity of proportion by reserving the seedbearing plants for a third volume. The third volume willnbsp;include the Pteridosperms, other than those briefly describednbsp;in the final chapter of the present volume, and other classes ofnbsp;Gymnosperms. I propose also to devote such space as isnbsp;available within the limits of a text-book to the neglectednbsp;subject of the geographical distribution of plants at different

stages in the history of the earth. It is my intention to complete Volume III with as little delay as possible. As Inbsp;have written elsewhere, the past history of the Floweringnbsp;plants needs special treatment, and anything more than a merenbsp;compilation can be adequately attempted only after considerable research and with the assistance of botanists possessingnbsp;a special knowledge of different families of Angiosperms. Thenbsp;need of a critical examination of available data in regard tonbsp;the geological history of this dominant group will not be lostnbsp;sight of.

I am well aware that while certain genera have received an undue share of attention in the present volume, othersnbsp;have been ignored or treated with scant consideration. Fornbsp;this inconsistency I have no excuse to offer, beyond the statement that the subject is a large one, and selection is necessarynbsp;even though the work consists of three volumes.

The publication in 1909 of a collection of excellent photographs of Palaeozoic Plants, with brief descriptive notes, by Mr Newell Arber, as one of a series of popular � Nature Books,�nbsp;bears striking testimony to the remarkable spread of interestnbsp;in the study of the vegetation of the past, which is one of thenbsp;outstanding features in the recent history of botanical science.

In the list of illustrations I have mentioned the source of all figures which have been previously published. I would,nbsp;however, supplement the statement of fact with an expressionnbsp;of thanks to corporate bodies and to individuals who havenbsp;allowed me to make use of blocks, drawings, or photographs.

I wish to thank my colleague, Mr A. G. Tansley, for placing at my disposal several blocks originally published in the pagesnbsp;of the New Phytologist. To Professor Bertrand of Lille and tonbsp;his son Dr Paul Bertrand I am indebted for several prints and

descriptive notes of specimens in their possession. My friends Dr Nathorst of Stockholm and Dr Zeiller of Paris have generously responded to my requests for information on variousnbsp;points. I wish especially to thank Dr Kidston for severalnbsp;excellent prints of specimens in his collection and for the loannbsp;of sections. I have profited by more than one examination ofnbsp;his splendid collection at Stirling. Professor Weiss has generously allowed me to borrow sections from the Manchesternbsp;University collections, more especially several which have beennbsp;reproduced in the chapter devoted to the genus Lepidodendron.nbsp;To Professor F. W. Oliver my thanks are due for the loannbsp;of sections from the collection under his charge at Universitynbsp;College. I have pleasure also in thanking Dr Scott, not onlynbsp;for lending me sections of a Lepidodendron and for allowingnbsp;me to use some drawings of Miadesmia originally made bynbsp;Mrs Scott for reproduction in his invaluable book, Studies innbsp;Fossil Botany, but for kindly undertaking the laborious tasknbsp;of reading the proofs of this volume. It would be unfair tonbsp;express my gratitude to Dr Scott for many helpful suggestionsnbsp;and criticisms, without explicitly stating that thanks to anbsp;friend for reading proofs must not be interpreted as an attemptnbsp;to claim his support for all statements or views expressed.nbsp;The General Editor of the Series, Mr A. E. Shipley, has alsonbsp;kindly read the proofs. I am under obligations also for assistance of various kinds to Prof. Thomas of Auckland, Newnbsp;Zealand, to Mr Boodle of Kew, to Mr D. M. S. Watson ofnbsp;Manchester, to Mr T. G. Hill of University College, and tonbsp;Mr Gordon of Emmanuel College, Cambridge. I am indebtednbsp;to the kind offices of Miss M. C. Knowles for the photographnbsp;of the specimen of Archaeopteris hibernica in the Irish Nationalnbsp;Museum, Dublin, reproduced on page 561.

Many of the illustrations are reproduced from drawings by my wife: those made from the actual specimens are distinguished by the addition of the initials M. S. I am gratefulnbsp;to her also for some improvements in the letter-press. For thenbsp;drawings made from sections and for some of the outlinenbsp;sketches I am responsible. I have availed myself freely ofnbsp;the facilities afforded by Professor McKenny Hughes in thenbsp;Sedgwick Museum of Geology for the examination of specimens under the charge of Mr Newell Arber, the Universitynbsp;Demonstrator in Palaeobotany. It is a pleasure to add that,nbsp;as on former occasions, I am indebted to the vigilance of thenbsp;Readers of the University Press for the detection of severalnbsp;errors which escaped my notice in the revision of the proofs.

CHAPTER XII

CHAPTER XIV

CHAPTER XVII

STIGMARIA. Pp. 227�247.

CHAPTER XVIII

Bothrodendron 248-264. a. B. minutifolium 251-253; h. B. punctatum 254, 255; c. B. Mltorkense 255-259. Anatomynbsp;of vegetative shoots of Bothrodendron 260-262 ; Cones ofnbsp;Bothrodendron 262-264.

General considerations ........

CHAPTER XIX

SEED-BEARING PLANTS CLOSELY ALLIED TO MEMBERS OF THE LYCOPODIALES. Pp. 271�279.

CHAPTER XX

PILIOALES. Pp. 280�323.

I. Leptosporangiate Filicales...... 283-316

Eufilicineae 284-316. Osmundaceae 285, 286; Schi-zaeaoeae 286-291; Matonineae 291-293; Loxsomaceae 293; Hymenophyllaceae 294; Oyatheaceae 294-296 ;nbsp;Dennstaedtiinae 296 ; Polypodiaceae 296 ; Parkeria-ceae 297 ; Dipteridinae 298.

The habit, leaf-form, and distribution of ferns 300-309; The anatomy of ferns 309-316.

III. Ophioglossales........ 321-323

CHAPTEE XXI

FOSSIL FERNS. Pp. 324�394.

Osmundaceae 324-346; Schizaeaceae 346-351; G-leichenia-ceae 351-355; Matonineae 355-363; Hymenophyllaceae 363-365; Cyatheaceae 365-375; Polypodiaceae 375-380; Dipteridinae 380-394.

CHAPTER XXII

MARATTIALES (FOSSIL). Pp. 395�411.

Ptychocarpus 397; Danaeites 398; Parapecopteris 398; Astero-theca 398-400; Hawlea 400 ; Soolecopteris 401,402; Disoo-pteris 402-404; Dactylotheca 404-406; Renaultia 406; Zeilleria 407; Urnatopteris 407; Marattiopsis 407-409;nbsp;Danaeopsis 409; Ndthorstia 410, 411.

CHAPTER XXIII PSARONIEAE. Pp. 412�426.

CHAPTER XXIV

OPHIOGLOSSALES (FOSSIL). Pp. 427�431. CHAPTER XXV

COENOPTERIDEAE. Pp. 432�472.

CHAPTER XXVI

HYDROPTERIDEAE AND SAGENOPTERIS.

Pp. 473�483.

CHAPTER XXVII

GENERA OP PTERIDOSPERMS, FERNS, AND

PLANTAE INCERTAE SEDIS. Pp. 484�580.

Taeniopter� 485-494; Weichselia 494-496; Glossopter� 496-512; Gangamopteris 512�517; Lesleyd 517-519; Neuvo-pteridium 519-523 ; Cardiopteris 523-525 ; Aphlebia 525-529; Sphenopteris 529-532; M(triopteris, DiplotMeina^ Palmatopteris 532-537; Cephalotheca 537; Thinnfeldianbsp;537-544; Lomatopteris 544�546; Cycadopteris 546; Ptilo-zamites 546-548; Ctenopteris 548-550; Dichopteris 550-552; Odontopteris 552�556; Callipteris 557-559; Calli-ptzridinm 560 ; Archaeopteris 560-565; Newropteris 565�nbsp;571; Cyclopter� 571, 572; Linopteris 572,573; Alethopterisnbsp;573-576; Lonohopteris 516 \ Pecopteris 576-580.

LIST OF ILLUSTRATIONS

Several of the illustrations are printed from blocks for which I am indebted to learned societies or to individuals. The sources fromnbsp;which cliches were obtained .are mentioned within square brackets.

Frontispiece. Thamnopteris Schlechtendalii (Eich.). From a photograph given to me by Dr Kidston and Mr Gwynne-Vaughan. (Page 329.)

Ulodendroid soar . nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;. t .

179

XVI

FIG.

187.

188,

190.

191.

192.

193.

194.

195.

196.

197.

198. 200.nbsp;201.nbsp;202.

203.

204, 206.nbsp;207,

209.

210. 211.nbsp;212.

213.

214.

215.

216.

217.

218.

219.

220. 221.nbsp;222.

223.

224.

225.

226. 227.

Sigillaria Brardi, S. laevigata, and Lepidodendron Carica sp. ......nbsp;nbsp;nbsp;nbsp;�

Miadesmia and Bothrodendron Angiopteris evecta and Cycas revolutanbsp;Osmunda cinnamomea, 0. regalis, and Todea harhara

Aneimia jlexuosa, A. phyllitidis, Hymenophyllum, Matonia pectinata, Thyrsopteris elegans, Oleichenia

Oleichenites Rostafiniskii, Qldchenia dicarpa, O. dichotoma Matonia pectinata ......��

Matonia pectinata .........

Thyrsopteris elegant, Cyathea spinulosa, Dicksonia ooniifolia, B. culcita, Davallia concinna, Alsophila excelsa

PAGE

293

294

230.	Bicksonia Bertervana ...... [Trustees of the British Museum.]	. nbsp;nbsp;nbsp;295
231.	Bipteris quinquefurcata, B. conjugata, D. Wallichii, and Poly-
	podium, qusrcifolium, ......	. nbsp;nbsp;nbsp;297
232.	Bavallia aculeata .......	. nbsp;nbsp;nbsp;299
233.	Polypodium Billardieri......	. nbsp;nbsp;nbsp;302
234.	Polypodium quercifolium.....	. nbsp;nbsp;nbsp;303
235.	Hemitelia capensis .......	. nbsp;nbsp;nbsp;304
236 a	236 h. Pteris aquilina ......	305, 306

237.

Oleichenite� hantonensis ........

[Council of the Palaeontographical Society.]

264. nbsp;nbsp;nbsp;Laccopteris elegans .........

[Council of the Royal Society.]

265. nbsp;nbsp;nbsp;Matonidiwm Wiesneri, Marattiopds marantacea, Gleichenites

gracilis, Laccopteris Qoepperti, and L. M�nsteri

266. nbsp;nbsp;nbsp;Laccopteris polypodioides.......

[Trustees of the British Museum.]

267. nbsp;nbsp;nbsp;Laccopteris .nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;. �......

[Trustees of the British Museum.]

268. nbsp;nbsp;nbsp;1 Laccopteris polypodioides.......

[Trustees of the British Museum.]

269. nbsp;nbsp;nbsp;Matonidium Goepperti........

[Editor of the Encyclopaedia Britannica.]

270. nbsp;nbsp;nbsp;Senftenhergia elegans, Oligocarpia Brongniartii, Trichomanes

sp., Hymenophyllum, timhridgense, Sphenoptens [Hymeno-phyllites) quadridactylites ......

271. nbsp;nbsp;nbsp;Coniopteris hymenophylloides ......

[Council of the Manchester Literary and Philosophical Society.]

272. nbsp;nbsp;nbsp;G. hymenophylloides .......

273. nbsp;nbsp;nbsp;Coniopteris qninqvdoha.......

274. nbsp;nbsp;nbsp;Coniopteris arguta ........

275. nbsp;nbsp;nbsp;Coniopteris arguta and G. hymenophylloides .

276. nbsp;nbsp;nbsp;Oncopteris Nettvalli .......

277. nbsp;nbsp;nbsp;Protopteris punctata .......

278. nbsp;nbsp;nbsp;Laccopteris polypodioides, L. M�nsteri, Bichsonia, Onychiopsis

Mantelli, Hausmannia Sewardi, H. Kohlmanni, and Protopteris Witteana ........

279. nbsp;nbsp;nbsp;Adiantides antiquus and A. Undsayoides

280. nbsp;nbsp;nbsp;Onychiopsis Mantelli .......

281. nbsp;nbsp;nbsp;Dictyophyllum exile.......

282. nbsp;nbsp;nbsp;Dictyophyllum Missoni, Rhizomopteris Schenki, Camptopteris

spiralis, and D. exile .......

283. nbsp;nbsp;nbsp;Dictyophyllum rugosum .......

[Trustees of the British Museum.]

284. nbsp;nbsp;nbsp;Thaumatopteris M�nsteri ......

285. nbsp;nbsp;nbsp;Clathropteris meniscoides......

286. nbsp;nbsp;nbsp;Clathropteris egyptiaca.......

[Editor of the Geological Magazine.]

287. nbsp;nbsp;nbsp;Camptopteris spiralis.......

288. nbsp;nbsp;nbsp;Hausmannia dichotoma .......

289. nbsp;nbsp;nbsp;Hausmannia sp.........

290. nbsp;nbsp;nbsp;Alethopteris lonchitica, Lonchopteris rugosa, Sphenopteris

Hoeninghausi, Parapecopteris -nev.ropteroides, and Peco-pteris {Dactylotheca) plumosa ......

291. nbsp;nbsp;nbsp;Ptyohocarpus unita, Asterotheca Sternhergii, Danaeites sare-

308. nbsp;nbsp;nbsp;Clepsydropsis antiqua, Etapteris Scotti, Diplolahis forensis,

309. nbsp;nbsp;nbsp;Diplolahis forensis, Botryopteris forensis, *Corynepteris coral-

310. nbsp;nbsp;nbsp;Metaclepsydropsis duplex, Stawropteris oldhamia, Ankyropteris

316. nbsp;nbsp;nbsp;317. Ankyropteris corrugata ......nbsp;nbsp;nbsp;nbsp;459, 460

318. nbsp;nbsp;nbsp;Etapteris Scotti .nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;. * 462

Asterochlaena laxa ......

[Editor of the New Phytologist.]

Sporocarp-like bodies (? Sagenopteris)

RegnelUdium diphyllxim, Sagenoptamp;ris rhoifolia Sagenopteris Phillipsi ........nbsp;nbsp;nbsp;nbsp;480

[Trustees of the British Museum.]

Sagenopteris Phillipsi .......

[Council of the Manchester Literary and Philosophical Society.]

Taeniopteris multinervis, Lesleya Belafondi .

Taeniopteris Garnoti, T. spatulata, T. coriacea Taeniopteris Carruthersi.....

[Annals of the South African Museum Taeniopteris vittata .....

Weichselia Mantelli, W. erratica

Glossopteris Browniana.....

[Council of the Geological Society of London.]

336. Glossopteris Browniana ....

[Trustees of the British Museum.]

Vertehraria indica......

Vertehraria indica, Onoclea struthiopteris Glossopteris fronds attached to rhizome.

341. Glossopteris indica, O. angustifolia .

[Trustees of the British Museum.]

Glossopteris axigustifolia var. taeniopteroides .

[Council of the Geological Society.]

Blechnoxylon talbragarense ....

Glossopteris retifera ...

[Trustees of the British Museum.]

Gangamopteris cyclopteroides ....

[Trustees of the British Museum.]

Arheria sp. nbsp;nbsp;nbsp;�

Leslya simplicinervis.....

Neuropteridium validum . nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;,

[Trustees of the British Museum.]

Neuropteridium intermedium, ....

Cardiopteris frondosa nbsp;nbsp;nbsp;.....

Gunnera manicata ......

Spherwpteris ohtusiloha, Pecopteris arhoreacens, Sphenopteris furcatanbsp;nbsp;nbsp;nbsp;.......

Sphenopteris afiinis......

Pcdmatopteris, Mariopteris, Diplotmema Zeilleri, Neuropteris macrophylla, N. heterophylla, N. Scheuchzeri, Alloiopteris

Essinghii ..........

Cephalotheea mirahilis........

Thinnfeldia odontopteroides, Ptilosamites

[Council of the Geological Society.]

Thinnfeldia odontopteroidea.....

[Council of the Geological Society.]

Thinnfeldia odontopteroidea .....

[Annals of the South African Museum.] Thinnfeldia rhomboidalia .nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;.

Lomatopteria jnrenaia, L. Schimperi-, Thinnfeldia rhomboidalia Ptilozamitea Heeri .

Ctenopteris cycadea Pichopteria viaianicanbsp;Alethopteria lonchitica, Mariopteria muricata, Odontopteris cf.nbsp;alpina

Odontopteris minor Odontopteris gemdna, Callipteridimn gigas, Callipteria Pellati,nbsp;C. lyratifolianbsp;Callipteria conferta .

Arohaeopteris hihernica Archaeopteris hihernica, A. archetypus, A.fissilia, A. fimhriatanbsp;Neuropteria with Cyclopteris leaflets

[From a block received from Mr Carruthera.] Neuropteria heterophylla .

Neuropteris macrophylla Neuropteria Scheuchzeri .

Linopteria neuropteroides Alethopteria Berliinbsp;Peoopteris arboreacena

ERRATA IN VOL. I

Page 16, line 4. For �The North American Tulip tree� read The Tulip tree of North America and China.

� nbsp;nbsp;nbsp;127, line 3 and 4 from bottom. For Ackyla and Palaeackyla read

� nbsp;nbsp;nbsp;145, lines 4 and 5. For � Upper Greensand � read Lower Eocene.

� nbsp;nbsp;nbsp;162, line 3 from bottom. For � Corallina barbata� read Cymopolia

� nbsp;nbsp;nbsp;170, line 20. For � sporangiaphore � read sporangiophore.

� nbsp;nbsp;nbsp;191, line 11 from bottom. Omit Chondrus crispus, which is one of

CHAPTER XIP.

The account of the Sphenophyllales given in the first volume ^ of this work roust be extended and somewhat modifiednbsp;in the light of recent work on the fertile shoots of hpheno-phyllum.

Sphenophyllostachys Dawsoni (Will.) was described as consisting of an axis bearing superposed whorls of bracts connate at the base in the form of a shallow funnel-shaped collar givingnbsp;off from the upper surface and close to the Axis of the cone twonbsp;concentric series of sporangiophores. Occasionally there arenbsp;three series, as represented in fig. 112. In another cype ofnbsp;strobilus, Sphenophyllostachys B�meri^ each sporangiophorenbsp;terminates in two pendulous sporangia (fig. 113, A; see also fig.nbsp;107, C, vol. I.). It has already been pointed out that the commonnbsp;occurrence of detached strobili necessitates their descriptionnbsp;under distinct specific names; it is only by a rare accidentnbsp;that we can assign fossil cones to their vegetative shoots. Therenbsp;nre, however, reasons for believing that Sphenophyllostachysnbsp;Bawsoni is the strobilus of the plant originally described bynbsp;Sternberg¹ from impressions of foliage-shoots as Rotulana cunei-folia. Another difficulty,presented by petrified material is thatnbsp;of determining, with certainty, whether two imperfect specimens,nbsp;differing from one another in features which do not appear to

nbsp;nbsp;nbsp;The full titles of books and papers referred to in footnotes distinguished

by the addition of A after the date are given in the Bibliography at the end of Volume I.

be of sufficient importance to warrant specific separation, are forms of one species or portions of specifically distinct cones.nbsp;It has been pointed out by Scott ^ that the strobilus known asnbsp;Sphenophyllostachys Dawsoni probably includes two distinctnbsp;species, one being the cone of Sphenophyllum cuneifoliumnbsp;Sternb., and the other the cone of S. myriophyllum Cr�p-.nbsp;The stem of S. myriophyllum agrees anatomically with the typenbsp;known as Sphenophyllum piurifoliatum Will, and Scott�.

In addition to the two types of cone already mentioned, Sphenophyllostachys Dawsoni and S. R�meri, others havenbsp;been described by Kidston from carbonised impressions. Onenbsp;of these is the fertile branch of Sphenophyllum majus*. Thenbsp;basal portions of the bracts of each whorl form a narrow collarnbsp;round the axis of the cone; the free portion of each bractnbsp;consists of a lamina divided into two equal bifid lobes bearing

on. its upper surface one group, or possibly two groups, of four sessile sporangia between the narrow coherent bases of thenbsp;laminae and the sinus between the terminal lobes (fig. 113, C).nbsp;Another characteristic feature is the greater length of thenbsp;internodes; this renders the cone less compact and less sharplynbsp;differentiated from the vegetative shoots than those of othernbsp;species. A specimen in Dr Kidston�s collection illustrates the

peculiar character of the fertile portion of this species; it consists of an axis bearing a succession of lax sporophylls succeeded above and below by whorls of sterile leaves. In this species,nbsp;therefore, we cannot speak of a compact strobilus at the end ofnbsp;a shoot of limited growth, but of axes in which sterile andnbsp;fertile leaves are borne alternately^, a condition recalling thenbsp;1 Bower (08) p. 404, fig. 221.

alternation of foliage leaves and sporophylls in Tmesipteris and in Lycopodium Selago.

Another form of cone, also from the Middle Coal Measures, is referred by Kidston to Sphenophyllum trichomatosum Stur^nbsp;(fig. 118, B): this is characterised by the more horizontalnbsp;position of the bracts, which � do not appear to be so much ornbsp;so suddenly bent upwards in their distal portion as in somenbsp;other species of Sphenophyllum,� and by sessile sporangianbsp;borne singly on the upper face of each bract.

A more recent addition to our knowledge of the fertile shoots of Sphenophyllum is due to Scott who has described anbsp;new type of cone under the name Sphenophyllum fertile�s. The

Fia. 114. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a node in longitudinal section, showing one sporophyll and the base of thenbsp;opposite one. v.l. ventral lobe of sporophyll; v.s. one of the segments intonbsp;which it divides; v.s'. stamp of another segment; d.l. dorsal lobe;nbsp;d.s., d.s'. segments of dorsal lobe.

petrified specimen on which the species was founded was discovered by Mr James Lomax in the Lower Coal Measures ofnbsp;Lancashire; it represents a portion of a cone 6 cm. long andnbsp;approximately 12 mm. broad. The axis contains a singlenbsp;vascular cylinder agreeing in essentials with the type of stemnbsp;structure known as Sphenophyllum plurifoliatum. The nodalnbsp;regions, which exhibit the slight swelling characteristic of thenbsp;genus, bear several (probably twelve) appendages connate at

the base and forming a narrow flange encircling the axis. Each bract, the base of which forms part of the narrow collarnbsp;surrounding the axis, consists of two lobes, ventral and dorsal,nbsp;divided palmately into several (sometimes four) segments ornbsp;sporangiophores (fig. 115). Each sporangiophore terminatesnbsp;distally in an oblong or oval lamina bearing two sporangia onnbsp;its adaxial face (fig. 114). The space between the axis and thenbsp;periphery of the cone is thus occupied by crowded peltatenbsp;laminae, each with its pair of sporangia. A single vascularnbsp;bundle supplies each sporangiophore and bifurcates in the

distal lamina into two branches which extend to the bases of the sporangia. The sporangia agree in structure with those ofnbsp;other species of Sphenophyllum: the spores are of one size andnbsp;olliptical, characterised by the presence of several sharp ridgesnbsp;or flanges encircling the spore-wall in the direction of the major-axis. Sphenophyllostachys fertilis differs from all previouslynbsp;recorded types in the absence of sterile bracts. The appendagesnbsp;of the cone-axis are all fertile, a striking contrast to thenbsp;differentiation into protective and sporangia-bearing bracts whichnbsp;constitutes a constant feature in the cones of Sphenophyllumnbsp;and Catamites. It is possible, as Scott suggests, that thenbsp;absence of sterile segments is the result of modification of the

more usual type of strobilus; instead of the dorsal and ventral lobes of the bracts sharing between them the duties of protection and spore-production, the whole of each bract is constructednbsp;on the plan of the maximum spore-output, the laminar terminations of the sporangiophores serving the purpose of protection.nbsp;The cone may be described as more specialised than the normalnbsp;type of strobilus for reproductive purposes^.

It has been stated, on evidence which is unsatisfactory, that Sphenophyllum possesses two kinds of spores. While regardingnbsp;the genus as homosporous on the evidence before us, it is

interesting to find that cases occur in which the spores in the same sporangium exhibit a marked difference in size. Attentionnbsp;has been called by Williamson and Scott ^ to variation in thenbsp;dimensions of spores: a more pronounced difference in size hasnbsp;been recorded by Mr Thoday� who gives 120/a as the maximumnbsp;and 90/4 as the minimum diameter of the spores in a cone ofnbsp;Sphenophyllostachys Dawsoni. The presence of several abortivenbsp;spores in the sporangium (fig. 116) containing the largernbsp;spores favours the view that this difference in size may be thenbsp;first step towards the development of heterospory.

It is clear that the types of strobilus designated Sphe phyllostachys (figs. 112�114) present a divergence of charactersnbsp;too great to be comprised under one genus; but in the absencenbsp;of fuller information, we cannot do otherwise than follow thenbsp;only logical custom of grouping them together as examples ofnbsp;strobili borne by plants which, in the present state of ournbsp;knowledge, are most conveniently referred to the genus Spheno-

phyllum.

This generic name was applied by Dr Scott* to a calcified cone obtained by Mr James Bennie in 1883 from the Lowernbsp;Carboniferous plant-beds of Pettycur near Burntisland on thenbsp;Firth of Forth. Cheirostrohns is distinguished from Spheno-phyllostachys by its greater breadth (3�5 cm.); externally itnbsp;agrees more closely with the fertile shoots of Lepidodetuironnbsp;than with those of Sphenophyllum. A single vascular cylindernbsp;having the form of a fluted Doric column (fig. Hf, B, x)nbsp;occupies the axis of the cone; it consists for the most part ofnbsp;reticulate tracheae which tend to assume a short or isodiametricnbsp;form in the central region; the smaller protoxylem tracheidsnbsp;with the spiral form of pitting constitute the sharp andnbsp;prominent ridges at the periphery of the xylem-cylinder. Innbsp;the outer part of the cylinder the metaxylem ^ consists exclusively of tracheae, but towards the centre of the axis these arenbsp;associated with numerous parenchymatous cells.

The xylem is therefore centripetal in origin as in Spheno-phyllum and in nearly all recent and fossil members of the Lycopodiales. In the type-specimen of Cheirostrobus thenbsp;vascular cylinder of the cone consists entirely of primary xylem,nbsp;but secondary xylem has been found in a more recently discovered specimen�. Secondary xylem occurs also in thenbsp;peduncle of the cone. No appreciable remains of phloem have

^ The term metaxylem may be conveniently applied to the primary xylem other than protoxylem; the latter is usually but by no means invariablynbsp;characterised by spiral thickening bands.

been found. The cortex consists of slightly elongated rather thick-walled tissue containing secretory sacs. Crowded superposed whorls of bracts (or sporophylls), usually twelve in eachnbsp;whorl, are borne on the axis and each sporophyll receives anbsp;single vascular bundle from one of the vertical ridges of thenbsp;xylem column (fig. If?, A, U). The members of each whorlnbsp;are connate at the base: from this narrow collar each sporophyll

branches into an upper or dorsal and a lower or ventral limb (fig. 117, A,/and s). Each limb divides palmately at a shortnbsp;distance from its origin into three slender segments, whichnbsp;extend in a horizontal direction and terminate in large laminarnbsp;expansions (fig. 117, B, s) to afford a protective covering to thenbsp;surface of the cone. The upper set of three segments, consti-

tuting sporangiophores (fig. 11^, A, B, �) or fertile div^ions of the sporophyll, expand distally into comparatively bulkynbsp;laminae; each of these bears on its adaxial face four diagonallynbsp;placed outgrowths which form the short pedicels of very longnbsp;and narrow sporangia. The three lower segments-the sterilenbsp;divisions of the sporophylls-(fig. HV, A, B, s) are similar tonbsp;the upper set except in their greater length and in the kiteshaped form of their distal laminae which are provided withnbsp;lateral lobes. The single vascular strand which supplies eachnbsp;sporophyll is represented at in fig. 117, B; at It' the strandnbsp;has divided into four, the three upper bundles in the figurenbsp;��Pply the sterile segments and the single lower bundlenbsp;ultimately divides into three which supply the fertile segments.

A pair of blunt processes (fig. A, s) extend downwards over the ends of the underlying fertile lamina and two slender prolongations extend upwards through several internodes.

An economical arrangement of the long and narrow sporangia and of the sporophyll-segments between the axis and thenbsp;periphery of the cone is rendered possible by the interlockingnbsp;of the sterile and fertile segments by means of a groove in thenbsp;upper face of the latter for the accommodation of the former.nbsp;The sporangia are characterised by their unusually long andnbsp;narrow form; the length of a sporangium may reachnbsp;1 centimetre. In the structure of the wall the sporangia ofnbsp;Gheirostrohus agree closely with those of Calamostachys^ andnbsp;^ph^nophyllostachys. The spores are of one size only. Thenbsp;vascular cylinder of the peduncle, originally described bynbsp;Williamson^ as the peduncle of a large Lepidostrobus (the conenbsp;of Lepidodendron), is characterised by the presence of a shortnbsp;radially disposed zone of secondary tracheids, a feature, asnbsp;Scott points out, which may extend into the axis of the cone.nbsp;It is noteworthy that the protoxylem elements are not alwaysnbsp;external, but occasionally occur internal to one or two of thenbsp;outermost metaxylem tracheae; the usual exarch� structure of

� Exarch � denotes that the protoxylem is on the outside of the primary xylem ; � endarch' that it is on the inner edge or in a central position; � mesarch �nbsp;that it is internal, either near the inner or the outer edge of the metaxylem.

the central cylinder is not therefore absolutely constant, but may be replaced by a mesarch arrangement.

The presence of a few sterile leaves on the peduncle below the fertile portion of the cone, which agree in their lobednbsp;laminae with the sporophylls, is the only fact which we possessnbsp;as to the form of the vegetative characters of the genus.

The above description is sufficient to indicate the extraordinary complexity and high degree of specialisation of Cheirostrobus. The sporophylls, with their trilohed segments,nbsp;and the crowded sporangia of exceptional length attached onlynbsp;by a narrow base constitute striking peculiarities of the genus.

It is unfortunate that we are still without any satisfactory evidence as to the nature of the plant the cones of which havenbsp;been made the type of a new genus and a new family.nbsp;Cheirostrobus affords an interesting example of a type ofnbsp;reproductive shoot constructed on a plan sui generis, and maynbsp;be classed with some other extinct genera as instances of thenbsp;production in the course of evolution of architectural schemesnbsp;which appear to have been ill adapted for competition withnbsp;equally efficient though much simpler types. But the discoverynbsp;of these isolated forms of restricted geological range amongnbsp;the relics of the Palaeozoic vegetation frequently supplies anbsp;key to phylogenetic problems. Cheirostrobus by its complexnbsp;combination of features characteristic of the Equisetales, thenbsp;Lycopodiales and the genus Sphenophyllum throws a welcomenbsp;light on the inter-relationships of groups which representnbsp;divergent series. The combination of morphological featuresnbsp;in this generalised type led the author of the genus to describenbsp;it as a descendant of an old stock which existed prior to thenbsp;divergence of the Equisetales and Lycopodiales.

The discovery of this new type of strobilus naturally led to a search among Lower Carboniferous plants for vegetative shootsnbsp;exhibiting characters conformable with the whorled and branchednbsp;leaves of Cheirostrobus. In Sphenophyllum we have a genusnbsp;obviously comparable with Cheirostrobus as regards the formnbsp;and disposition of the leaves, hut the differences between thenbsp;cones and the striking similarity of the vascular cylinder of thenbsp;latter to that of Lepidodendron demonstrate conclusively that

we xnust look elsewhere for the vegetative members of the plant which produced cones of the Cheirostro unbsp;nbsp;nbsp;nbsp;^ name

branched with fine serrated edges (ng. gt; nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;.

leaves with sporangia on their lower surfaces, u � �nation is not sufficiently well preserved to render possi e anbsp;of structural details. It has been suggested y oonbsp;Pseudobornia may possibly be referable to the p enopnbsp;and that the stem of Cheirostrobus �may have anbsp;in common with� Nathorst�s genus*. The beds in wnbsp;stems occur are of Upper Devonian age, whi enbsp;was found in Lower Carboniferous rocks; thisnbsp;age is not, however, a serious objection to the va i i y onbsp;comparison. We cannot do more than express t e viewnbsp;Pseudobornia, so far as can be ascertained without an enbsp;nation of petrified material or of more perfect imprenbsp;strobili, exhibits vegetative features not inconsistent winbsp;morphological characters of the fertile shoots nownnbsp;Cheirostrobus.

The institution of a special group-name for the receptio Sphenophyllum is justified by the sum of its morp o ogicnbsp;features, which do not sufficiently conform to thosenbsp;existing group of Pteridophytes to warrant its inc usion mnbsp;system of classification based on recent genera. In the casenbsp;Cheirostrobus we are limited to the characters of the cone

its peduncle. The suggestion that the Devonian fossils known as Pseudohornia may represent the foliage shoots of a plantnbsp;closely related to Gheirostrobus has still to be proved correct.nbsp;Although we may find justification in the highly complex andnbsp;peculiar structure of Gheirostrobus for the recognition of thenbsp;genus as a type of still another group of Pteridophytes, it wouldnbsp;be unwise to take this step without additional knowledge.

The undoubted similarity between Gheirostrobus and Sphenophyllum coupled with striking points of difference favoursnbsp;the inclusion of the two genera in distinct families placed, fornbsp;the present at least, in the group Sphenophyllales.

It has recently been proposed to include the family Psilotaceae, comprising the two recent genera Psilotum andnbsp;Tmesipteris, as another subdivision of the Sphenophyllales.nbsp;This proposal had been made by Professor Thomas^ primarilynbsp;on the ground that the sporophylls of Tmesipteris and Psilotumnbsp;appear to afford the closest parallel among existing plants tonbsp;the peculiar form of sporophyll characteristic of the Sphenophyllales. The morphological interpretation of the sporophyllsnbsp;of both Sphenophyllum and Gheirostrobus has been the sourcenbsp;of considerable discussion^. If we regard each sporophyll as anbsp;leaf with two lobes, one fertile and one sterile, except in thenbsp;case of Sphenophyllostachys fertilis in which both are fertile, annbsp;obvious comparison may be made with the fern Ophioglossum;nbsp;but the difference between a single fern frond, consisting of anbsp;comparatively large sterile lamina bearing a fertile branchnbsp;composed of a long axis with two rows of sporangia embeddednbsp;in its tissues, and the whorled sporophylls of Sphenophyllum isnbsp;considerable.

A brief reference may be made to the principal reasons which have led to the suggestion that the Psilotaceae should be included

(fig. 120, B). The synangium is seated on y given off from its sporophyll atnbsp;nbsp;nbsp;nbsp;of as the

is prolonged and forms an obvious feature^ ^ correspond that the Tmesipterisnbsp;nbsp;nbsp;nbsp;^nbsp;nbsp;nbsp;nbsp;In the

phore bearing one or more sporangia, nbsp;nbsp;nbsp;. ,nbsp;nbsp;nbsp;nbsp;^ -unnhprl

sterile portion bearing a septate sporangium or bilocular synangium on a very short ventral lobe, rrnbsp;his account of the development and structure^of t e spor pnbsp;of Tmesipteris, drew attention to the comparatree y fi ^nbsp;occurrence of abnormal sporophylls and spoke � ^ � P ,nbsp;unstable. More recently Professor Thomas o uc � . �nbsp;nbsp;nbsp;nbsp;�

carefully examined living plants, with the result t a var of different kinds are proved to be exceedinglynbsp;finds that sporophylls occur which exhibit repeate inbsp;nbsp;nbsp;nbsp;,

of the axis (fig. 120, D, F) and thus each may bear four i of two leaf-lobes and three synangia, one at the rst ornbsp;one at each of the forks of the second order�.

Other abnormalities occur in which the synangium is a distinct stalk instead of being more or less sesnbsp;point from which the leaf-lobes diverge. A t ir �nbsp;departure from the normal is that in which there is no syna �nbsp;on the bilobed sporophyll, its place being taken by a eanbsp;The deduction from the occurrence of thesenbsp;nbsp;nbsp;nbsp;^nbsp;nbsp;nbsp;nbsp;r

lobe, as Scott suggested. Professor Thomas draws attention to the resemblance between Tmesipteris sporophylls and the foliage-leaves of Sphenophyllum, which are either simple with dicho-tomously branched veins or the lamina is deeply divided intonbsp;two or more segments. In some types of Sphenophyllostachysnbsp;the bracts are simple (8. Dawsoni), but in others {Sphenophyllumnbsp;majus, fig. 113, C) they are forked like the foliage-leavesnbsp;and bear a close resemblance to the abormal sporophylls ofnbsp;Tmesipteris. Moreover, in Sphenophyllostachys R�meri (fig.nbsp;113, A) each ventral lobe of a sporophyll bears two sporangia,nbsp;a condition almost identical with that represented by thenbsp;occasional occurrence of a synangium on a comparativelynbsp;long stalk in Tmesipteris. Similarly the more elaboratenbsp;sporophylls of Cheirostrobus may be compared with the branchednbsp;sporophylls of Tmesipte^'is (fig. 120). This agreement betweennbsp;the sporophylls of the Palaeozoic and recent genera acquiresnbsp;additional importance from the very close resemblance betweennbsp;the exarch stele of Sphenophyllum and that of the genusnbsp;Fsilotum, which conforms to the Palaeozoic type not only innbsp;the centripetal character of the primary xylem and in its exarchnbsp;structure, but also in the occasional occurrence of secondarynbsp;xylemand in the stellate form of its transverse section. Thenbsp;occasional mesarch structure of the stele of Cheirostrobus findsnbsp;a parallel in the mesarch xylem groups in the stem of Tmesipteris.nbsp;It is thus on the strength of these resemblances that Thomasnbsp;and Bower would remove the Psilotaceae from the groupnbsp;Lycopodiales and unite them with Sphenophyllum and Cheirostrobus in the Sphenophyllales. While admitting the validitynbsp;of the comparison briefly referred to above, I prefer to retainnbsp;the Psilotaceae as a division of the Pteridophyta including onlynbsp;Fsilotum and Tmesipteris.

In his recent book on The Origin of Land Flora, Prof. Bower raises objection to the use of the term ventral lobe in speaking ofnbsp;the sporangium-bearing stalk or sporangiophore borne on thenbsp;sporophyll of Sphenophyllum. He points out that the use ofnbsp;this term implies the derivation of the sporangiophore bynbsp;metamorphosis of part of a vegetative leaf, an opinion untenable

concession to a non-committal attitude we m y least regard a sporangiophore as an organ smgener

phyllales are descendants of primitive ferns is no ^ lacks and his comparison o{ Sphmophyllum with Anhatrypnbsp;force m view of o�r ignorioe as to the natnie of the '�Pnbsp;organs of the latte, genns. That thenbsp;nbsp;nbsp;nbsp;quot;!

existing genus Tmesipteris, though helpful P ^ornho-on true homology, cannot he considered as sett mg e logical value of the sporangiophores of Sphenop y

^'^Tdon^'propose to discuss at length the'iifferent views in regard to the morphological nature of the sporangiopnbsp;Sphenophyllum. The comparison, which we owe innbsp;instance to Scott, with the synangium of the Psi ota es wnbsp;short stalk, though not accepted by Lignier as anbsp;based on true homology, is one which appeals to manynbsp;and is probably the best so far suggested. The furthernbsp;whether these sporangiophores are to be called fo lar ornbsp;structures is one which has been answered by severanbsp;nbsp;nbsp;nbsp;�

but it is improbable that we shall soon arrive at a decision to be accepted as final. Discussions of this kind tend to assnbsp;an exaggerated importance and frequently carry wit t cmnbsp;implication that every appendage of the nature of anbsp;phore can be labelled either shoot or leaf. Wenbsp;question from an academic standpoint and run a nsnbsp;ignoring the fact that the conception of stem and lea is a ^nbsp;on morphological characteristics, which have been evo venbsp;the result of gradual differentiation of parts of one orig y

homogeneous whole. There is much that is attractive in the view recently propounded by Mr Tansley that a leaf is not annbsp;appendicular organ differing ah initio from the axis on which itnbsp;is borne, but that it is in phylogenetic origin a � branch-systemnbsp;of a primitive undifferentiated sporangium-bearing thallusb�nbsp;Admitting the probability that this view is correct, our faithnbsp;in the importance of discussions on the morphological nature ofnbsp;sporangiophores is shaken, and we realise the possibility thatnbsp;our zeal for formality and classification may lead to resultsnbsp;inconsistent with an evolutionary standpoint^.

1 nbsp;nbsp;nbsp;Tansley (08) p. 26, who refers to similar views held by Potonie and bynbsp;Hallier.

2 nbsp;nbsp;nbsp;On the morphology of Sporangiophores, see also Benson (08^) andnbsp;Scott, D. H. (09) p. 623.

CHAPTER XIII.

The two recent genera Psilotum and Tmesipteris are usually spoken of as members of the family Psxlotaceae wnbsp;included as one of the subdivisions of the Lycopodiales. isnbsp;probable, as Scott^ first suggested, that these two plants arenbsp;more nearly allied than are any other existing types to thenbsp;Palaeozoic genus Sphenophyllum.

We may give expression to the undoubted resemblances between Tmesipteris and Psilotum and the Sphenophyllales bynbsp;including the recent genera as members of that group, originallynbsp;founded on the extinct genus Sphenophyllum-, this is thenbsp;course adopted by Thomasquot; and by Bower�; or we maynbsp;emphasise the fact that these two recent genera differ innbsp;certain important respects from Lycopodium and Selaginella bynbsp;removing them to a separate group, the Psilotales. The latternbsp;course is preferred on the ground that the inclusion of Psilotumnbsp;and Tmesipteris in a group founded on an extinct and necessarilynbsp;imperfectly known type, is based on insufficient evidence andnbsp;carries with it an assumption of closer relationship than hasnbsp;been satisfactorily established.

The genus Tmesipteris (fig. 120, A) is represented by a 1^''nbsp;nbsp;nbsp;nbsp;^ tannensis Bertr. ^ which usually occurs as an

on the stems of tree-ferns in Australia, New Zealand, and Polynesia. Psilotum, with two species P- triquetrum

* Dangeard (91) and Bertrand, ( nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;-nbsp;nbsp;nbsp;nbsp;_

Sw. (fig. 118) and P. complanatum Sw., flourishes in moist tropical regions of both hemispheres, growing either on soilnbsp;rich in organic substances or as an epiphyte. Both generanbsp;are considered to be more or less saprophytic.

Psilotum. The common tropical species P. triquetrum (fig. 118) is characterised by an underground rhizome whichnbsp;forms a confused mass of dark brown branches covered withnbsp;filamentous hairs as substitutes for roots and gives off erectnbsp;repeatedly forked aerial shoots. In P. complanatum^ the habit

is similar to that of the more abundant and better-known species, but the pendulous shoots are characterised by thei�nbsp;broader and flatter form. In both species the function ofnbsp;carbon-assimilation is performed by the outer cortex of thenbsp;green branches, as the small size of the widely-separatenbsp;foliage leaves renders them practically useless as assimilatingnbsp;organs.

The sporophylls consist of a short axis terminating in two small divergent forks and bearing on its adaxial surface anbsp;trilocular or in rare cases a bilocular synangium (fig. Hd, Anbsp;and B). The walls of the loculi are composed of several layersnbsp;lt;^f cells and dehiscence takes place along three lines radiatingnbsp;from the centre of the synangium. Professor Thomas' hasnbsp;recorded �fairly numerous instances in Psilotum of a secondnbsp;dichotomy of one branch of the first fork, or, less frequently, ofnbsp;both branches�: instead of one synangium subtended by thenbsp;two slender leaflets of the forked sporophyll-axis. there may benbsp;two synangia and three leaf-lobes or three synangia and fournbsp;leaf-lobes. The occurrence of both thesd abnormalites innbsp;Psilotum and Tmesipteris shows a decided tendency in thenbsp;Psilotales to a repeated dichotomy of the sporophylls^.

A single stele� with a fluted surface occupies the axis of an aerial shoot (fig. 119, A); the axial region is occupied by anbsp;core of elongated mechanical elements (s), which may occasionally extend to the periphery of the xylem and break thenbsp;continuity of the band of scalariform tracheae (fig. 119, A, a).nbsp;The tracheae form the arms of an irregularly stellate stele andnbsp;each arm is terminated by protoxylem elements (fig. 119, B, pa;).nbsp;The rays of the xylem cylinder, which may be as many as sixnbsp;or eight in the upper part of the aerial shoots, become reducednbsp;m number as the rhizome is approached, assuming a diarchnbsp;structure near the junction. In the rhizome the xylem formsnbsp;an approximately triangular group of tracheae without anynbsp;core of mechanical elements. Three to four layers of paren-

Another form of abnormality in the sporophylls of Psilotum has recently been described by Miss Sykes. Sykes (OS^).

chyma succeeded externally by an ill-defined phloem (fig. 119, A, p) surround the xylem and a fairly distinct endodermis (fig. 119,nbsp;A and B, e) encloses the whole. To Mr Boodle^ is due the

adaxial surface an elongated bilocular synangium attached to a very short stalk (fig. 120, B). Reference has already been

-px

made to the divergent opinions as to the morphological nature of the sporophylls or sporangiophores, hut recent investigations

and a dorsal lobeh The drawings ^^�^�tnrdeSribed by and F, illustrate some of the frequennbsp;nbsp;nbsp;nbsp;Zealand

occupies an internal position nbsp;nbsp;nbsp;�nbsp;nbsp;nbsp;nbsp;of a few layers of the

a solid strand without protoxylem elements an con clearly to that of Psilotum.nbsp;nbsp;nbsp;nbsp;' n mnbsp;nbsp;nbsp;nbsp;no

In this short account of the anatomy o� Tmesip mention is made of the effect produced on t e s �nrmlvnbsp;departure of leaf-traces and of vascular stan snbsp;branches. Miss Sykes^ in a recently publis e pap , ^nbsp;genus has shown that the exit of a leaf-trace oes n �nbsp;the continuity of the xylem of the stele, whi e e enbsp;sporophyll-trace is marked by an obvious gap-nbsp;nbsp;nbsp;nbsp;�

is adduced in support of the conclusion that t is i e which at first sight appears to be one of morpholo^ca �1*nbsp;ance, is in reality merely a question of degree annbsp;nbsp;nbsp;nbsp;tPan

the earlier preparation for the formation of sporop y leaf-traces.� Miss Sykes gives her adherence to the view a

the �sporophylls� of JVnesipteris are branches and not eaves,

despite the arguments advanced this interpretation me less probable than that which recognises thenbsp;a foliar organ. Prof. Lignier� has pointed out that inbsp;Sykes�s conclusion as to the axial nature of the sporop ynbsp;Tmesipteris is accepted, it diminishes the force of t e conbsp;1 Sykes (08).nbsp;nbsp;nbsp;nbsp;^ iud. (08).nbsp;nbsp;nbsp;nbsp;* Liguier (08).

parison between the sporophylls of that genus and Spheno-phyllum as those of the latter can hardly be regarded as other than foliar organs.

Both members of the Psilotales may, as Boodle has suggested, be regarded as descendants of a common parent innbsp;which the aerial stems possessed a fluted or stellate cylinder ofnbsp;mesarch xylem. There can be no doubt as to the significancenbsp;of the morphological resemblances between the Psilotales andnbsp;the genera Sphenophyllum and Cheirostrobus, but the positionnbsp;of Tmesipteris and Psilotum in the plant-kingdom may probablynbsp;be best expressed by adopting the group-name Psilotales rathernbsp;than by transferring the recent genera to the Sphenophyllales.nbsp;One of the most striking differences between the Psilotales andnbsp;the genus Lycopodium is in the form of the sporophylls andnbsp;sporangia; in Lycopodium a single sporophyll bears a unilocularnbsp;sporangium, but in the Psilotales the sporophyll may benbsp;described as a bilobed structure homologous with a foliage-leaf, bearing a sporangiophore which consists of a short stalknbsp;terminating in a bilocular or trilocular synanguim; the shortnbsp;stalk receives a special branch from the vascular bundle of thenbsp;sterile portion of the sporophylP.

A search through palaeobotanical literature reveals the existence of a very small number of specimens which have beennbsp;identified as representatives of the Psilotales. An inspection ofnbsp;the material or published drawings leads one to the conclusionnbsp;that practically no information of a satisfactory kind isnbsp;available in regard to the past history of the two southernnbsp;genera Psilotum and Tmesipteris, which are regarded by somenbsp;botanists as relics of an ancient branch ^ of pteridophytes.

In 1842 M�nster^ instituted the genus Psilotites for a small impression of a slender branched axis from Jurassic rocks nearnbsp;Mannheim in Germany which he named Psilotites filiformis;

agree. Goldenbergs species quot; nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;pjjg of axes:

the Saarbr�cken coal-field is founded on i p some of these are dichotomonsly branche anbsp;nbsp;nbsp;nbsp;g^bly leaf-

and S. Alleni Lesq.^ a Tertiary species, a their very close resemblance in form, nerva ion,

with a midrib and numerous anastomosing a ^ whereas in Tmesipteris the lamina of the Teanbsp;nbsp;nbsp;nbsp;outlines

without lateral branches. An enlarged drawing o e of the epidermal cells would correspond closely winbsp;reticulations in the fossil leaves and it may be t anbsp;been some confusion between veins and cell-out ^ recentnbsp;case there would seem to be no reason for the use onbsp;generic name�.nbsp;nbsp;nbsp;nbsp;.

Among other fossils assigned to the Psdota es w Marion�s genus Gomphostrobus from the Permian o mnnbsp;Germany^ Marion placed this plant in the Conifera es onbsp;strength of its resemblance to Walchia and Araucan �nbsp;Potoni� is inclined to recognise in the leaves and monospnbsp;sporophylls characters suggestive of Lycopodiaceous a

� Lesquereux (78) PL v. fig. 11. nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;, � labelled

� Since this -was written I have had an opportunity of seeing � recently Tmesipteris from the Tertiary plant-beds of Elorissant in a oo ec ^nbsp;nbsp;nbsp;nbsp;^

acquired by the British Museum : the specimen bears no resem ane of the recent genus.nbsp;nbsp;nbsp;nbsp;wxtii

The latter author in 1891 h in ignorance of Marion�s proposal to adopt the name Gomphostrobus, instituted a genus Psiloti-phyllum for the sporophylls of a species originally described bynbsp;Geinitz^ as Sigillariostrohus bifidus, but he subsequently adoptednbsp;Marion�s designation and with some hesitation included thenbsp;French and German specimens in the Psilotales. As statednbsp;elsewhere�, Potoni�s arguments in favour of his view hardlynbsp;carry conviction, and it is probably more in acdordance withnbsp;truth to deal with Gomphostrobus in the chapter devoted tonbsp;the Coniferales.

The generic title Psilophyton, instituted by the late Sir William Dawson^, has become familiar to geologists as that ofnbsp;a Pre-Carboniferous plant characteristic of Devonian andnbsp;Silurian rocks in Canada, the United States of America, andnbsp;Europe. From the botanist�s point of view the name standsnbsp;for miscellaneous remains of plants of different types and innbsp;many cases unworthy of record. The genus was founded onnbsp;impressions of branched axes from the Devonian strata of Newnbsp;Brunswick resembling the rachis and portions of lateral pinnaenbsp;of ferns or the forked slender twigs of a Lycopod. The type-species Psilophyton princeps Daws, as represented on somewhatnbsp;slender evidence in Dawson�s restoration, which accompanies thenbsp;original description of the genus and has since been copied bynbsp;several authors, is characterised by the possession of a horizontalnbsp;rhizome bearing numerous rootlets and giving off dichotomouslynbsp;branched aerial shoots with spinous appendages, compared withnbsp;rudimentary leaves, and terminating in slender branchletsnbsp;bearing pendulous oval � spore-cases � from their tips. Some ofnbsp;the branchlets exhibit a fern-like vernation. The plant isnbsp;spoken of by Dawson as apparently a generalised type�, resembling in habit and in its rudimentary leaves the recentnbsp;genus Psilotum and presenting points of contact with ferns.

Specimens were found in an imperfectly petrified state showing a central cylinder of scalariform tracheae surroun e y a ronbsp;cortical zone of parenchyma and fibrous tissue. �

Among other species described by the author o t e we need only mention Psilophyton robustius, characterised bynbsp;vegetative shoots and � spore-cases � similar to those ^nbsp;species ; but, as Solms-Laubach' has pointed out, the petnhednbsp;sections referred by Dawson to P. robustius^ are of an entire ynbsp;different anatomical type from that of P. princeps .

British fossils from the Old Red Sandstone from t e nor of Scotland, Orkney and Caithness, originally figured by Hugnbsp;Miller and compared by him with algae but more espema ynbsp;with recent Lycopods, were subsequently placed by Carrut ersnbsp;in the genus Psilophyton as P. Dechianum, the specific designation being chosen on the ground that the Scotch specimens^arenbsp;specifically identical with fossils described by Goeppert* asnbsp;Haliserites Dechianus.nbsp;nbsp;nbsp;nbsp;,

Various opinions have been expressed in regard to the nature of the Devonian species Haliserites Dechianus Goepp.nbsp;with which Carruthers^ identified Miller�s Old Red Sandstonenbsp;plant: reference may be made to a paper by White� containingnbsp;figures of dichotomously branched impressions described asnbsp;species of Thamnocladus which he includes among the algae.

In describing some Belgian impressions of Devonian age as Depidodendron gaspianum Daws. Cr�pin'^ states that Carruthersnbsp;has come to regard the specimens named by him Psilophytonnbsp;Dechianum as branches of a Lepidodendron; he also quotesnbsp;_ Carruthers as having expressed the opinion that the namenbsp;Psilophyton had been employed by Dawson for two kinds ofnbsp;fossils, some being twigs of Lepidodendron while others, identi-by Dawson �nbsp;nbsp;nbsp;nbsp;...nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;-j?

8 mm. broad, bears a close resemblance to a fern rhizome covered with ramental scales such as that of a species ofnbsp;Davallia. Other Belgian specimens described by GilkineO asnbsp;Lepidodendron hurnotense, like Crepin�s species, are no doubtnbsp;generically identical with some of the Scotch and Canadiannbsp;fossils placed in the genus Psilophyton, though Penhallow^nbsp;considers that the species Lycopodites Milleri is more correctlynbsp;referred to Lycopodites than to Psilophyton.

A more recent paper on the Geology of the Perry basin in South-eastern Maine by Smith and White� contains a criticalnbsp;summary of the literature on Psilophyton and drawings ofnbsp;specimens. The latter afford good examples of Pre-Carboniferousnbsp;plant fragments, such as are often met with in various partsnbsp;of the world, which conform in habit to the New Brunswicknbsp;specimens made by Dawson the type of his genus.

An examination of material in the Montreal Museum and of Hugh Miller�s specimens in the Edinburgh collection leads menbsp;to share the opinion of Count Solms-Laubach that the namenbsp;Psilophyton has been applied to plants which should not benbsp;included under one generic title. As Kidston^ pointed out,nbsp;the Canadian species Psilophyton robustius is not genericallynbsp;distinct from British and Belgian specimens referred to Lepidodendron] it may possibly be identical with the Bohemian plantsnbsp;on which Stur founded his genus Hostinella^. The Devoniannbsp;plants described by Stur have since been examined by Jahn�nbsp;who regards them as vascular plants, and not as algae to whichnbsp;Stur referred them; he mentions two species of Psilophytonnbsp;but gives no figures.

The � spore-cases � of Dawson may be found to be the micro-sporangia or perhaps the small seeds of some pteri-dosperm; the forked axes with a smooth surface and othersnbsp;figured by Miller and by Dawson, with the surface covered withnbsp;scales suggesting the ramenta of a fern, may be the rachises ornbsp;rhizomes of filicinean plants. Other specimens may be Lepidodendron twigs, as for example the petrified fragments figured by

^ Smith and White (05) p. 58, Pis. v. vi. nbsp;nbsp;nbsp;^ Kidston (86^) p. 232.

Dawson as Psilophyton princeps; while the stem identified as �f*- 't'ohustius is most probably that of a Gymnosperm. It isnbsp;doubtful whether a useful purpose is served by retaining thenbsp;genus Psilophyton. It was in the first instance instituted onnbsp;the assumption, which cannot be upheld, that the abundantnbsp;niaterial in the New Brunswick beds bore a sufficiently closenbsp;resemblance to the rhizome and aerial branches of Psilotum.

ophyton has served as a name for miscellaneous plant ragments, many of which are indeterminable. Dr Whitenbsp;concludes his account of the sfenus with the following words^;

^ ^ ne examination of such so-called Psilophyton material as �ce seen shows the existence in America of two or morenbsp;groups, represented by several fairly well-marked species whichnbsp;possess stratigraphical value, and which should be carefullynbsp;^n^osed and illustrated. It is probable also that additionalnbsp;thesT^*^^ throwing light on the structure and relationships ofnbsp;disconbsp;nbsp;nbsp;nbsp;remarkable early types of land-plants will be

hand^^'�^'^ �cine locality. The inspection of the material in I aubnbsp;nbsp;nbsp;nbsp;tbe need , as was pointed out by Solms-

^^6 revision of the material referred by various .,�1nbsp;nbsp;nbsp;nbsp;^si'i'ophyton, together with a thorough re-examination

Canad' ^ ^ thorough re-examination has been made of the exist Vinbsp;nbsp;nbsp;nbsp;with a view to determine whether there

continue to make use of this name for Pre-with ^ fossils which are too incomplete to be assigned attent'^^^^^^*quot;^nbsp;nbsp;nbsp;nbsp;^ definite group of plants. Dr White draws

Island ^ genus Cephalotheca^ from Devonian rocks of Bear extendnbsp;nbsp;nbsp;nbsp;-Arctic regions, a comparison which might be

tvnp�'^u I recognised as examples of well defined generic e onging to more than one group of plants.

CHAPTEE XIV.

The recent members of the Lycopodiales are considered apart from the extinct genera in order that our examination ofnbsp;the latter may be facilitated by a knowledge of the salientnbsp;characteristics of the surviving types of this important sectionnbsp;of the Pteridophyta. A general acquaintance with the extinct asnbsp;well as with the recent genera will enable us to appreciate thenbsp;contrasts between the living and the fossil forms and to realisenbsp;the prominent position occupied by this group in the Palaeozoicnbsp;period, a position in striking contrast to the part played bynbsp;the diminutive survivors in the vegetation of the present day.nbsp;In the account of the recent genera special attention is drawnnbsp;to such features as afford a clue to the interpretation of thenbsp;fossils, and the point of view adopted, which at times maynbsp;appear to lead to an excessive attention to details, is necessarilynbsp;somewhat different from that represented in botanical textbooks'.

A. HOMO SPORE AE.

B. HETEROSPOREAE.

The existing plants included in the Lycopodiales are in nearly all cases perennial herbaceous pteridophytes, exhibiting

' For a general account of recent Lycopodiales see Pritzel (02); Campbell (05); Bower (08).

in their life-histories a well marked alternation of generations. The sporophyte (asexual generation) is characterised by thenbsp;relatively small size of the leaves except in the genus Isoetesnbsp;(fig. 182) and in the Australian and New Zealand genusnbsp;Phylloglossum. The stems are usually erect or trailing, pendulous in epiphytic species or small and tuberous in Isoetesnbsp;and Phylloglossum. The repeated forking of the shootsnbsp;(monopodial and dichotomous branching) is a prominent featurenbsp;of the group. The vascular tissue of the stem usually assumesnbsp;the form of a single axial strand (stele) (fig. 125), hut thenbsp;shoots of some species of Selaginella often contain two or morenbsp;distinct steles (fig. 131). The group as a whole is characterisednbsp;by the centripetal development of the xylem composed almostnbsp;entirely of scalariform tracheids: secondary xylem and phloemnbsp;of a peculiar type occur in Isoetes, and the production of secondarynbsp;xylem elements in a very slight degree has been noticed in onenbsp;species of Selaginella (S. spinosay. The roots are constructed onnbsp;a simple plan, having in most cases only one strand of spiralnbsp;protoxylem elements (monarch structure). In Lycopodium, innbsp;which stem and root anatomy are more nearly of the same typenbsp;than in the majority of plants, several protoxylem strands may benbsp;present. The sporangia are axillary or, more frequently, bornenbsp;on the upper surface of sporophylls, which are either identicalnbsp;with or more or less distinct from the foliage leaves; in thenbsp;latter case the sporophylls often occur in the form of a wellnbsp;defined strobilus (cone) at the tips of branches.

The gametophyte (sexual generation) is represented by prothalli which, in the homosporous genera, may live underground as saprophytes, or the upper portion may developnbsp;chlorophyll and project above the surface of the ground as annbsp;irregularly lobed green structure {e.g. Lycopodium cernuumy.nbsp;In the heterosporous forms the prothalli are much reduced andnbsp;do not lead an independent existence outside the spore by thenbsp;niembrane of which they are always more or less enclosed. Thenbsp;sexual organs are represented by antheridia and archegonia;

the male cells are provided with two cilia except in Isoetes which has multiciliate antherozoids like those of the ferns.

The existing Lycopods, though widely distributed, never grow in sufficiently dense masses to the exclusion of othernbsp;plants to form a conspicuous feature in the vegetation of anbsp;country. The inconspicuous r�le which they play among thenbsp;plant-associations of the present era affords a striking contrastnbsp;to the abundance of the arborescent species in the Palaeozoicnbsp;forests of the northern hemisphere.

Lycopodiaceae. Lycopodium, represented by nearly 100 species, forms a constituent of most floras: epiphytic speciesnbsp;predominate in tropical regions, while others flourish on thenbsp;mountains and moorlands of Britain and in other extra-tropicalnbsp;countries. For the most part Lycopodium exhibits a preferencenbsp;for a moist climate and appears to be well adapted to habitatsnbsp;where the amount of sunlight is relatively small and thenbsp;conditions of life unfavourable for dense vegetation. Mountainsnbsp;and islands constantly recur as situations from which speciesnbsp;have been recorded. Some species are essentially swamp-plants, e.g. Lycopodium immdatum, a British species, andnbsp;L. cruentum from the marshes of Sierra Nevada. A varietynbsp;of the American species, L. alopecuroides (var. aquaticum)nbsp;affords an instance of a submerged form, which has beennbsp;collected from an altitude of 12�14,000 ft. on the Andes andnbsp;Himalayas. It is noteworth}^ that a considerable variety ofnbsp;habitats is represented within the limits of the genus and thatnbsp;many species are sufficiently hardy to exist in circumstancesnbsp;which would be intolerable to the majority of flowering plantstnbsp;The British species frequently spoken of as Club Mosses,nbsp;include Lycopodium Selago, L. annotinium, L. clavatum, L. al-pinum, and L. inundatum.

Selaginellaceae. The species of Selaginella, over 300 in number, are widely spread in tropical and subtropical forests,nbsp;growing on the ground with trailing, suberect or erect stemsnbsp;climbing over taller and stouter plants or as pendulous epiphytesnbsp;on forest trees.

shoots during periods of drought and '^?�^^rast to those which species adapted to conditions in mavke

type apart, iffering in habit at in eertain o�� f the other members of the Lycopodia es.nbsp;nbsp;nbsp;nbsp;balance

of evidence, including resemblances between soe es ^ Lycopodiaceous plants, would seem to favour ite re �nbsp;aberrant genus of the group Lycopodiales. i-bomenbsp;permanently submerged, others occur innbsp;nbsp;nbsp;nbsp;^ g^jp

and elsewhere in Central and Northern Europe America. Isoetes hystrix*, a land-form occurs mnbsp;North-East France, Spain and Asia Minor.

Drummondii of Australia and New Zealand, thoug lu 1 �u from the point of view of its probable claim to e co �

not be dealt with in detail. A complete individual, w^cti does not exceed 4 or 5 cm. in length, consists of a verynbsp;tubercle or protocorm hearing a rosette of slen er snbsp;nbsp;nbsp;nbsp;_

leaves and prolonged distally as a simple naked axis w ic tops the foliage leaves and terminates in a compact c

1 The Bose of Jericho is Amstatiea Hierochuntina L. a Crucifra Baker (87) A. p. 34.nbsp;nbsp;nbsp;nbsp;^ Xmes (88).nbsp;nbsp;nbsp;nbsp;^ Scott and HiU (00).

Lycopodium. It would be out of place in a volume devoted mainly to fossil plants to attempt a comprehensive account ofnbsp;the general morphology of recent species, and indeed ournbsp;knowledge of the anatomical characters of the genus is stillnbsp;somewhat meagre. For purposes of comparison with extinctnbsp;types, it is essential that some of the more important morphological features of existing species should be .briefly considered.nbsp;The additions made to our knowledge of the gameophyte^nbsp;of European and tropical species during the last two decadesnbsp;have revealed a striking diversity in habit.

In several species, grouped round the widely distributed type Lycopodium Selago Linn., the comparatively short, erect ornbsp;suberect, shoots form fairly compact tufts; the ordinary foliage-leaves function as sporophylls, and the sporangia are not localisednbsp;on special portions of shoots. From this type, we pass to othersnbsp;in which the fertile leaves tend to be confined to the tips ofnbsp;branches, but hardly differ in form from the sterile. A furthernbsp;degree of specialisation is exhibited by species with well-definednbsp;cones composed of leaves (or bracts), the primary function ofnbsp;which is to bear sporangia and to afford a protective coveringnbsp;to the strobilus^

Lycopodium rufescens Hook. An Andian species with stout dichotomously branched erect stems bears on the youngernbsp;shoots crowded leaves with their thick and broadly triangularnbsp;laminae pointing upwards, but on the older and thick shootsnbsp;the laminae are strongly reflexed (fig. 121, A). The lower partnbsp;of the specimen represented in fig. 121, A, shows tangentially elongated scars and persistent leaf-bases or cushionsnbsp;left on the stem after the removal of the free portions of thenbsp;leathery leaves, a surface-feature which also characterises thenbsp;Palaeozoic genus Lepidodendron. The reflexed leaves andnbsp;persistent leaf-cushions are clearly seen in the piece of old stemnbsp;of Lycopodium dichotomum Jacq., a tropical American species

1 For Phylloglossum, see Bertrand (82); Bower (91), (08) ; Campbell (05). ^ Treub (84�90); Bruchmann (98); Lang (99).

quot;'� nbsp;nbsp;nbsp;ta a. o�M,. H.,w� �4 Bo,.�nbsp;nbsp;nbsp;nbsp;M.B.,

reproduced in fig. 121, B. Such species as L. erythraeum Spring, and others with stiff lanceolate leaves exhibit a strikingnbsp;resemblance to the more slender shoots of some recent conifers,nbsp;more especially Araucaria excelsa, A. Balansae, Cryptomeria,nbsp;Dacrydium and other genera.

In Lycopodium tetragonum Hook., (fig. 121, C), a species from the Alpine region of the Andes, the long, pendulous andnbsp;repeatedly forked branches bear four rows of fleshy ovate leavesnbsp;and simulate the vegetative characters of certain conifers.

L. squarrosum Forst. (fig. 122) a tropical species from India, Polynesia, and other regions, is characterised by its stout stemsnbsp;reaching a diameter of 2'5 cm., bearing long pendulous branchesnbsp;with large terminal cones composed of sporophylls differing butnbsp;slightly from the foliage leaves. The plant represented in thenbsp;photograph serves as a good illustration of the practical identitynbsp;in habit between Palaeozoic and recent genera.

L. Balhousianum Spring, from the mountains of the Malay Peninsula and Borneo, has larger leaves of finer texture with a

distinct midrib reaching a length of 2�3 cm. (fig. 121, E). Another type is illustrated by L. nummularifolium Blume, alsonbsp;a Malayan species, in which the leaves are shorter, broadlynbsp;oblong or suborbicular, and the branches terminate in narrownbsp;and often very long strobili (sometimes reaching a length of

�1 � f'nntrast to the foliag� 30 cm.) with small bracts in strikingnbsp;nbsp;nbsp;nbsp;slender

tropical

cylindrical cones with broadly ovate ^ sporop y gjjgcMruw habit characterises the North American ^ specinbsp;nbsp;nbsp;nbsp;ip^gth.

species, is worthy of notice as exhibiting a pecu consisting of a very small lamina, 3 mm. m leng ,nbsp;top of a long decurrent base, which forms a narrownbsp;cushion, bearing^ some resemblance to thenbsp;cushions of certain species ofnbsp;nbsp;nbsp;nbsp;an

es of Lycopodium tetragonum, to the very slend subulate leaves of such a species as L. verticiUatu7n, Lmn-long and broader lamina of L. Dalhousianum (fig- 1�- �nbsp;obvious that fragments of the various types preserve a

for twigs of conifers. As Dr Bommer^ has pointed out in his interesting paper on � Les causes d�erreur dans I�etude desnbsp;empreintes v�g�tales� some dicotyledonous plants may alsonbsp;simulate the habit of Lycopods: he cites Phyllachne clavigeranbsp;Hook (Candolleaceae), Tafalla graveolens Wedd (Compositae)nbsp;and Lavoisiera lycopodioides Gard. (Melastomaceae). Anothernbsp;point illustrated by fig. 121 is the close agreement in habit andnbsp;in the form of the leaves and leaf-cushions between the recentnbsp;plants and the Palaeozoic Lepidodendreae.

In his masterly essay � On the vegetation of the Carboniferous Period, as compared with that of the present day � Sir Joseph Hooker called attention to the variation in the shapenbsp;and arrangement of the leaves in the same species of Lycopodium. The three woodcuts which he publishes of Lycopodiumnbsp;densum, a New Zealand species, afford striking examples of thenbsp;diversity in habit and leaf-form and justify his warning � thatnbsp;if the species of Lepidodendron were as prone to vary in thenbsp;foliage as are those of Lycopodium, our available means fornbsp;distinguishing them are wholly insufficient^.�

As we have already noticed, there is a considerable diversity among recent species, both as regards habitat andnbsp;habit; in the anatomy of the stem also corresponding variationsnbsp;occur within the limits of a well-defined generic type of stele.nbsp;In species with creeping stems, such as L. clavatum^, thenbsp;stele exhibits an arrangement of vascular tissue characteristicnbsp;of the plagiotropic forms. The xylem consists of more or lessnbsp;horizontal plates of scalariform tracheae, each surrounded bynbsp;small-celled parenchyma, alternating with bands or groups ofnbsp;somewhat ill-defined phloem. The protoxylem and protophloem elements occupy an external position (exarch), pointingnbsp;to a centripetal development of the metaxylem. This centripetal or root-like character of the primary xylem is an importantnbsp;feature in recent as in fossil Lycopods. The close agreementnbsp;between the roots and stems of recent species in the dispositionnbsp;of the vascular elements also denotes a simpler type of anatomy

than occurs in the majority of vascular plants in which stem and root have more pronounced structural peculiarities. Anbsp;pericycle, 2�6 cells in breadth, encloses the xylem and phloem

ws.

Fig. 125. A. Lycopodium dickotomum. Transverse section of stem: Zc, leaf-cushion ; It, leaf-trace ; B, roots.

H. nbsp;nbsp;nbsp;L. cernuum: b, branch of stele; c�cquot;, cortex; s, space in

bands and this is succeeded by an endodermis, 2�3 cells broad, With vaguely defined limits. In L. clavatum, as in L. alpinum,nbsp;another British species, the broad cortex is differentiated into

three fairly distinct regions; abutting on the endodermis is a zone several layers broad of thick-walled cells constituting annbsp;inner cortex modified for protection and support; the centralnbsp;region consists of larger and thinner-walled cells adapted fornbsp;water-storage and aeration; beyond this is an outer corticalnbsp;zone of firmer and thicker elements. The prominent leaf-basesnbsp;or leaf-cushions (fig. 125, A, Ic) give to the surface of a transversenbsp;section a characteristic appearance which presents the closestnbsp;agreement with that of the younger shoots of Lepidodendron.nbsp;From the peripheral protoxylem groups small strands of xylemnbsp;are given off, which follow a steeply ascending course throughnbsp;the cortex to the single-veined leaves. The leaf-traces, in severalnbsp;species at least, are characterised by a mesarch structurenbsp;(fig. 125, F, G), the spiral protoxylem elements occupying annbsp;approximately central position. The mesophyll of the leavesnbsp;varies in regard to the extent of differentiation into a palisadenbsp;and spongy parenchyma; in all cases there is a single vascularnbsp;bundle occasionally accompanied by a secretory duct.

In erect stems of Lycopodium, as represented by L. cernuum (figs. 123,125, H, I), L. Dalhousianum, L. squarrosum (fig. 122)nbsp;and many others, the stele presents a characteristic appearancenbsp;due to the xylem plates being broken up into detached groupsnbsp;or short uniseriate bands with the interspaces occupied bynbsp;phloem islands. This type of structure bears a superficialnbsp;resemblance to that in the single stele of certain species of thenbsp;fern Lygodium^, but it is distinguished by the islands of phloemnbsp;scattered through the stele. In other species the xylem tendsnbsp;to assume the form of a Maltese cross {e.g. L. serratum Thbg.)nbsp;or it may be disposed as V-shaped and sinuous bands terminating in broad truncate ends composed of protoxylem elements.nbsp;This form of the xylem and the distribution of the phloemnbsp;groups are shown in fig. 125, D, E, drawn from a section of anbsp;plant of Lycopodium saururus Lam.^ collected by Mr A. W. Hillnbsp;at an altitude of 15,000 feet on the Andes of Peru. Thenbsp;position of the protoxylem is shown fig. 125, E, px.

� This species is figured under the name Lycopodium crassum by Hooker and Greville (31) PI. 224. See also Brongniart (37) PI. i. fig. 1.

leaf-bases, and presents the appearance of an irregular reticulum. This arrangement of the mechanical tissue in the outer cortexnbsp;is comparable with that in stems of some species of Sigillaria.

In certain species of Lycopodium, the roots h which arise endogenously from the axial vascular cylinder, instead ofnbsp;passing through the cortex of the stem by the shortest route,nbsp;bend downwards and bore their way in a more or less verticalnbsp;direction before emerging at or near the base of the aerialnbsp;shoot. The transverse section of L. dichotomurn represented innbsp;fig. 125, A, shows several roots (R) in the cortex; they consistnbsp;of a xylem strand of circular or crescentric form accompanied bynbsp;phloem and enclosed by several layers of root-cortex. The rootsnbsp;of Lycopodium do not always present so simple a structure asnbsp;those of L. dichotomurn; the xylem may have an irregularlynbsp;stellate form with as many as ten protoxylem groups.

Reproductive Shoots^. In Lycopodium Relayo the foliage leaves serve also as sporophylls and, as Professor Bower^ hasnbsp;pointed out, the branches exhibit to some extent a zonalnbsp;alternation of sterile and fertile leaves; in other species, innbsp;which foliage leaves and sporophylls are practically identical,nbsp;the sporangia occur sporadically on the ordinary leaves. Innbsp;species with well-defined terminal cones the lower sporophyllsnbsp;may bear arrested sporangia and thus form transitional stagesnbsp;between sterile and fertile leaves, a feature which occurs alsonbsp;in the male and female flowers of many recent Araucarieae^.nbsp;The sporangia� (fig. 126, D, F) are usually reniform andnbsp;compressed in a direction parallel to the surface of the cone-scales ; they are developed from the upper surface and close tonbsp;the base of the fertile leaf to which they are attached by anbsp;short and thick stalk {e.g. L. inundatum) or by a longer andnbsp;more slender pedicel {L. Phlegmaria, fig. 126, E). On maturitynbsp;the sporangia open as two valves in the plane of compression

1 Strasburger (73) p. 109; Brongniart (37) PI. 8; (39) A. PI. 32; Brongniart figures stems of L. Phlegmaria and other species showing roots in the cortex.nbsp;See also Goldenberg (55); Bruchmann (74); Saxelby (08).

^ Since this was written a comparative account of the sporophylls of Lycopodium has been published by Miss Sykes. [Sykes (08�).]

and the line of dehiscence is determined in some species at least by the occurrence of smaller cells in the wall. In

fiG. 126. A. Lycopodium cernuum, longitudinal section of strobilus; a, band of lignified cells.

D. nbsp;nbsp;nbsp;L. clavatum. Part of radial longitudinal section of strobilus ;

transverse sections of cones in which the sporangia are strongly saddle-shaped, the sporophylls may appear to bear two sporangia.

This is well shown in the section of a cone of L. clavatum shown in fig. 126, F. The sporangia a and b are cut through in annbsp;approximately median plane showing the irregular outline ofnbsp;the sterile pad (p) of tissue in the sporogenous cavity. Thosenbsp;at c and d have been traversed at a lower level and the twonbsp;lobes of the saddle-shaped sporangia are cut below the attachmentnbsp;to the sporophyll. The distal laminae of the sporophylls, cutnbsp;at different levels, are seen at the periphery of the cone.

In longitudinal radial section of some cones the sporangia appear to occupy an axillary position, but in others (e.g.nbsp;L. clavatum') they are attached to the horizontal portion of thenbsp;sporophyll almost midway between the axis of the cone and thenbsp;upturned distal end of the sporophyll (fig. 126, D). The wallnbsp;of a sporangium frequently consists of 2�3 cell-layers and innbsp;some cases {e.g. L. dichotomum), it may reach a thickness of sevennbsp;layers, resembling in this respect the more bulky sporangianbsp;of a certain type of Lepidodendroid cone. The sporogenousnbsp;tissue is separated from the stalk of the sporangium by a massnbsp;of parenchymatous tissue which may project as a prominentnbsp;pad (fig. 126, D, F, p) into the interior of the sporogenousnbsp;cavity. This basal tissue (the subarchesporial pad of Bower i)nbsp;has been observed in L. clavatum to send up irregular processesnbsp;of sterile cells among the developing spores, suggesting anbsp;comparison with the trabeculae which form a characteristicnbsp;feature of the sporangia of Isoetes and with similar sterilenbsp;strands noticed by Bower'^ in Lepidostrobus (cone of Lepido-dendron).

Each sporophyll is supplied by a single vascular bundle which according to published statements never sends a branchnbsp;to the sporangium base. The fertile tips of the foliage shootsnbsp;of L. cernuum (figs. 126, A�C) afford good examples ofnbsp;specialised cones. The surface of the cone is covered by thenbsp;broadly triangular laminae of sporophylls (fig. 126, G) which innbsp;their fimbriate margins resemble the Palaeozoic cone-scalesnbsp;described by Dr Kidston� as Lepidostrobus fimbriatus. Thenbsp;distal portions of the sporophylls are prolonged downwards

(fig. 126, A) to afford protection to the lower sporangia, their efficiency being increased by the lignified and thicker wallsnbsp;(A, a) of the cells in the lower portion of the laminar expansion.nbsp;The cells of the sporangial wall are provided with strengtheningnbsp;bands which in surface-view (fig. 126, B) present the appearancenbsp;of prominent pegs. Since the appearance of Miss Sykes�snbsp;paper on the sporangium-bearing organs of the Lycopodiaceae,nbsp;Dr Langi has published a more complete account of thenbsp;structure of the strobilus of Lycopodium cernuum in which he

records certain features of special interest. The importance of these morphological characters is increased by their agreement,nbsp;as shown by Lang, with those of the Palaeozoic cone Spencerites^.nbsp;The sporophylls of a cone (12 mm. long by 3 mm. in diameter)nbsp;of Lycopodium cernuum show an abrupt transition from thenbsp;foliage leaves, but like these they occur in alternate whorls of five.nbsp;A large sporangium is attached to the upper face of each sporo-phyll close to the base of the obliquely vertical distal lamina

(fig. 127); each sporophyll,which is supplied with a single vascular bundle, has a large mucilage-cavity (??i) in its lower region.nbsp;�The mucilaginous change� in the sub-sporangial portion ofnbsp;a sporophyll � extends to the surface involving the epidermis, sonbsp;that this portion of the sporophyll-base may be described asnbsp;consisting of a mass of mucilage bounded below by a structurelessnbsp;membrane^.� Dehiscence of the sporangia occurs at the middlenbsp;of the distal face (fig. 127, x). As seen in the radial sectionnbsp;(fig. 127, ma) the outer margin of the base of the sporophyllnbsp;bears a short outgrowth. The leaf-bases of each whorl hangnbsp;down between the sporangia of the alternating whorl below, and

the base of each sporophyll is coherent with the margins of the two sporophylls of the next lower whorl between which it lies,nbsp;the sporangia being thus closely packed and lying in anbsp;pocket �open only on the outer surface of the cone.� Fig. 128nbsp;represents a transverse section through a cone in the plane AAnbsp;of fig. 127 ; this traverses the sporangia and their subtendingnbsp;bracts (b) of one whorl and the dependent bases of thenbsp;sporophylls of the next higher whorl in the region of thenbsp;mucilage-sacs (m), which are bounded at the periphery by thenbsp;i Lang (08) p. 357.

Pig. 129. Transverse section of the cone of Lycojpodwm cermwni m BB of fig. 12V.nbsp;nbsp;nbsp;nbsp;Lang.)

sporangial stalks. The upper portions of . ^ , miicilagi-next lower whorl, which project upwards

at c and eatemal to them, at a. the section hM cat throng the outer persistent portions of these sporop y f � la opnbsp;As Lang points out, this highly complexnbsp;nbsp;nbsp;nbsp;^ rnmna

of a plant met with in exposed situations in t e rop � is also of importance from a morphological stan poinnbsp;hihiting an agreement with the extinct type of ycoponbsp;represented by Spencerites.

Selaginellaceae.

Selaginella differs from Lycopodium in the production of two kinds of spores, megaspores and microspores, and, in the

great majority of species, in the dimorphic character of the foliage leaves, which are usually arranged in four rows, thenbsp;laminae of the upper rows being very much smaller than those ofnbsp;the lower (fig. 130, 1�3). The smaller leaves are shown morenbsp;clearly in fig. 130, la. It is obvious from an examination of a

Selaginella shoot, such as is shown in fig. 130, that in fossil specimens it would often be almost impossible to recognise thenbsp;existence of two kinds of leaves. Some species, e.g. Selaginellanbsp;spinosa^, the sole British representative of the genus, arenbsp;homophyllous and agree in this respect with most species ofnbsp;Lycopodium. Another feature characteristic of Selaginella, as

contrasted with Lycopodium, is the presence of a ligule in both foliage leaves and sporophylls. This is a colourless thin laminanbsp;attached by a comparatively stout foot to the base of a pitnbsp;on the upper surface and close to the lower edge of the leafnbsp;(fig. 130, 4,1; fig. 131, E, F, 1).

In an erect species, such as S. grandis Moore^ (fig. 130 and fig. 131, G) from Borneo, the main shoots, which may attain anbsp;height of 2�3 feet, bear small and inconspicuous leaves ofnbsp;one kind, but the lateral and repeatedly forked shoots arenbsp;heterophyllous. The passage from the homoph3dlous to thenbsp;heterophyllous arrangement is shown in the transition from thenbsp;erect to the dorsiventral habit of the lateral shoots (fig. 130, 2).nbsp;The monopodially or dichotomously branched shoots producenbsp;long naked axes at the forks; these grow downwards to thenbsp;ground where they develop numerous dichotomously forkednbsp;branches. For certain reasons these naked aerial axes werenbsp;named rhizophores and have always been styled shoots, the termnbsp;root being restricted to repeatedly forked branches which thenbsp;rhizophores produce in the soil. It has, however, been shown bynbsp;Professor Harvey-Gibson^ that there is no sufficient reason fornbsp;drawing any morphological distinction between rhizophores andnbsp;roots, the term root being applicable to both.

Our knowledge of the anatomy of Selaginella, thanks chiefly to the researches of Harvey-Gibson�, is much more completenbsp;than in the case of Lycopodium. The stems, which may benbsp;either trailing or erect, are usually dorsiventral, and it is noteworthy that different shoots of the same plant or even the samenbsp;axis in different regions may exhibit considerable variation innbsp;the structure and arrangement of the vascular tissue. In thenbsp;well-known species, Selaginella Martensii, the stem, which isnbsp;partly trailing, partly ascending, possesses a single ribbon-shapednbsp;stele composed of scalariform tracheids with two marginalnbsp;protoxylems formed by the fusion of the leaf-traces of thenbsp;dorsal and ventral leaves respectively. As in Lycopodium thenbsp;metaxylem tracheae are as a rule scalariform, but reticulatenbsp;xylem elements are by no means unknown. The tracheal band.

of another species�S. Willdenown represen chlorophyll-A pericycle composed of one or nbsp;nbsp;nbsp;is suspended

cuticular bands. The cortex is usually V,Vk-walled fibres fairly distinct regions. Mechanicalnbsp;nbsp;nbsp;nbsp;� ^ cortex consists of

thinner-walled parenchyma, the elements o^^ inner zone, smaller and rather more compactly arrange iirnbsp;nbsp;nbsp;nbsp;presence

many recent and fossil members of the Lycopp ^ nbsp;nbsp;nbsp;^nbsp;nbsp;nbsp;nbsp;cortex

shoots of Selaginella spinosa^ present a distinct type axial anatomy. The upper part of the ascending stem asnbsp;nbsp;nbsp;nbsp;, ^

as affording an exception, in the endarch structure of the xylem, to the usual exarch plan of the stelar tissues. Thisnbsp;species is the only one in which any indication of the production of secondary xylem elements has so far been recorded.nbsp;Bruchmann^ has shown that, in the small tuberous swellingnbsp;which occurs at the base of the young shoot (hypocotyl), anbsp;meristematic zone is formed round the axial vascular strand andnbsp;by its activity quot;a few secondary tracheids are added to thenbsp;primary xylem. With this exception Selaginella appears tonbsp;have lost the power of secondary thickening, the possessionnbsp;of which constitutes so striking a feature of the Palaeozoicnbsp;Lycopods. Another type is represented by S. maequalifolia,nbsp;an Indian species, the shoots of which may have either anbsp;single stele or as many as five, each in its separate lacuna.nbsp;The homophyllous S. laevigata var. Lyallii Spr., a Madagascannbsp;species, affords a further illustration of the variation in plan ofnbsp;the vascular tissues within the genus. There is a considerablenbsp;difference in structure between the erect and creeping shoots ;nbsp;in the former there may be as many as 12�13 steles, whichnbsp;gradually coalesce before the vertical axis joins the creepingnbsp;rhizome to form one central and four peripheral steles. In thenbsp;rhizome there is usually a distinct axial stele without proto-xylem, surrounded by an ill-defined lacuna and enclosed by anbsp;cylindrical stele (solenostele)^ usually two tracheae in widthnbsp;with four protoxylem strands on its outer edge. The continuitynbsp;of the tubular stele is broken and, in transverse section, itnbsp;assumes the form of a horseshoe close to the base of an erectnbsp;shoot to which a crescentric vascular strand is given off.nbsp;Harvey-Gibson� has figured a section of the rhizome of thisnbsp;type in which the axial vascular strand is represented by anbsp;slight ridge of tracheae (fig. 131, G, t) projecting towards the

^ The term solenostele, first used by Van Tieghem and revived by Gwynne-Vaughan, may be applied to a stem in which the vascular tissue has the form of a hollow cylinder with phloem and endodermis on each side of the xylem. Asnbsp;each leaf-trace is given off the continuity of the vascular tube is interrupted.nbsp;See Gwynne-Vaughan (01) p. 73.

Leaves. The leaves of Selaginella usu y reticulum of loosely arranged cells, hut innbsp;nbsp;nbsp;nbsp;, vascular

bundle consists of a few small annular or spira rac ' the apex of the lamina the protoxyleni elementsnbsp;nbsp;nbsp;nbsp;foliage

sheath is separated from the vascular bundle ^ � � trans-or more layers of cells, and in some species these formed into short tracheids. The ligule is regar e y _nbsp;G�bson� as a specialised ramentum which serves t e nnbsp;function of keeping moist the growing-point and

Cones. Thquot; temmal p�rti�m of fte btonchos of �� usually bear smaller leaves of uniform size whic rinc

sporophylls, but in this genus the fertile shoots do not generally form such distinct cones as in many species of Lycopodium.nbsp;In S.grandis (figs. 130, 3; 131, G) the long and narrow strobilinbsp;consist of a slender axis bearing imbricate sporophylls in fournbsp;rows : each sporophyll subtends a sporangium situated betweennbsp;the ligule and the axis of the shoot. The sporangium may benbsp;developed from the axis of the cone or, as in Lycopodium, fromnbsp;the cells of the sporophylls In some species the lower sporophylls bear only megasporangia, each normally containing fournbsp;megaspores, the microsporangia being confined to the uppernbsp;part of the cone. This distribution of the two kinds of sporangianbsp;is, however, by no means constant^: in some cases, e.g. S.

rupestris, cones may bear megasporangia only, and in the cone of S. grandis, of which a small piece is represented in fig. 131, G,nbsp;all the sporangia were found to contain microspores.

The occurrence of two kinds of spores in Selaginella constitutes a feature of special importance from the point ofnbsp;view of the relationship between the Phanerogams, in whichnbsp;heterospory is a constant character, and the heterosporousnbsp;Pteridophytes. One of the most striking distinctions betweennbsp;the Phanerogams and the rest of the vegetable kingdom lies innbsp;the production of seeds. Recent work has, however, shown thatnbsp;seed-production can no longer be regarded as a distinguishingnbsp;feature of the Gymnosperms and Angiosperms. Palaeozoicnbsp;plants which combined filicinean and cycadean features resembled the existing Phanerogams in the possession of highlynbsp;specialised seeds. This discovery adds point to the comparisonnbsp;of the true seed with structures concerned with reproduction innbsp;seedless plants, which in the course of evolution gave rise to thenbsp;more efficient arrangement for the nursing, protection, andnbsp;ultimate dispersal of the embryo. In the megaspore ofnbsp;Selaginella we have, as Hofmeister was the first to recognisenbsp;in 1851, a structure homologous with the embryo-sac of thenbsp;Phanerogam. The embryo-sac consists of a large cell producednbsp;in a mass of parenchymatous tissue known as the nucellusnbsp;which is almost completely enclosed by one or more integuments. Fertilisation of the egg-cell within the embryo-sacnbsp;1 Bower (08) p. 315.nbsp;nbsp;nbsp;nbsp;^ Hieronymus (02).

\�rten tlTseed and its parent Mangrove swamps, continuity be wnbsp;nbsp;nbsp;nbsp;^^gga,gporangium of

examining withered decayed strobih o ^^^^^.^onths after had been partially covered with the soi ornbsp;nbsp;nbsp;nbsp;plants were

found with cotyledons and roots nbsp;nbsp;nbsp;tnbsp;nbsp;nbsp;nbsp;� it seems

protruding from the imbricate sporophyl s nbsp;nbsp;nbsp;^nbsp;nbsp;nbsp;nbsp;from the

occasional occurrence of one instead of four mega. � sporangium; a condition which affords anot er connbsp;between the heterosporous Pteridophytes, on t e onnbsp;nbsp;nbsp;nbsp;^

through the winter and fertilisation occurs in � ^nm spring. After the embryo has been formed thenbsp;� becomes sunken in a shallow pit formed y i- i� pt isnbsp;outgrowth of the sporophyll around the pe me.

suggested that this outgrowth may he comparable with the integument which grows up from the sporophyll in the fossilnbsp;genus Lepidocarpon^ and almost completely encloses thenbsp;sporangium. In the drawings given by Miss Lyon no featuresnbsp;are recognisable which afford a parallel to the integumentnbsp;of Lepidocarpon. I have, however, endeavoured to show, by anbsp;brief reference to this author�s interesting account of the twonbsp;Californian species, that the physiological and morphologicalnbsp;resemblances between the megasporangia of Selaginella and thenbsp;integumented ovules of the seed-bearing plants are sufficientlynbsp;close to enable us to recognise possible lines of advance towardsnbsp;the development of the true seed.

Professor CampbelP records an additional example of a Selaginella�probably S. Bigelovii�from the dry region ofnbsp;Southern California in which the spores become completelynbsp;dried up after the embryo has attained some size, remaining innbsp;that state until the more favourable conditions succeeding thenbsp;dry season induce renewed activity.

The genus Isoetes is peculiar among Pteridophytes both in habit and in anatomical features. In its short and relativelynbsp;thick tuberous stem, terminating in a crowded rosette ofnbsp;subulate leaves like those of Juncus and bearing numerousnbsp;adventitious roots, Isoetes presents an appearance similar tonbsp;that of many monocotyledonous plants. The habit of thenbsp;genus is well represented by such species as Isoetes lacustrisnbsp;and I. echinospora^ (fig. 132) both of which grow in freshwaternbsp;lakes in Britain and in other north European countries.nbsp;The latter species bears leaves reaching a length of 18 cm.nbsp;The resemblance in habit between this isolated member ofnbsp;the Pteridophytes and certain Flowering plants, although innbsp;itself of no morphological significance, is consistent with thenbsp;view expressed by Campbell that Isoetes may be directly relatednbsp;to the Monocotyledons^

There is as a rule little or no difference between the foliage leaves and sporophylls; in I. lacustris the latter are rather

spg-

larger and in the terrestrial species I. hystrix^ the sterile leaves are represented by the expanded basal portions only, whichnbsp;persist like the leaf-bases of Lepidodendron as dark brownnbsp;scales to form a protective investment to the older part of thenbsp;stem. The innermost leaves are usually sterile; next to thesenbsp;are sporophylls bearing megasporangia, and on the outside arenbsp;the older sporophylls with microsporangia. The long andnbsp;slender portion of the leaf becomes suddenly expanded closenbsp;to its attachment to the stem into a broad base of crescenticnbsp;section which bears a fairly conspicuous ligule (figs. 132, B, I,nbsp;133, E, 1) inserted by a foot or glossopodium in a pit near thenbsp;upper part of the concave inner face. The ligule is usuallynbsp;larger than that of Selaginella, though of the same type. Thenbsp;free awl-like lamina contains four large canals bridged across atnbsp;intervals by transverse diaphragms, and in the axial region anbsp;single vascular bundle of collateral structure. Other vascularnbsp;elements, in the form of numerous short tracheids occur belownbsp;the base of the transversely elongated ligule.

Stomata are found on the leaves of I. hystrix, I. Boryana^, and in other species which are not permanently submerged.nbsp;Both microsporangia and megasporangia are characterised bynbsp;their large size and by the presence of trabeculae or strands ofnbsp;sterile tissue (fig. 133, E, H, t) completely bridging across thenbsp;sporangial cavity or extending as irregular ingrowths among thenbsp;spore-producing tissue. Similar sterile bands, though lessnbsp;abundant and smaller, are occasionally met with in the stillnbsp;larger sporangia of Lepidostrobus; these may be regarded as anbsp;further development of the prominent pad of cells whichnbsp;projects into the sporangial cavity in recent species ofnbsp;Lycopodium (fig. 126, D, p). The sporangia are attached by anbsp;very short stalk to the base of a large depression in the leaf-base below the ligule, from the pit of which they are separatednbsp;by a ridge of tissue known as the saddle, and from this ridge anbsp;veil of tissue (the velum) extends as a roof over the sporangialnbsp;chamber (fig. 133, E, v). In most species there is a large gapnbsp;between the lower edge of the velum and that of the sporangial

pit, but in I. hystrix this protective membrane is separated from the base of the leaf by a narrow opening, the resemblancenbsp;of which to the micropyle of an ovule suggested to one of thenbsp;older botanists the employment of the same term¹. Mr T. G.nbsp;HilP has called attention to the presence of mucilage canals innbsp;the base of the sporophylls of I. hystrix, which he compares withnbsp;the strands of tissue known as the parichnos accompanying thenbsp;leaf-traces of Lepidodendron and Sigillaria in the outer cortexnbsp;of the stem. The transverse section shown in fig. 133, H and I,nbsp;shows two of these mucilage canals in an early stage ofnbsp;development; a strand of parenchymatous elements distinguished by their partially disorganised condition and morenbsp;deeply stained membranes (fig. 133, I) runs through thenbsp;spandrels of the sporophyll tissue close to the upper surface.nbsp;There is a close resemblance between the structure of thesenbsp;partially formed mucilage-canals and the tissue which has beennbsp;called the secretory zone in Lepidodendron stems. Fig. 133, H,nbsp;also shows a large microsporangium with prominent trabeculaenbsp;(t) lying below the velum. A longitudinal section (fig. 133, E)nbsp;through a sporophyll-base presents an appearance comparablenbsp;with that of an Araucarian cone-scale with its integumentednbsp;ovule and micropyle. The megaspores are characterised bynbsp;ridges, spines, and other surface-ornamentation^. Though usuallynbsp;unbranched, the perennial stem of Isoetes (fig. 132) has in rarenbsp;cases been found to exhibit dichotomous branching, a feature, asnbsp;Solms-Laubach^ points out, consistent with a Lycopodiaceousnbsp;affinity. The apex is situated at the base of a funnel-shapednbsp;depression. The stem is always grooved ; in some species twonbsp;and in others three deep furrows extend from the base up the sidesnbsp;of the short and thick axis towards the leaves: from the sides ofnbsp;these furrows numerous slender roots are given off in acropetalnbsp;succession. A stele of peculiar structure occupies the centre ofnbsp;the stem; cylindrical in the upper part (fig. 133, A), it assumesnbsp;a narrow elliptical or, in species in which there are three furrows,nbsp;a triangular form in the lower portion of the tuberous stem.

^ For figures, see Motelay and Vendry�s (82); Bennie and Kidston (88) PI- VI.nbsp;nbsp;nbsp;nbsp;4 Solms-Laubach (02).

A. nbsp;nbsp;nbsp;Transverse section of stem: cr, cortex; x, xylem; e, cambium;nbsp;a, thin-walled tissue; It, leaf-traces; b, dead tissue.

E. nbsp;nbsp;nbsp;Longitudinal radial section of sporonhyll-base: v, velum; I,nbsp;ligule; bb, vascular bundle; m, megaspores; t, sterile tissue.

H. nbsp;nbsp;nbsp;Transverse section of sporophyll, showing sporangium withnbsp;trabeculae, t; leaf-trace, (It), and two groups of secretory cells.

the laminae of the leaves have been removed from the summit affords an example of a species with two furrows. Thenbsp;drawing shows the widely gaping sides of the broad furrownbsp;with circular root-scars and a few simple and dichotomouslynbsp;branched roots. A short thick column of parenchymatousnbsp;tissue projects from a slightly eccentric position on the base ofnbsp;the stem.

The primary vascular cylinder' consists of numerous spiral, annular or reticulate tracheids (fig. 133, A, x), which are eithernbsp;isodiametric or longer in a horizontal than in a verticalnbsp;direction, associated with parenchyma. Lower in the stemnbsp;crushed and disorganised xylem elements are scattered throughnbsp;a still living trabecular network of parenchymatous tissue.nbsp;From the axial cylinder numerous leaf-traces (fig. 133, A, It)nbsp;radiate outwards, at first in a horizontal direction and thennbsp;gradually ascending towards the leaves. The vascular cylindernbsp;is of the type known as cauline; that is, some of the xylem isnbsp;distinct in origin from that which consists solely of the lowernbsp;ends of leaf-traces. As in Lycopodium the development of thenbsp;metaxylem is centripetal.

Von Mohb, and a few years later Hofmeister�, were the first botanists to give a satisfactory account of the anatomy ofnbsp;Isoetes, but it is only recently^ that fresh light has been thrownnbsp;upon the structural features of the genus the interest of whichnbsp;is enhanced by the many points of resemblance between thenbsp;recent type and the Palaeozoic Lepidodendreae. A strikingnbsp;anatomical feature is the power of the stem to produce secondarynbsp;vascular and non-vascular tissue ; the genus is also characterised by the early appearance of secondary meristematicnbsp;activity which renders it practically impossible to draw anynbsp;distinct line between primary and secondary growth. Anbsp;cylinder of thin-walled tissue (fig. 133, A, a) surroundsnbsp;the primary central cylinder and in this a cambial zone, c, isnbsp;recognised even close to the stem-apex; this zone of dividingnbsp;cells is separated from the xylem by a few layers of rectangularnbsp;cells to which the term prismatic zone has been applied.

The early appearance of the cambial activity on the edge of the vascular cylinder is shown in fig. 133, C, which represents partnbsp;of a transverse section of a young stem. A leaf-trace, It, is innbsp;connexion with the primary xylem, x', which consists of shortnbsp;tracheids, often represented only by their spiral or reticulatelynbsp;thickened bands of lignified wall, and scattered parenchyma.nbsp;Some of the radially elongated cells on the sides of the leaf-trace are seen to be in continuity on the outer edge of thenbsp;stele, at st, with flattened elements, some of which are sieve-tubes. The position of a second leaf-trace is shown at It'.nbsp;External to the sieve-tubes the tissue consists of radiallynbsp;arranged series of rectangular cells, some of which have alreadynbsp;assumed the function of a cambium (c). The tissue producednbsp;by the cambium on its inner edge consists of a varying amountnbsp;of secondary xylem composed of very short spiral tracheids;nbsp;a few of these may be lignified (fig. 133, A, while othersnbsp;remain thin.

Phloem elements, recognisable by the presence of a thickened reticulum enclosing small sieve-areas (fig 133, B, s) are fairlynbsp;abundant, and for the rest this intracambial region is composednbsp;of thin-walled parenchyma. In longitudinal section thesenbsp;tissues present an appearance almost identical with thatnbsp;observed in a transverse section. Fig. 133, B represents anbsp;longitudinal section, through the intracambial zone and thenbsp;edge of the stele, of a younger stem' than that shown innbsp;fig. 133, A. Most of the radially disposed cells internal to thenbsp;meristematic region are parenchymatous without any distinctivenbsp;features; a few scattered sieve-tubes (s) are recognised by theirnbsp;elliptical sieve-areas and an occasional tracheid can be detected.nbsp;The cambium cuts off externally a succession of segmentsnbsp;which constitute additional cortical tissue (fig. 133, A, cr)nbsp;of homogeneous structure, composed of parenchymatous cellsnbsp;containing starch and rich in intercellular spaces. As the stemnbsp;grows in thickness the secondary cortex reaches a considerablenbsp;breadth and the superficial layers are from time to timenbsp;exfoliated as strips of dead and crushed tissue (fig. 133, A, b).nbsp;The diagrammatic sketch reproduced in fig. 133, A, serves tonbsp;illustrate the arrangement and relative size of the tissue-regions

in an Isoetes stem. In the centre occur numerous spirally or reticulate tracheae scattered in parenchymatous tissue whichnbsp;has been considerably stretched and torn in the peripheralnbsp;region of the stele; the radiating lines mark the position of thenbsp;leaf-traces (It) in the more horizontal part of their course. Thenbsp;zone between the cambium (c) and the edge of the centralnbsp;cylinder consists of radially disposed secondary tissue of short,nbsp;and for the most part unlignified, elements including sieve-tubes and parenchyma; the secondary xylem elements consistnbsp;largely of thin-walled rectangular cells with delicate spiralnbsp;bands, but discontinuous rows of lignified tracheae (x^) occurnbsp;in certain regions of the intracambial zone. The rest of thenbsp;stem consists of secondary cortex (cr) with patches of deadnbsp;tissue (b) still adhering to the irregularly furrowed surface.nbsp;The structure of the cambium and its products is shown innbsp;the detailed drawing reproduced in fig. 133, D. Many of thenbsp;elements cut off on the inner side of the cambium exhibit thenbsp;characters of tracheids : most of these are unlignified, but othersnbsp;have thicker and lignified walls (tr).

I. hystrix appears to be exceptional in retaining its leaf-bases, which form a complete protective investment and prevent the exfoliation of dead cortex. Each leaf-trace consists of a fewnbsp;spiral tracheids accompanied by narrow phloem elements directlynbsp;continuous with the secondary phloem of the intracambial zone.nbsp;Dr Scott and Mr Hill have pointed out that a normal cambiumnbsp;is occasionally present in the stem of I. hystrix during the earlynbsp;stages of growth,' this gives rise to xylem internally. Thenbsp;few phloem elements observed external to the cambium may benbsp;regarded as primary phloem, a tissue not usually represented innbsp;an Isoetes stemb The occasional occurrence of this normalnbsp;cambium, may, as Scott and Hill suggest, be a survival from anbsp;former condition in which the secondary thickening followednbsp;a less peculiar course. The lower leaf-traces become more ornbsp;less obliterated as the result of the constant increase in thicknessnbsp;of the broad zone of secondary tissues through which they pass.

^ Miss Stokey (09), in a paper which appeared since this account was written, criticises the conclusions of Scott and Hill (00).

arranged in parallel series on each side of the median line of the two or three furrows. The three arms of the triangular stele ofnbsp;I. hystrix and the two narrow ends of the long axis of the stelenbsp;of I. lacustris, which in transverse section has the form of anbsp;flattened ellipse, are built up of successive root-bases. A rootnbsp;of Isoetes (fig. 133, G) possesses one vascular bundle, x, with anbsp;single strand of protoxylem, px, thus agreeing in its monarchnbsp;structure with the root-bundle in Selaginella and many speciesnbsp;of Lycopodium. The cortical region of the root consists of a fewnbsp;layers of outer cortex succeeded by a large space, formed bynbsp;the breaking down of the inner cortical tissue, into which thenbsp;vascular bundle projects (fig. 133, F). The peculiarity of thenbsp;roots in having a hollow cortex and an eccentric vascular bundlenbsp;was noticed by Von Mohlh In the monarch bundles, as in thenbsp;fistular cortex and dichotomous branching, the roots of Isoetesnbsp;present a striking resemblance to the slender rootlets of thenbsp;Palaeozoic Stigmaria (see Page 246). The longitudinal sectionnbsp;through the base of a root of Isoetes lacustris shown in fig. 133, F,nbsp;affords a further illustration of certain features common to thenbsp;fossil and recent types.

FOSSIL LYCOPODIALES.

Isoetaceae

The geological history of this division of the Pteridophyta is exceedingly meagre, a fact all the more regrettable as it is bynbsp;no means improbable that in the surviving genus Isoetes wenbsp;have an isolated type possibly of considerable antiquity andnbsp;closely akin to such extinct genera as Pleuromeia andnbsp;Sigillaria. If Saporta�s Lower Cretaceous species Isoetesnbsp;ChoffatP, or more appropriately Isoetites Ghoffati, is correctlynbsp;determined, it is the oldest fossil member of the family andnbsp;indeed the most satisfactory among the more than doubtfulnbsp;species described as extinct forms of Isoetes.

The generic name Isoetites was first used by Munster^ in the description of a specimen, from the Jurassic lithographic slatesnbsp;of Solenhofen in Bavaria, which he named Isoetites crociformis.nbsp;The specific name was chosen to express a resemblance of thenbsp;tuberous appearance of the lower part of the imperfectly preserved and indeterminable fossil to a Crocus conn.

Impressions of Isoetes-like leaves from the Inferior Oolite of Yorkshire figured by Phillips^ and afterwards by Bindley� asnbsp;Solenites Murrayana were compared by the latter author withnbsp;Isoetes and Pilularia, but these leaves are now generallynbsp;assigned to Heer�s gymnospermous genus Czekanowskia. Annbsp;examination of the structure of the epidermal cells of thesenbsp;Jurassic impressions convinced me that they resemble recentnbsp;coniferous needles more closely than the leaves of any Pterido-phyte. The genus Czekanowskia^ is recognised by severalnbsp;authors as a probable member of the Ginkgoales.

The late Marquis of Saporta founded this species on two sets of impressions from the Urgonian (Lower Cretaceous) ofnbsp;Portugal which, though not found in actual organic connexion,nbsp;may possibly be portions of the same plant. Small relativelynbsp;broad tuberous bodies reaching a breadth of 1 cm. are comparednbsp;with the short and broad stem of Isoetes, which they resemblenbsp;in bearing numerous appendages radiating from the surface likenbsp;the roots of the recent species; on the exposed face of the stemnbsp;occur scattered circular scars representing the position of rootsnbsp;which were detached before fossilisation. Other impressionsnbsp;are identified as the basal portions of sporophylls bearingnbsp;sporangia: these suggest the expanded base of the fertile leavesnbsp;of Isoetes with vertically elongated sporangia, some of whichnbsp;have a smooth surface while in others traces of internal structurenbsp;are exposed; the interior consists of an irregular network withnbsp;depressions containing carbonised remains of spores.

While recognising a general resemblance to the sporophylls of Isoetes, certain differences are obvious : there is no ligule innbsp;the fossil leaves nor are there any distinct traces of vascularnbsp;strands such as occur in the leaves of recent species. Thenbsp;form of the sporangium, more elongated than in the majoritynbsp;of recent forms, is compared by Saporta with that in a southnbsp;European species Isoetes setacea Spr.

Such evidence as we have lends support to the inclusion of these Portuguese fossils in the genus Isoetites, but apart fromnbsp;the fact that we have no proof of any connexion between thenbsp;stems and supposed sporophylls, the resemblance of the latternbsp;to those of Isoetes is, perhaps, hardly sufficient to satisfy allnbsp;reasonable scepticism.

The generic name Isoetopsis was used by Saporta as more appropriate than Isoetes for some Eocene fossils from Aix-en-Provence which are too doubtful to rank as trustworthy evidencenbsp;of the existence of the recent genus. The species, Isoetopsisnbsp;subaphylla^ is founded on impressions of small scales, 4 mm.nbsp;long, bearing circular bodies which are compared with sporangianbsp;or spores.

Other records of fossils referred to Isoetes need not be described as they have no claim to be regarded as contributionsnbsp;towards the past history of the genus. Heer�s Miocene speciesnbsp;Isoetites Scheuzeri and I. Braunii linger^ from Switzerland arenbsp;based on unsatisfactory material and are of no importance.

The generic name Pleuromeia, was suggested by Corda� for a fossil from the Bunter Sandstone, the original description ofnbsp;which was based by Munster^ on a specimen discovered in anbsp;split stone from the tower of Magdeburg Cathedral.

The majority of the specimens have been obtained from the neighbourhood of Bernburg, but a few examples are recordednbsp;from Commern and other German localities; all are now includednbsp;under the name Pleuromeia Sternbergi. Germar, who published

one of the earlier accounts of the species, states that Corda dissented from Munsters choice of the name Sigillaria andnbsp;proposed the new generic title Pleuromeia. One of the bestnbsp;descriptions of the genus we owe to Solms-Laubach^ whosenbsp;paper contains references to earlier writers. Illustrations havenbsp;been published by M�nster, Germar^, Bischofr, Solms-Laubachnbsp;and Potonie^.

Pleuromeia Sternbergi is represented by casts of vegetative and fertile axes, but the preservation of the latter is notnbsp;sufficiently good to enable us to draw any very definitenbsp;conclusions as to the nature of the reproductive organs. Castsnbsp;of the stems reach a length of about 1 metre and a diameter ofnbsp;5�6 cm., or in some cases 10 cm.; all of them are in a morenbsp;or less decorticated state, the degree of decortication beingnbsp;responsible for difPerences in the external features which lednbsp;Spieker� to adopt more than one specific name.

Fig. 134, A, represents a sketch, made some years ago, of a specimen in the Breslau Museum which contains several examplesnbsp;of this species, among others those described by Germar innbsp;1852. The cylindrical cast (38 cm. long by 12 cm. in circumference), which has been slightly squeezed towards the uppernbsp;ond, bears spirally arranged imperfectly preserved leaf-scars andnbsp;the lower end shows the truncated base of one of the shortnbsp;�tigmaria-like arms characteristic of the plant. As shownnbsp;clearly in ^ specimen originally figured by Bischof and morenbsp;recently by Potonie^, the stem-base is divided by a doublenbsp;^ichotomy into four short and broad lobes with blunt apices andnbsp;cot upwards like the arms of a grappling iron (fig. 134, D).

^ Solms-Laubach (99). nbsp;nbsp;nbsp;^ Germar (52).nbsp;nbsp;nbsp;nbsp;� Bischof (53).

6 nbsp;nbsp;nbsp;(01) p, 754; (04) Lief ii. � Goeppert, in E�mer (54) PL xv. fig. 7.

The surface of this basal region is characterised by numerous circular scars (fig. 134, D; 4 scars enlarged) in the form of

C. nbsp;nbsp;nbsp;D. L^f-scars and base of stem: a, vascular tissue. (After Solms-

slightly projecting areas with a depression in the centre of each. These are undoubtedly the scars of rootlets, remains ofnbsp;which are occasionally seen radiating through the surrounding

rock. As seen in fig. 134, D, a, the fractured surface of a basal area may reveal the existence of an axial vascular cylindernbsp;giving off slender branches to the rootlets.

The bulbous enlargement at the base of the Brown seaweed Laminaria bulbosa Lam.' simulates the swollen base of Pleuromeia; but a confusion between these two plants is hardlynbsp;likely to occur. Above the Stigmaria-like base the graduallynbsp;tapered axis, in the less decorticated specimens, bears spirallynbsp;disposed transversely elongated areas consisting of two triangular scars between which is the point of exit of a leaf-trace.nbsp;The form of the leaf-scars is best seen on the face of a mouldnbsp;figured by Solms-Laubach (fig. 134, C): in this case the twonbsp;triangular areas appear as slight projections separated by anbsp;narrow groove marking the position of the vascular bundle ofnbsp;the leaf. The curved lines above and below the leaf-scarnbsp;probably mark the boundary of the leaf-base. The twonbsp;triangular scars are compared by Solms-Laubach and bynbsp;Potoni� with the parichnos-scars of Sigillaria and Lepido-dendron (c� fig. 146, C.), but the large size of the Pleuromeianbsp;scars constitutes an obvious difference though possibly not anbsp;distinction of importance.

The occurrence of a vertical canal filled with carbonaceous material in some of the stems throws light on the internalnbsp;structure: the canal, which is described by Solms-Laubach asnbsp;having a stellate outline in transverse section recalls the narrownbsp;central cylinder of a Lepidodendron stem, and this comparisonnbsp;is strengthened by the presence of obliquely ascending groovesnbsp;which represent leaf-traces passing through the cortex. Innbsp;specimens which have lost more of the cortical tissues thenbsp;surface is characterised by spirally disposed, discontinuousnbsp;vertical grooves representing portions of leaf-traces precisely asnbsp;they appear in similar casts of Lepidodendron. There is nonbsp;direct evidence of the existence of secondary wood in the stem,nbsp;but, as Potoni� has pointed out, the greater transverse elongationnbsp;of the leaf-scars in the lower part of a cast (fig. 134, A) pointsnbsp;to the production of some secondary tissue either in thenbsp;vascular cylinder or cortex, or possibly in both regions.

In some specimens of Pleuromeia the upper portion is clothed with crowded and imbricate sporophylls which reach anbsp;length of 2'5 cm., a maximum breadth of 2'7 cm., and a thicknessnbsp;of 1 mm. Each sporophyll has a thin wing-like border, andnbsp;on the lower face are several parallel lines. Solms-Laubachnbsp;describes the sporangium or ovule as attached to the lowernbsp;surface of the sporophyll and this opinion has been confirmed bynbsp;Fitting^ who has also brought forward satisfactory evidence innbsp;favour of the sporangial nature of the reproductive organs.nbsp;Fitting found numerous spores in the Bunter Sandstone nearnbsp;Halle; these are flattened circular bodies 0'5�0'7 mm. in diameter with a granulated surface and the three converging linesnbsp;characteristic of spores produced in tetrads. The comparisonnbsp;made by this author between the sporophylls of Pleuromeia,nbsp;which bore the sporangia on the lower surface instead of on thenbsp;upper as in other lycopodiaceous plants, and the pollen-sacs ofnbsp;Conifers, is worthy of note in reference to the possible relationship between Conifers and Lycopods.

A comparison of the Isoetes stem represented in fig. 132, A, with the base of a Pleuromeia shows a striking similarity, but,nbsp;as Fitting points out, the Stigmaria-like arms of the fossil contained a vascular cylinder whereas the blunt lobes of Isoetesnbsp;consist exclusively of cortical tissue, the roots being given offnbsp;from the grooves between the lobes of the tuberous stem.

The position of Pleuromeia must for the present be left an open question; it is, however, clear that the plant bears a closenbsp;resemblance in the form of its base to the Stigmarian branchesnbsp;of Lepidodendron and Sigillaria. The vegetative shoot appearsnbsp;to be constructed on a plan similar to that of these twonbsp;Palaeozoic genera, but the strobilus is of a different type. Itnbsp;would seem probable that Pleuromeia, may be closely allied tonbsp;Isoetes and to the arborescent Lycopods of Palaeozoic floras.nbsp;It is not improbably a link in a chain of types which includesnbsp;Sigillaria on the one hand and Isoetes on the other.

It is not improbable that a specimen from the Lower Bunter of Commern which Blanckenhorn made the type of anbsp;new species, Sigillaria oculina (fig. 134, B) is specifically

identical with Pleuromeia Sternhergi. An examination of a cast of the type-specimen in the Berlin Bergakademie led menbsp;to regard the fossil with some hesitation as a true Sigillaria,nbsp;but a more extended knowledge of Pleuromeia lends support tonbsp;the view adopted by Potonid^ that Blanckenhom�s plant is notnbsp;generically distinct from Pleuromeia Sternhergi. The resemblance between Sigillaria oculina and some of the Palaeozoicnbsp;species of Sigillaria emphasised by Weiss ^ has given rise to thenbsp;belief that the genus Sigillaria persisted into the Triassic era; itnbsp;is, however, highly probable that the Bunter specimen has nonbsp;claim to the generic name under which it has hithertobeen known.

The Bunter Sandstone in which Pleuromeia is the sole representative of plant-life, at least in certain localities, isnbsp;usually considered to be a desert formation. We may not be farnbsp;wrong in accepting Fitting�s suggestion that in this isolatednbsp;species we have a relic of the sparse vegetation which was ablenbsp;to exist where the presence of lakes added a touch of life to thenbsp;deadness of the Triassic desert.

Pleuromeia is recorded by Fliche as a rare fossil in the Middle Trias of France in the neighbourhood of Lun�ville''.

The history of our knowledge of fossil representatives of the Lycopodiales, as also of the Equisetales, affords a strikingnbsp;illustration of the danger of attempting to found a classificationnbsp;on such differences as are expressed by the terms herbaceousnbsp;and arborescent in the sense in which they are usuallynbsp;employed. As we have seen*, the presence of secondary wood innbsp;stems of the Palaeozoic plant now known as Catamites led sonbsp;competent a botanist as Adolphe Brongniart to recognise anbsp;distinct generic type Calamodendron, which he placed in thenbsp;Gymnosperms, reserving the designation Calamities for speciesnbsp;in which no indication of secondary thickening had been found.

Similarly, the genus Sigillaria was regarded as a Gymno-sperm because it was believed to be distinguished from

Lepidodendron by the power of forming secondary vascular tissues; the latter genus, originally thought to be alwaysnbsp;herbaceous, was classed with the Pteridophytes. At the timenbsp;when this unnatural separation was made between stems withnbsp;secondary wood and those in which no secondary wood wasnbsp;known to exist, botanists were not aware of the occurrence ofnbsp;any recent Pteridophyte which shared with the higher plantsnbsp;the power of secondary growth in thickness provided by meansnbsp;of a meristematic zone. It is true that the presence or absencenbsp;of a cambium does not in practice always coincide with thenbsp;division into herbaceous and arborescent plants: no one wouldnbsp;speak of a Date-Palm as a herbaceous plant despite the absencenbsp;of secondary wood.

The danger which should be borne in mind, in adopting as a matter of convenience the term herbaceous as a sectionalnbsp;heading, is that it should not be taken to imply a completenbsp;inability of the so-called herbaceous types to make secondarynbsp;additions to their conducting tissues. The specimens on whichnbsp;the species of Lycopodites and Selaginellites, (genera whichnbsp;may be designated herbaceous,) are founded are preserved asnbsp;impressions and not as petrifications; we can, therefore, basenbsp;definitions only on habit and on such features as are shownnbsp;by fertile leaves and sporangia. We are fully justified innbsp;concluding from evidence adduced by Goldenberg more thannbsp;fifty years ago and from similar evidence brought to light bynbsp;more recent researches, that there existed in the Palaeozoic eranbsp;lycopodiaceous species in close agreement in their herbaceousnbsp;habit with the lycopods of present-day floras. It has beennbsp;suggested¹ that the direct ancestors of the genera Lycopodiumnbsp;and Selaginella are represented by the species of Lycopoditesnbsp;and Selaginellites rather than by Lepidodendron and Sigillaria,nbsp;the arborescent habit of which has been rendered familiar bynbsp;the numerous attempts to furnish pictorial reproductions of anbsp;Palaeozoic forest. Until we are able to subject the speciesnbsp;classed as herbaceous to microscopical examination we cannotnbsp;make any positive statement as to the correctness of this view.

but such facts as we possess lead us to regard the suggestion as resting on a sound basis.

Palaeobotanical literature abounds in records of species of Lycopodites, Lycopodium, Selaginella and Selaginites, whichnbsp;have been so named in the belief that their vegetative shootsnbsp;bear a greater resemblance to those of recent lycopodiaceousnbsp;plants than to the foliage shoots of Lepidodendron. Manynbsp;of these records are valueless: Lepidodendra, twigs of Both-rodendron^ species of conifers, fern rhizomes, and Aphlebiae^nbsp;have masqueraded as herbaceous lycopods. It is obvious thatnbsp;an attempt to identify fossils presenting a general agreement innbsp;habit and leaf-form with recent species of lycopods must benbsp;attended with considerable risk of error. Recent Conifersnbsp;include several species the smaller branches of which simulatenbsp;the leafy shoots of certain species of Lycopodium andnbsp;Selaginella, and it is not surprising to find that this similaritynbsp;has been responsible for many false determinations. Amongnbsp;Mosses and the larger foliose Liverworts there are species whichnbsp;in the condition of imperfectly preserved impressions, mightnbsp;easily be mistaken for lycopodiaceous shoots: an equallynbsp;close resemblance is apparent in the case of some floweringnbsp;plants, such as New Zealand species of Veronica, Tafallanbsp;graveolens (a Composite), Lavoisiera lycopodiodes Card.� (anbsp;species of Melastomaceae), all of which have the habit ofnbsp;Cupressineae among the conifers as well as of certain lycopodiaceous plants. It may be impossible to decide whether fossilnbsp;impressions of branches, which are presumably lycopodiaceous,nbsp;bear two kinds of leaves^ like the great majority of recentnbsp;species of Selaginella. Selaginella grandis, if seen from thenbsp;under surface, would appear to have two rows of leaves onlynbsp;and might be confused with a small twig of such a conifer asnbsp;Dacrydium Kirkii, a New Zealand species.

The New Zealand conifers Dacrydium cupressinum Soland., and Podocarpus dacrydioides Rich, closely simulate species ofnbsp;Selaginellites and Lycopodites: in the British Museum a

specimen of the latter species bears a label describing it as Lycopodium arboreum (Sir Joseph Hooker and Dr Solander;nbsp;1769). The twigs of the Tasmanian conifer Microcachyrsnbsp;tetragona Hook. f. are very similar in habit to shoots of thenbsp;recent Lycopodium tetragoniim (fig. 121, C).

In the description of examples of Lycopodites and Selaginel-lites I have confined myself to such as appear to be above suspicion either because of the presence of spore-bearing organsnbsp;or, in a few cases, because the specimens of sterile shoots arenbsp;sufficiently large to show the form of branching in addition tonbsp;the texture of the leaves. The two generic names Lycopoditesnbsp;and Selaginellites are employed for fossil species which therenbsp;are substantial grounds for regarding as representatives ofnbsp;Lycopodium and Selaginella. The designation Selaginellites isnbsp;adopted only for species which afford evidence of heterospory;nbsp;the name Lycopodites, on the other hand, is used in a comprehensive sense to include all forms�whether homophyllous or heterophyllous�which are not known to be heterosporous. Thisnbsp;restricted use of the generic name Selaginellites is advocatednbsp;by Zeiller^, who instituted the genus, and by Halle ^ in hisnbsp;recent paper on herbaceous lycopods.

The generic term Lycopodites was used by Brongniart in 1822� in describing some Tertiary examples of slender axesnbsp;clothed with small scale-like leaves which he named Lycopoditesnbsp;squamatus. These are fragments of coniferous shoots. In thenbsp;Prodrome d�une histoire des v�g�taux fossiles^ Brongniartnbsp;included several Palaeozoic and Jurassic species in Lycopoditesnbsp;and instituted a new genus Selaginites, expressing a doubt as tonbsp;the wisdom of attempting to draw a generic distinction betweennbsp;the two sets of species. In a later work� he recognised only onenbsp;undoubted species, Lycopodites falcatus. The first satisfactorynbsp;account of fossils referred to Lycopodites is by Goldenberg�

who gave the following definition of the genus;�� Branches with leaves spirally disposed or in whorls. Sporangia in thenbsp;axil of foliage leaves or borne in terminal strobili.�

It was suggested by Lesquereux^ that Goldenberg�s definition, which was intended to apply to herbaceous species, should be extended so as to include forms with woody stemsnbsp;but which do not in all respects agree with Lepidodendron.nbsp;Kidston^ subsequently adopted Lesquereux�s modification ofnbsp;Goldenberg�s definition. We cannot draw any well-defined linenbsp;between impressions of herbaceous forms and those of smallnbsp;arborescent species. We use the name Lycopodites for suchnbsp;plants as appear to agree in habit with recent species ofnbsp;Lycopodium and Selaginella and which, so far as we know, werenbsp;not heterosporous: it is highly probable that some of thenbsp;species so named had the power of producing secondary wood,nbsp;a power possessed by some recent Pteridophytes which nevernbsp;attain the dimensions of arborescent plants.

It has been shown by Halle�, who has re-examined several of Goldenberg�s specimens which have been acquired by thenbsp;Stockholm Palaeobotanical Museum, that some of his speciesnbsp;of Lycopodites are heterosporous and therefore referable tonbsp;Zeiller�s genus Selaginellites.

In 1869 Renault described two species of supposed Palaeozoic Lycopods as Lycopodium punctatum and L. Renaultii^, the latter name having been suggested by Brongniart to whomnbsp;specimens were submitted. These species were afterwardsnbsp;recognised by their author as wrongly named and werenbsp;transferred to the genus Heterangium^, a determination whichnbsp;is probably correct; it is at least certain that the use of thenbsp;name Lycopodium cannot be upheld.

We have unfortunately to rely on specimens without petrified tissues for our information in regard to the history ofnbsp;Lycopodites and Selaginellites. Among the older fossils referrednbsp;to Lycopodites are specimens from Lower Carboniferous rocksnbsp;at Shap in Westmoreland which Kidston originally described

as Lycopodites Vanuxemi^, identifying them with Goeppert�s Sigillaria Vanuxemi^ founded on German material. In a laternbsp;paper Kidston transferred the British specimens of vegetativenbsp;shoots to a new genus Archaeosigillaria^

The plant so named was discovered in Lower Carboniferous strata of Eskdale, Dumfries, Scotland; it is represented bynbsp;imperfectly preserved shoots bearing a terminal strobilus andnbsp;was originally described by Kidston as apparently possessing twonbsp;kinds of foliage leaves borne in whorls. The larger leaves havenbsp;an ovate cordate lamina with an acuminate apex, while thenbsp;smaller leaves, which are less distinct, are transverselynbsp;elongated, and simulate sporangia in appearance. Dr Kidston�snbsp;figure of this species has recently been reproduced bynbsp;Professor Bower� who speaks of the supposed smaller leaves asnbsp;sporangia, a view with which the author of the species agrees.nbsp;It would appear that this identification is, however, basednbsp;solely on external resemblance and has not been confirmed bynbsp;the discovery of any spores. Assuming the sporangial naturenbsp;of these structures, this Palaeozoic type represents, as Bowernbsp;points out, a condition similar to that in some recent species ofnbsp;Lycopodium in which sporangia are not confined to a terminalnbsp;strobilus but occur also in association with ordinary foliagenbsp;leaves. The strobilus consists of crowded sporophylls which arenbsp;too imperfect to afford any definite evidence as to theirnbsp;homosporous or heterosporous nature. As Solms-Laubach�nbsp;points out, this type recalls Lycopodium Phlegmaria amongnbsp;recent species.

Professor Penhallow^ instituted this name for a specimen measuring 8 cm. long by 6 mm. in breadth, collected by Mr

Eeid from the Old Eed Sandstone of Caithness, consisting of an axis bearing narrow lanceolate leaves some of which bearnbsp;sporangia at the base.

The species, figured by Geinitz as Lycopodites Guthieri^, from the Coal-Measures of Saxony is probably a true representative of the genus. The Saxon specimens are heterophyllous ; the larger lanceolate and slightly falcate leavesnbsp;arranged in two rows, are 4�5 mm. long while the smallernbsp;leaves are one half or one third this size; some of thenbsp;dichotomously branched shoots terminate in long and narrownbsp;strobili not unlike those of Zeiller�s species Selaginellitesnbsp;Suissei^. Kidston� has included under this specific name somenbsp;fragments collected by Hemingway from the Upper Coal-Measures of Eadstock, Somersetshire, but as only one form ofnbsp;� leaf is seen the reasons for adopting Goeppert�s designation arenbsp;perhaps hardly adequate.

Under this name Kidston describes a small specimen, obtained by Hemingway from the Middle Coal-Measures ofnbsp;Barnsley in Yorkshire, consisting of a slender forked axisnbsp;bearing oval-acuminate leaves approximately 5 mm. long withnbsp;a finely ciliate margin. Associated with the leaves were foundnbsp;spores which Kidston regards as megaspores.

This species, originally described by Goldenberg from the Coal-Measures of Saarbr�cken has been re-examined by Halle�nbsp;who is unable to confirm Goldenberg�s statement as to hetero-

Halle has founded this species on specimens, from the Coal-Measures of Zwickau in Saxony, characterised by dimorphic

lanceolate leaves in four rows, the larger being 4�-6 mm. long: the smaller leaves have a ciliate edge. A comparison is madenbsp;with the recent species Selaginella arabica Baker, 8. revolutaBak.,nbsp;and 8. arniata Bak. in which the leaves are described as ciliate.nbsp;In the absence of sporangia and spores the species is placed innbsp;the genus Lycopodites.

Lycopodites lanceolatus (Brodie). Fig. 136.

1901 nbsp;nbsp;nbsp;Naiadita lanceolata, Sollas� (figures showing habit of the

Pig. 136. Lycopodites lanceolatus (Brodie). (After Miss Sollas. x 40.) a, Sporangium wall; b, leaf,nbsp;c, remains of tubular elements in stem.

Specimens referred to this species were originally recorded by Brodie from Rhaetic rocks in the Severn valley, the namenbsp;Naiadita being chosen as the result of Bindley�s comparison ofnbsp;the small and delicate leaves with those of recent species of thenbsp;Monocotyledonous family Naiadaceae. The species may benbsp;described as follows:

Plant slender and moss-like in habit. The axis, which is delicate and thread-like, bears numerous linear acuminate ornbsp;narrow ovate leaves reaching a length of approximately 5 mm.nbsp;Under a low magnifying power the thin lamina of the leavesnbsp;is seen to have a superficial layer of polygonal or rectangularnbsp;cells arranged in parallel series (fig. 136 b). There is no trace ofnbsp;midrib or stomata. The sporangia are more or less sphericalnbsp;and short-stalked, situated at the base of the foliage leaves andnbsp;containing numerous tetrads of spores. The spores have anbsp;diameter of 0'08 mm.

Buckman founded additional species on differences in the shape of the leaves but, as Miss Sollas has pointed out, suchnbsp;differences as he noticed may be detected on the same axis.nbsp;It was stated in an earlier chapter^ that Starkie Gardner, onnbsp;insufficient evidence, proposed to place Brodie�s plant amongnbsp;the Mosses. The discovery by Mr Wickes of new material atnbsp;Pylle hill near Bristol afforded an opportunity for a re-examination of the species; this was successfully undertaken by Missnbsp;Sollas who was able to dissolve out spores from the matrix bynbsp;dilute hydrochloric acid, and to recognise the remains of internalnbsp;structure in the slender axes by exposing successive surfacesnbsp;with the aid of a hone. It was found that sporangia occurrednbsp;at the base of some of the leaves containing numerous tetradsnbsp;of spores, the individual spores having a diameter of 0-08 mm.,nbsp;apparently twice as large as those of any recent species ofnbsp;Lycopodium. Fig. 136 shows a sporangium, a, at the base ofnbsp;a leaf, h. Indications of tubular elements were recognised innbsp;the stem and it is noteworthy that although the outlines ofnbsp;epidermal cells on the leaves are well preserved no stomata werenbsp;found. The leaves of the recent American species Lycopodium alopecuroides Linn. var. aquaticum Spring^, which livesnbsp;1 Vol. I. p. 240.nbsp;nbsp;nbsp;nbsp;2 Sollas (01) p. 311.

under water, possess stomata. It is probable that in Lycopodites lanceolatus the leaves had a very thin lamina and may havenbsp;been similar in structure to those of recent Mosses; the plantnbsp;possibly lived in very humid situations or grew submerged.nbsp;Miss Sollas�s investigations afford a satisfactory demonstrationnbsp;of the lycopodiaceous nature of this small Rhaetic species: asnbsp;I have elsewhere suggested^, the generic name Lycopodites shouldnbsp;be substituted for that of Naiadita. Examples of this speciesnbsp;may be seen in the British Museum.

The Rhaetic species from Scania, Lycopodites scanicus Nath.^ {in litt.), recently re-described by Halle and originally referrednbsp;by Nathorst to Oleichenia affords another example of thenbsp;occurrence of a small herbaceous lycopod of Rhaetic age.

� Schimper (70) A. p. 9. nbsp;nbsp;nbsp;� Young and Bird (22) A. PI. ii. fig. 7.

Inferior Oolite rocks of the Yorkshire coast bearing � small round crowded leaves,� which was afterwards described bynbsp;Lindley from additional material obtained from Clough tonnbsp;near Scarborough as Lycopodites falcatus. The example represented in fig. 137 shows the dichotomously branched shootsnbsp;bearing two rows of broadly falcate leaves. A careful examination of the type-specimen^ revealed traces of what appeared to benbsp;smaller leaves, but there is no satisfactory proof of heterophylly.nbsp;No sporangia or spores have been found. This British speciesnbsp;has been recorded from Lower Jurassic or Rhaetic rocks ofnbsp;Bornholm^ and a similar though probably not identical type,nbsp;Lycopodites Victoriae^, has been recognised in Jurassic stratanbsp;of Australia (South Gippsland, Victoria). An Indian plantnbsp;described by Oldham and Morris^ from the Jurassic flora of thenbsp;Rajmahal hills as Araucarites (?) gracilis and subsequentlynbsp;transferred by Feistmantel to Schimper�s genus Gheirolepis^nbsp;may be identical with the Yorkshire species. The Jurassicnbsp;fragments described by Heer from Siberia as Lycopoditesnbsp;tenerrimus^ may be lycopodiaceous, but they are of no botanicalnbsp;interest.

Other examples of Mesozoic Lycopods have been recorded, but in the absence of well-preserved shoots and sporangia theynbsp;are noteworthy only as pointing to a wide distribution of Lycopodites in Jurassic and Cretaceous floras'^.

From Tertiary strata species of supposed herbaceous lycopods have been figured by several authors, one of the best of whichnbsp;is Selaginella Berthoudi Lesq.� from Tertiary beds in Colorado.nbsp;This species agrees very closely in the two forms of leafnbsp;with Selaginella grandis, but as the specimens are sterile wenbsp;have not sufficient justification for the employment of thenbsp;generic name Selaginellites.

� Nathorst (90) A. PI. ii. fig. 3. Saporta (94) Pis. xxiii.�xxvi. � Kuowlton (98) p. 136.

This generic name has been instituted by Zeiller^ for specimens from the coal basis of Blanzy (France). It is appliednbsp;to heterosporous species with the habit of Selaginella: Zeillernbsp;preferred the designation Selaginellites to Selaginella on thenbsp;ground that the type species differs from recent forms in havingnbsp;more than four megaspores in each megasporangium. It is,nbsp;however, convenient to extend the term to all heterosporousnbsp;fossil species irrespective of the spore-output.

This species was described in Zeiller�s preliminary note^ as Lycopodites Suissei, but he afterwards transferred it to the genusnbsp;Selaginellites. In habit the plant bears a close resemblance tonbsp;Lycopodites macrophyllus of Goldenberg; the shoots, 1.�3 mm.nbsp;thick, are branched in a more or less dichotomous fashion andnbsp;bear tetrastichous leaves. The larger leaves reach a length ofnbsp;4�6 mm. and a breadth of 2�3 mm ; the smaller leaves arenbsp;described as almost invisible, closely applied to the axis, oval-lanceolate and 1�2 mm. long with a breadth of 0�5�075 mm.nbsp;Long and narrow strobili (15 cm. by 8�10 mm.) terminate thenbsp;fertile branches; these bear crowded sporophylls with a triangular lamina and finely denticulate margin. Oval sporangianbsp;were found on the lower sporophylls containing 16�24 sphericalnbsp;megaspores 0'6�0'65 mm. in diameter. The outer membranenbsp;of the spore is characterised by fine anastomosing ridges andnbsp;thin plates radiating from the apex and forming an equatorialnbsp;collarette. The microspores have a diameter of 40�60yu, andnbsp;the same type of outer membrane as in the megaspores. Thenbsp;megaspores of the recent species Selaginella caulescens, asnbsp;figured by Bennie and Kidston resemble those of the Palaeozoicnbsp;type in the presence of an equatorial flange. It is interestingnbsp;to find that, in spite of the occurrence of 16�24 megasporesnbsp;in a single sporangium the size of the fossil spores exceedsnbsp;that of the recent species.

1 Zeiller (06) p. 141, Pis. xxxix. xli. nbsp;nbsp;nbsp;� Zeiller (00) p. 1077.

The drawing reproduced in fig. 138 is a copy of that of the type-specimen : another specimen, named by Goldenberg, is figured by Halle in his recently published paper. The leaves appearnbsp;to be distichous; no smaller leaves have been detected, thoughnbsp;Halle is inclined to regard the plant as heterophyllous. Thenbsp;sporophylls, borne in slender terminal strobili, are smaller thannbsp;the foliage leaves and spirally disposed (fig. 138; smallernbsp;specimen). Halle succeeded in demonstrating that some ofnbsp;the sporangia contained a single tetrad of spores, each sporenbsp;having a diameter of 0'4�0'5 mm. No microspores were found,nbsp;but it is clear that the species was heterosporous and that itnbsp;agrees with recent species in having only four spores in thenbsp;megasporangium.

The shoots of this species resemble the recent Lycopodium complanatum', they differ from those of Selaginellites pritnaevusnbsp;in their long and narrow branches which bear two forms of leaf.nbsp;The longer leaves, arranged in opposite pairs, are slightly falcate;nbsp;the smaller leaves are appressed to the axis and have a triangularnbsp;cordate lamina. Another peculiarity of this species is thenbsp;occurrence of sporangia in the axil of the foliage leaves, a featurenbsp;characteristic of the recent Lycopodium Selago. In recentnbsp;species of Selaginella the sporophylls are always in strobili. Nonbsp;microspores have been found nor the walls of niegasporangia,nbsp;but tetrads of raegaspores were isolated by Halle; the spores havenbsp;three radiating ridges (fig. 135, B) connected by an equatorialnbsp;ridge. Halle estimates the number of spores (0�45 mm. innbsp;diameter) in a sporangium at 20 to 30. In size as in numbernbsp;the spores exceed those of recent species and agree more nearlynbsp;with the megaspores of S. Suissei.

It would seem to be a general rule that the spores (megaspores) of the fossil herbaceous species exceeded considerably in

dimensions those of recent forms and on the other hand were smaller than those of the Palaeozoic arborescent species.

There can be little doubt that some of the Mesozoic and Tertiary species included under Lycopodites agree more closelynbsp;with the recent genus Selaginella than with Lycopodium, butnbsp;this does not constitute an argument of any importance againstnbsp;the restricted use of the designation Selaginellites which wenbsp;have adopted. From a botanical point of view the variousnbsp;records of Lycopodites and Selaginellites have but a minor importance; they are not sufficiently numerous to throw any lightnbsp;on questions of distribution in former periods, nor is the preservation of the material such as to enable us to compare the fossilnbsp;with recent types either as regards their anatomy or, except innbsp;a few cases, their sporangia and spores. The Palaeozoic speciesnbsp;are interesting as revealing less reduction in the number of sporesnbsp;produced in the megasporangia. Among existing Pteridophytesnbsp;the genus Isoetes agrees more closely than Selaginella, as regardsnbsp;the number of megaspores in each sporangium, with such fossilsnbsp;as Selaginellites Suissei and S. elongatus.

It would seem that in most Palaeozoic species heterospory had not reached the same stage of development as in the recentnbsp;genus Selaginella in which the megaspores do not exceed fournbsp;in each sporangium. In Selaginellites primaevus, however, thenbsp;heterospory appears to be precisely of the same type as innbsp;existing species.

The generic name Lycostrobus has recently been instituted by Nathorst^ for certain specimens of a lycopodiaceous strobilus,nbsp;from the Rhaetic strata of Scania, which he formerly referred tonbsp;the genus Androstrobus'^.

The fossil described under this name is of special interest as affording an example of a Mesozoic lycopodiaceous cone comparable in habit and in size with some of the largest examplesnbsp;^ Nathorst (08).nbsp;nbsp;nbsp;nbsp;^ Nathorst (02^ p. 5, PI. r. fig. 1.

of Palaeozoic Lepidostrobi, the cones of Lepidodendron. The Swedish fossil from Upper Rhaetic strata of Helsingborg (Scania)nbsp;was originally designated Androstrobus Scotti, the generic namenbsp;being adopted in view of the close resemblance of the form ofnbsp;the strohilus to the male flower of a Cycad. A more completenbsp;examination has shown that the bodies, which were thought to

he pollen-sacs�though Nathorst recognised certain differences between them and the pollen-sacs of lycopods�are thenbsp;megaspores of a lycopod. Microspores have also been identified.nbsp;The axis of the cone has a breadth of 2 cm. with a pedunclenbsp;which may be naked or provided with a few small scales; the

sporophyll region of the axis reached a length of at least 12 cm. The spirally disposed sporophylls terminate in a rhombic distalnbsp;end which may represent the original termination or theynbsp;may have been prolonged upwards as free laminae. Eachnbsp;sporophyll bears on its upper face a single large sporangiumnbsp;containing either megaspores or microspores: the megaspores,nbsp;0�55�0'60 mm. in diameter, are finely granulate and bear smallnbsp;warty thorns or more slender pointed appendages. The microspores, after treatment with eau de Javeile, were found tonbsp;measure 36�44/a while others which had been treated withnbsp;ammonia reached 54/a in diameter. Nathorst describes thenbsp;microspores as occurring in spherical groups or balls, whichnbsp;it is suggested may be compared with the groups of sporesnbsp;separated by strands of sterile tissue (trabeculae) in the largenbsp;sporangia of Isoetes (cf. fig. 133, H). If this comparison is soundnbsp;it would point to a more complete septation of the sporangiumnbsp;in Lycostrobus than in any recent species of Isoetes. The sizenbsp;of the strobilus would seem to indicate the persistence intonbsp;the Rhaetic era of an arborescent lycopodiaceous type; but thenbsp;appearance and manner of preservation of the axis is interpreted by Nathorst as evidence of a herbaceous rather thannbsp;a woody structure. He is disposed to regard Isoetes as thenbsp;most nearly allied existing genus.

The comparison made by Nathorst with Isoetes is based on a resemblance between the spores of the two genera and on thenbsp;evidence, which is not decisive, of the existence of sterilenbsp;strands of tissue in the sporangia of Lycostrobus. Thisnbsp;similarity is however hardly of sufficient importance to justifynbsp;the inclusion of the Rhaetic strobilus in the Isoetaceae. Innbsp;size and in the arrangement and form of the sporophylls thenbsp;cone presents a much closer resemblance to Lepidodendron thannbsp;to Isoetes. It is probably advisable to regard this Rhaeticnbsp;type simply as a lycopodiaceous genus which we are unable,nbsp;without additional information, to assign to a particular position.

The opinion expressed by Professor Fliche^ that the plant described by Schimper and Mougeot as Caulopteris tessellata,nbsp;a supposed tree-fern stem, from Triassic rocks of Lorraine, is

more probably a large lycopodiaceous stem, either a Lepido-dendron or a new genus, is worthy of note in reference to Nathorst�s account of Lycostrohus.

In habit the fossil strobilus may be compared with the triassic genus Pleuromeia, but the position of the sporangia onnbsp;the sporophylls constitutes a well-marked difference. Thenbsp;most important result of Nathorst�s skillful treatment of thisnbsp;interesting fossil by chemical microscopic methods is thenbsp;demonstration of the existence of a large heterosporous type ofnbsp;lycopodiaceous cone in a Rhaetic flora.

Under this generic name M. Fliche^ has briefly described a fertile lycopodiaceous shoot from the Triassic rocks of Epinal innbsp;France; the type species Poecilitostachys Hangi consists of anbsp;cylindrical axis, 10 cm. x 5 mm., deprived of leaves and terminating in a rounded receptacle bearing a capitulum of bracts ornbsp;fertile leaves. Detached megasporangia containing small globularnbsp;bodies found in association with the capitulum are comparednbsp;with the megasporangia of Isoetes.

CHAPTER XV.

Among the best known plants in the Palaeozoic floras are the genera Lepidodendron and Sigillaria, types which are oftennbsp;spoken of as Giant Club-Mosses or as ancestors of existingnbsp;species of Lycopodium and Selaginella. Of these genera, butnbsp;more particularly of Lepidodendron, we possess abundantnbsp;records in a condition which have made it possible to obtainnbsp;fairly complete information not only in regard to habit andnbsp;external features but as to the anatomical characters of bothnbsp;vegetative and reproductive shoots. The structure of Lepidodendron differs too widely from that of recent Club-Mosses (speciesnbsp;of Lycopodium) to justify the statement that this prominentnbsp;member of the Palaeozoic vegetation may be regarded as a directnbsp;ancestor of any living plant. There is at least no doubt thatnbsp;Lepidodendron and Sigillaria must be included in the Pterido-phyta. The description by Dr Scott ^ of the genus Lepidocarpon,nbsp;founded on petrified specimens of strobili, demonstrated thenbsp;existence of a type of lycopodiaceous plant in the Carboniferousnbsp;period distinguished from all living representatives of thenbsp;group by the possession of integumented megaspores, which maynbsp;fairly be styled seeds. Lepidocarpon and another seed-bearingnbsp;plant Miadesmia are described under a separate heading asnbsp;lycopodiaceous types characterised by an important morphological feature, which among recent plants constitutes anbsp;differentiating character between the Pteridophytes and thenbsp;Phanerogams.

The genus Lepidodendron included species comparable in size with existing forest trees. A tapered trunk rose verticallynbsp;to a height of 100 feet or upwards from a dichotomouslynbsp;branched subterranean axis of which the spreading branches,nbsp;clothed with numerous rootlets, grew in a horizontal directionnbsp;probably in a swampy soil or possibly under water. Anbsp;description by Mr Rod way ^ of Lycopods on the border of anbsp;savannah in Guiana forming a miniature forest of Pine-likenbsp;Lycopodiums might, with the omission of the qualifyingnbsp;adjective, be applied with equal force to a grove of Lepidodendra.nbsp;The equal dichotomy of many of the branches gave to the tree anbsp;habit in striking contrast to that of our modern forest trees, but,nbsp;on the other hand, in close agreement with that of such recentnbsp;species of Lycopodium as L. cernuum (fig. 123), L. ohsciirumnbsp;(fig. 124) and other types. Linear or oval cones terminatednbsp;some of the more slender branches (fig. 188) agreeing in size andnbsp;form with the cones of the Spruce Fir and other conifers or withnbsp;the male flowers of species of Araucaria, e.g. A. imbricata.nbsp;Needle-like leaves, varying considerably in length in differentnbsp;species, covered the surface of young shoots in crowded spiralsnbsp;and their decurrent bases or leaf-cushions formed an encasingnbsp;cylinder continuous with the outer cortex. The fact that leavesnbsp;are usually found attached only to branches of comparativelynbsp;small diameter would seem to show that Lepidodendron, thoughnbsp;an evergreen, did not retain its foliage even for so long a periodnbsp;as do some recent conifers.

By the activity of a zone of growing tissue encircling the cylinder of wood the main trunk and branches grew in thicknessnbsp;year by year; the general uniformity in size of the secondarynbsp;conducting elements affords no indication of changing seasons.nbsp;As the branches grew stouter and shed their leaves the surface ofnbsp;the bark resembled in some degree that of a Spruce Fir and othernbsp;species of Picea, in which the leaf-scars form the upper limit of

prominent peg-like projections, which, at first contiguous and regular in contour, afterwards become less regular and separatednbsp;by grooves (fig. 140) and at a later stage lose their outline as thenbsp;hark is stretched to the tearing point (fig. 140, C), The leaflessnbsp;branches of Lepidodendron were covered with spirally disposednbsp;oval cushions less peg-like and larger than the decurrent leaf-bases of Picea, which show in the upper third of their length anbsp;clean-cut triangular area and swell out below into two prominentnbsp;cheeks separated by a median groove and tapering withnbsp;decreasing thickness to a pointed base, which in some formsnbsp;{e.g. Lepidodendron Veltheimianum, fig. 185, C, D), is prolongednbsp;as a curved ridge to the summit of a lower leaf-cushion.

ABC

Fig. 140. Picea excelsa. Shoots of different ages showing changes in the appearance of the leaf-cushions: a leaf attached to a cushion in fig. A. (Slightly enlarged.)

A portion of the cushion below the triangular leaf-scar often shows transverse gaping cracks or depressions (fig. 185, C) suchnbsp;as occur on a smaller scale on the older cushions of a Fir twignbsp;(fig. 140). Secondary thickening, as in recent trees, is not confinednbsp;to the vascular cylinder but at an early stage, frequently beforenbsp;there are any signs of secondary wood, the outer region of thenbsp;broad cortex becomes the seat of active cell-formation whichnbsp;results in the addition of a considerable thickness to the bark.nbsp;At a later stage of increase in girth, the leaf-cushions are stretched

apart and the original surface-features become obliterated by vertical cracks and by the exfoliation of the superficial tissues^.

Some species of Lepidodendron produced branches characterised by spiral or vertical series of scars; these in older shoots were replaced by depressions having a diameter of several inchesnbsp;and comparable in appearance, as also perhaps in manner ofnbsp;formation, with the scars left on the stem of a Kauri Pinenbsp;{Agathis australis)^ on the abscission of lateral branches by anbsp;natural process. These shoots, known as Ulodendron, arenbsp;described in a subsequent section. (Page 128.)

A fully-grown Lepidodendron must have been an impressive tree, probably of sombre colour, relieved by the encircling felt ofnbsp;green needles on the young pendulous twigs. The leaves ofnbsp;some species were similar to those of a fir while in others theynbsp;resembled the filiform needles of the Himalayan Pine {Finnsnbsp;longifolia). The occasional presence of delicate hyphae in thenbsp;tissues of Lepidodendron demonstrates susceptibility to fungalnbsp;pests.

Architecturally, if one may use the term, Lepidodendron owed its power of resistance to the bending force of the windnbsp;to its stout outer bark formed of thick-walled elements producednbsp;by the activity of a cylinder of cortical meristem (figs. 148,172,nbsp;etc.). The vascular axis, of insignificant diameter in proportionnbsp;to the size of the stem (figs. 152, 153, 172, 181, A), must havenbsp;played a subordinate part, from a mechanical point of view, asnbsp;compared with the solid mass of wood of a Pine or an Oak.

Within the compass of a text-book it is impossible, even if it were desirable, to include an account of the majority of thenbsp;species of the widely distributed Palaeozoic genus Lepidodendron.nbsp;In spite of the great number of known species of this commonnbsp;member of Carboniferous floras, our knowledge of the type as anbsp;whole is deficient in many points, and such information as wenbsp;possess needs systematising and extending by comparativenbsp;treatment based on a re-examination of available data.

1 nbsp;nbsp;nbsp;A good example of an old Lepidodendron stem (L. aculeatmn) is figured bynbsp;Zalessky (04) PI. i. fig. 3.

features characteristic of Lepidodendron stems it is essential to have some knowledge of the internal structure.

A dual system of terminology has been unavoidably adopted for species of Lepidodendron: the majority of specific names havenbsp;been assigned to fossils known only in the form of casts or impressions, while petrified fragments, which unfortunately seldomnbsp;show the surface-features, have received another set of names.nbsp;A glance at the older palaeobotanical literature reveals thenbsp;existence of several generic designations, which fuller informationnbsp;has shown to have been applied to lepidodendroid shootsnbsp;deprived of some of their superficial tissues before fossilisationnbsp;and differing considerably in appearance from the more completenbsp;branches of the same species'. It has in some instances beennbsp;possible to correlate the two sets of specimens, casts or impressions, showing external features, and petrified fragments.nbsp;We may reasonably expect that future discoveries will enablenbsp;us to piece together as definite specific types specimens atnbsp;present labelled with different names.

A well-preserved leaf-cushion of a Lepidodendron�the most obvious distinguishing feature of the genus�is rhomboidal ornbsp;fusiform and vertically elongated (fig. 146, C, E; fig. 185, C, D):nbsp;in exceptional cases it may reach a length of 8 cm. and a breadthnbsp;of 2 cm. The cushion as a whole represents a prominent portionnbsp;of the stem or branch comparable with the elevation on the twignbsp;of a Spruce Fir and the leaf-base of a Lycopodium (cf. fig. 121, A,nbsp;lower portion) which appears in a transverse section of a branchnbsp;as a rounded prominence (cf. Lycopodium, fig. 125, A and H).nbsp;Disregarding differences in detail, a typical Lepidodendronnbsp;leaf-cushion is characterised by a clearly defined smoothnbsp;area often situated in the middle region (fig. 146, C, s).nbsp;This is the leaf-scar or place of attachment of the base ofnbsp;the leaf which was cut off by an absciss-layer while the branchnbsp;was comparatively young, as in recent forest trees and in somenbsp;species of Ferns. On the leaf-scar are three smaller scars ornbsp;cicatricules, the central one is circular or more or less triangularnbsp;in outline, the two lateral scars being usually oval or circular.nbsp;The central pit marks the position of the single vascular bundle

which constituted the conducting tissue connecting the leaf with the main vascular system of the stem. The two lateralnbsp;scars (figs. 145, K,p-, 146, C, s; 147, p) represent the exposed endsnbsp;of two strands of tissue, the-forked branches of a strand whichnbsp;pass from the middle cortex of the stem into the leaf; this isnbsp;known as the parichnos, a name proposed by Professor Bertrandnbsp;in 18911

The specimen shown in fig. 141 shows the linear leaves attached to their respective cushions.

Fig. 141. Lepidodendron Sternbergii. From a specimen in the British Museum (No. v. 1235) from the Coal-Measures of Shropshire.nbsp;(Nat. size.)

The lamina has a well-defined median keel on the lower surface and on either side a groove in which sections of petrifiednbsp;leaves have demonstrated the occurrence of stomata (cf fig. 142).

All Lepidodendron leaves, so far as we know, possessed a single median vein only. In some species, as for example innbsp;Lepidodendron longifolium Brongn., they have the form of long

and slender acicular needles very similar to those of Pinus longifoliuni', in L. Sternhergii (fig. 141) they are much broadernbsp;and shorter. In external form as in internal structure it is oftennbsp;impossible to distinguish between the leaves of Lepidodendronnbsp;and Sigillaria. The distinguishing features enumerated by thenbsp;late M. Renault cannot be employed, with any great degree ofnbsp;confidence, as diagnostic characters. In transverse section thenbsp;lamina of a Lepidodendron leaf presents the same appearance asnbsp;that of the Sigillarian leaves represented in fig. 142. Near the

base the free part of the leaf is usually subrhomboidal in section with short lateral wings, a ventral keel and two stomatalnbsp;grooves (fig. 142, A, B, g). The form and arrangement of stomata are shown in fig. 143, A, which was drawn from a piece of anbsp;leaf shown in surface-view in a section lent to me by Professornbsp;Weiss. It should, however, be pointed out that the leaf cannotnbsp;be certainly identified with Lepidodendron rather than with

Sigillaria, but as the leaves of these two genera are constructed on the same plan the identification is of secondary importance.

The single xylem bundle consists of primary tracheae only, at least in such laminae as have been identified as Lepidodendroid.nbsp;Surrounding the xylem strand occur delicate parenchymatousnbsp;cells in some cases accompanied by darker and thicker-wallednbsp;elements. As in Sigillaria, the leaves of which are more fullynbsp;described on page 210, a fairly broad sheath of wider and shorternbsp;scalariform or spiral transfusion tracheids surrounds the con-

ducting strand (figs. 142, f; 143, B, C, t). As Renault shows in the case of Lepidodendron esnostense'^, the small leaves of which arenbsp;1�5�2 mm. broad at the base and several centimetres long, thenbsp;stomatal grooves and keel die out towards the apex when thenbsp;lamina assumes a more nearly circular form (fig. 143, C).

The area of the cushion excluding the leaf-scar is spoken of by some writers as the field. Below the leaf-scar the kiteshaped cushion tapers to a gradually narrowing basal position:nbsp;in Lepidodendron Veltheimianuni, a species characteristic of

' Renault (96) A. Pis. xxxiii. xxxiv. p. 178. For a good section of another Lepidodendron leaf, see Scott (08) p. 160, figs. 64, 65.

Lower Carboniferous strata, it is seen to be continuous, as a ridge with sloping sides, with a lower cushion (fig. 185).

Below a leaf-scar the cushion frequently shows a pair of oval areas on which a fine pitting may be detected in well-preservednbsp;impressions, these oval scars, as seen in fig. 185, D, are practicallynbsp;continuous at the upper end with the parichnos scars on thenbsp;leaf-scar area; this is explained by the fact that these infra-foliarnbsp;scars also owe their existence to patches of lacunar, aerenchy-matous tissue in close connexion with the parichnos^.

Shortly before entering the base of the leaf-lamina the parichnos divides into two arms which diverge in the outernbsp;cortical region right and left of the vascular bundle, and passingnbsp;obliquely upwards they come close to the surface of the leaf-cushion just below the leaf-scar. The diagram�fig. 144, B�nbsp;shows a leaf-trace,(f,in the leaf-cushion,as seen in a diagrammaticnbsp;drawing of a vertical radial section of a stem, the dotted lines,nbsp;p, p, show the two parichnos arms which are represented asnbsp;impinging on the surface of the leaf-cushion at p', and thennbsp;bending upwards to pass into the leaf-base right and left of thenbsp;vascular bundle or leaf-trace. For convenience the arms of thenbsp;parichnos are represented in one plane though actually innbsp;different vertical planes.

Fig. 144, A, shows the difference between a view of the original surface of a Lepidodendron, as at a, where a leaf-cushionnbsp;with a leaf-scar is seen, and a view of an impression representingnbsp;the outer cortex, 6, a short distance below the surface. Thenbsp;surface b, in fig. 144, A, corresponds to the face d�e in the diagrammatic longitudinal section fig. 144, B; the outline of eachnbsp;cushion is clearly visible and in the centre is seen the leaf-trace.nbsp;It, with its parichnos.

The surface-features, a (fig. 144, A), have been impressed on the rock, c, (fig. 144, B) in which the specimen was entombednbsp;and by the removal of the cast of the stem, that is the thickness bnbsp;toje in fig. 144, B, the form of the leaf-cushion is revealed. Thenbsp;presence of the two infra-foliar parichnos scars at p' (fig. 144, A)nbsp;j^.lex�plained by the diagram, fig. 144, B, p'.

Lepidodendron leaf-cushion has been worked out in detail by Weiss who shows that, at least in some species, the two armsnbsp;do not bend downwards as shown in the diagram, fig. 144, B,nbsp;but pursue a straight gradually ascending course as seen innbsp;fig. 145, A. Just below the leaf-scar region of the cushion eachnbsp;arm comes into association with a group of lacunar, aerenchy-matous tissue, such as occurs in the roots of certain Mangroove

A. nbsp;nbsp;nbsp;Leaf-cushion and leaf-scar seen in surface-view at a ; on the restnbsp;of the specimen a slightly lower surface is exposed. (Afternbsp;Stur.)

B. nbsp;nbsp;nbsp;Diagrammatic longitudinal section to explain the differencesnbsp;between its two surfaces a and b shown in fig. A.

The shaded portion c represents the rock matrix, the surfaces ab, ed, mark the outer and inner edge of the outer portion ofnbsp;the bark of the Lepidodendron stem.

plants, and it is this aerenchyma which is exposed on the two oval depressions below the leaf-scar. The structure of thisnbsp;aerenchyma is shown in fig. 145, B; it consists in this speciesnbsp;{L. Hickii Wats.) of stellate cells which wouldnbsp;efficient aerating system. Probably, as Weissnbsp;patches of aerenchyma were originally coverednbsp;provided with stomata, and it is owing to the dlt;nbsp;superficial layer that the two oval scars often ft

feature on Lepidodendron leaf-bases ^ The diagram reproduced in fig. 144, B, may be taken as practically correct, as the patchesnbsp;of aerenchyma described by Weiss do not differ essentially fromnbsp;the parichnos tissue.

The parichnos scars are shown on the leaf-scar and cushion in fig. 146, C. In the lower leaf-cushion shown in fig. 146, E,nbsp;the infra-foliar parichnos syars, p, are clearly seen, but thenbsp;preservation of the leaf-scar is i^t sufficiently good to show them

cya^c

Fig. 145. A. Diagrammatic surface-view and longitudinal section Lepidodendron leaf-cushion.

on that part of the fossil. In the upper cushion (fig. 146, E) the position of the parichnos arms is shown on the leaf-scar, but thenbsp;infra-foliar parichnos scars are hidden by two small spiralnbsp;shells. The genus Spirorbis, to which these shells are referred,nbsp;appears to have persisted from the Silurian epoch to the presentnbsp;day. The comparatively frequent occurrence of Spirorbis shellsnbsp;on the leaves and other parts of Palaeozoic plants, has recently

1 For a fuller account of the parichnos, see Hill, T. G. (06) and other papers quoted by F. E. Weiss (07).

been dealt with in a paper by Barrois* who discusses in detail the habitats of these small animals from the point of view of thenbsp;conditions under which the plants were preserved. In a note bynbsp;Malaquin appended to Barrois� paper the belief is expressednbsp;that Spirorbis lived on pieces of Palaeozoic plants which laynbsp;under water.

The fact that with one exception all the Spirorbis shells on the specimen of Lepidodendron, of which two leaf-cushions arenbsp;shown in fig. 146, E, occur on the large parichnos scars onnbsp;the cheeks of the cushions, suggests the possibility that thenbsp;escape of gases from the parichnos tissue may have renderednbsp;the position attractive to the Spirorbis. It can hardly benbsp;accidental that the shells occur on the parichnos strands. Thisnbsp;fact recalls the view held by Binney^ and accepted with favournbsp;by Darwin� that Lepidodendron and other coal-forest trees maynbsp;have lived with the lower parts of the stems in sea water.

Above the leaf-scar is a fairly deep triangular or cresentic pit (fig. 146, C, 1) known as the ligular pit from the occurrencenbsp;on younger shoots of a delicate organ like the ligule of Isoetesnbsp;(fig. 132) embedded in a depression in the upper part of the leaf-cushion. The ligule was first figured in Lepidodendron bynbsp;Solms-Laubach^and described in English material by Williamsonnbsp;under the name of the adenoid organ�.

In some Lepidodendron stems a second triangular depression may occur above the ligular pit, the meaning of which is notnbsp;clear: this has been called the triangulum by Potoni�. Stur�nbsp;suggested that it may represent the position occupied by anbsp;sporangium in Lepidodendron cones.

It is important to remember that as a branch increases in girth the leaf-cushions are capable of only a certain amount ofnbsp;growth: when the limit is reached-they are stretched farthernbsp;apart and thus the narrow groove which separates them is converted in older stems into a comparatively broad and flatnbsp;channel, thus altering the surface characters.

� Darwin (03) vol. ii. pp. 217, 220. Solms-Laubach (92) PL ii. figs. 2, 4.nbsp;nbsp;nbsp;nbsp;� Williamson (93) p. 10.

� Potoni� (05) Lief, iii., p. 41. nbsp;nbsp;nbsp;^ Stur (75) A. Heft ii. p. 277.

^*�V~-y-p

A, B. From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion 3 cm. broad).

C. nbsp;nbsp;nbsp;From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion 4 cm. long.

E. nbsp;nbsp;nbsp;From a specimen in the Bunbury Collection, Cambridge Botanynbsp;School, showing Spirorhis shells (leaf-cushion 2 cm. long).

F. nbsp;nbsp;nbsp;From a section in the Williamson Collection, British Museumnbsp;No. 1, 973.

G. nbsp;nbsp;nbsp;H, I. From sections in the Cambridge Botany School Collection.

Another feature worthy of notice in reference to the leaf-cushions of Lepidodendron is the occurrence in rare instances of alternate zones of larger and smaller cushions. This variationnbsp;in the size of the leaf-cushions is by no means uncommon in thenbsp;closely allied germs Sigillaria; in Lepidodendron it has beennbsp;described by Potoni�' in L. volkmannianuni and more recentlynbsp;by Mr Leslie and myself^ in a South African species L.nbsp;vereenigense.

Owing to the natural exfoliation of the superficial layers of the outer bark at a certain stage in the growth of the plant, ornbsp;in some instances no doubt as the result of post-mortem decay,nbsp;which destroys the delicate cells of the meristematic zone in thenbsp;outer cortex, isolated leaf-cushions and strips of the externalnbsp;surface are occasionally met with as carbonised impressions.

The appearance presented by a Lepidodendron stem which has been deprived of its superficial tissues may be dealt withnbsp;more intelligibly after we have become familiar with the anatomical characters.

Before proceeding further with the genus Lepidodendron a short account may be intercalated of the external features of anbsp;lepidodendroid type of stem which it is customary to describenbsp;under a distinct generic title Lepidophloios. This name is convenient for diagnostic purposes though it seems clear that apartnbsp;from the form of the leaf-cushion (fig. 146, A) we are at presentnbsp;unable to recognise any well-defined differences between thenbsp;two forms Lepidodendron and Lepidophloios. For generalnbsp;purposes the name Lepidodendron will be used as includingnbsp;plants possessing leaf-cushions of the type already described asnbsp;well as those with the Lepidophloios form of cushion.

The generic name Lepidophloios was first used by Sternberg� for a Carboniferous species which he had previously describednbsp;as Lepidodendron laricinum. In 1845 Corda^ instituted thenbsp;name Lomatophloios for specimens possessing the same external

1 PotoDi� (05) fig. 4. nbsp;nbsp;nbsp;^ Seward and Leslie (08) PI. x. figs. 1 and 2.

characters as those for which Sternberg had chosen the name Lepidophloios. The leaf-cushions of Lepidophloios differ fromnbsp;those of the true Lepidodendron in their relatively greater lateralnbsp;extension (cf. fig. 146, A and C), in their imbricate arrangementnbsp;and in bearing the leaf, or leaf-scar, at the summit. In somenbsp;species referred to Lepidophloios the cushions are howevernbsp;vertically elongated and in this respect similar to those ofnbsp;Lepidodendron: an example of this type is afforded by Lepidophloios Dessorti a French species described by Zeillerb Innbsp;younger branches the cushions may be directed upwards havingnbsp;the leaf-scar at the top; but in the majority of specimensnbsp;the cushions are deflexed as in figs. 146, D; 160, A. The shootnbsp;of Lycopodium dichotomum shown in fig. 121, B, with the leavesnbsp;in the reversed position bears a close resemblance to a branchnbsp;of Ijepidophloios.

The photograph of Lepidophloios scoticus Kidst.'* reproduced in fig. 160, A, illustrates the dichotomous branching of thenbsp;stem and the form of the cushions with the leaf-scars pointingnbsp;downwards. In the fertile branch of the same species shown innbsp;fig. 160, B, the leaf-scars face upwards.

In most species the cushions are simply convex without a median keel, but in some cases a median ridge divides thenbsp;cushion into two cheeks as in the genus Lepidodendron. Thenbsp;leaf-scar bears three small scars, the larger median scar markingnbsp;the position of the leaf-trace, while the lateral scars are formednbsp;by the two arms of the parichnos: in some examples of deflexednbsp;cushions, though not in all, a ligular pit occurs on the cushion anbsp;short distance above the leaf-scar.

The drawing reproduced in fig. 146, A, showing the leaf-scar on the upper edge of the cushion should have been reversednbsp;with the leaf-scars pointing downwards. This figure representsnbsp;part of the surface of a specimen consisting of the outer cortexnbsp;of a stem with leaf-cushions 3 cm. broad. The thickness ofnbsp;this specimen is 4 cm.: a section through the line ah is represented in fig. 146, D (reproduced in the correct position, withnbsp;the leaf-scars, sc, pointing downwards): internal to the cushionsnbsp;is a band of secondary cortex (the shaded strip on the outernbsp;1 Zeiller (92) A.nbsp;nbsp;nbsp;nbsp;^ Kidston (93) p. 561, Pis. i. and ii.

edge of the section) which was formed on the outside of the phellogen. The phellogen is a cylinder of actively dividingnbsp;cells in the outer part of the cortex of the stem, oftennbsp;spoken of as the cork-cambium or cortical meristem, whichnbsp;produces a considerable amount of secondary cortical tissue onnbsp;its inner face and a much smaller amount towards the stemnbsp;surface. This delicate cylinder frequently forms a natural linenbsp;of separation between the outer shell of bark and the rest ofnbsp;the stem. In the specimen before us, the thin-walled cells ofnbsp;the phellogen were ruptured before petrification and the outernbsp;shell of bark was thus separated as a hollow cylinder from thenbsp;rest of the stem: this cylinder was then flattened, the two innernbsp;surfaces coming into contact. Fig. 146, D, represents a sectionnbsp;of one half of the thickness of the flattened shell.

This separation of the outer cortex, and its preservation apart from the rest of the stem, is of frequent occurrence innbsp;fossil lycopodiaceous stems. The flattened outer cortical shellnbsp;of a Lepidophloios, specifically identical with that shown innbsp;fig. 146, A and D, was erroneously described by Dr C. E. Weissnbsp;in 1881 as a large lepidodendroid cone^.

Fig. 146, B, affords a view of the inner face of the specimen of which the outer surface is seen in fig. 146, A: the surface shownnbsp;in the lower part of the drawing, on which the boundaries ofnbsp;the cushions are represented by a reticulum, corresponds tonbsp;the inner edge of the strip of secondary cortical tissue represented by the vertically shaded band in fig. 146, D.

The shaded surface in fig. 146, B, represents a slightly deeper level in the stem which corresponds to the outer edge ofnbsp;the vertically shaded band of fig. 146, D: the narrow taperednbsp;ridges (fig. 146 B) represent the leaf-traces passing through thenbsp;secondary cortex, and the fine vertical shading indicates thenbsp;elongated elements of which this strip of secondary cortex isnbsp;composed.

In the longitudinal section diagrammatically reproduced in fig. 146, D, cut along the line ah of fig. 146, A, the parenchymatousnbsp;tissue of the stout cushions has been partially destroyed, as atnbsp;a; at s is seen the section of a Stigmarian rootlet which has

found its way into the interior of a cushion. Each leaf-trace is accompanied by a parichnos strand as in the true Lepidodendron',nbsp;at the base of the leaf-cushion the parichnos branches intonbsp;two arms which diverge slightly right and left of the leaf-trace,nbsp;finally entering the base of the leaf lamina as two lateral strandsnbsp;(fig. 147, p). At one point in fig. 146, D the section has shaved anbsp;leaf-trace represented by a black patch resting on the parichnos

just above the line ef, but it passes through one of the parichnos arms p' which debouches on to the leaf-scar sc at p. Hadnbsp;the section been cut along the line cd of fig. 146, A the leaf-trace would have been seen in a position similar to that occupiednbsp;by the parichnos p' in fig. 146, D.

Fig. 147, A, affords a good example of a tangential section through a Lepidophloios leaf-cushion, 1 cm. broad, like that repre-

sented in fig. 146, A, showing the vascular bundle It, the tw'o parichnos strands, p, composed of large thin-walled cellsnbsp;(cf. Isoetes, fig. 133, H, I), and the ligular pit near the uppernbsp;edge of the section enclosing the shrunken remains of the ligulenbsp;(fig. 147, B, 1).

Fig. 147, B shows the form of the tangentially elongated leaf-cushions of Lepidophloios and their spiral disposition.

Fig. 146, F, represents a section similar to that shown in figs. 147, A and B, but in this case the leaf-trace. It, and thenbsp;parichnos strands, p, lie in a cavity formed by the destructionnbsp;of some of the leaf-cushion tissue. It is worthy of notice thatnbsp;the parichnos cells have resisted decay more successfully thannbsp;the adjacent tissue of the cushion.

The diagrammatic sketches reproduced in fig. 146, H and I, were made from a transverse section similar to one originallynbsp;figured by Williamsonb fig. 146, H, corresponding in position tonbsp;the line gh in fig. 146, A, passes through the ligular pit, I, andnbsp;cuts across the parichnos in the act of branching; the leaf-tracenbsp;passes outwards beyond the Y-shaped parichnos strand. In thenbsp;other section, fig. 146,1, the parichnos is shown in a horizontalnbsp;plane and the leaf-trace. It, appears in oblique transversenbsp;section. In both sections and in fig. 146, G the shaded band atnbsp;the base represents the secondary cortical tissue external to thenbsp;phellogen.

The transverse section represented in fig. 146, G shows in the left-hand cushion, a, the exit of the two parichnos arms and thenbsp;leaf-trace between them: it illustrates also the various formsnbsp;assumed by lepidodendroid leaf-cushions when cut across atnbsp;different levels.

In the earlier literature dealing with the anatomy of Lepidodendron and Sigillaria the presence or absence of secondary vascular tissue was made the criterion of generic distinction andnbsp;the distinguishing feature between the classes Pteridophytes and

Gymnosperms, Lepidodendron being relegated to the former class because it was supposed to have no power of forming secondarynbsp;wood, while Sigillaria, characterised by a considerable development of such tissue, was classed by Brongniart and afterwardsnbsp;by Renault as a Gymnosperm. Binney^ in 1865 recognised thatnbsp;the two types of stem pass into one another, but. it wasnbsp;Williamson^ who provided complete demonstration of the fallacynbsp;of the Brongniartian view.

These two undoubted Pteridophytes agree very closely in anatomical structure and both are now recognised as arborescentnbsp;genera of Lycopodiaceous plants. In a paper published bynbsp;Lomax and Weiss in 1905� a specimen is described from thenbsp;Coal-Measures of Huddersfield, in which a, decorticated stemnbsp;with the anatomical characters of Binney�s Sigillaria vascularisnbsp;gives off a branch having the anatomical structure which it hasnbsp;been customary to associate with the species Lepidodendronnbsp;selaginoides, so-called by Sternberg and founded by him onnbsp;impressions showing well-preserved external characters.

In 1862 Binney^ described petrified specimens of vegetative shoots from the Lower Coal-Measures of Lancashire under thenbsp;names Sigillaria vascidaris and Lepidodendr-on vasculare.nbsp;These were afterwards recognised as different states of thenbsp;same species. A few years after the publication of Binney�snbsp;paper Carjxrfhers� identified Binney�s species Lepidodendronnbsp;vasculare with Sternberg�s L. selaginoides. The evidence onnbsp;which this identification rests has not 'been 'stated, but manynbsp;writers have retained this specific designation for the well-definednbsp;type of anatomical structure first described by Binney asnbsp;L. vasculare. The use of the specific name selaginoides is, however, open to objection. The species Lepidodendron selaginoides,nbsp;as pointed out by Kidston�, is probably identical with the plantnbsp;which Brongniart had named L. Sternbergii before the institutionnbsp;of Sternberg�s species, and we are not in possession of convincingnbsp;evidence as to the connection of L. Sternbergii (=L. selaginoides) with specimens possessing the anatomy of Binney�s type.

Binney�s designation is therefore retained for the anatomical type described in the following pages ^

The most detailed account hitherto published of the anatomy of Lepidodendron vasculare is that by the late M. Hovelacque^nbsp;based on material from the Lower Coal-Measures of England.

The small shoot, represented somewhat diagrammatically in fig. 148, A, illustrates the anatomical features of a typicalnbsp;example of the species; the shoot has a diameter of 2'5 cm. andnbsp;its central cylinder (x�sc) is 2'5 mm. in width.

Noticeable features are (i) the small size of the central cylinder (or stele) in proportion to the diameter of the branch,nbsp;(ii) the production at a comparatively early stage of growth of anbsp;zone of secondary wood, x^, which gradually assumes the formnbsp;of a complete cylinder of unequal breadth, surrounding thenbsp;primary xylein, x, (iii) the formation of a secondary corticalnbsp;tissue by a meristematic cylinder (phellogen, pT) situated closenbsp;to the leaf-cushion region of the outer cortex. On the outernbsp;edge the stele consists of narrow tracheae some of which show innbsp;longitudinal section the spiral form of thickening characteristicnbsp;of most protoxylem elements: towards the centre of the stele the

Fig. 149. Lepidodendron vasculare, a, immature tracheae; m, meristem; mr, medullary ray; x, xylem.

B. nbsp;nbsp;nbsp;Short tracheae in the centre of the stele. (From a specimennbsp;from the Halifax Hard bed in Dr Kidston�s Collection.)

diameter of the tracheae gradually increases and parenchymatous cells become associated with the elongated scalariform elements.nbsp;In the central region the stele is composed of parenchymatousnbsp;tissue arranged in vertical series of short cells, interspersed wdthnbsp;short tracheae distinguished by the greater thicknessof theirwallsnbsp;and by their scalariform and reticulate thickening bands. Somenbsp;of these short tracheae are shown in vertical section in fig. 149, B :nbsp;the fine and broken lines connecting adjacent thickening bandsnbsp;probably represent the remains of the original wall. Thesenbsp;delicate bands, which have been figured in various species

of lepidodendroid plants^ are worthy of notice in connexion with the recent work of Mr Gwynne-Vaughan'� who has shownnbsp;that in many recent ferns the scalariform bands in the xylemnbsp;elements are not connected by a thin pit-closing membrane, butnbsp;are separated from one another by open spaces. In the Lepido-dendron tracheae we seem to have a stage in which the intervening membrane is in process of absorption. It is, however,nbsp;possible that the threads may be the result of contraction andnbsp;splitting of the membrane during drying or decay.

The stele of Lepidodendron vasculare, before the addition of any secondary xylem, may be described as a protostele, a termnbsp;originally proposed by Professor Jeffrey�, in which the central partnbsp;of the conducting strand of xylem elements has been convertednbsp;into rows of parenchyma and short tracheids, the latter beingnbsp;better adapted to storage than to conduction. It is probablenbsp;that this type of stelar anatomy, which distinguishes L. vascularenbsp;from other species, represents a comparatively primitive arrangement forming a transition between the stele of L. esnostense,nbsp;which consists of a solid rod of tracheids, and the stele ofnbsp;L. Harcourtii (fig. 179, A) and other species in which the xylemnbsp;forms a cylinder enclosing a large parenchymatous pith.

Parenchymatous cells occur in contact with the outer edge of the xylem-cylinder some of which are distinguished by annbsp;irregular reticulate pitting. The tangential section represented in fig. 148, B, illustrates the appearance of a shoot ofnbsp;L. vasculare in which no secondary xylem is present: the centralnbsp;strand of tissue consists of the parenchyma abutting on thenbsp;xylem with several leaf-traces (It) passing upwards in an almostnbsp;vertical course from the outer edge of the stele.

The secondary xylem (fig. 148, A, x^') consists of radially arranged scalariform tracheae with associated rows of parenchymatous cells which form medullary rays (fig. 149, mr).nbsp;Leaf-traces pass through the medullary rays in the secondarynbsp;xylem cylinder in a direction at right angles to the primary xylemnbsp;stele from which they are given off, but at the outer edge of the

� Solms-Laubach (92) PI. ii. fig. 6; Seward and Hill (00) PI. iv. tig. 26. See p. 910 of the latter paper for other references.

- Gwynne-Vaughan (08). nbsp;nbsp;nbsp;� Jeffrey (98). See also Tansley (08) p. 37.

secondary xylem they bend suddenly upwards and for a time follow a steep and almost vertical course.

In well-preserved longitudinal sections the outermost secondary xylem tracheae' are seen to be succeeded by a fewnbsp;narrow and vertically elongated elements (fig. 149, A, a),nbsp;which represent young unlignified tracheae: these are followednbsp;by shorter parenchymatous cells (m) forming part of a meriste-matic zone from which the secondary xylem receives additions.

Returning to fig. 148, A; the zone of secondary wood, x^, composed of scalariform tracheids and medullary rays, is succeeded by a few layers of parenchymatous cells and beyond thisnbsp;is a broader zone, sc, to which the term secretory zone hasnbsp;been applied'; this is made up of small parenchymatous cellsnbsp;varying in size and of larger spaces which appear to have beennbsp;formed by the disorganisation of thin-walled elements. Thenbsp;whole zone presents a characteristic appearance due to thenbsp;association of small cells, large clear spaces, and a certain amountnbsp;of dark-coloured material suggestive of tissue disorganisationnbsp;and secreted products. The anatomical characters of thenbsp;secretory zone are shown in the photograph, fig. 168, A, sc.nbsp;Several leaf-traces are seen in transverse section in the secretorynbsp;zone (black dots in fig. 148, A, sc; fig. 154, C, It): each tracenbsp;consists of a strand of narrow tracheae accompanied by a fewnbsp;encircling layers of small parenchymatous cells. As a tracenbsp;continues its steeply ascending course through the secretory zone,nbsp;it becomes associated with a strand of that tissue and assumes thenbsp;form of a collateral vascular bundle, the outer part of which doesnbsp;not consist of typical phloem but of shorter elements derivednbsp;from the secretory zone. Beyond the secretory zone we find anbsp;more homogeneous tissue composed of parenchymatous elementsnbsp;slightly extended tangentially (figs. 148, A, c'; fig. 168, A, c);nbsp;this is spoken of as the inner cortical region. In the greatnbsp;majority of sections of L. vascidare as of other species of thenbsp;genus, the broader middle cortex (fig. 148, c^) is occupied bynbsp;mineral matter, introduced subsequent to decay of the tissue;nbsp;or it is represented by patches of delicate tissue composed ofnbsp;loosely arranged parenchymatous cells varying considerably innbsp;1 Seward (99) p. 144.

size and shape, some being small, oval or polygonal elements while others have the form of sinuous hypha-like tubes.

In this middle cortical region may be seen leaf-traces passing outwards in an almost horizontal course (fig. 148, A, It): afternbsp;leaving the inner cortex the leaf-traces bend somewhat abruptlynbsp;outwards to follow a more direct path through the middle andnbsp;outer cortex. The ring of tissue, s, seen in the middle cortexnbsp;of fig. 148, A, belongs to a Stigmarian rootlet.

The outer cortex (fig. 148, A and B, cf) consists of homogeneous parenchyma which is stronger and more resistant to decay than the looser middle cortex. The leaf-traces, as shownnbsp;in fig. 148, B, pass through this region in a rather steeplynbsp;ascending direction: each is seen to be enclosed by a spacenbsp;originally occupied by a strand of middle cortical tissue whichnbsp;accompanies lepidodendroid leaf-traces on their under side andnbsp;has already been described as the parichnos, (pp. 97,100�103;nbsp;figs. 146, 147).

The surface of the stem shown in section in fig. 148, A, is composed of broad leaf-cushions. A single leaf-trace with itsnbsp;parichnos passes into each cushion, but in the neighbourhoodnbsp;of the base of a cushion the parichnos bifurcates (cf. fig. 146,nbsp;H, I) and the arms diverge slightly to the right and left finallynbsp;passing beyond the cushion into the lamina of the leaf, theirnbsp;position being shown, as already explained, by the two smallnbsp;lateral scars on the leaf-scar area.

The diagrammatic sketch of a radial longitudinal section through a leaf-cushion represented in fig. 150 illustrates thenbsp;relation of the leaf-trace to the leaf-cushion. The trace consistsnbsp;of xylem, x, above and a strand of the secretory zone, st, below;nbsp;the parichnos tissue was originally present on the under side ofnbsp;the leaf-trace at a. The external surface, be, marks the limitnbsp;of the leaf-scar through the middle of which passes the vascularnbsp;strand It.

The lower gap a has been' formed by the tearing of thin-walled cells of the phellogen, the meristematic tissue from which a considerable amount of secondary cortical tissue ornbsp;phelloderm has been produced at pd. On the outside of thenbsp;cushion, c, the cells are somewhat crushed and distinguished

This section also illustrates another characteristic feature of Lepidodendron, namely the presence of a ligule and a ligularnbsp;pit: the former is represented by a carbonised patch of tissue andnbsp;the latter extends from the surface of the cushion at b, justnbsp;above the leaf-scar, almost to the level of the leaf-trace, It. Anbsp;comparison of this section with figs, 146 and 147 will makenbsp;clear the relation of the several parts of the cushion and leaf-scar.

\--pd

the delicate meristematic zone or phellogen which arises close to the bases of the leaf-cushions; the phellogen has already produced a few rows of radially disposed elements, represented bynbsp;short radial lines in the drawing, which constitute secondarynbsp;cortical tissue.

In older shoots the amount of the secondary cortical tissue developed on the inner side of the phellogen is considerable (cf figs. 152, 153).

The structure of the cortex of a shoot in -which secondary growth, both in the stele and in the outer cortex, has progressed further than in the specimen shown in fig. 148 isnbsp;represented in fig. 151.

The section (fig. 151, A) measures 7 x 3'8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylindernbsp;of secondary wood (fig. 151, E, x and x��). The almost smoothnbsp;surface of the primary wood (fig. 151, E, x) is succeeded by thenbsp;secondary xylem, x^, characterised at its inner edge by thenbsp;tapered ends of the radial rows of scalariform tracheids between

which occur several delicate parenchymatous cells (fig. 151, E, a). The occurrence of such isodiametric elements, often exhibitingnbsp;a delicate spiral thickening band, is a characteristic feature ofnbsp;the boundary between primary and secondary -wood in lepido-dendroid stems. The secondary wood is penetrated by numerousnbsp;medullary rays and in some of them are seen strands of narrownbsp;spirally thickened tracheae�the leaf-traces�which are innbsp;organic continuity with the exarch protoxylem of the primarynbsp;wood. The leaf-traces are oval and mesarch. The space, c^,nbsp;(fig. 151, A) originally occupied by the delicate middle cortex,nbsp;is succeeded by a shell of outer cortex composed chiefly of

secondary tissue (phelloderm, pd) passing towards the inner boundary of this region into the primary outer cortex g (fig. 151,nbsp;A and C). The radially disposed elements which make up thenbsp;bulk of the phelloderm are associated with concentric rows ofnbsp;secretory strands, represented by tangentially arranged dots innbsp;fig. 151, A: on the outer edge of the phelloderm a few patches ofnbsp;primary cortex are still preserved, as at c, fig. A. One of thesenbsp;is shown on a larger scale in fig. B; at m the phelloderm isnbsp;interrupted by a gap beyond which the cells have thinner wallsnbsp;and show signs of recent division; this is probably the positionnbsp;of the phellogen. The tissue h, fig. 151, B, consists of secondarynbsp;cortex succeeded beyond d by the parenchymatous tissue of thenbsp;leaf-cushion, in which the remains of a ligule, I, are seen in thenbsp;ligular pit. This section corresponds in position to a linenbsp;drawn across fig. 150 at the level of h. In this specimen wenbsp;have two kinds of secondary cortical tissue: that formed externalnbsp;to the phellogen, from m to d in fig. 151, B, is less in amountnbsp;than that produced internal to the phellogen. We cannot makenbsp;any satisfactory statement as to the nature of this secondarynbsp;tissue, whether or not any of it agreed in composition with thenbsp;cork which is usually formed external to the phellogen in recentnbsp;plants. As the stem of a Lepidodendron grew in girth the leaf-cushions became separated by intervening depressions composednbsp;of the secondary cortex formed external to the phellogen, but atnbsp;a later stage the cushions were thrown off, leaving the outernbsp;edge of the phelloderm as the superficial tissue. This exposednbsp;tissue became fissured as growth and consequent stretchingnbsp;continued, producing the appearance seen on the surface ofnbsp;the still older stem represented in fig. 153.

The inner edge of the phelloderm seen at e in fig. 151, C, passes suddenly into the inner primary region of the outer cortexnbsp;(fig. 151, A and C, g) which comprises two types of parenchymatous tissue, patches of isodiametric cells, g, g, alternatingnbsp;with radially arranged areas consisting of tangentially elongatednbsp;elements (fig. C, �, �; fig. D) which extend as wedges into thenbsp;phelloderm.

The longitudinal section represented in fig. 152, B, shows an equal bifurcation of a stem in which no secondary xylem is

present; in the lower part of the section the xylem and the outgoing leaf-traces are seen in radial section and at the uppernbsp;end of each arm the leaf-traces alone, It, are exposed, as innbsp;fig. 148, B. It is interesting to notice the large amount of

A. nbsp;nbsp;nbsp;From a section (10-5 x 9 cm.) in the Cambridge Botany Schoolnbsp;Collection.

phelloderm which has been produced in the fork of the branch, at pd, where greater strength is required.

The section represented diagrammatically in fig. 152, A, has lost the outermost part of the cortex together with the leaf-

cushions; it consists largely of secondary cortex composed of radially disposed phelloderm cells and tangentially placednbsp;secretory strands (represented by the discontinuous black linesnbsp;in the drawing); the dotted region in the central part of thenbsp;axis is composed of primary cortical parenchyma, and the twonbsp;spaces surrounding the steles contain portions of the lacunarnbsp;middle cortex. Each stele posseses a narrow crescentic zone

of secondary xylem; the amount is greater in the case of the right-hand stele, of which a small piece is shown on a largernbsp;scale; the striking contrast in size between the outer and morenbsp;internal secondary tracheae is no doubt the expression of somenbsp;unfavourable condition of growth. The position of the secretorynbsp;zone beyond the secondary xylem is shown at sc, fig. 152, A.

An example of a large and partially decorticated stem is afforded by the specimen (16 x 7'5 cm.) shown in fig. 153. Thenbsp;irregularly ribbed surface is formed of rather thick-wallednbsp;phelloderm, in which occur tangentially arranged rows ofnbsp;secretory strands. The tapered form of the secondary cortexnbsp;as it abuts internally on the primary cortex is shown very clearlynbsp;in the drawing (cf. fig. 151, C). The stele in this much oldernbsp;stem consists mainly of secondary wood.

An interesting example of a small shoot, the largest diameter of which is 2'8 cm., is shown in fig. 154, A: the sectionnbsp;was cut a short distance above the bifurcation of the stele intonbsp;two approximately equal branches. The outer part of thenbsp;cortex consists of phelloderm, pd, with the usual rows ofnbsp;secretory tracts, and primary outer cortex g; the middle cortexnbsp;is represented by patches of parenchyma with a few leaf-traces.nbsp;To one of the steles, s' (fig. 154, A), a crescent-shaped band of

secondary xylem has been added; the other stele, s, possesses no fully developed secondary elements.

Fig. 154, B and C, illustrates the anatomical features immediately external to the primary xylem of the smaller stele, s. The comparatively broad band of radially disposed parenchyma, m,nbsp;is connected with the outermost elements of the xylem by anbsp;few rather dark and small crushed parenchymatous cells. Thenbsp;band m, which we may speak of as the meristematic zone,nbsp;clearly consists of cells in a state of division; it is in this regionnbsp;that the secondary xylem is produced. Beyond the leaf-trace,nbsp;(fig. 154, C It), occurs a portion of the secretory zone, some of the

smaller cells of which show signs of disorganisation; but most of this tissue has been destroyed (fig. 154, B, sc). The outer edge ofnbsp;the secretory zone is shown in fig. 154, D abutting on the cellsnbsp;of the inner cortex, c. The leaf-trace shown in the inner cortexnbsp;in fig. 154, B illustrates the more oval or tangentially extendednbsp;form of the xylem in this region, in contrast to the more circularnbsp;outline which it exhibits on the inner side of the secretorynbsp;zone.

The transverse section, part of which is reproduced in fig. 168, A, illustrates a characteristic feature, namely thenbsp;juxtaposition of the outermost tracheae of the secondary xylem

and ranch smaller cells of the meristematic zone� This is seen in fig. 155, which shows a small piece of fig. 168, A, on a largernbsp;scale. In plants with a normal cambium the segments cut offnbsp;from the initial layer fib on to the elements of the xylem ornbsp;phloem to which they are to form additions, but in Lepido-dendron it seems to be a general rule to find each of the mostnbsp;external lignified elements abutting on a group of two or threenbsp;much smaller cells. It is difficult to believe that the meristemnbsp;shown in fig. 155, m, could produce secondary xylem elementsnbsp;equal in size to those already formed: in all probability hadnbsp;growth continued there would have been a marked differencenbsp;between the size of the secondary tracheids, as in fig. 152,nbsp;where there was no doubt some cause which interfered withnbsp;normal cambial activity. This disparity in size between thenbsp;secondary xylem elements and the adjacent parenchymatousnbsp;tissue of the meristematic zone is by no means exceptional andnbsp;may be described as the general rule. It is at least certainnbsp;that in Lepidodendron vasculare, as in other species, the secondarynbsp;xylem was succeeded by a broad band of parenchymatousnbsp;tissue, from which new tracheae and medullary-ray elementsnbsp;were produced, and not by a narrow cambium such as occursnbsp;in recent plants.

V. Lepidodendron sterns as represented by casts and impressions of partially decorticated specimens.

The differentiation of the outer cortex of a Lepidodendron into comparatively thin-walled and more resistant tissue hasnbsp;been the cause of unequal decay and the consequent formationnbsp;of shrinkage cavities. In addition to the unequal resistingnbsp;power of contiguous tissues, another important factor in determining the nature of casts and impressions is the existence ofnbsp;the cylinder of delicate cells in the outer cortex of stems andnbsp;branches. As already pointed out, this meristematic cylindernbsp;or phellogen constitutes a natural line of separation, as in thenbsp;case of the cambium layer between the wood and the externalnbsp;tissues in a fresh Sycamore twig. The result of the separationnbsp;of an outer shell of bark from the rest of the stem and the

results of unequal decay in the more superficial tissues, have necessarily led to the preservation of the same specific typenbsp;under a variety of forms.

Our knowledge of the anatomy of Lepidodendron stems enables us to recognise in fossils of very different appearancenbsp;specimens in various conditions of preservation of one andnbsp;the same type. Such names as Knorria, Bergeria and Aspi-diaria are examples of generic titles instituted before anynbsp;adequate knowledge of Lepidodendron anatomy was available.

Differences in age as well as different degrees of decortication have contributed in no small measure to the institution of generic and specific names which more recently acquired knowledge has shown to be superfluous.

The designation Knorria, after a certain G. W. Knorr of NUrnberg, was proposed by Sternberg in 1826�^ for casts ofnbsp;Palaeozoic stems of a type figured more than a century earliernbsp;by Volkmann^. Goeppert, in his earlier works, published drawings of fossil stems which he referred to Sternberg�s genus: onenbsp;species he at first called Didymophyllum Schollini. He afterwards* described some specimens which showed that thenbsp;features characteristic of Knorria may occur on partially decorticated stems with leaf-cushions of the true Lepidodendronnbsp;type. His specimens, preserved in the Breslau Museum,nbsp;demonstrate the accuracy of his drawings and conclusions.nbsp;Goeppert, and after him Balfourquot;*, drew attention to the differentnbsp;appearances presented by branches of Araucaria imhricata whennbsp;preserved with the surface intact and after partial decortication,nbsp;as illustrating possible sources of error in the determination ofnbsp;fossil stems.

Although it is now a well-established fact that fossils bearing the name Knorria are imperfect lepidodendroid stems, the use of the term may be conveniently retained for descriptivenbsp;purposes. The specimen from the Commentry coal-field of

2 Goeppert (52) A. p. 196. See also Kidston (01) p. 50. Pis. XXX. XXXI. Lief, i and ii.nbsp;nbsp;nbsp;nbsp;Balfour (72) A.

France, shown in fig. 156, affords some excuse for the institution of several generic names for different states of preservation or decortication of one species. The cortical level exposed at enbsp;is characterised by spirally disposed peg-like ridges with truncated apices: it is this form of cast which is usually designatednbsp;Knorria. The ridges vary in size and shape in different types

fy.

/-I

Fig. 156. A dichotomously branched Lepidodendroid stem (Knorria mirabilis Een. and Zeill.). (After Eenault and Zeiller.) (J nat. size.) Thenbsp;original specimen is in the Natural History Museum, Paris.nbsp;a�g, surface features exposed as the result of different degrees ofnbsp;decortication. (See vol. i. p. 102, fig. 23).

of stem; they may be narrow as shown at e, fig. 156, or short and broad with rounded distal ends. In some cases they arenbsp;forked at the apex, as in the partially decorticated specimennbsp;of Lepidodendron Veltheimianum represented in fig. 185, A.

The Knorria state represents the impression or cast of the outer cortical region too deep below the leaf-cushion region to

retain any indications of the cushion-form; the ridges are the casts of the spaces produced in the cortex by the decay of thenbsp;sheath of delicate cells surrounding each leaf-trace and by thenbsp;decay of the thin-walled cells of the parichnos. The occasionalnbsp;forked apex of a ridge is the expression of the fact that the castnbsp;was made at the region where the parichnos divides into two armsnbsp;(c� p. 100). In certain specimens it is possible to connect thenbsp;Knorria casts with associated lepidodendroid stems which maynbsp;be determined specifically; but when we have no evidence as tonbsp;surface-features the fossils may be designated casts of lepidodendroid stems in the Knorria condition. Such casts are illustrated by numerous drawings in palaeobotanical literaturehnbsp;h. Bergeria.

This is another name first used by Sternberg in his classic work, Die Flora der Vorwelt, for casts of lepidodendroid plantsnbsp;such as Steinhauer^ had previously figured as Phytolithusnbsp;cancellatus. Brongniart� recognised that the application of thenbsp;generic title Lepidodendron should be extended to includenbsp;specimens referred by Sternberg to Bergeria, and a few yearsnbsp;later Goldenberg^ realised that this name does not standnbsp;for well-defined generic characters. The correctness of thesenbsp;views was, however, first satisfactorily demonstrated by Car-ruthers� and by FeistmanteP.

If a Lepidodendron stem loses its superficial layers of outer cortex and in this condition is embedded in sand or mud, thenbsp;cast is distinguished from that of a perfect stem by the absencenbsp;of the leaf-scars and by other features. It may, however, stillnbsp;show spirally disposed areas, corresponding approximately tonbsp;the original leaf-cushions, which are characterised by a smallnbsp;depression or pit either at the apex or near the centre of eachnbsp;oval area: the pit marks the position of the leaf-trace and itsnbsp;parichnos strand. In some cases the exposed surface may benbsp;smooth without any indication of leaf-cushions, while narrow

2 nbsp;nbsp;nbsp;Steinhauer (18) A. PI. iv. fig. 5.nbsp;nbsp;nbsp;nbsp;^ Brongniart (49) A. p. 42.

spirally arranged grooves represent the obliquely ascending vascular bundles passing through the cortex to the leaves.

Fig. 185, B, shows the Bergeria state of Lepidodendron Veltheimianum, which differs from the Knorria condition in thenbsp;fact that decortication had not extended below the level atnbsp;which the form of the leaf-cushions could be recognised. It isnbsp;clear that no sharp line can be drawn in all cases between thenbsp;different degrees of decortication as expressed by the termsnbsp;Knorria and Bergeria.

A list of synonyms of Knorria, Bergeria, and Aspidiaria forms of stem and a detailed treatment of their characteristicnbsp;features may be found in a recent work by Potonieh

In one of the earliest English books on fossil plants, the Antediluvian Phytology by Artisa specimen from the Carboniferous sandstone of Yorkshire is figured as Aphyllumnbsp;cristatum, and a similar fossil is described as A. asperum.nbsp;These are impressions of Lepidodendron stems in which thenbsp;characteristic leaf-cushions are replaced by smooth and slightlynbsp;convex areas with a narrow central ridge. To this type ofnbsp;specimen Presl gave the name Aspidiaria^, under the impression, shared by subsequent writers, that the supposed externalnbsp;features were entitled to generic recognition.

It is to Sturquot;* that we owe the first satisfactory interpretation of fossils included under the name Aspidiaria: he showed thatnbsp;on the removal of the projecting convex areas from some of hisnbsp;specimens a typical Lepidodendron leaf-cushion was exposednbsp;(fig. 144, A, a). The Aspidiaria condition (fig. 144, A, h) represents the inner face of the detached shell of outer bark of anbsp;Lepidodendron stem, while in the Bergeria casts we have anbsp;view of the external face of a stem deprived of its superficialnbsp;tissues.

In a Lepidodendron stem embedded in sediment the more delicate portions of the leaf-cushions would tend to shrink away

from the internal and more resistant tissues of the outer cortex, thus producing spaces between each cushion; further decaynbsp;would cause rupture of the leaf-traces and the superficial tissuesnbsp;would thus be separated from the rest of the stem. Thenbsp;tendency of Lepidodendron stems to split along the line ofnbsp;phellogen in the outer cortex is seen in fig. 148, A, g. Thenbsp;deposition of sediment on the exposed inner face of this corticalnbsp;shell would result in the production of a specimen of thenbsp;Aspidiaria type: the reticulum enclosing the spirally disposednbsp;convex areas is formed by the impression of the firmer tissuenbsp;between the leaf-cushions.

This generic name was suggested by Lindley and Hutton* for two specimens from the English Coal-measures characterisednbsp;by leaf-cushions like those of a Lepidodendron, hnt distinguishednbsp;by the presence of two vertical rows of large and more or lessnbsp;circular cup-shaped scars. These authors, while recognisingnbsp;the possibility that the fossils might be identical with Lepidodendron, regarded them as generically distinct. The genericnbsp;title Ulodendron, though no longer denoting generic rank, isnbsp;still applied to certain shoots of lycopodiaceous plants whichnbsp;may belong to the genera Lepidodendron, Bothrodendron, andnbsp;accoi�ding to some authors^, also to Sigillaria.

The large specimen from the Belgian coal-measures, represented in fig. 211, affords a good example of the Ulodendron form of shoot of the genus Bothrodendron, which is described onnbsp;page 249. The specimen shown in fig. 157 shows the Ulodendron shoot of Lepidodendron Veltheimianum.

Casts of large Ulodendron scars are occasionally met with as separate fossils bearing a resemblance to an oval shell.

^ Kidston (85). In this important paper Dr Kidston gives a full account of the history of our knowledge of Ulodendron.

of a Ulodendron stem under the name Phytolithus parmatus and a similar stem specifically identical with that shown innbsp;fig. 157 was figured by Ehodeh one of the earliest writers on

in offering suggestions as to the meaning of these large cuplike depressions, and there is still difference of opinion as to their significance. Lindley and Hutton^ described them as thenbsp;scars of branches or masses of inflorescence. Sir Josephnbsp;Hooker^ speaks of a specimen of Ulodendron, shown to him bynbsp;Mr Dawes, on which a large organ, supposed to be a cone, wasnbsp;inserted in one of the depressions, but he was unable to arrivenbsp;at any conclusion as to the real nature of the fossil. Whilenbsp;most authors have seen in the scars pressure-areas formed by thenbsp;pressure of sessile cones against the surface of a growing branch,nbsp;others, as for example Geinitz�, have described the depressionsnbsp;as branch-scars. Carruthers^ regarded the scars as those ofnbsp;adventitious roots and Williamson referred to them as thenbsp;scars of reproductive shoots. The depressions vary considerablynbsp;in size. The Belgian example shown in fig. 211 possesses scarsnbsp;9 cm. in diameter. A specimen of Botkrodendron in the Manchester Museum from the Lancashire Coal-Measures, to whichnbsp;Williamson� has referred, hears two rows of scars 11�12 cm.nbsp;in diameter on a stem 112 cm. in girth and 233 cm. long. Thenbsp;scars occur in two alternate series, on opposite faces of thenbsp;axis, the distance between the successive scars in the same rownbsp;being 29 cm. The surface-features of this large stem are notnbsp;preserved.

Before considering the nature and origin of the scars it is important to remember the considerable size to which theynbsp;may attain; other points of importance are the occurrence,nbsp;either in the centre of each depression or in an excentricnbsp;position, of an umbilicus or slightly projecting boss, in thenbsp;centre of which is a pit formed by the decay of an outgoingnbsp;vascular strand. The sloping sides of the scars sometimesnbsp;bear elevations resembling leaf-cushions like those on the restnbsp;of the stem surface. In the specimen shown in fig. 157 thenbsp;lower margin of each cup shows indistinctly the outlines of whatnbsp;appear to be leaf-cushions, while the rest of the sloping face isnbsp;characterised by radial ridges, which may be due to bracts ornbsp;leaves.

1 Lindley and Hutton (31) A. Hooker (48), p. 427. nbsp;nbsp;nbsp;* Geinitz (55) A.

It is obvious that in these cups we have the scars of some lateral organ, hut the evidence afforded by specimens of whichnbsp;the depressions contain the remains of such organs is by nonbsp;means conclusive. A Ulodendron has been figured by D�Arcynbsp;Thompson', in which the lower part of a lateral organ isnbsp;attached by a narrow base to one of the scars, but the preservation is not sufficiently good to enable us to decide whethernbsp;the organ is a cone or a vegetative shoot. Kidston^ hasnbsp;described other examples showing portions of organs in connexion with the scars, but an examination of the specimens innbsp;his collection failed to convince me that his interpretation ofnbsp;them as strobili is correct.

The phenomenon known as cladoptosis, as shown on a stem of the Conifer Agathis^ and certain Dicotyledonous trees suchnbsp;as Castilloa, suggests a possible explanation of the Ulodendronnbsp;scars. This comparison was made by Shattock^ in 1888, but henbsp;did not accept the resemblance as a real one. An objectionnbsp;may be urged to the cladoptosis hypothesis that in Ulodendronnbsp;the branch, whether vegetative or reproductive, was notnbsp;attached to the whole of the depressed area. On the othernbsp;hand, a lateral branch originally attached by a narrow basenbsp;may have continued to increase in diameter until its basenbsp;became slightly sunk in the bark of the stem, thus producing anbsp;cup-like depression which, on the fall of the branch, wouldnbsp;retain traces of the original surface-features of the stem.

Mr Watson^ of Manchester recently published a paper on Ulodendron scars, in which he adduces fresh and, as it seemsnbsp;to me, satisfactory arguments in favour of the branch-scar hypothesis. Fig. 158, from one of Mr Watson�s blocks,nbsp;illustrates the nature of his evidence. He points out that innbsp;the obverse half of a large specimen of Bothrodendron in thenbsp;Manchester Museum, the umbilicus consists of a cylindricalnbsp;hole, 18 mm. deep and 8 mm. in diameter, surrounded by anbsp;projecting ring of mineral material which doubtless representsnbsp;some portion of the original plant; on the reverse half of the

specimen the continuation of the ring is seen as a prominent cone fitting into the cup-like depression in the obverse half: the

Fis. 15 conical cast shows that numerous small vascular strands werenbsp;given off from this ring of tissue, and these strands have the

same arrangement and size as the dots which are found on typical Ulodendron scars. He interprets the ring surroundingnbsp;the umbilicus as the remains of the primary wood and thenbsp;small strands as leaf-traces supplying the branch.

In the diagrammatic section shown in fig. 158 the outer cortex of the main stem is represented by oc 1; this consists ofnbsp;secondary tissue. The corresponding tissue in the branch isnbsp;seen at oc2. The stele of the stem is shown at Tr. St. andnbsp;that of the branch at Br. St.; It, It, mark the position of thenbsp;leaf-traces. If we assume the branch to be detached alongnbsp;the line LS, the depression would show numerous spirallynbsp;arranged dots representing the points of exit of leaf-traces andnbsp;the vascular axis would be exposed in the umbilicus. Thisnbsp;explanation appears to me to be in harmony with the surface-features of Ulodendron scars on both Bothrodendron andnbsp;Lepidodendron stems. The occasional occurrence of leaf-cushions on a portion of a Ulodendron scar is a difficulty on thenbsp;cladoptosis hypothesis. Assuming that true leaf-cushions occur,nbsp;their presence may, as Watson suggests, be due to the foldingnbsp;back of a piece of the outer cortex of the branch which hasnbsp;been � crushed down on to the area of the scar!�

Since this account was written a note has been published by M. Renier^ in which he describes a specimen of Bothrodendron from Li�ge, one face of which shows a projecting Ulo-dendroid scar with an excentric umbilicus. On the other facenbsp;is a dichotomously branched shoot with surface-features corresponding to those on the scar; the evidence that the scarnbsp;represents the base of the branch is described as indisputable.

Stur^ held the view that the depressions on Ulodendron stems represent the places of attachment of special shoots comparablenbsp;with the bulbils of Lycopodium Selago, or, it may be added,nbsp;with the short branches occasionally produced on Gycas stems.nbsp;If the depressions were formed by the pressure of the basesnbsp;of cones, it is clear that the size of the cavity must be an indexnbsp;of the diameter of the cone. The larger Ulodendron scarsnbsp;exceed in diameter the base of any known lepidodendroid stro-bilus. Another obvious difficulty, which has not been over-

1 Watson (08) p. 10. nbsp;nbsp;nbsp;^ Renter (08).nbsp;nbsp;nbsp;nbsp;^ gtm- (75) a. Heft ii.

looked by Kidston who holds that the scars were produced by sessile cones,is that in LepidodendronVeltheimianum strobili werenbsp;borne at the tips of slender branches ; the same difficulty isnbsp;presented by Bothrodendron (Fig. 213). It is unlikely that twonbsp;types of strobili were produced on the same plant, particularlynbsp;as the cone of L. Veltheimianum was heterosporous.

The cones of certain species of Finns remain attached to the tree for many years and their bases become embedded innbsp;the stem; this is particularly well shown in the drawing of a

cone of Pinus clausa (fig. 159), for which I am indebted to Mr Sudworth, Dendrologist in the United States Forest Service.nbsp;Mr Sudworth has drawn my attention to P. attenuata and P.nbsp;niuricata in illustration of the same phenomenon^. Thenbsp;example shown in fig. 169 cannot, however, be matched by anynbsp;known specimen of Ulodendron; in the case of the depressionsnbsp;on the stem of a Pine the cone-base fits the circular scar,nbsp;but in the fossil stems it is practically certain that this wasnbsp;not the case.

There can be little doubt that certain Palaeozoic Lycopods shed their branches by a method similar to that employed bynbsp;the Kauri Pine of New Zealand and by some species of Dicotyledons. The evidence adduced in the case of Bothrodendronnbsp;punctatum is a strong argument in favour of extending thenbsp;same explanation to other Ulodendron shoots.

The branched axis with Lepidophloios leaf-cushions, represented in fig. 160, A, illustrates a special form of shoot described by Lindley and Hutton^ under the generic namenbsp;Halonia. The original specimens referred to this genus are

decorticated axes showing remains of Lepidodendroid leaf-cushions. The spirally disposed circular scars in the specimen of Halonia {Lepidophloios scoticus^) shown in fig. 160 constitutenbsp;the characteristic feature of the genus; they may have thenbsp;form, as in fig. 160, A, of circular discs with a central umbilicusnbsp;marking the position of a vascular strand, or, as in the sandstone cast of Halonia tortuosa shown in fig. 161^ they may

appear as prominent tubercles. The latter example illustrates the condition characteristic of partially decorticated stems.

In 1883 Williamson� described a specimen, now in the Leeds Museum, which convinced him that Halonia is merely a specialnbsp;form of Lepidodendron concerned with the production of fertilenbsp;shoots or strobili. Feistmantel^ also recognised that Halonia

regularis is identical in the form of the cushions with the type known as Lepidophloios laricinus. It is worthy of note thatnbsp;under the name Halonia, Feistmanteh figured a piece of decorticated axis characterised by two rows instead of the usualnbsp;spiral series of large cup-shaped scars. Recent researches have,nbsp;however, tended to break down the distinction between Ulo-dendron and Halonia founded respectively on the biseriatenbsp;and spiral arrangement of the scars or tubercles.

The interpretation of Halonial branches as cone-bearing members of Lepidodendroid plants has passed into a generallynbsp;accepted statement of fact, but, so far as I know, only onenbsp;specimen has been figured in which strobili are seen attachednbsp;to an Halonia axis. This specimen, described by Grand�Eury^nbsp;from the coal-field of Gard, is hardly sufficiently well-preservednbsp;to constitute a demonstration of the correctness of the generallynbsp;received view, which, as is not unusual, has been repeated bynbsp;one writer after another without due regard being paid to thenbsp;nature of the evidence on which the statement is based. Itnbsp;may, indeed, be correct to describe Halonial branches as conebearing, but there are certain considerations which make onenbsp;pause before unhesitatingly accepting this explanation. Thenbsp;vascular strand which passes from the central cylinder of thenbsp;shoot to the tubercle or scar is composed of a solid rod ofnbsp;xylem distinguished from the main stele by the absence of anbsp;pith. In such petrified peduncles as have been discovered thenbsp;stele is of the medullated type. The common occurrence ofnbsp;strobili terminating slender branches of lepidodendroid plants,nbsp;though not a fatal objection to their attachment to Halonialnbsp;shoots, shows that in many cases the cones were borne at the tipnbsp;of leafy shoots. It may be that some of the Halonial scars arenbsp;in origin like those of the Ulodendron axes of Bothrodendronnbsp;and mark the position of deciduous vegetative branches.

The first account of the anatomy of Halonia we owe to Dawes^; this was followed by a fuller description by Binney^.nbsp;The history of our knowledge of this type of branch has beennbsp;given by Carruthers�, who expressed the opinion that Halonia

� Dawes (48). nbsp;nbsp;nbsp;^ Binney (72); see also Seward (99).nbsp;nbsp;nbsp;nbsp;� Carruthers (73^).

is merely a fertile condition of Lepidophloios and possibly of other lepidodendroid plants. He was also inclined to regardnbsp;the Halonial tubercles as younger stages of the larger scarsnbsp;characteristic of the genus Ulodendron. Williamson�s, contributions to our knowledge of Halonia are of primary importance;nbsp;he supplied further proof of the Lepidodendroid nature of thesenbsp;branches and advanced our knowledge of their anatomy. Innbsp;an early paper^ he expressed the view that the differences onnbsp;which Halonia and Ulodendron are separated are such as resultnbsp;from a difference in age and are not of generic importance.nbsp;In the last memoir, of which he was sole author, published bynbsp;the Koyal Society^ Williamson brought forward further evidence in support of this well-founded opinion.

That the fossils known as Halonia are branches of a lepidodendroid plant is at least certain, and it is probable thatnbsp;the lateral branches which they bore were fertile, though satisfactory proof of this is lacking. We know also that Halonianbsp;branches are characterised by the Lepidophloios form of leaf-cushion ; there is, however, no sufScient reason to assume thatnbsp;such branches were never attached to stems with the cushionsnbsp;of the Lepidodendron form. The further question, namelynbsp;whether Williamson was correct in his contention as to thenbsp;absence of any essential distinction between Ulodendron andnbsp;Halonia, does not admit of an unchallenged answer. In 1903nbsp;Weiss'* described the anatomy of a specimen of a biseriatenbsp;Halonia branch of Lepidophloios. The form of the leaf-cushionsnbsp;is unfortunately not very well preserved, but Weiss figuresnbsp;other specimens with two rows of tubercles on which the leaf-cushions are sufficiently distinct to justify a comparison withnbsp;those of Lepidophloios. He believes with Williamson that it isnbsp;the presence of tubercles in place of scars which distinguishesnbsp;Halonia from Ulodendron, and that the arrangement of thenbsp;tubercles or scars is a matter of little importance. He expresses the opinion justified by the evidence available that thenbsp;absence or presence of tubercles is merely due to accidents ofnbsp;preservation or, one may add, to difference in age. Kidston^nbsp;dissents from Weiss�s description of his specimen as a biseriatenbsp;1 Williamson (72).nbsp;nbsp;nbsp;nbsp;^ ibid. (93).nbsp;nbsp;nbsp;nbsp;3 Weiss, F. E. (03).nbsp;nbsp;nbsp;nbsp;� Kidston (05).

Halonia-, he regards it as a Ulodendron branch of Sigillaria discophora (K�nig). Until specimens with more clearly preserved external features are forthcoming it is impossible tonbsp;settle the point in dispute, but on the facts before us therenbsp;would seem to be a prima fade case in favour of Weiss�snbsp;contention.

The designation Halonia may be retained as a descriptive term for Lepidodendroid shoots characterised by spirally disposed scars or tubercles and bearing leaf-cushions of the Lepi-dophloios type. In the case of specimens showing prominentnbsp;tubercles, the superficial tissues are usually absent and, as innbsp;the fossil represented in fig. 161, the name Halonia does notnbsp;necessarily imply the presence of leaf-cushions of a particularnbsp;type.

The type already described under the name Lepidodendron vasculare differs from those dealt with in the following pagesnbsp;chiefly in the anatomy of the stele. The simplest and probablynbsp;most primitive type of Lepidodendron stem is that in whichnbsp;the xylem forms a solid rod; the type of stele most frequentlynbsp;represented is that of L. Harcourtii, L. fidiginosum, and othernbsp;species in which the diameter of the stele is greater and anbsp;cylinder of primary xylem encloses a comparatively largenbsp;parenchymatous pith.

This species was founded by Renault on petrified specimens from the Culm beds of Esnost in France. The surface of anbsp;young twig bears prominent leaf-cushions of elongated rhom-boidal form similar to those of Lepidodendron ohovatumnbsp;(fig. 173) and other species. In older branches the primarynbsp;cortex is replaced by a considerable thickness of radially disposed secondary cortical tissue which, as shown in tangentialnbsp;section, consists of a reticulum of elongated pointed elements

with comparatively thick walls enclosing meshes filled with large-celled parenchyma. It is worthy of note that if such anbsp;branch were exposed to decay, the earlier destruction of thenbsp;more delicate tissue in the meshes of the secondary cortexnbsp;would produce a series of oval depressions, corresponding tonbsp;the parenchymatous areas, separated by a projecting reticulumnbsp;of the more resistant elements: a cast of this partially decayednbsp;surface would be indistinguishable from that of some typesnbsp;of Sigillaria or of a Lyginodendron. The inner regions ofnbsp;the cortex of the type-specimens have not been preserved.nbsp;The xylem, which is the only part of the stele represented,nbsp;has the form of a protostele or solid cylinder of scalariformnbsp;tracheids with peripheral groups of narrower protoxylem elements which mark the points of exit of the leaf-traces: in anbsp;branch 1�2 cm. wide the xylem column has a diameter ofnbsp;3 mm. The small leaves (fig. 143, B, C), similar to those of anbsp;Sigillaria, are sub-rhomboidal in section near the base andnbsp;approximately circular near the apexh The mesophyll consistsnbsp;of palisade cells having the appearance of typical chlorophyll-tissue. The heterosporous strobili attributed to this species borenbsp;microsporangia on the upper and megasporangia on the lowernbsp;sporophylls; the megaspores, of which a considerable numbernbsp;occur in each megasporangium, are identical in size with thosenbsp;of another Culm form, Lepidodendroyi rhodumnense. Some ofnbsp;these have retained traces of prothallus tissue, and in one sporenbsp;Renault figures what he regards as an archegonium; thenbsp;drawing is by no means convincing.

The species from the Culm of Combres (Loire) agrees in its solid xylem cylinder and in the differentiation of the secondarynbsp;cortex, as also in the association of two kinds of spore, withnbsp;Lepidodendron esnostense. A comparison of the leaves of thenbsp;two types reveals certain differences which may be of specificnbsp;rank, but, apart from minor differences, these Culm speciesnbsp;may be classed under one anatomical type.

This Devonian species was founded on a specimen 3 X 2'5 cm. broad at the base, which shows the stumps of fournbsp;branches recalling the dichotomously branched arms of Stig-maria and Pleuromeia. If these are in reality the remains ofnbsp;Stigmaria-like horizontal branches the species affords annbsp;interesting example of a Lepidodendron axis with a subterranean rhizome of the type which has been found in severalnbsp;Sigillarian stems. In the upper end of the axis the stelenbsp;consists of a solid strand of xylem which is not sufficientlynbsp;well preserved to show the position of the protoxylem groups.nbsp;A transverse section taken near the base reveals a type ofnbsp;stele differing from that at the upper end in being composednbsp;of radially disposed tracheids and in its resemblance to thenbsp;stele of Stigmaria.

1871. nbsp;nbsp;nbsp;Lepidodendron Harcourtii, Biimey, Palseont. Soc., p. 48, PI. vii.nbsp;fig. 6.

1881. Lepidodendron Harcourtii, Williamson, Phil. Trans. Roy. Soc., Vol. 172, p. 288, Pis. XLix-Lii.

1893. Lepidophloios fuliginosus, Kidston, Trans. Roy. Soc. Edinburgh, Vol. XXXVIII. p. 548.

The name Lepidodendron fuliginosum was proposed by Williamson in 1887 for petrified stems previously included bynbsp;him in Witham�s species L. Harcourtii, but subsequentlynbsp;recognised as a distinct type characterised by � the greater uniformity in the composition of the entire cortex � and by othernbsp;features some of which do not constitute distinctive characters.nbsp;The species agrees with L. Harcourtii and with L. Veltheimi-anum in having a medullated stele; it is distinguished not

only by the more frequent preservation of the middle cortex, a fact due to a difference in minute structure, but chiefly bynbsp;the peculiar structure of the secondary tissue added to thenbsp;stele; this is in part composed of radial series of parenchymatous cells and of a varying amount of tracheal tissue thenbsp;elements of which are narrower than in other species and arenbsp;characterised also by their sinuous vertical course. As is pointednbsp;out in the sequel, the anatomical features of L. fuliginosum, asnbsp;at present understood, are not confined to one type of Lepido-dendron stem. Specimens have been described with leaf-cushions of the form characteristic of L. aculeatum, L. ohovatumnbsp;and Lepidophloios combined with the anatomical features ofnbsp;Williamson�s species: it is possible that the two speciesnbsp;L. ohovatum and L. aculeatum are not really distinct^, but it isnbsp;certain that shoots with both the Lepidodendron and Lepidophloios cushions may have the same type of anatomicalnbsp;structure.

A more detailed knowledge of the structural features of Lepidodendron shoots may enable us to define anatomicalnbsp;species with more exactness than is possible at present. Therenbsp;can, however, be little doubt that well-marked anatomicalnbsp;features may be associated with more than one specific form ofnbsp;shoot as defined by the form of the leaf-cushions.

Solms-Laubach proposed the name Lepidodendron William-soni for the anatomical type L. fuliginosum of Williamson, but the latter name has been generally adopted.

In the following account special attention is directed to the nature and origin of the secondary stelar tissue and to thenbsp;secretory zone, as difference of opinion exists as to the interpretation of these features. Among the best examples of shootsnbsp;of Lepidodendron fuliginosum without secondary tissue or innbsp;which it is feebly developed are those originally described bynbsp;Binney. The stele includes a large parenchymatous pith, thenbsp;cells of which frequently show signs of recent division, a featurenbsp;observed also in the pith of the large stem of L. W�nschianum,nbsp;represented in figs. 181, 182. The primary xylem cylinder hasnbsp;an irregularly crenulate outer edge like that of L. W�nschianumnbsp;1 They are regarded as identical by Fischer (01).

and L. Harcourtii and the protoxylem elements occupy an exarch position. Isodianietric reticulately-pitted elements arenbsp;met with both on the inner and outer edge of the xylem.

Figs. 162 and 163 illustrate the structure of the outer portion of the xylem and adjacent tissues in a section of a shootnbsp;3'8 cm. X 2'5 cm. in diameter, which is in the act of branching,nbsp;as shown by the occurrence of two steles of equal size.nbsp;A figure of the complete section will be found in Binney�snbsp;memoir^, and additional illustrations were published in 1899^.

rows of polygonal cells with dark contents and associated with isodiametric tracheae: these pass into clearer parenchymatousnbsp;tissue, a, characterised by the arrangement of the cells innbsp;vertical series, to which the term meristematic zone has beennbsp;applied. The secretory zone, s, abutting on the meristematicnbsp;zone, consists of more or less disorganised parenchymatous cellsnbsp;1 Binney (72) PI. xiii. fig. 1.nbsp;nbsp;nbsp;nbsp;^ geward (99).

and broader and more elongated spaces; it is interrupted here and there by an outgoing leaf-trace, as at It 1 and It 2 in fig. 162.nbsp;The secretory zone is succeeded by a homogeneous inner cortexnbsp;like that described in L. vasculare; part of this region is seennbsp;at the upper edge of fig. 162. The broad middle cortex, whichnbsp;is separated from the inner cortex by a sharply definednbsp;boundary, is composed of rather small lacunar parenchymatous tissue consisting of sinuous tubular elements interspersednbsp;among isodiametric cells of various sizes (fig. 166, p). Innbsp;the middle cortical region the leaf-traces pursue an almost

horizontal course; one is shown in fig. 164, in oblique longitudinal section, in a reversed position; the xylem, x, should be on the inner side of the secretory tissue, s. The clear spacenbsp;between the two parts of the vascular bundle was originallynbsp;occupied by a few layers of parenchymatous cells, as seen in thenbsp;transverse sections, figs. 165 and 166. In some specimens thenbsp;leaf-traces pass through the middle cortex in a much morenbsp;vertical course, as shown by the section represented in fig. 165.nbsp;This section illustrates the structure of a typical leaf-trace withnbsp;unusual clearness; it shows the tangentially elongated group ofnbsp;xylem, the strand of tissue which occupies the position of phloem,

s (to which the term secretory zone is applied), the compact parenchyma between the two parts of the bundle, and surrounding the whole a narrow sheath sharply contrasted by thenbsp;smaller and more uniform size of the cells from the middlenbsp;cortex, a few cells of which are seen in the photograph. Thenbsp;middle cortex shows a well-defined junction with the morenbsp;compact outer cortical region, which consists of primarynbsp;parenchyma passing externally into a zone of phelloderm composed of thick-walled and more elongated cells. A noticeablenbsp;feature in many Lepidodendron shoots is the occurrence of anbsp;circle of strands of secretory cells often surrounding fairly large

ducts just internal to the edge of phelloderm: similar strands form irregularly concentric circles, as was pointed out in thenbsp;case of L. vasculare, in the phelloderm itself.

Fig. 166 shows a leaf-trace in the outer cortex accompanied by its crescent-shaped parichnos, p, derived from the middlenbsp;cortex and by means of which the outer cortex and the laminanbsp;of the leaves are connected with the inner region of the shoot.nbsp;This lacunar middle cortex and parichnos doubtless constitute annbsp;aerating tissue-system which after leaf-fail is exposed directlynbsp;to the air at the ends of the parichnos arms on the leaf-scars.

Some of the sections in the Binnej Collection (Sedgwick Museum, Cambridge) show early stages in the production ofnbsp;secondary xylem; in the section represented in fig. 167 thenbsp;secretory zone is succeeded on its inner face by a zone of radiallynbsp;elongated cells, to, which are clearly in a meristematic condition.nbsp;The same section show^s also the more radially extended formnbsp;of the xylem of a leaf-trace wdth its internal protoxylem, px, in

contrast to the tangentially elongated form which is assumed during its passage through the cortex (cf figs. 165, 166).

Some sections of Lepidodmdron fuliginosum in the Manchester University Collection are of special interest from the point of view of the method of secondary thickening. Innbsp;the section reproduced in fig. 168, B, the meristematic zonenbsp;is seen to consist in part of radially elongated elements, m,nbsp;with parallel cross-walls evidently of recent origin. The samenbsp;tissue is shown also in fig, 168, C, a, D, a, and in fig. 169, A, a

This band of meristem, which we may speak of as the cambium, occurs in the outer region of the meristematic zone immediatelynbsp;internal to the secretory zone, sc.

The result of the activity of this cambium band is the production of secondary parenchyma and tracheal tissue. Innbsp;fig. 179, E, drawn from a portion of the section represented in

fig. 168, B, a projecting arm of primary xylem is seen at x; this is followed by 2�3 layers of parenchymatous cells, some ofnbsp;which have dark contents, and beyond this is seen a group ofnbsp;secondary elements, tr-, cut across somewhat obliquely, whichnbsp;are evidently products of the cambial cells on the inner margin

of the secretory zone, sc. The longitudinal section (fig. 169, D) shows the cambial cells, a, next the secretory zone, sc, passingnbsp;internally into crushed and imperfectly preserved elongatednbsp;elements which are presiimably miniature tracheae, and thesenbsp;are succeeded by older and more completely lignified xylem

elements, ir. In larger shoots the amount of secondary tissue is considerably greater ; it may consist almost entirely of short-celled parenchyma (fig. 168, C, from x to sc), or it may includenbsp;a large proportion of radially disposed and vertically elongatednbsp;tracheae (fig. 168, D, x^, and fig, 170, A, a^), or it may consist of

Fig. 169, A, is a diagrammatic sketch of the tissues�1 mm. wide�between the primary xylem, x, and the inner cortex.nbsp;'I'he primary xylem is succeeded by short parenchymatous cellsnbsp;followed by a zone of radially elongated elements passingnbsp;occasionally into rows of narrow scalariform tracheae, some ofnbsp;which, owing to their sinuous longitudinal course (fig. l7l, C),nbsp;are seen in oblique section, as at C, fig. 169, A. At its outernbsp;edge this secondary tissue, consisting of parenchyma andnbsp;tracheae, passes into the cambial band (fig. 169, B, a).

A nbsp;nbsp;nbsp;B

The radial longitudinal section represented in fig. 168, C, is taken from the fossil described by Weiss as a biseriate Halonia;nbsp;it agrees sufficiently closely in structure with others referred tonbsp;Lepidodendron fiiUgmosum to be classed as an example of thisnbsp;anatomical type. A complete transverse section of the stemnbsp;measures 9x6�3cm.; the breadth of the tissues between thenbsp;edge of the primary xylem and the outer edge of the secretorynbsp;zone is 2�5 mm. The middle cortical region, characterised bynbsp;the sooty appearance, which led Williamson to choose thenbsp;specific name fuligmosum, is traversed by the leaf-traces and isnbsp;sharply differentiated from both the inner and outer cortex.

1 As Miss Stokey (09) points out the production of parenchyma internal to the cambium of L. fuUginosum is a feature shared by Isoetes. See also Scottnbsp;and Hill (00), p. 424.

The longitudinal section (lig. 168, C) shows the outer edge of the primary xylem, x, abutting on a band of dark and small-celled parenchyma which passes into the broad zone of secondarynbsp;tissue, m, the inner region of which consists of fairly thick-walled elements in radial series passing externally into thenbsp;thin-walled cells of the cambial region, a, on the inner edge ofnbsp;the secretory zone, sc. This section shows also the interruption

of the secretory zone by an out-going leaf-trace. It, the lower part of which, sc, is continued downwards into the secretory zone. Thenbsp;exit of a leaf-trace produces a gap in the secretory zone of thenbsp;stem, but not in the xylem. If we applied the term phloem to thenbsp;secretory zone�a course adopted by Prof. F. E. Weiss and somenbsp;other authors, but which I do not propose to follow�we shouldnbsp;speak of a phloem foliar-gap as a characteristic feature of a

Lepidodendron shoot. This applies to other species of the genus as well as to L. fuliginosum.

Fig. 171, A, shows more clearly the broad zone of secondary parenchyma with the thinner-walled cambial region, a; the latternbsp;is represented on a larger sca,le in fig. 171, B. The sectionnbsp;shown in fig. 168, D, and in fig. 170, A, affords an example of anbsp;stem in which the secondary tissue consists largely of narrownbsp;scalariform tracheae, the primary stele has a diameter ofnbsp;1 cm.; the secondary xylem, forms a fairly broad zone ofnbsp;parenchyma and tracheal elements through which leaf-tracesnbsp;pass vertically, a fact of some interest in comparison with the

horizontal course which they pursue through the medullary rays in the normal secondary wood of L. vasculare and L. Wmischi-anum. The secondary tracheae pass gradually into thin-wallednbsp;cambial cells (a, fig. 168, D ; 170, A) with parallel tangentialnbsp;walls. Fig. 171, C, shows the sinuous course of the secondarynbsp;tracheae as seen in longitudinal section, and a few smallnbsp;groups of parenchymatous cells, mr, which may be of the naturenbsp;of medullary rays, enclosed between the winding scalariformnbsp;tracheae.

The secretory zone of Lepidodendron fuliginosum agrees essentially with that of other species; it usually presents the

appearance shown in fig. 168, B, sc; fig. 169, B and C ; fig. 170, B (longitudinal section) ; fig. 171, D, sc. The comparatively large clear spaces which characterise this tissue, as seennbsp;in fig. 168, B, appear to owe their origin to groups of smallnbsp;cells which gradually break down and give rise to spaces containing remnants of the disorganised elements, as in fig. 171, D,nbsp;and fig. 169, B, b. The secretory tissue seen in fig. 170, B,nbsp;consists of large and small parenchymatous cells without any ofnbsp;the broad sacs or spaces such as are shown in fig. 169, C.

Fig. 172 represents a diagrammatic sketch of a transverse section (4 x 3'4 cm. in diameter) of a young shoot from thenbsp;Lower Coal-Measures of Lancashire figured by Williamson^ innbsp;1881 as Lepidodendron Harcourtii. It shows the featuresnbsp;characteristic of L. fuliginosum and is of importance as affording an example of a shoot giving off a branch from the stele tonbsp;supply a lateral axis of the type characteristic of Halonia. Thenbsp;exit of the branch-stele forms a gap in the main stele; anbsp;ramular gap as distinguished from a foliar gap. The outgoingnbsp;vascular strand is at first crescentic, but becomes graduallynbsp;converted into a solid stele. The primary xylem of the mainnbsp;stele (black in the figure) consists of a ring six tracheae innbsp;breadth; this is succeeded by a few layers of dark parenchymatous cells and a band of radially elongated elements, a, whichnbsp;abuts on the secretory zone. The middle lacunar cortex, c^,nbsp;with Stigmaria rootlets, s, is fairly well preserved. In the outernbsp;cortex occur several leaf-traces. It, accompanied by spaces originally occupied by the parichnos strand, p. A band of secondarynbsp;cortex, consisting chiefly of phelloderm, is seen at pd. Thenbsp;prominent leaf-cushions, some of which show the parichnos, jp,nbsp;appear to be of the Lepidophloios type.

It remains to consider the external characters of Lepidoden-droid shoots possessing the anatomical features represented by the comprehensive species Lepidodendron fuliginosum.

Certain sections exhibiting this type of structure were described by Binney in 1872 as Halonia regularise on evidencenbsp;supplied by Mr Dawes, who stated that they were cut from anbsp;1 Williamson (81) A. PI. lii. p. 288. (Will. Coll. No. 379.)nbsp;nbsp;nbsp;nbsp;^ Binney (72).

specimen bearing Halonia tubercles. The section represented in fig. 172 is no doubt from an Halonia axis. In 1890 Cashnbsp;and Lomax ^ stated that they had in their possession a stem ofnbsp;the L. fuliginosum type with the external features of Lepi-dophloios; this identification has been confirmed by Kidston^nbsp;and Weiss�. It is, however, equally clear that certain speciesnbsp;with the elongated leaf-cushions of Lepidodendron must benbsp;included among examples of shoots with the anatomicalnbsp;characters of L. fidiginosum.

Dr Scotf* published in 1906 a short account of the structure of a specimen from the Lower Coal-Measures of Lancashire, thenbsp;external features of which were identified by Kidston withnbsp;those of Lepidodendron ohovatum Sternb. Dr Scott generouslynbsp;alloAved me to have drawings made from his specimen; thesenbsp;are reproduced in fig. 173. The form of the leaf-cushion is bynbsp;no means perfect; there is a well-marked median ridge, andnbsp;the small circular scar near the upper end of some of thenbsp;cushions may represent the ligular cavity. At the base of thenbsp;leaf-cushions a cortical meristem has produced a zone of

secondary cortex; at c a second meristem is seen in the oneer cortex: the dark dots in the cortex mark the positions of leaf-trace bundles. The inner cortex, d, is a more compact tissue

surrounding the imperfectly preserved secretory zone. From the medullated stele a lateral branch, h, is being given off; itsnbsp;crescentic form becoming changed to circular as it passes nearernbsp;to the surface.

A type of Lepidodendroyi, L. Hickii, founded on anatomical characters by Mr Watsonh is believed by him to possess leaf-cushions like those of L. obovatum; if this is so, it is interesting,nbsp;as he points out, to find two distinct anatomical types associatednbsp;with one species. Watson thinks it probable that the � species �nbsp;L. obovatum includes at least two widely different species. Thisnbsp;merely emphasizes the importance of correlating structure andnbsp;external characters as far as available data permit.

The specimen, of which part of the surface is shown in fig. 174, is in all probability L. aculeatum Sternb. This wasnbsp;described by me in detail in The Annals of Botany (1906) asnbsp;another example of the co-existence of the Lepidodendroyi full-ginosum type of anatomy with a true Lepidodendron. Thenbsp;locality of the specimen is not known. The leaf-cushionsnbsp;are lA cm. long with tapered upper and lower ends; a ligularnbsp;cavity may be recognised on some parts of the fossil, also faintnbsp;indications of leaf-trace scars. The tubercles (fig. 174, A�C, t)nbsp;probably represent leaf-traces which the shrinkage of the superficial tissues has rendered visible in the lower part of theirnbsp;course. The circular scar, s (fig. B), on the partially decorticatednbsp;surface is apparently a wound. The stele is sufficiently

well preserved to justify its reference to L. fuliginosum. The irregularly crenulated edge of the primary xylem, x (fig. 175), isnbsp;succeeded by a broad band of parenchyma (the meristematicnbsp;zone), m, and beyond this are remnants of the secretory zone, s.nbsp;The structure of the leaf-traces corresponds with that of othernbsp;specimens of the type, but the much steeper course of thesenbsp;vascular strands. It, It' (fig. 176), is a feature in which thisnbsp;example differs from most of those referred to L. fuliginosum.nbsp;Such evidence as is available would seem to point to the

Fio. 177. Stigmaria radiculosa (Hick). (From sections in the Manchester University Collection.)

absence of trustworthy criteria enabling us to separate, on anatomical grounds, Lepidophloios and Lepidodendron^.

We have no proof of the nature of the subterranean organs of Lepidodendron fuliginosum, though it is not improbable that thenbsp;specimens described below may be correctly assigned by Weissnbsp;to that species. Prof. Weiss^has made an interesting contribution to our knowledge of a type first described by Hick� undernbsp;the name Tylophora radicidosa, a designation which he after-

wards altered to Xenophyton radiculosum^ and for which we may now substitute Stigmaria radiculosa (Hick). Prof. Williamsonnbsp;expressed the opinion that Xenophyton exhibited considerablenbsp;affinity with Stigmaria ficoides and Weiss�s further study of thenbsp;species leads him to regard Hick�s plant as probably the Stig-marian organ of Lepidodendroji fidiginosum. The diagrammaticnbsp;transverse section represented in fig. 177, A (4'5 cm. in diameter),nbsp;shows an outer cortex of parenchyma, (f, consisting in part of

radial rows of secondary tissue and of a band of compact parenchyma bounded by the wavy line a; at sc is a series of secretory strands exactly like those in a corresponding position in Lepido-dendron fidiginosum and other species of the genus. The greaternbsp;part of the organ is occupied by a lacunar and hyphal middlenbsp;cortex identical in structure with that shown in fig. 178, B, drawnnbsp;from a rootlet. At d, fig. 177, A, the middle cortex has been

invaded by a narrow tongue of outer cortical tissue. The stele is characterised by a large pith filled with parenchyma; in Stig-maria ficoides'- the general absence of pith-tissue has led to thenbsp;inference that the stele was hollow. The xylem is represented bynbsp;a ring of bundles separated by broad medullary rays; each bundlenbsp;contains a few small, apparently primary, elements on its innernbsp;edge but is mainly composed of radial rows of secondary tracheaenbsp;x�% fig. 177, B. On the outer face of the secondary xylem occur anbsp;few smaller and thinner walled cells, c, having the appearancenbsp;of meristematic tissue; from these additional tracheae werenbsp;added to the xylem. This meristematic zone occurs, as in thenbsp;stems of Lepidodendron, immediately internal to the secretorynbsp;tissue, sc, at O�-, fig. 177, B, is seen the inner cortical tissue.

In surface-view a specimen figured by Hick^ shows a number of circular scars agreeing in shape and arrangement with thenbsp;rootlet scars of Stigmaria ficoides. At b in fig. 177, A, the basalnbsp;portion of a rootlet is shown in organic connexion with the outernbsp;cortex. The rootlet bundles are given off from the stele as innbsp;other examples of gt;Slt;f^mana; each bundle consists of a triangularnbsp;strand of xylem with an endarch protoxylem at the narrow endnbsp;accompanied by a portion of the secretory tissue as in thenbsp;leaf-traces. As in Stigmaria ficoides the rootlets are attachednbsp;to the outer cortex above a cushion of small cells. It is interesting to find that rootlet-bundles, as seen in tangential sectionnbsp;of the main axis, are associated with a parichnos strand, butnbsp;this is on the xylem side of the vascular strand, whereas in thenbsp;case of leaf-traces the parichnos is on the other side of thenbsp;bundle.

Fig. 178, A, represents a transverse section of a rootlet (6 mm. in diameter) associated with Stigmaria radiculosa andnbsp;probably belonging to this species. The xylem strand x is composed of a group of tracheae with a single protoxylem strand,nbsp;px, at the pointed end and with small metaxylem elements atnbsp;the broad end next the space originally occupied by the so-called phloem. A parenchymatous sheath, c', surrounds thenbsp;bundle, and beyond this is the broad middle cortex a smallnbsp;portion of which is shown on a larger scale in fig. 178, B; as Weissnbsp;^ See p. 240.nbsp;nbsp;nbsp;nbsp;^ Hick (93) PI. xvi. fig. 1.

points out, some of the outermost cells of the lacunar cortex (m) are clearly in a state of meristematic activity.

The preservation of the middle cortex and the small quantity of secondary xylem are characters which this Stigmaria shares with Lejndodendron fuliginosum, and although decisivenbsp;evidence is still to seek, we may express the opinion thatnbsp;Weiss�s surmise of a connexion between Stigmaria radiculosanbsp;and Lepidodendron fuliginosum is probably correct.

In 1831 Mr Witham^ published an anatomical description of a fragment of a Lepidodendron which he named Lepidodendronnbsp;Harcourtii after Mr C. G. V. Vernon Harcourt from whom thenbsp;specimen was originally obtained. The fossil was found in rocksnbsp;belonging to the Calciferous series in Northumberland. Withamnbsp;reproduced the account of this species in his classic worknbsp;on Fossil Vegetables^, and Lindley and Hutton��, who examinednbsp;Mr Harcourt�s material, published a description of it in theirnbsp;Fossil Flora in which they expressed the view that Lepidodendron is intermediate between Conifers and Lycopods. Adolphenbsp;Brongniart^ included in his memoir on Sigillaria elegans annbsp;account of Withara�s species based on material presented to thenbsp;Paris Museum by Mr Hutton and Robert Brown. Dr Kidston�nbsp;has shown that the actual transverse section figured by Withamnbsp;is now in the York Museum; a piece of stem in the samenbsp;Museum, which is not the specimen from which Witham�snbsp;section was cut, supplied the transverse section figured bynbsp;Brongniart. The figures given by Lindley and Hutton do notnbsp;appear to have been made from the York specimens. In 1887nbsp;Williamson� published a note in which he pointed out that somenbsp;of the specimens described by him as L. Harcourtii should benbsp;transferred to a distinct species, which he namp;mamp;AL. fuliginosum.nbsp;Subsequently in 1893 he gave a fuller account of Witham�snbsp;species; it has, however, been shown by Dr Kidston and by

Mr Watson^ that certain specimens identified by Williamson as L. Harcourtii differ sufficiently from that type to be placed innbsp;another species, for which Watson proposes the name L. Hickii.

A paper on L. Harcourtii published by Bertrand^ in 1891 extends our knowledge of this type in regard to several anatomical details. It was recognised by Williamson that thenbsp;absence of secondary wood in shoots possessing the anatomicalnbsp;characters of L. Harcourtii is a feature to which no greatnbsp;importance should be attached. It is possible that the largenbsp;stems from the Isle of Arran described by Williamson*� asnbsp;Lepidodendron W�nschianum, in which the secondary wood isnbsp;well developed, may be specifically identical with the smallernbsp;specimens from Northumberland and elsewhere which are recognised as examples of Witham�s type.

The diagrammatic sketch shown in fig. 179, A, was made from a section figured by Williamson in 1893^; it has a diameter ofnbsp;9 X 8'5 cm. The stele is of the medullated type like that ofnbsp;L. W�nschianu7n, and the outer edge of the primary xylem isnbsp;characterised by sharp and prominent projecting ridges similarnbsp;to those of L. fidiginositm hut rather more prominent. Parenchymatous cells succeed the xylem, as in other species, but innbsp;this case there is no indication of meristematic activity; beyondnbsp;this region occur occasional patches of a partially destroyednbsp;secretory zone. Remains of a lacunar tissue are seen in thenbsp;middle cortical region; also numerous leaf-traces. It, consistingnbsp;of a tangentially elongated xylem strand accompanied by anbsp;strand of secretory zone tissue enclosed in a sheath of delicatenbsp;parenchyma. In the inner part of the outer cortex, c*�, the leaf-traces lie in a space originally occupied by the parichnos; innbsp;the outer portion of the same region a band of secondarynbsp;cortex, pd, has been formed; immediately internal to thisnbsp;occur numerous patches of secretory tissue, represented by smallnbsp;dots in the drawing close to pd; one is shown on a larger scalenbsp;in fig. B.

The position of the phellogen is seen at a; external to this are radial rows of rather large cells with dark contents.

it serves to illustrate the general appearance of the xylem surface met with in both species, L. Harcourtii and L. fuliginosum. A tangential longitudinal section, taken through thenbsp;line ah in fig. C, is represented in fig. 179, D. The xylem ofnbsp;the- leaf-traces It, consisting chiefly of scalariform tracheae,nbsp;alternates with patches of crushed and delicate parenchyma which

immediately abut on the primary xylem; at p, p, the section passes through some of the projecting arms of the xylemnbsp;cylinder; at m is seen a patch of meristematic zone tissue.nbsp;This section together with the similar section of Lepidoden-dron vasculare described on a previous page demonstrates thatnbsp;the projecting ridges of the primary xylem form apparentlynbsp;vertical bands: they are not characterised by a lattice-worknbsp;arrangement as described by Bertrand and by other authorsnbsp;who have accepted his conclusions. If a reticulum of intersecting ridges were present on the face of the xylem cylinder itsnbsp;existence would be revealed by such a section as that represented in fig. 179, D.

Reference was made in Volume I. to the occurrence of large stems of a Lepidodendron in volcanic beds of Calciferous sandstone age in the island of Arrant These were discovered andnbsp;briefly described by Mr W�nscb in 1867^ and afterwards namednbsp;by Carruthers Lomatophloyos W�mchianus^. Mr Carruthers

1 Volume 1. p. 89. For other references to these stems, see Seward and Hill (00) p. 918.

visited the locality and published an account of the peculiar method of preservation of the plant remains^ It is, however, tonbsp;Williamson^ that we owe the more complete description of thesenbsp;Arran stems. Portions of large stems from the Arran beds arenbsp;preserved in the British Museum, the Sedgwick Museum, Cambridge, and in the Manchester Museum. The section of onenbsp;of these is shown in fig. 180; an outer shell of bark encloses anbsp;mass of volcanic ash in which are embedded several woodynbsp;cylinders originally described as �internal piths�,� and by Carru-thers as young stems produced from spores which had germinatednbsp;in the hollow trunk of a large tree. The true interpretation wasnbsp;supplied by Williamson who showed that a stem of the dimensionsnbsp;of that represented by the outer cortex, e, fig. 180, must havenbsp;possessed a single stele of the size of those seen in the interiornbsp;of the hollow trunk. The additional woody cylinders, or steles,nbsp;were derived from other stems, and carried, probably by water,nbsp;into the partially decajmd trunk. In addition to large Lepido-dendron stems Williamson described smaller shoots as wellnbsp;as an Halonial branch and made brief reference to somenbsp;cones described by Binney^ in 1871 from the same locality.

The following account of Lepidodendron W�nschianum is based on an exceptionally fine specimen discovered by Mr T.nbsp;Kerr of Edinburgh in Calciferous sandstone volcanic ashes atnbsp;Dalmeny in Linlithgowshire. The material from this localitynbsp;described by Mr Hill and myself� was generously placed in mynbsp;hands by Dr Kidston of Stirling. Fig. 181, A, shows anbsp;transverse section, 33 cm. in diameter, consisting of a shell ofnbsp;outer cortical tissue enclosing a core of light-coloured volcanicnbsp;ash; on the decay of the more delicate middle cortex the cylindrical stele dropped to one side of the hollow trunk. The stele,nbsp;fig. 182, has a diameter of 6'5 cm.; the centre is occupied bynbsp;concentric layers of silica, s, surrounded externally by thenbsp;remains of a parenchymatous pith, p, made up of isodiametricnbsp;and sinuous hypha-like elements like those in the middle cortexnbsp;of Lepidodendron shoots. On the inner edge of the primary

xylem,occur several isodiametric tracheae with fine scalariforni and reticulate thickening bands like those in the central region

of the stele of Lepidodendron vasculare: it is probable that these elements are vestiges of conducting tissue which in ancestralnbsp;forms formed a solid and not a medullated stele.

The primary xylem is limited externally by an unequally fluted surface with exarch protoxylem elements; it is, however,

noteworthy that there is not always a very clearly defined difference between the small protoxylem and the large centri-petally developed tracheae. Immediately beyond the primarynbsp;xylem occur numerous thin-walled parenchymatous cells withnbsp;spiral and reticulate pitting; beyond these is the broad zone of

secondary xylem, x^, composed of scalariform tracheae and numerous medullary rays consisting of one, two, or several rowsnbsp;of radially elongated elements with spiral and reticulate pitting.nbsp;In tangential sections the rays are seen to vary considerabl}quot;nbsp;in size, some being made up of a single row of cells while othersnbsp;are longer and broader; through the latter leaf-traces passnbsp;horizontally. Portions of medullary rays are seen at mr innbsp;fig. 181, C and E.

The leaf-traces given off from projecting ridges on the outer edge of the primary xylem pass upwards for a short distancenbsp;and then bend outwards through a broad medullary ray; onnbsp;reaching the limit of the secondary xylem they again bendnbsp;sharply upwards, appearing in transverse section at It fig. 181, B.nbsp;Each leaf-trace consists at first of long tracheae accompanied bynbsp;numerous thin-walled spiral and reticulate parenchymatous cellsnbsp;derived from the tissue in contact with the outer edge of thenbsp;primary wood. Fig. 181, B, shows a leaf-trace near the edge ofnbsp;the secondary xylem; it consists of a group of primary tracheae,nbsp;with narrower protoxylem elements, px, near the outer margin,nbsp;almost completely enclosed b}' radially disposed series of smallernbsp;and more delicate tracheae. These secondary elements of thenbsp;leaf-trace are apparently added during its passage throughnbsp;the medullary ray, but additions are also made to this tissuenbsp;by the meristematic zone, m, fig. 181, B and E. In contactnbsp;with the outermost tracheae of normal size at the edge of thenbsp;secondary xylem there are some smaller lignified elements, asnbsp;at a, fig. 181, E, and at T, fig 183; this juxtaposition of largenbsp;and small tracheae has been referred to in the description ofnbsp;L. vasculare.

Prof. Williamson^, in his account of the Arran specimens of this species, expressed the opinion that the trees probabh^nbsp;perished �in consequence of the mephitic vapours which fillednbsp;the atmosphere�; it maybe that in the striking difference in thenbsp;diameter of the conducting elements on the margin of the woodnbsp;we have evidence of approaching death.

of delicate parenchymatous cells (m, figs. B and E, 181; C, figs. 183, 184) forming the meristematic zoneh The longitudinalnbsp;section represented in fig. 184 shows some recently formed narrownbsp;tracheae, T, and beyond these the meristematic zone composed ofnbsp;thin-walled short cells, C, arranged in horizontal rows. It is thisnbsp;small-celled tissue to which the name phloem has been appliednbsp;by some authors^, a term which seems to me to be misleading

and inappropriate. In passing through this zone of dividing cells the leaf-traces become surrounded by an arc of meristem fromnbsp;which elements are added to the radially placed rows of secondarynbsp;tracheae. Beyond the meristematic region portions of the

1 nbsp;nbsp;nbsp;The term meristematic zone is used because some of the cells in thisnbsp;region are in a state of active division, though the inner portion may consistnbsp;of permanent tissue.

secretory zone are preserved, consisting of large sacs or spaces and small dark cells as seen in figs. 181, B, E, sc, F; 183, 184.nbsp;This tissue has the same structure as in i. vasculare and innbsp;L. faliginosum: it is a striking fact that there are no indicationsnbsp;of any additions to the secretory zone even in stems with suchnbsp;a large amount of secondary xylem as in the Dalmeny specimennbsp;(fig. 182, If the secretory zone were of the nature of phloemnbsp;we should expect to see signs of additions made to it in the

course of growth. In this connexion it is worth mentioning that in the recent fern Botrychium (Ophioglossaceae) secondarynbsp;xylem is formed in the stem, but apparently no additions arenbsp;made to the phloem. The structure of the secretory zone tissuenbsp;as seen in the longitudinal section fig. 184, S, is also a seriousnbsp;difficulty in the way of accepting the designation phloem as

employed by Scott and Weiss. Between the secretory zone and the outer cortical region, no tissues have been preserved. Thenbsp;shell of bark consists chiefly of radial rows of elongated cellsnbsp;with rather thick walls characterised by the occurrence of smallnbsp;intercellular spaces and by tangentially placed bands of secretorynbsp;cells and sacs (fig. 181, D, s). Immediately internal to thenbsp;secondary cortex or phelloderm occur groups of secretory tissuenbsp;as shown in the section of L. Harcourtii (fig. 179, B).

The large tree shown in transverse section in fig. 181, A, has lost its leaf-cushions; the bark, as seen in the lowernbsp;part of the photograph, presents a fissured appearance likenbsp;that with which we are familiar on an old Oak or Elm stem.nbsp;A radial longitudinal section through the phelloderm revealednbsp;the existence of a crushed leaf-trace passing outwards in annbsp;approximately horizontal course accompanied by a strand ofnbsp;parenchymatous tissue^ having the characteristic structure of anbsp;parichnos. It is probable that the surface of this partiallynbsp;decorticated stem differed in appearance from that of an oldnbsp;Sigillaria (c� fig. 198) in the much smaller and less conspicuousnbsp;parichnos strands.

In addition to the large stems of L. W�nschiaiium from Arran and Dalmeny numerous examples of smaller axes fromnbsp;the former locality are represented in the Williamson collectionnbsp;(British Museum). Some of the twigs are characterised by anbsp;solid stele (protostele) giving off numerous leaf-traces accompanied by short spirally thickened tracheids like those whichnbsp;occur at the outer edge of the primary xylem in the larger stem;nbsp;these extend into the leaf where they are arranged round thenbsp;vascular bundle like the transfusion tracheids^ in many recentnbsp;conifers. The surface of these smaller shoots bears large leaf-cushions which are seen in longitudinal section to have the formnbsp;characteristic of Lepidophloios. It is worthy of note that anbsp;section of a bifurcating axis of this species from the Calciferousnbsp;Sandstone of Craigleith (British Museum Collection^), although itsnbsp;diameter is 19 x 14 cm., shows no signs of secondary wood. Thisnbsp;late appearance of secondary xylem and other anatomical features

suggest the possibility of the specific identity of L. W�nschianum and L. Harcourtii''.

In 1871 Binney� described a specimen of a heterosporous cone, Lepidostrobus W�nschianus, from Arran exhibiting the ordinarynbsp;features of lepidodendroid strobili; this was probably borne bynbsp;Lepidodendron W�nschianum.

The diagrammatic sketch reproduced in fig. 186, C, was made from the transverse section of a small twig, slightlynbsp;less than 2 cm. in its longest diameter, originally figured bynbsp;Williamson^ in 1872. Earlier in the same year Carruthers^ published a short account of the same form based on specimensnbsp;collected by Mr Butterworth from the Coal-Measures ofnbsp;Lancashire near Oldham, but both authors refrained from instituting a new specific name. In a later publication Williamsonnbsp;spoke of the type as Lepidodendron macrophyllum^. Williamson�s species has nothing to do with Lycopodites macrophyllus ofnbsp;Golden berg**. The most striking feature of this rare form isnbsp;the large size of the leaf-cushions, which are of the L^epidophloiosnbsp;type, in proportion to the diameter of the shoot. The stelenbsp;consists of a ring of xylem, all of which is primary in the sectionsnbsp;so far described, enclosing a parenchymatous pith: a Stigmariannbsp;rootlet is shown at s.

8. Lepidodendron Veltheiniianum Sternb. (General account). Figs. 157, 185, 186, A, B.

1820. nbsp;nbsp;nbsp;� Schuppenpflanze,� Rhode, Beit, zur Pflaiizenkunde der Vor-

1825. Lepidodendron Veltheiniianum, Sternberg, Flora der Vorwelt, PI. Lil. fig. 5.

1836. Pachyphloeus tetragonus, Goeppert, Diefossilen Farnkrauter, PI. XLIII. fig. 5.

1852. Sagenaria Veltheimiana, Goeppert, Foss. Flora des Ubergangs-gebirges. Pis. xvii�xxiv.

1 Seward and Hill (00) p. 922. nbsp;nbsp;nbsp;^ Binney (71) p. 56, PL xi. figs. 2a�2c.

1886. Lepidodendron Veltheimianum, Kidston, Catalogue of Palaeozoic plants, British Museum, p. 160.

1901. Lepidodendron Veltheimianum, Potoni�, Silur und Culm Flora, p. 116, figs. 72�76.

1904. Lepidodendron Veltheimianum, Zalessky, M�m. Com. G�ol. Russie, PI. IV. figs. 4, 5.

1906. Lepidodendron Veltheimi, Potoni�, K�nigl. Preuss. geol. Landes-anstalt. Lief. iii.

The above list may serve to call attention to a few synonyms^ of this plant, and to a selection of sources from which fullnbsp;information may be obtained as to the history of our knowledgenbsp;of this characteristic and widely spread Lower Carboniferous

Lepidodendron Veltheimianum is represented by casts of stems, the largest of which hitherto described reaches a lengthnbsp;of 5�22 metres with a maximum diameter of 63 cm.; thisnbsp;specimen, figured by Stur�', consists of a tapered main axisnbsp;giving off smaller lateral shoots, some of which exhibit dichotomous branching. Fig. 185, C and D, represent the externalnbsp;features of a well-preserved cast and impression respectively.nbsp;Oblique rows of prominent cushions wind round the surface ofnbsp;the stem and branches: each cushion is prolonged upwards andnbsp;downwards in the form of a narrow ridge with sloping sidesnbsp;which connects adjacent cushions by an ogee curve. Atnbsp;the upper limit of the broader kite-shaped portion of thenbsp;cushion the ligular pit forms a conspicuous feature ; immediately below this is the leaf-scar with its three small scars,�nbsp;the lateral parichnos strands and the central leaf-trace. Thenbsp;two oval areas shown in fig. 185, D, just below the lower edgenbsp;of the leaf-scars, represent the parichnos arms which impingenbsp;on the surface of the cushions on their way to the leaves, asnbsp;explained on a previous page. It is possible that these areasnbsp;were visible on the living stem as strands of loose parenchymanbsp;comparable with the lenticel-like pits on the stipules ofnbsp;Angiopteris^ and the leaf-bases of Cyatheaceous ferns, or it

may be that their prominence in the specimen before us is the result of the decay of a thin layer of superficial cortex which hid

them on the living tree. Fig. 185, B, illustrates the appearance of a stem in a partially decorticated condition {Bergeria state).nbsp;A further degree of decortication is seen in fig. 185, A, whichnbsp;represents the Knorria condition.

Fig. 157 shows a Ulodendron axis of this species; in the lower part the specimen illustrates the partial obliteration ofnbsp;the surface features as the result of the splitting of thenbsp;outer bark consequent on growth in thickness of the tree.nbsp;By an extension of the cracks, shown in an early stage innbsp;fig. 157, the leaf-cushions would be entirely destroyed and thenbsp;surface of the' bark would he characterised by longitudinalnbsp;fissures simulating the vertical grooves and ridges of a Sigil-larian stem. The large stumps of trees shown in the frontispiece to Volum� I. are probably, as Kidston^ suggests, trunksnbsp;of L. Veltheimianum in which the leaf-cushions have beennbsp;replaced by irregular longitudinal fissures. In old stems ofnbsp;Sigillaria the enlarged parichnos areas constitute a characteristicnbsp;feature (p. 205), but it does not follow that the absence of largenbsp;parichnos scars is a distinguishing feature of all Lepidodendra.

In this species, as in others, the form of the* leaf-cushion exhibits a considerable range of variation dependent on thenbsp;thickness of the shoot; the contiguous cushions of youngnbsp;branches become stretched apart as the result of increasingnbsp;girth of the whole organ, and casts of still older branches maynbsp;exhibit very different surface-features^. The leaves as seennbsp;on impressions of slender branches are comparatively short,nbsp;reaching a length of 1�2 cm. It is important to notice thatnbsp;leafy twugs of this species may bear terminal cones ^ resemblingnbsp;in form those of Picea excelsa and other recent conifers, thoughnbsp;differing essentially in their morphological features.

The fossil stumps of trees represented in the frontispiece to Volume I, bear horizontally spreading and dichotomouslynbsp;branched root-like organs having the characters of Stigmarianbsp;ficoides*. Geinitz has suggested that Stigmaria inaequalisnbsp;G�pp. may be the underground portion of Lepidodendron Velthei-mianum.

It is unfortunately seldom possible to connect petrified Lepidodendron cones with particular species of the genus basednbsp;on purely vegetative characters, but it is practically certainnbsp;that we are justified in recognising certain strobili describednbsp;by Williamson* from the Calciferous Sandstone series of Burntisland on the Firth of Forth as those of Lepidodendron Velt-heimianum. Williamson believed that the cone which henbsp;described belonged to the plant with shoots characterised by thenbsp;anatomical features of his species Lepidodendron hrevifoliinnnbsp;(=L. Veltheimianum),a, conclusion which is confirmed by Kidston^.nbsp;The cone of L. Veltheimianum, which reached a diameter ofnbsp;at least 1 cm. and a length of 4 cm., agrees in essentials withnbsp;other species of Lepidostrobus ; the axis has a single medullatednbsp;stele of the same general type as that of the vegetative shootsnbsp;of Lepidodendron fuliginosum and L. Harcourtii. The sporo-phylls are described by Williamson as spirally disposed, andnbsp;Scott notices that in some specimens they are arranged innbsp;alternate whorls ; as in recent Lycopods both forms of phyllo-taxis may occur in the same species. The heterosporous naturenbsp;of this strobilus, to which Scott first applied the name Lepi-dostrobus Veltheimianus, is clearly demonstrated by the twonbsp;longitudinal sections contributed by Mr Carruthers and figurednbsp;by Williamson in 1893�,

Each sporophyll, attached almost at right angles to the cone-axis, bears a radially elongated sporangium seated on thenbsp;median line of its upper face;. its margins are laterally expanded as a thin lamina; from the middle of the lower face anbsp;narrow keel extends downwards between two sporangia belonging to a lower series. From the base of a sporangium a mass ofnbsp;sterile tissue penetrates into the spore-producing region as innbsp;the large sporangia of Lsoetes (cf. fig. 191, H, a, and fig. 133, H).nbsp;The distal and free portion of the sporophylls is bent upwardsnbsp;as a protecting bract. Some of the sporangia in the uppernbsp;part of the cone produced numerous microspores, while 8�16

3 nbsp;nbsp;nbsp;Williamson (93), PI. viii. figs. 51, 52. See also figs. 67�69 given bynbsp;Scott (00).

megaspores occur in the lower sporangia. The megaspores, having a mean diameter of 0'8 mm. �quite 40 times the size of thenbsp;microspores h� are characterised by tubular capitate appendages,nbsp;and by a conspicuous three-lobed projection (fig. 191, E)^ which,nbsp;as Scott suggests, may represent the outer spore-wall whichnbsp;has split as the result of germination. It is not improbable, as shown in fig. 191, I, that this cap was present

before germination. The megaspores represented in fig. 191,1, illustrate their characteristic form as seen in a sectionnbsp;of a megasporangium, Sm; the open beak-like portion of thenbsp;larger spore is probably the apical region which has split alongnbsp;the three-rayed lines. These lines form a characteristic feature of

2 nbsp;nbsp;nbsp;Scott [(08) p. 187] suggests that the projection may have formed a passagenbsp;for the admission of the microspores, or of the spermatozoids which they produced.

both recent and extinct spores and denote their origin in tetrads. The spore shown in fig. 191, Eh illustrates the externalnbsp;features. The apical region of the prothallus of a megasporenbsp;of Lepidodendron Veltheimianiim described by Mr Gordon^nbsp;consists of smaller cells than those occupying the greater partnbsp;of the spore-cavity, a differentiation which he compares withnbsp;that of the prothallus of Selaginella.

There can be little doubt that the petrified shoots described by Williamson� from the Calciferous Sandstone beds of Burntisland as Lepidodendron hrevifolium are identical with specimensnbsp;possessing the external features of L. Veltheimianuni. In 1872nbsp;Dawson expressed the opinion that Williamson�s species shouldnbsp;be referred to L. Veltheimianum, and evidence subsequentlynbsp;obtained confirms this view. The stele of this species is ofnbsp;the medullated type, differing from that of L. fuliginosum andnbsp;L. Harcourtii in the absence of prominent ridges on the externalnbsp;surface of the primary xylem, and from L. vasculare in thenbsp;possession of a parenchymatous pith. In younger twigs thenbsp;cortex consists of fairly homogeneous tissue, but in oldernbsp;branches there is a greater distinction between a delicatenbsp;middle cortex and a stronger outer cortex. Fig. 186, A,nbsp;represents a stem in which the vascular cylinder is composednbsp;of a primary xylem ring, x, I'o mm. broad, succeeded bynbsp;a zone of secondary wood 1'2 cm. in breadth. The junctionnbsp;between the primary and secondary xylem is shown on a largernbsp;scale in fig. 186, B. The tissues abutting on the secondarynbsp;xylem have not been preserved; the outer cortex, whichnbsp;consists chiefly of secondary elements, is divided superficiallynbsp;into unequal ridges corresponding to the leaf-cushions whichnbsp;have been more or less obliterated as the result of growthnbsp;in thickness of the stem.

In 1869 Mr Carruthers described some specimens of vegetative stems and isolated sporangia, collected by Mr Plant in Brazil, as Flemingites Pedroanus^. From a more recent account

published by Zeiller' it is clear that Carruthers� species is a true Lepidodendrmi ] an examination of the type-specimens innbsp;the British Museum confirms this determination. The contiguous leaf-cushions have rounded angles similar in formnbsp;to those of Lepidodendron Veltheimianum and L. dichotomum,nbsp;but it is not unlikely that the Brazilian plant is specificallynbsp;distinct from European species. A figure of one of the specimensnbsp;on which Carruthers founded the species is given by Arber^ innbsp;his Glossopteris Flora. The Brazilian plant is chiefly interestingnbsp;as affording proof of the existence of Lepidodendron in thenbsp;southern hemisphere; the species has also been recognisednbsp;in South Africa from material collected by Mr Leslie atnbsp;Vereeniging�.

As Zeiller'* has suggested, it is not improbable that the fossils described by Renault� from Brazil as Lycopodiopsisnbsp;Derbyi may be the petrified stems of Lepidodendron Pedro-anum. The structure of the central cylinder of Renault�snbsp;species is of the type represented by L. Harcourtii �, thenbsp;xylem forms a continuous ring and does not consist of separatenbsp;strands of tracheae as Renault believed.

Specimens described under this name are interesting rather on account of their extended geographical range and geologicalnbsp;antiquity than on botanical grounds. The drawings reproducednbsp;in fig. 187 illustrate the characteristic appearance of this Lowernbsp;Carboniferous and Upper Devonian type, as represented by anbsp;specimen recently described� from the Lower Karroo (DwjLa)nbsp;series, which is probably of Carboniferous age, near Orangenbsp;River Station, South Africa. The surface is divided into polygonal or rhomboidal areas (figs. A and B) 8�9 mm. long andnbsp;7�8 mm. broad, arranged in regular series and representingnbsp;leaf-scars, comparable with those of Sigillaria Brardi and othernbsp;species, or possibly partially decorticated leaf-cushions. A short

2 nbsp;nbsp;nbsp;Arber (05) PI. i. fig. 2.nbsp;nbsp;nbsp;nbsp;� Seward and Leslie (08).

distance below the apex of each area there is a more or less circular prominence or depression (fig. 187, B) and on a few ofnbsp;the areas there are indications of a groove (fig. A, g) extendingnbsp;from the raised scar to the pointed base, as at g, g.

In examining the graphitic layer on the surface of the South African specimen shown in fig. 187, A, use was made ofnbsp;a method recently described by Professor Nathorst'. A few

drops of collodion were placed on the surface, and after a short interval the film was removed and mounted on a slide. Thenbsp;addition of a stain facilitated the microscopic examination andnbsp;the drawing of the collodion film. The cell-outlines (fig. 187, C)nbsp;on the surface of the polygonal areas may be those of thenbsp;epidermis, but they were more probably formed by a sub-epidermal tissue; the scar, which interrupts the continuity ofnbsp;the flat surface, may mark the position of a leaf-base, or,nbsp;^ Nathorst (07) ; Bather (07); (08).

assuming a partial decortication to have occurred prior to fossilisation, it may represent a gap in the cortical tissue causednbsp;by the decay of delicate tissue which surrounded the vascularnbsp;bundle of each leaf in its course through the cortex of thenbsp;stem. If the impression were that of the actual surface of anbsp;Lepidodendron or a Sigillaria, we should expect to find tracesnbsp;of the parichnos appearing on the leaf-scar as two small scars,nbsp;one on each side of the leaf-bundle. In specimens from Veree-niging described in 1897^ as Sigillaria Brardi, which bear anbsp;superficial resemblance to that shown in fig. A, the parichnos isnbsp;clearly shown. On the other hand, an impression of a partiallynbsp;decorticated Lepidodendroid stem need not necessarily show thenbsp;parichnos as a distinct feature: owing to its close associationnbsp;with the leaf-trace in the outer cortex, before its separation innbsp;the form of two diverging arms, it would not appear as anbsp;distinct gap apart from that representing the leaf-bundle.nbsp;The absence of the parichnos may be regarded as a point innbsp;favour of the view that the impression is that of a partiallynbsp;decorticated stem. Similarly, the absence of any demarcationnbsp;between a leaf-cushion and a true leaf-scar such as characterisesnbsp;the stems of Lepidodendra and many Sigillariae is also favourable to the same interpretation.

In 1872 Mr Carruthers^ described some fossils from Queensland, some of which appear to be identical with that shown in fig. 187 under the name Lepidodendron nothum, Unger�, anbsp;species founded on Upper Devonian specimens from Thuringia.nbsp;The Queensland plant is probably identical with Dawson�snbsp;Canadian species, Leptophloeum rhomhicumf'. In 1874 M�Coy�nbsp;instituted the name Lepidodendron australe for some Lowernbsp;Carboniferous specimens from Victoria, Australia: these arenbsp;in all probability identical with the Queensland fossils referrednbsp;by Carruthers to Unger�s species, but as the identity of thenbsp;German and Australian plants is very doubtful� it is better tonbsp;adopt M�Coy�s specific designation.

Krasser^ has described a similar, but probably not specifically identical, type from China; from Devonian rocks of Spitzbergennbsp;Nathorsf* has figured, under the name Bergeria, an example ofnbsp;this form of stem, and Szajnocha� has described other specimensnbsp;from Lower Carboniferous strata in the Argentine.

Lepidodendron australe has been recorded from several Australian localities^ from strata below those containing thenbsp;genus Glossopteris and other members of the Glossopteris, or, asnbsp;it has recently been re-christened, the Gangamopteris� Flora.

The generic name Lepidostrohus was first used by Brong-niart� for the cones of Lepidodendron, the type-species of the genus being Lepidostrohus ornatus, the designation given bynbsp;the author of the genus to a Lepidostrohus previously figurednbsp;by Parkinson^ in his Organic Remains of a Former World.nbsp;The generic name Flemingites proposed by Carruthers� in 1865,nbsp;under a misapprehension as to the nature of spores which henbsp;identified as sporangia, was applied to specimens of true Lepido-strohi. Brongniart also instituted the generic name Lepido-phyllum for detached leaves of Lepidodendron, both vegetativenbsp;and fertile; the specimen figured by him in 1822 as Filicitesnbsp;(Glossopteris) duhius^, and which was afterwards made the type-species of the genus, was recognised as being a portion of thenbsp;lanceolate limb of a large single-veined sporophyll belonging tonbsp;a species of Lepidostrohus.

In an unusually large Lepidophyllum, or detached sporophyll of Lepidostrohus, in the Manchester University Museum, thenbsp;free laminar portion reaches a length of 8 cm.

It is not uncommon to find Lepidodendron preserved in the form of a shell of outer cortex, which has become separatednbsp;along the phellogen from the rest of the stem; as the result of

^ Krasser (00) PI. ii. fig. 1. nbsp;nbsp;nbsp;^ Nathorst (94) A. PI. ii. fig. 8.

^ Szajnocha (91) p. 203. nbsp;nbsp;nbsp;^ See Etheridge (90); David and Pittman (93).

^ Parkinson (11) A. PI. ix. fig. 1, p. 428. nbsp;nbsp;nbsp;� Carruthers (69^).

compression the cylinder of bark may assume the appearance of a flattened stem covered with leaf-cushions. A specimen preserved in this way was described by E. Weiss as a cone of Lomato-phloios macrolepidotus Gold.,and is quoted bySolms-Laubach andnbsp;other authors� as an example of an unusually large Lepidostrobiis.nbsp;An examination of the type-specimen in the Bergakadernie ofnbsp;Berlin convinced me that Weiss had mistaken the partiallynbsp;destroyed leaf-cushions for sporophylls, and Stigmarian rootlets,nbsp;which had invaded the empty space, for sporangia 2.

In external appearance some species of Lepidostrobus bear a superficial resemblance to the cone of a Spruce Fir (Piceanbsp;excelsa), but the surface of a lycopodiaceous strobilus is usuallynbsp;covered by the overlapping and upturned laminae whichnbsp;terminate the more or less horizontal sporangium-bearingnbsp;portion of the sporophyll.

Fig. 188 affords a good example of a long and narrow Lepidostrobus. This specimen from the Middle Coal-Measuresnbsp;of Lancashire has a length of 23 cm.; like other Lepidostrobinbsp;it is borne at the tip of a slender shoot. The fossil is sufficiently well preserved to show the characteristic radiallynbsp;elongated form of the large sporangia and the long and upturned distal portions of the sporophylls.

We may briefly describe Lepidostrobus as follows:�Cylindrical strobili consisting of an axis containing a single cylindrical stele which agrees generally with that of the vegetativenbsp;shoots of L. Harcourtii and other species. The amount ofnbsp;parenchymatous pith varies in different forms ; in some thenbsp;primary xylem is almost solid. The middle cortical region,nbsp;which has usually been destroyed before fossilisation, possessesnbsp;the loose lacunar structure characteristic of this region in thenbsp;vegetative branches. The thicker walled outer cortex is continued at the periphery into crowded, usually spirally disposednbsp;sporophylls, each of which consists of a more or less horizontalnbsp;pedicel, which may be characterised by a keel-like mediannbsp;ridge on its lower surface, while to the central region of thenbsp;upper face is attached a large radially elongated sporangium.nbsp;One of the chief differences between a Lepidodendron conenbsp;� Bower (08) p. 305.nbsp;nbsp;nbsp;nbsp;^ Seward (90); Potoni� (93^).

Fig. 188. Lepidostrobus. Middle Coal-Measures, Bardsley, Lancashire. From a specimen in the Manchester Museum. (J nat. size.)

be heterosporous. In the latter the megasporangia borne on the lower sporophylls usually contain several megaspores as in

Isoetes (cf. fig. 133, E). Beyond the distal end of the sporangium the sporophyll becomes broader in a horizontal plane and isnbsp;bent upwards as a lanceolate limb; it may also be prolonged anbsp;short distance downwards as a bluntly triangular expansion.

Fig. 189 is an accurate representation of a transverse section, 6 mm. in diameter, of what is no doubt the apicalnbsp;portion of a Lepidostrobus from the Coal-Measures of Shore,nbsp;Lancashire. The section cuts across the upturned free laminaenbsp;above the level of the apex of the cone-axis. Each lamina

contains a small vascular bundle composed of a few tracheae and some thin-walled cells surrounded by delicate mesophyll tissue.nbsp;Immediately in front of the distal end of a sporangium anbsp;small ligule is borne on the upper face of the sporophyllnbsp;(fig. 191, A, B, 1) occupying the same position as in Selaginellanbsp;(cf. fig. 131, F). Strands of vascular tissue pass in a steeplynbsp;ascending course from the xylem to the pedicels of sporo-phylls, finally curving upwards and ending in the upper limb.

Each vascular bundle consists of a strand of xylem, apparently of mesarch structure, accompanied by a few layers of parenchyma on its outer face and by a group of cambiform elements,nbsp;the whole being enclosed in a sheath of parenchyma continuousnbsp;with the inner cortex of the cone axis. The vascular bundle isnbsp;accompanied by a parichnos in the outer cortex and in thenbsp;sporophyll.

There can be little doubt that the Palaeozoic Lepidodendra, like Lycopodium cernuum (fig. 123) and other recent Lycopods,nbsp;usually bore their cones at the tips of slender shoots. Thenbsp;fertile shoot of Lepidophloios scoticus shown in fig. 160, B, affordsnbsp;one of several instances supporting this statement; similarnbsp;examples are figured by Brongniarth Morris^, and by more recentnbsp;writers. The apparently sessile cone figured by Williamson*nbsp;from a specimen in the Manchester Museum is certainly notnbsp;in situ, but is accidentally associated with the stem.

The general absence of secondary wood in the steles of Lepidostrobi is, as Dr Kidston^ points out, consistent with thenbsp;view that the cones were shed on maturity and that fertilisationnbsp;probably took place on the ground, or perhaps on the surface ofnbsp;the w'ater where the slender hairs of the megaspores (fig. 191,nbsp;F, I) may have served to catch the microspores.

Reference has already been made to the belief on the part of some palaeobotanists that the large scars of JJlodendronnbsp;represent attachment-surfaces of sessile cones, and reasons havenbsp;been given against the acceptance of this view.

There is considerable range in the size of Lepidostrobi. An incomplete specimen, 33 cm. long and 6 cm. broad, whichnbsp;may have been 50 cm. in length, is described by Renault andnbsp;Zeiller* from the Gommentry Coal-field. The larger conesnbsp;afford a striking demonstration of the enormous spore-outputnbsp;of some species of Lepidodendron.

Among the earliest accounts of the anatomy of Lepido-strobus are those by Hooker� and Binneyh One of the specimens

described by the former author (fig. 190) affords an interesting example of an unusual manner of fossilisation ; a hollow stem oinbsp;Lepidodendron is filled with sedimentary material containingnbsp;several pieces of Lepidostrohi in an approximately verticalnbsp;position.

The fact that Lepidostrohi usually occur as isolated specimens renders it impossible in most cases to refer them to particular species of Lepidodendron. Neither external featuresnbsp;nor anatomical characters afford satisfactory criteria by whichnbsp;to correlate vegetative and fertile shoots; in some measure thisnbsp;is due to the imperfection of our knowledge as regards thenbsp;range of structure within the limits of species; it is also due

to lack of information as to the extent to which the transition from sterile to fertile portions of a shoot is accompanied bynbsp;anatomical differences. Prof. Williamson wrote: �I have fornbsp;many years endeavoured to discover some specific characters bynbsp;which different Lepidostrobi can be distinguished and identified,nbsp;but thus far my efforts have been unsuccessful*.� In a fewnbsp;cases, such as those mentioned in the description of Lepidoden-dron Veltheimianum and L. W�nschianum, it has been possiblenbsp;to correlate cones and vegetative shoots.

The most complete account we possess of the anatomy of Lepidodendron cones is that by Mr Maslen^ who first demonstrated the occurrence of a ligule on the sporophylls, and thusnbsp;supplied a missing piece of evidence in support of the generallynbsp;accepted view as to the homology of the sporangium-bearingnbsp;members and foliage leaves.

1811. nbsp;nbsp;nbsp;� Strobilus,� Parkinson, Organic Remains, Vol. l. p. 428,

1875. Lepidostrohus variabilis, Feistmantel, Palaeontographica, Vol. LXIII. PI. XLIV.

Under this specific name are included strobili from Upper Carboniferous rocks which, in spite of minor differences, may be'nbsp;considered as one type. The cylindrical cones vary considerablynbsp;in size, some reaching a length of 50 cm. or more. The sporophylls are attached by a pedicel, 4�8 mm. long, at right anglesnbsp;to the axis, while the distal portion forms an oval lanceolatenbsp;limb 10�20 mm. in length. The sporangia are 4�8 mm. long.

The branched example figured by Lindley and Hutton' as a variety {L. ornatus var. didymns) illustrates a phenomenonnbsp;not uncommon in both Palaeozoic and recent lycopodiaceousnbsp;strobili.

Williamson� instituted this term for strobili previously described by Binney', without adequate evidence, as the�cones

of Lepidodendron Harcourtii. In shape and in the main morphological features this type resembles L. variahilis, whichnbsp;is however known only in the form of casts and impressions.nbsp;A cone of L. oldhamius, 2�3 cm. in diameter, possesses anbsp;medullated stele consisting of a ring of primary xylem (fig.nbsp;191, D, x) with exarch protoxylem and no secondary elements.nbsp;Maslen found several short tracheae at the periphery of thenbsp;xylem and states that these led him to compare the cone with thenbsp;vegetative shoots of Lepidodendron vasculare, but the commonnbsp;occurrence of such elements in different types of shoot rendersnbsp;them of little or no specific value. The inner cortex is likenbsp;that of vegetative shoots of Lepidodendron and the middlenbsp;cortex, which was no doubt of the type described in Lepido-strobus Brownii, is represented by a gap in the sections, beyondnbsp;which is the stronger outer cortex (fig. 191, D) passing into thenbsp;horizontal pedicels of the sporophylls. The section of the axisnbsp;reproduced in fig. 191, D, was figured by Binney^ as Lepidodendron vasculare. The leaf-traces, several of which are seennbsp;in the middle cortical region in fig. D, It, consist of a strandnbsp;of scalariform tracheae, with a mesarch protoxylem, succeedednbsp;by a few parenchymatous cells; beyond these there is usuallynbsp;a small gap which was originally occupied by a strand of thin-walled cells. It is important to note that in one sporophyll-trace figured by Maslen^ there is a strand of thin-wallednbsp;elongated elements abutting on the xylem, which he describesnbsp;as phloem. This tissue is certainly more like true phloem thannbsp;any which has hitherto been described in the leaf-traces ofnbsp;vegetative shoots. The state of preservation is not, however,nbsp;sufficiently good to enable us to recognise undoubted phloemnbsp;features.

In such cones as I have examined no tissue has been seen which shows the histological features characteristic of thenbsp;secretory zone of vegetative shoots; the � phloem � (Maslen)nbsp;occupies the position in the sporophyll bundle which in thenbsp;vascular bundles of foliage leaves is occupied by a dark-cellednbsp;and partially disorganised tissue in continuity with the secretory zone of the main stele. It may be that in the strobili

1 Binney (71) PI. viii. figs. 2, 4. nbsp;nbsp;nbsp;^ Maslen (99) PI. xxxvi. fig. 11.

this tissue occurred in a modified form, but even assuming that the section figured by Maslen shows true phloem, an assumption based on slender evidence, this is not sufficient justificationnbsp;for the application of the term phloem to a tissue occupyingnbsp;a corresponding position in vegetative shoots and distinguishednbsp;by well-marked histological features.

The sporophyll-traces, as seen in the outer cortex in fig. 191, D, are partially surrounded by a large crescentic space, p,nbsp;which was originally occupied by the parichnos. The sporangianbsp;are attached along the middle line of the sporophyll and, as innbsp;Lepidostrobus Brownii, a cushion of parenchyma projects intonbsp;the lower part of the sporangial cavity (fig. 191, A, a; C, a).

The diagrammatic sketch of part of a section in the Binney Collection reproduced in fig. 191, B, shows the position of thenbsp;ligule, 1. No megaspores have been discovered in any specimens of this type; the microspores, which occur both singlynbsp;and in tetrads, have a length of 0'02�0'03 mm.

The drawing shown in fig. 191, A, based on a section in the Binney Collection, illustrates the general arrangement of thenbsp;parts of a typical Lepidostrobus. I have made use of thisnbsp;sketch instead of that given by Maslen, as his figure conveysnbsp;the idea that the sporophylls are superposed, whereas, whethernbsp;they are verticillate or spiral, a radial longitudinal sectionnbsp;would not cut successive sporangia in the same plane.

In 1843 a specimen of a portion of a petrified cone was purchased by the British Museum, assisted by the Marquis ofnbsp;Northampton and Robert Brown, for �30 from a Frenchnbsp;dealer. This fossil, from an unknown locality, was brieflynbsp;described by Brown in 1851' and named by him Triplosporites,nbsp;but in a note added to his paper he expressed the opinion thatnbsp;the generic designation Lepidostrobus would be more appropriate. Brongniart afterwards named the cone Triplosporitesnbsp;Brownii'^, and Schimper� described it in his Trait� as Lepido-

strobus Brownii. The type-specimen is preserved in the British Museum and the Paris Museum possesses a piece of the samenbsp;fossil.

The central axis of the cone has a stele of the type characteristic of Lepidodendroii fuliginosum and L. Harcourtii,nbsp;and the xylem is surrounded by a thin-walled tissue describednbsp;by Bower^ as possibly phloem; but in the absence of longitudinal sections it is impossible to say how far the tissuenbsp;external to the xylem agrees with that in Lepidodendron stems.nbsp;The sporophylls consist of a horizontal portion, to the uppernbsp;face of which the radially elongated sporangia are attached, onenbsp;to each sporophyll; beyond the distal end of the sporangiumnbsp;the sporophyll bends sharply upwards as a fairly stout lamina.nbsp;The wall of the sporangium is composed of several layers ofnbsp;cells, as shown in a drawing published by Bower^; in thenbsp;interior occur groups of microspores, and from a ridge of tissuenbsp;which extends along the whole length of the sporangiumnbsp;irregular trabeculae of sterile tissue project into the sporangialnbsp;cavity, as in Isoetes (fig. 191, H: cf. fig. 133, H).

Further information in regard to Lepidostrobus Brownii has recently been supplied by Prof. Zeiller^, who recognises thenbsp;existence of a ligule, and draws attention to some interestingnbsp;histological features in the tissue of the sporophylls^

The calcareous nodules from the Coal seams of Yorkshire and Lancashire are rich in isolated spores, many of which arenbsp;undoubtedly those of Lepidostrobi. Examples of spores werenbsp;figured by Morris� in 1840, and their occurrence in coal hasnbsp;been described by several authors, one of the earliest accountsnbsp;being by Balfour�. The drawings of Palaeozoic and recentnbsp;spores published by Kidston and Bennie�� demonstrate a strikingnbsp;similarity between the megaspores of existing and extinctnbsp;Lycopods, the chief difference being the larger size of the fossils.

^ Bower (93). nbsp;nbsp;nbsp;^ Bower (94) PI. xlviii. fig. 93.nbsp;nbsp;nbsp;nbsp;� Zeiller (09).

�* Zalessky has recently (08) described a large species of cone, Lepidostrobus Bertrandi, 5 cm. in diameter.

� Morris (40). nbsp;nbsp;nbsp;� Balfour (57).nbsp;nbsp;nbsp;nbsp;� Kidston and Bennie (88).

The general generic name Triletes, originally used by Reinschb is a convenient term by which to designate Pteri-dophytic spores which cannot be referred to definite types.

It is usual to find more than four megaspores in each mega-sporangium in Palaeozoic and not infrequently, as we have seen, in Mesozoic lycopodiaceous strobili, but in some Palaeozoic cones,nbsp;e.g. Bothrostrobus (fig. 216) and Lepidostrobus foliaceus^, a singlenbsp;tetrad only appears to have reached maturity.

The occurrence of long simple or branched and sometimes capitate hairs is a common feature of Carboniferous megasporesnbsp;(fig. 191, E, F, I). It is possible that these appendages servednbsp;to catch the microspores, thus facilitating fertilisation. Anbsp;peculiar form of megaspore has been described by Mrs Scott�,nbsp;and assigned by her to Lepidostrobus foliaceus, the megasporangium of which apparently contained only four spores. Asnbsp;shown in fig. 191, G, a large bladder-like appendage characterisednbsp;by radiating veins is attached to the thick spore-coat; it isnbsp;suggested that this excrescence may be compared with thenbsp;� swimming � apparatus of the recent water-fern Azolla. Thenbsp;epithet swimming which it is customary to apply to thenbsp;appendages of Azolla megaspores would seem to be inappropriate if Campbelb is correct in stating that spores of Azollanbsp;are incapable of floating.

1880. Lepidostrohtis insignis, Williamson, Phil. Trans. R. Soc., 23- 502, PI. XV. figs. 8�12.

1889. Lepidodendron Spenceri, Williamson, Phil. Trans. R. Soc., p. 199, PI. VII. figs. 20�22; PI. viii. fig. 19.

Another type of lycopodiaceous strobilus, differing sufficiently from Lepidostrobus to deserve a special generic designation,

is that originally described by Williamson^ from the Lower Coal-Measures of Yorkshire, as a type of Lepidostrobus, L. in-signis, but afterwards� more fully investigated and assigned tonbsp;a new genus by Scott I It should be pointed out that in anbsp;later publication Williamson spoke of the Ij^copodiaceous axis,

which he suspected might belong to his L. insignis, as possibly worthy of recognition as a distinct generic type.

Of the two species included by Scott in his genus Spencerites only one, S. insignis, need be considered. Since the publica-

tion of Scott�s paper our knowledge of this type has been extended by Miss Berridge^ and by Prof Lang^.

The axis of the strobilus has a stele characterised by a pith of elongated elements, most of which have thin walls;nbsp;the xylem cylinder possesses about twenty protoxylem strandsnbsp;forming more or less prominent exarch ridges. The cortexnbsp;exhibits a differentiation comparable with that in the shoots ofnbsp;Lepidodendron. The sporophylls are arranged in alternatingnbsp;verticils, each whorl consisting of ten members: the narrownbsp;horizontal pedicel of a sporophyll, containing a single vascularnbsp;bundle, as shown in fig. 192, is expanded distally into anbsp;prominent upper lobe bearing a cushion of small and delicatenbsp;cells, to which the sporangium is attached, and prolonged obliquelynbsp;upwards as a free leaf-like lamina. The lower blunt prolongation of the sporophylls appears to form a thick dorsal lobe, but,nbsp;as Lang has pointed out, it is highly probable that the presentnbsp;form of the dorsal lobe is of secondary origin, and is � due to thenbsp;disappearance of a mucilage cavity from a large sporophyllnbsp;base�.� As Miss Berridge remarks, the vascular bundle of thenbsp;sporophyll does not give ofif a branch to the ventral lobe andnbsp;sporangium. In attachment, in shape, and in the structure ofnbsp;the wall the sporangia differ markedly from those of Lepidostrobi.nbsp;The spores, which also constitute a characteristic feature ofnbsp;the genus, have a maximum diameter of 0T4 mm.; they arenbsp;described as oblate spheroids with a broad hollow wing runningnbsp;round the equator (fig. 192) comparable with the air-sacs of thenbsp;pollen of Pinus. Scott points out that the spores of Spenceritesnbsp;are intermediate in size between the microspores of Lepidodendron and the megaspores of Lycopodium; it is difficult thereforenbsp;to decide to which category they should be referred. Spenceritesnbsp;is clearly distinct from Lepidostrohus; the absence of a ligule,nbsp;the manner of attachment of the sporangia, and the form andnbsp;size of the spores, are characteristic features.

A comparison of Spencerites with the strobili of Lycopodium cernuum (figs. 123, 126�129) has recently been made by Lang,nbsp;who draws attention to the striking agreement as regards generalnbsp;plan and even detailed structural features between the Palaeozoic

1 Berridge (05). nbsp;nbsp;nbsp;^ Lang (08).nbsp;nbsp;nbsp;nbsp;� Lang (08) p. 364.

and the recent type of strobilus. It is interesting to find, as Lang points out, that in the original account of the fossil conenbsp;by Williamson, the view is expressed that the sporangiophoresnbsp;were confiuent. An examination of the section figured bynbsp;Williamson^ led Lang to confirm this opinion. It would benbsp;out of place to enter here into a detailed comparison ofnbsp;Spencerites insignis and the cone of Lycopodium, but the resemblances are considered by Lang to be sufficiently close to suggestnbsp;that the striking similarity may be indicative of relationship^.

It is worthy of notice that the radial section of Spencerites (fig. 192) presents a fairly close resemblance to a correspondingnbsp;section through a cone-scale of Agathis (Kauri Pine)l In eachnbsp;case the megasporangium is attached by a narrow pedicel tonbsp;the sporophyll and the latter has a similar form in the twonbsp;plants, though the extent of the resemblance is somewhatnbsp;lessened by Lang�s more complete account of the Palaeozoicnbsp;type. If the Spencerites sporangia possessed an integumentnbsp;the similarity with the Agathis ovule would of course be muchnbsp;closer; recent palaeobotanical investigations have shown thatnbsp;ovules and sporangia are not separated by impassable barriers.

[Since this Chapter was set up in type a paper has appeared by Dr Bruno Kubart on a new species of Spencerites spore,nbsp;S.membranaceus, from the Ostrau-Karwiner Coal-basin (Austria).nbsp;The spores are larger than those of jS. insignis and in some thenbsp;cells of a prothallus are preserved. Kubart figures a section ofnbsp;a spore containing a group of seven cells, a central cell, whichnbsp;he regards as an antheridial mother-cell, surrounded by six wall-cells. Kubart (90).]

* Lang (08) p. 367. Since this was written a paper has been published by Mr Watson on a new type of Lycopodiaceous cone from the Lower Coal-Measuresnbsp;(Mesostrobus): in an appendix he criticises Dr Lang�s views in regard to Spencerites. [Watson, Annals of Botany, Vol. xxni. p. 379, 1909.]

CHAPTEH XVI.

Inquot; view of the close resemblance between Lepidodendron and Sigillaria, another lycopodiaceous plant characteristic ofnbsp;Carboniferous and Permian floras, a comparatively brief description of the latter genus must suffice, more particularly asnbsp;Lepidodendron has received rather an undue share of attention.nbsp;Sigillaria, though abundantly represented among the relics ofnbsp;Palaeozoic floras, especially those preserved in the Coal-Measures, is rare in a petrified state, and our knowledge of itsnbsp;anatomy is far from complete. In external form as in internalnbsp;structure the difference between the two genera are not such asnbsp;enable us to draw in all cases a clearly defined line of separation.

In the Antediluvian Phytology, Artis^ figured a fossil from the Carboniferous sandstones of Yorkshire which he callednbsp;Euphorhites vulgaris on account of a superficial resemblancenbsp;to the stems of existing succulent Euphorbias. Rhode^ alsonbsp;compared Sigillarian stems with those of recent Cacti. Thenbsp;specimen described by Artis is characterised by regular verticalnbsp;and slightly convex ribs bearing rows of leaf-scars in spiral series,nbsp;like those on the cushions of Lepidodendron. A few years earliernbsp;Brongniart� had instituted the genus Sigillaria* for plants withnbsp;ribbed but not jointed stems bearingquot; disc-like impressions� (leaf-scars) disposed in quincunx; the type-species named by the authornbsp;of the genus Sigillaria scutellata is identical, as Kidston� points

For generic names wholly or in part synonymous with Sigillaria, see White (99) p. 230.

out, with Euphorhites vulgaris of Artis and with the plant afterwards figured by Brongniart as S. pachyderma'-. Brongniart in 1822 figured another type of stem characterised by the absencenbsp;of ribs and by prominent spirally arranged cushions bearingnbsp;relatively large leaf-scars like the upper part of the specimennbsp;shown in fig. 203; this he named Clathraria Brardii, a well-known and widely distributed Carboniferous and Permian speciesnbsp;now spoken of as Sigillaria Brardi' (�gs. 196, A�C ; 203). Anbsp;third type of stem figured by Brongniart as Syringodendronnbsp;striatum'' agrees with Sigillaria scutellata in having ribs, butnbsp;differs in the substitution of narrow oval ridges or depressionsnbsp;for leaf-scars; this is now recognised as a partially decorticatednbsp;Sigillaria, in whiph the vascular bundle of each leaf is represented by a narrow ridge or depression. The name Syringodendron, originally used by Sternberg, is conveniently appliednbsp;to certain forms of Sigillarian stems which have lost theirnbsp;superficial tissues. A fourth generic name, Favularia, wasnbsp;instituted by Sternberg� for Sigillarian stems with ribs coverednbsp;with contiguous leaf-scars of hexagonal form and prominentnbsp;lateral angles (fig. 193, A; fig. 200, G).

The generic or subgeneric title Bhytidolepis, also instituted by Sternberg, is applied to ribbed Sigillarian stems such asnbsp;8. scutellata, S. rugosa(�g. 193, B), S. mammillaris (fig. 195), ornbsp;S. laevigata (fig. 196, D). Goldenberg^ proposed the namenbsp;Leiodermaria for smooth Sigillarian stems with leaf-scars notnbsp;in contact with one another (fig. 196, C).

The shoot system of Sigillaria consisted of a stout stem tapering upwards to a height of 100 feet� or more as an unbranched column, with its dome-shaped apexquot; covered withnbsp;linear grass-like leaves or, in some species, such as Sigillarianbsp;BrardF, S. EugeniF, etc., the main trunk was occasionally dividednbsp;by apparently equal dichotomy. The younger portions of thenbsp;stem or branches were in some species clothed with leavesnbsp;separated by a narrow zigzag groove surrounding their hexa-

gonal bases, -while in other forms each leaf was seated on a more or less prominent cushion having the form illustrated bynbsp;Sigillaria McMuHriei (fig. 194) or by the example represented innbsp;fig. 200, H; or as in the ribbed species shown in figs. 193, B, and

Fig. 194. Sigillaria UcMurtriei Kidst. From a specimen from the Upper Coal-Measures of Eadstock, in the British Museum (V. 952). Nat. size.

195, the leaves in vertical series were separated from one another by longer portions of the ribs. As in Lepidodendron the cushionsnbsp;are frequently characterised by irregular transverse wrinklingsnbsp;and other^ surface-ornamentation which in some instance^ at

Fig. 195. Sigillaria mammillaris. (Ehytidolepis form.) From a specimen in the Manchester Museum, p, pariohnos; I, ligule-pit; t, leaf-trace;nbsp;c, cushion ; s, leaf-scar.

least may have been produced as the result of post-mortem shrinkage of superficial tissue. � From the rarity of shoots withnbsp;the foliage attached, it would seem that the leaves persisted fornbsp;a comparatively short time and were cut off by an absciss-layernbsp;leaving behind a well-marked leaf-scar area. The linear leaves,

^ For an account of the various external features made use of in the classification of Sigillarias, see Koehne (04).

reaching in rare cases a length of one metre (e.g. 8. lepidoden-drifolia) but usually much shorter, possessed a single median bundle, and the lower face was characterised by two stomatalnbsp;grooves and a median keel. It is not uncommon to find leaf-bases of Sigillaria detached from the stem and preserved asnbsp;separate impressions. The term Sigillariophyllum used by

Grand�Eury^ may be applied to detached leaves, though it is by no means easy to distinguish between the foliage of Sigillarianbsp;and Lepidodendron. A comparison of a typical species ofnbsp;Sigillaria, such as 8. rugosa (fig. 193, B) or Brardinbsp;(fig. 196, A�C) with a typical Lepidodendron reveals obvious

differences in the form of the leaf-cushion, hut in some cases the distinction becomes purely arbitrary.

Immediately above the centre of the upper boundary of a Sigillarian leaf-scar a ligule pit may often be detected, asnbsp;shown in fig. 195, I, and in some cases, e.g. a specimen figurednbsp;by Germar� (fig. 196, A) as Sigillaria spinulosa (identical withnbsp;S. Brardi), some circular scars with a central pit surrounded bynbsp;a raised rim occur on the surface of the stem, either singlynbsp;or in pairs, near the leaf-scars; these, it is suggested, maynbsp;represent the position of adventitious roots or, as Germarnbsp;thought, of some deciduous spinous processes. The leaf-scarsnbsp;are frequently hexagonal in shape, with the lateral anglesnbsp;either rounded (fig. 200, F) or sharply pointed (fig. 200, G, H);nbsp;each scar bears three smaller scars as in Lepidodendron, anbsp;central circular, oval or crescentic leaf-trace scar and largernbsp;oval or slightly curved scars formed by the two parichnos armsnbsp;(fig. 195, p). The larger size of the parichnos arms, the individual cells of which may often be detected as a fine punctation,nbsp;is a distinguishing feature of the genus, but otherwise thenbsp;structure is very similar to that in Lepidodendron. As shownnbsp;in figs. 195, 200, F, G, the three scars may occur nearer thenbsp;upper than the lower margin of the leaf-base area.

Lepidodendron Wortheni^ (fig. 196, E), described from North America by Lesquereux�, by Zeiller^ from France, and bynbsp;Kidston� from the Upper and Middle Coal-Measures of England,nbsp;may be quoted as a Lepidodendron bearing a close resemblancenbsp;to Sigillaria. The shoots bear cushions two or three times asnbsp;long as broad and without the usual median division, but withnbsp;numerous irregular and discontinuous transverse wrinklings.nbsp;Lepidodendron Peachii Kidston� affords another example of anbsp;form agreeing both with Sigillaria and with Lepidodendron.nbsp;An Upper Devonian type described by White�� as Archaeo-sigillar'ia primaeva affords a strikmg instance of the combinationnbsp;on one stem of Sigillarian and Lepidodendroid leaf-cushions.

� Kidston (01) p. 46. nbsp;nbsp;nbsp;� Kidston (85).nbsp;nbsp;nbsp;nbsp;^ White, D. (07^).

The difference between the original surface of a Sigillaria stem and that of partially decorticated specimens is seen innbsp;figs. 196, C and D; in fig. C the bark of Sigillaria Brardinbsp;shows the characteristic wrinklings of the superficial tissue,nbsp;while at a slightly lower level the leaf-scars are replaced bynbsp;the parichnos casts, a, and fine longitudinal striations represent

the elongated phelloderm cells laid bare by the exfoliation of the surface-layers. Similarly, in the rib of Sigillaria laevigatanbsp;(fig. 196, D) the parichnos arms, p, and the longitudinal striations are exposed at the lower level, while the surface is spioothnbsp;and bears rows of widely separated leaf-scars.

The older part of a Sigillarian stem may present an appearance very different from that of the younger shoots. The leaf-cushions may be stretched apart as the result of elongationnbsp;and increase in girth, while in some cases the arrangement of thenbsp;leaf-scars may vary on the same axis as the result of inequalitiesnbsp;in growth or changing climatic conditions. The contiguousnbsp;arrangement of the leaf-scars and narrow cushions characteristicnbsp;of the Clathrarian form of stem, as was first demonstrated bynbsp;Weiss^, and afterwards illustrated by Zeiller^ and Kidston, maynbsp;be gradually replaced (on the same specimen) by a more distantnbsp;disposition of the leaf-scars separated by a smooth interveningnbsp;surface of bark. The specimen of S. Brardi reproduced in partnbsp;in fig. 203, and first figured by Kidston, affords an examplenbsp;of three � species � on one piece of stem, S. Brardi Brongn.nbsp;8. denudata Goepp. and 8. rhomhoidea Brongn.*

The piece of Carica stem, represented in fig. 197, illustrates the danger of trusting to the disposition of leaves as a specificnbsp;criterion.

Similarly, in the ribbed forms the degree of separation of the leaf-scars is by no means uniform in a single species!nbsp;Some authors have adopted a two-fold classification of Sigillarian stems proposed by the late Prof Weiss* of Berlin, whonbsp;divided the Sigillariae into (A) Sub-Sigillariae, comprisingnbsp;Leiodermariae and Cancellatae, and (B) Eu-Sigillariae, including Favulariae and Rhytidolepis. Grand�Eury* adopts thenbsp;terms Rhytidolepis and Leiodermaria for ribbed and smoothnbsp;stems respectively, the type to which the name Clathraria wasnbsp;applied by Brongniart being in some cases at least the youngnbsp;form of Leiodermarian stems. While recognising the artificialnbsp;distinction implied by such terms as Rhytidolepis, Leiodermaria,nbsp;and other sub-generic titles, we may conveniently speak of thenbsp;two main types of 8igillaria stems as ribbed and smooth.

Still older stems of 8iyillaria are not uncommon from which the leaf-scars and other superficial tissues have been exfoliated,nbsp;leaving exposed a longitudinally fissured surface of secondary-cortex characterised by pairs of considerably enlarged parichnos

strands (fig. 198) which are sometimes partially or wholly fused into one {Syringodendron state�of Sigillaria). The singlenbsp;or double nature of the elliptical or circular parichnos areas isnbsp;doubtless due to the degree of exfoliation, which may extendnbsp;sufficiently deep into the cortex to reach the level of thenbsp;parichnos before the single strand has bifurcated (cf. Lepido-dendron, p. 100). In the Museums of Manchester, Newcastle,nbsp;and other places casts of large Sigillaria stems may be seen,nbsp;which illustrate the differences in breadth and regularity of thenbsp;vertical ribs, and in the size and shape of the parichnos areasnbsp;in different regions of a partially decorticated stem. A cast of anbsp;ribbed species in the Manchester Museum, having a length ofnbsp;185 cm. and a breadth of 56 cm., shows in the upper portionnbsp;straight vertical grooves and broad ribs bearing pairs of parichnosnbsp;scars 11 mm. long; in the lower portion the ribs tend tonbsp;become obliterated and the parichnos scars, 2 cm. in length,nbsp;may be partially fused and arranged in much less regular verticalnbsp;series. A feature of these older ribbed Sigillarian stems is thenbsp;increase in the number of the ribs from below upwards. Kidston *nbsp;has described a specimen in the Sunderland Museum, 6 feetnbsp;6 inches long, with a circumference at the slightly bottle-shapednbsp;base of 5 feet. On the lower portion of the stem there arenbsp;29 broad ribs; about one-third the height many of thesenbsp;bifurcate, producing as many as 40 ribs in the upper partnbsp;where the cast has a circumference of 3 feet. The increasenbsp;in number of the ribs is due in part to bifurcation, but alsonbsp;to the intercalation of new ones. As Kidston points out, thisnbsp;example shows that as a stem grew in length additional leavesnbsp;were developed at the apex. A similar stem, which illustratesnbsp;very clearly the increase in the number of ribs from belownbsp;upwards, may be seen in the Newcastle Museum.

Grand�Eury^ has described an example of an old stem of a ribless species of Sigillaria, Syringodendron hioculatum, bearingnbsp;single and double parichnos areas of nearly circular form andnbsp;with a diameter of 1�2 cm. In a specimen figured by Renaultnbsp;and Roche'* {Syringodendron esnostense) from the Culm strata

in France, the parichnos scars reach a length of 3 cm. As seen in the fragment of a ribbed Sigillaria represented in fig. 198,nbsp;the large parichnos areas exhibit a distinct surface pitting innbsp;contrast to the fine longitudinal striation of the rib; thenbsp;difference in surface-appearance is due to the nature of thenbsp;tissue, which in the parichnos consists of fairly large parenchymatous elements with groups of secretory cells S and in thenbsp;exposed cortex of elongated elements. The vertical line in thenbsp;middle of fig. 198, which occurs in the middle of the rib, hasnbsp;probably been formed by splitting of the bark.

Grand�Eury�s description of fossil forests of Sigillariae in the rocks of the St Etienne^ district affords a striking picture ofnbsp;these arborescent Pteridophytes; he speaks of the stems of somenbsp;of the trees as swollen like a bottle at the base, characterisednbsp;by the Syringodendron features and terminating below innbsp;short repeatedly forked roots of the type known as Stigmari-opsis. Other specimens of Sigillaria stumps show a markednbsp;decrease in girth towards the base; this tapered form isnbsp;regarded by Grand�Eury as the result of the development ofnbsp;aerial columnar stems from underground rhizomes.

The nature of the root-like organs of Sigillaria is dealt with in the sequel: a brief reference may, however, be made

to the occurrence of stumps of vertical trunks which pass downwards into regularly forked and spreading arms. These arms lie almost horizontally in the sand or mud like the underground

rhizomes of Phragmites and other recent plants growing in swampy situations where water is abundant and where deeper

penetration of the soil would expose them to an insufficient supply of oxygen^. It is certain that Sigillaria had no tap-root,nbsp;hut was supported on spreading subterranean organs bearingnbsp;spirally disposed long and slender rootlets which absorbed waternbsp;from a swampy soil.

The regularity of the leaf-scar series on a Sigillarian stem may be interrupted by the occurrence of oval scars with anbsp;central scar and surrounding groove (fig. 193, E); these occurnbsp;in zones at more or less regular intervals on the stem, as seennbsp;in the partially decorticated cast represented in fig. 199.nbsp;Zeiller has pointed out that the rows of oval or circular scars,nbsp;which mark the position of caducous stalked strobili, may occurnbsp;between the leaf-scars in vertical series, each of which maynbsp;include as many as 20 scars, while in other cases a single seriesnbsp;of such cone-scars may encircle the stem^. The zones arenbsp;usually of uneven breadth, as in S. Brardi, and their occurrencenbsp;produces some deformation of the adjacent leaf-scars.

By the earlier writers Sigillaria was compared with succulent Euphorbias, Cacti, and Palms; Brongniart� at first included undoubted Sigillarian stems among Ferns, but afternbsp;investigating an agatised stem from Autun, he referred Sigillaria to the Gymnosperms^ on the ground that it had thenbsp;power of producing secondary wood. It was then supposednbsp;that Lepidodendron possessed only primary xylem, and thatnbsp;the presence of a vascular meristem in Sigillaria necessitatednbsp;its separation from the lycopodiaceous genus Lepidodendronnbsp;and its inclusion in the higher plants. By slow degrees it wasnbsp;recognised, as in the parallel case of the genus Calamites, thatnbsp;the presence or absence of secondary vascular tissue is anbsp;character of small importance. Williamson, whose anatomicalnbsp;researches played the most important part in ridding the mindsnbsp;of palaeobotanists of the superstition that secondary growth innbsp;thickness is a monopoly of the Phanerogams, spoke in 1883 ofnbsp;the conflict as to the affinities of Lepidodendron and Sigillaria asnbsp;virtually over but leaving here and there �the ground-swell of a

stormy past\� In 1872 the same author had written; � If then I am correct in thus bringing the Lepidodendra and Sigillariaenbsp;into such close affinity, there is an end of M. Brongniart�s theory,nbsp;that the latter were gymnospermous exogens, because the crypto-gamic character of the former is disputed by no one; we mustnbsp;rather conclude as I have done that the entire series represents,nbsp;along with the Calamites, an exogenous group of Cryptogams innbsp;which the woody zone separated a medullary from a corticalnbsp;portion^.�

In 1879 Renault� expressed the opinion that Brongniart by his investigation of the anatomy of Sigillaria elegans hadnbsp;established in a manner � presque irrefutable � that Sigillarianbsp;must be classed as a Gymnosperm showing affinity with thenbsp;Cycads.

In 1855 Goldenberg^ described some strobili which he regarded as those of Sigillaria and recognised their close resemblance to a fertile plant of Isoetes. He was led to thenbsp;conclusion, which had little influence on contemporary opinion,nbsp;that Sigillaria is related to Isoetes and must be classed amongnbsp;Pteridophytes. To these long and narrow strobili Schimpernbsp;gave the name Sigillariostrohus^. In 1884 Zeiller� supplied confirmation of Goldenberg�s view by the discovery of cones bornenbsp;on pedicels with Sigillarian leaf-scars, thus demonstrating thenbsp;generic identity of cones and vegetative shoots, which Golden-berg had connected on the evidence of association. Zeiller�snbsp;more recent work�� and the still later researches of Kidston�nbsp;have added considerably to our knowledge of the morphologynbsp;of Sigillarian cones. Grand�Eury�s remark made so recently asnbsp;1890� that opinion in regard to the Gymnospermous nature ofnbsp;Sigillaria is losing ground every day, bears striking testimonynbsp;to the pertinacity with which old beliefs linger even in the facenbsp;of overwhelming proof of their falsity.

It is remarkable, in view of the abundance of vegetative shoots, how rarely undoubted Sigillarian strobili have been

found; this may, however, be in part due to a confusion with Lepidostrobi which so far as we know do not differ in importantnbsp;respects from Sigillariostrobih

There can be no doubt that Sigillaria usually produced its cones on slender pedicels which bore a few leaves or bracts innbsp;irregular verticils, or in short vertical series on comparativelynbsp;stout stems, an arrangement reminding us of the occurrence ofnbsp;flowers on old stems of Theobronia and other recent Dicotyledons.nbsp;As Renault^ pointed out the fertile shoots are axillary in origin.

Dr Kidston� is of opinion that certain species of Sigillaria bore cones sessile on large vegetative shoots characterised bynbsp;two opposite rows of cup-like depressions like those in thenbsp;Ulodendron form of Lepidodendron Veltheimianum (fig. 157).nbsp;He has described the Ulodendron condition of two species,nbsp;Sigillaria discophora (K�nig) and S. Taylori (Carr.); the cuplike depressions may have a diameter of several centimetresnbsp;and are distinguished from those of Bothrodendron by thenbsp;almost central position of the umbilicus. The specimensnbsp;which he figures as ^S. discophora are identified by him withnbsp;the stem figured by K�nig as Lepidodendron discophoruni andnbsp;by Lindley and Hutton* as Ulodendron minus. We havenbsp;already dealt with the nature of Ulodendron shoots, expressingnbsp;the opinion that in spite of the often quoted specimen describednbsp;by D�Arcy Thompson�, in which a supposed cone occurs in one ofnbsp;the cups, there is no satisfactory case of any undoubted conenbsp;having been found attached to the large Ulodendron scars. Itnbsp;is more probable that the Ulodendron depressions representnbsp;the scars of branches, either elongated axes, or possibly innbsp;some cases deciduous tuberous shoots which served as organsnbsp;of vegetative reproduction. A specimen figured by Kidstonnbsp;as Sigillaria Taylori from the Calciferous sandstone of Scotland�nbsp;bears a row of slightly projecting �appendicular organs�nbsp;attached to a Ulodendron axis; but these furnish no proof ofnbsp;their strobiloid nature. The main question is, are these Ulodendron shoots correctly identified by Kidston as Sigillarian?

The surface of the specimens shows crowded rhomboidal scars surrounded in some cases by a very narrow border ornbsp;cushion; the general appearance is, as Kidston maintains, likenbsp;that of Sigillaria Brardi in which the leaf-scars are contiguousnbsp;(e.g. fig. 203, upper part). None of the leaf-scars exhibitnbsp;the three characteristic features, the leaf-trace and parichnosnbsp;scars, but only one small scar appears on each leaf-base area.nbsp;In a more recent paper Kidston figures a small piece of anbsp;stem from Kilmarnock, which he identifies as Sigillaria dis-cophora, showing the three characteristic scars on the leaf-base area. There is no doubt as to the Sigillarian nature of thisnbsp;specimen, but it is not clear if the piece figured is part of anbsp;Ulodendron shoot

Prof Zeiller^ retains the older name Ulodendron minus Lind, and Hutt. in place of Konig�s specific designation and dissentsnbsp;from Kidston�s identification of Ulodendron minus and U. majusnbsp;of Lindley and Hutton as one species; he is also inclined tonbsp;refer these Ulodendron axes to Lepidodendron. In spite of thenbsp;superficial resemblance to Sigillaria of the specimens describednbsp;by Kidston, and which I have had an opportunity of examining,nbsp;I venture to regard their reference to that genus as by nonbsp;means definitely established. We must recognise the difficultynbsp;in certain cases of drawing any satisfactory distinction betweennbsp;Sigillaria and Lepidodendron based on external features, andnbsp;while giving due weight to the conclusions of so experienced anbsp;palaeobotanist as my friend Dr Kidston, I venture to think wenbsp;are not in a position to state with confidence that Sigillarianbsp;possessed Ulodendron shoots.

The leaves of Sigillaria agree closely with those of Lepidodendron] they are either acicular (fig. 200, D) like Pine needles or broader and flatter like the leaves of Podo-carpus. Their attachment to comparatively thick branches�

shows that they persisted, in some cases at least, for several years as in Araucaria imbricata. The lower surface of thenbsp;lamina was characterised by a prominent keel (fig. 142, Anbsp;and C) which dies out towards the apex; on either side of itnbsp;are well-defined stomatal grooves (figs. 142, g, g; 143, A, 200,nbsp;D, g). The upper face may be characterised by another groovenbsp;(fig. 142, B) but without stomata. The occurrence of thenbsp;stomatal grooves, the abundance of transfusion tracheaenbsp;(fig. 142, t) surrounding the vascular bundle, and the presence of strengthening hypodermal tissue suggest that thenbsp;leaves of Sigillaria were of a more or less pronounced xerophi-lous type and had a fairly strong and leathery lamina. Thenbsp;mesophyll tissue consists either of short parenchymatous cellsnbsp;or of radially elongated palisade-like elements and has the loosenbsp;or lacunar arrangement characteristic of the aerating systemnbsp;in recent leaves; the slight development or absence of palisade-tissue may indicate exposure to diffuse light of no great intensity.

In most species there is a single vein, but in others the xylem forms a double strand (fig. 142, B). Sections of the lamina nearnbsp;the apical region present a more circular form, owing to thenbsp;gradual obliteration of the upper groove and lower keel and tonbsp;the dying out of the stomatal grooves.

The transverse section of the leaf diagrammatically represented in fig. 142, A, A', shows the two stomatal grooves, g, and a prominent keel; the single vein consists of a small groupnbsp;of primary tracheae, x, some delicate parenchyma, and a brownnbsp;patch of imperfectly preserved tissues, a, resembling the secretory zone tissue of a Lepidodendron. The whole is surroundednbsp;by a sheath of rather wide and short thinner-walled spiral ornbsp;reticulate tracheids, which may be spoken of as transfusionnbsp;tracheae, t, and compared with similar elements in the leavesnbsp;of many recent Conifers. To this tissue Renault applies thenbsp;epithet �water-bearing� and it is very likely that this may havenbsp;been its function. The shaded portions of the lamina, innbsp;fig. 142, A, represent the distribution of thicker-walled hypo-dermal tissue. The section of a leaf 3 mm. wide shown innbsp;fig. 142, C, shows an almost identical structure; the transfusionnbsp;tracheae are richly developed especially on the sides and lower

(fig. 200, E) the xylem consists of both primary and secondary elements {x, x^), but in the lamina the latter is poorly if at allnbsp;represented. In the lamina of the leaves of S. Brardi thenbsp;primary xylem forms a narrow slightly curved band with twonbsp;lateral groups of narrower, presumably protoxylem elements,nbsp;this is surrounded by delicate parenchyma styled by Renault, onnbsp;very slender evidence, phloem (� liber �). Some dark cells belownbsp;the xylem are described as sclerous tissue, and surrounding thenbsp;bundle is a sheath of transfusion tracheae (dotted area innbsp;fig. 200, E). It is possible that the elements spoken of withnbsp;hesitation by Renault as secondary xylem are transfusionnbsp;tracheae.

There has probably been some confusion in the minds of authors between sclerous tissue and dark secretory tissue innbsp;Sigillarian leaves; the crescentic band, a, shown in fig. 142, B,nbsp;which corresponds in position with the sclerous tissue ofnbsp;Renault in S. Brardi leaves, appears to be of the nature ofnbsp;secretory tissue.

The diagram shown in fig. 142, B, illustrates a type of leaf very like those already described, except that there are twonbsp;xylem strands, x. The difference between the double strand andnbsp;the single bundle seen in figs. 142, A, C and 200 E, is comparatively small, but it is a real distinction. This type of leafnbsp;(fig. 142, B) was originally described by Renault� under thenbsp;generic title Sigillariopsis. The genus was founded on a Frenchnbsp;petrified specimen consisting of part of a ribbed stem possessingnbsp;a stele of the Sigillarian type and characterised by separatenbsp;primary xylem strands, like those of S. Brardi described bynbsp;Brongniart in 1839. Renault considered the presence of twonbsp;xylem strands in the leaf a sufficient reason for the institutionnbsp;of a new genus and named the specimen Sigillariopsis Decaisnei.nbsp;Prof. Bertrand of Lille kindly photographed for me Renault�snbsp;type-specimen and sent several prints with explanatory notes.nbsp;The transverse section of the leaves shows very clearly the twonbsp;xylem strands; each strand consists of a triangular group ofnbsp;primary tracheae with the protoxylem apex pointing towardsnbsp;the lower surface of the lamina. Below each primary strandnbsp;� Renault (79) Pis. xii. xiii. p. 270 ; (96) A. p. 245.

of centripetal xylem is an arc composed of a few small tracheae which Renault and Bertrand describe as secondary xylem; itnbsp;is, however, not clear from the photomicrographs that thesenbsp;are of secondary origin, their position and appearance remindingnbsp;one of the primary centrifugal xylem of a cycadean foliarnbsp;bundle. Below this centrifugal xylem is another arc of imperfectly preserved elements described by Renault as anbsp;protective sheath and by Bertrand as glandular tissue; thenbsp;latter term is probably the more correct as the tissue may wellnbsp;correspond to the secretory-zone tissue of Lepidodendron stems.nbsp;Fairly large groups of transfusion tracheids occur on the flanksnbsp;of the xylem. Prof Bertrand points out that one of his sections,nbsp;cut nearer the apex of a leaf than that figured by Renault withnbsp;a single xylem strand, contains a double strand and thus showsnbsp;the latter�s description to be an incorrect interpretation of thenbsp;imperfectly preserved tissues.

The Sigillariopsis type of leaf was recognised by Scott* in English material on which he founded the species Sigillariopsis sulcata. In a section which he has recently figured^ anbsp;lacuna below the two xylem strands is described as � representingnbsp;secretory tissue � ; a band of transfusion tracheae almost encirclesnbsp;the pair of bundles.

In a note published in 1907, Kidston^ demonstrated the association of Sigillariopsis leaves with an undoubted Sigil-larian stem of the Rhytidolepis type and expressed hisnbsp;conviction that Renault�s genus is identical with Sigillaria.nbsp;The correctness of Kidston�s conclusion has been proved bynbsp;Arber and Thomas* who found that the leaf-traces of Sigillarianbsp;scutellata bifurcate during their course through the outer regionnbsp;of the cortex and enter the leaf as two distinct strands ofnbsp;primary xylem. In the section from Dr Kidston�s collectionnbsp;shown in fig. 142, B, the lamina, 4 mm. wide, consists mainly ofnbsp;thin-walled assimilating tissue composed of radially elongatednbsp;cells abutting at the periphery on hypodermal mechanical tissue,nbsp;except at the edges of the stomatal grooves which are boundednbsp;by the small-celled epidermis. A broad sheath of thicker-walled

elements, s, surrounds numerous scattered transfusion tracheae, t, and below the two xylem strands, cc, which are embedded innbsp;delicate parenchyma there is a crescentic band of dark tissue, a,nbsp;resembling the smaller strand, a, in fig. 142, A', and the secretory zone tissue of a Lepidodendron stem.

Reference has already been made to the manner of occurrence of strobili on Sigillarian stems; it remains to describe the structure of these reproductive shoots. Sigillariostrohus, thenbsp;name given to Sigillarian strobili, may be defined in generalnbsp;terms as follows:

Cylindrical cones, rarely dichotomously branched^ as in species of Lycopodium and Selaginella, which may reach anbsp;length of 30 cm. {e.g. Sigillariostrohus nohilis Zeill.^) and anbsp;diameter 2�5 cm.; peduncle long and slender, sometimesnbsp;bearing acicular bracts or, after leaf-fail, characterised by leaf-cushions and leaf-scars like those on vegetative shootsnbsp;(fig. 201, E). The stalked cones are borne in irregular verticilsnbsp;and in some species in vertical series, the fertile zones beingnbsp;separated by comparatively long sterile portions of the stemnbsp;(fig. 199). The cones were deciduous and, in certain cases ifnbsp;not in all, the individual sporophylls became detached fromnbsp;the cone-axis on maturity. The slender axis bore spiral ornbsp;verticillate imbricate sporophylls attached at right angles ornbsp;more or less obliquely. The basal rhomboidal portion borenbsp;spores on its upper surface (fig. 201, F), presumably enclosednbsp;in a somewhat radially elongated sporangium (fig. B) and wasnbsp;prolonged distally into a narrow lanceolate free portion, in somenbsp;species with a ciliate border (fig. D). The sporangia probablynbsp;produced megaspores and microspores, but such spores as havenbsp;been recognised appear to belong to the former category. Thenbsp;designation Triletes is applied to isolated spores of Sigillaria ornbsp;to those of Lepidodendron.

^ Goldenberg (55) ; Kidston (97). nbsp;nbsp;nbsp;^ Zeiller (88) A. PI. xo. 1, p- 598.

species, from the Coal-field of Valenciennes, the pedicel bore acicular leaves or bracts attached to the upper portion of leaf-cushions arranged in vertical series (fig. E). The cones reachednbsp;a length of 16 cm. and a breadth of 2�5�5 cm.; the sporophyllsnbsp;are borne in alternating verticils with 8�10 in each whorl.nbsp;Several megaspores (2 mm. in diameter) appear to have beennbsp;produced in tetrads in each sporangium.

Kidston described this species from the Middle Coal-Measures of England: it is similiar in habit and in the form of the sporophylls to S. Tieghemi, but rather smaller, and the morenbsp;definitely rhomboidal sporophylls have a ciliate margin.

cone was probably heterosporous, but megaspores alone have so far been discovered. The sporophylls bear a close resemblancenbsp;to those of Lycopodium cernuum (fig. 126, C). In some of thenbsp;illustrations of this type given by Kidston the naked cone-axisnbsp;�with its numerous sporophyll-scars is clearly shown, remindingnbsp;one of the naked axes of the cones of the Silver Fir {Abiesnbsp;pectinata) or Cedar after the fall of the scales.

Our knowledge of Sigillarian cones is too incomplete to admit of a detailed comparison with the strobili of Lepidodendronnbsp;or with those of recent Pteridophytes. There can, however, benbsp;little doubt that Goldenberg�^ was correct in his selection ofnbsp;Isoetes as the most nearly allied recent plant so far as the fertilenbsp;leaves are concerned. It would seem that the sporangia werenbsp;comparatively delicate structures which have left no clearlynbsp;defined remains of their walls in the carbonised specimens;nbsp;Kidston, indeed, speaks of the hollow bases of the sporophylls asnbsp;holding the spores, but this is hardly likely to have been thenbsp;case. Our knowledge of the anatomy of Sigillariostrobus isnbsp;practically nil, but in one specimen of a Sigillaria elegansnbsp;stem Kidston^ describes the structure of the tissues as seen innbsp;a transverse section of a scar of a fertile shoot; from this wenbsp;learn that the stele was composed exclusively of primarynbsp;tracheids forming a solid strand without a pith. It is probablenbsp;that the cones of Sigillaria were heterosporous, but in nonbsp;instance have undoubted microspores been discovered; thenbsp;megaspores in each megasporangium were fairly numerous as innbsp;Isoetes (fig. 133, E). In one species, Sigillariostrobus majornbsp;(Germar), from Permian rocks of France and Germany, Zeiller�nbsp;states that the whole of a single cone bore megaspores (0'8�1nbsp;mm, in diameter) only; this is, however, not opposed to the ideanbsp;of heterospory, as we find instances in Selaginella of strobilinbsp;bearing one kind of spore only (cf. p. 56).

In a few instances, it has been possible to correlate cones with certain species of Sigillaria, but in most cases the strobilinbsp;occur as isolated fossils.

The first account of the anatomy of Sigillaria we owe to Brongniart' who published a description of the internal structurenbsp;of an agatised stem, about 4 cm. in diameter, from Autun, whichnbsp;he referred to Sigillaria elegam. It has, however, been shownnbsp;by Zeiller^ and by Renault that this petrified fragment belongsnbsp;to Brongniart�s species S. Menardi, which is probably a youngnbsp;form of S. Brardi. Brongniart�s specimen, now preserved innbsp;the Paris Natural History Museum, is a very beautiful examplenbsp;of a silicified plant; on part of the surface are preserved thenbsp;hexagonal contiguous leaf-scars, like those shown in fig. 193, A,nbsp;and on the polished transverse section is seen a relatively largenbsp;stele consisting of a ring of secondary xylem surroundingnbsp;a series of crescentic groups of primary xylem (fig. 200, A)nbsp;enclosing a wide pith occupied by concentric layers of silica. Anbsp;portion of the outer cortex is preserved, and this is separatednbsp;from the stele by a broad space filled with siliceous rock. Thenbsp;main features of this type may be described in a few words.nbsp;The primary xylem differs from that of such Lepidodendronnbsp;stems as have been described in being made up of groups ofnbsp;scalariform and occasionally reticulate (fig. 200, C) tracheae,nbsp;having a plano-convex or more or less crescentic form as seen innbsp;transverse section. These primary strands, in contact with onenbsp;another laterally, have their narrowest elements on the outernbsp;edge. The leaf-traces are given off from the middle of thenbsp;abaxial face of each xylem strand (fig. 202, C, It); these passnbsp;obliquely outwards through medullary rays and then, as innbsp;Lepidodendron, turn sharply upwards before bending outwardsnbsp;again on their way to the leaves. Each leaf-trace consists of anbsp;group of primary tracheae to which a few secondary tracheae arenbsp;added during the passage through the secondary wood. Thenbsp;secondary xylem forms a continuous cylinder of tracheae withnbsp;scalariform bands on both radial and tangential walls; thenbsp;medullary rays are numerous and consist of long and narrow

series, usually one cell broad, of parenchymatous cells with occasional short rays one or more cells in depth.

The slightly greater breadth of the rays between each primary xylem strand tends to divide the secondary woodnbsp;into bundles corresponding in breadth to the primary groups.nbsp;The outer cortex closely resembles that of Lepidodendron,nbsp;it consists internally of radial series of secondary, elongatednbsp;and rather stout, elements abutting on the parenchymatous ^nbsp;tissue of the leaf-cushions.

The next contribution to our knowledge of the anatomy of Sigillaria was made by Renault and Grand�Eury^ who describednbsp;the structure of Sigillaria spinulosa Germar^, a species nownbsp;recognised as the Leiodermarian condition of S. Brardi, andnbsp;probably, therefore, not specially distinct from the specimennbsp;described by Brongniart in 1839 as 8. elegans. In Brongniart�snbsp;fossil the leaf-cushions are in contact (Clathrarian form of S.nbsp;Brardi: fig. 203, upper part) whereas in the specimen nownbsp;under consideration the leaf-scars are further apart (Leiodermarian form of 8. Brardi, fig. 203, lower part, and fig. 196, C).nbsp;It may be, as Scott suggests, that these two specimens are notnbsp;specifically identical but closely allied, an opinion based onnbsp;certain anatomical differences�; we may, however, include bothnbsp;under the comprehensive name 8. Brardi.

The primary xylem (fig. 200, B, x), is in some regions separated into distinct strands, in others it forms a continuousnbsp;band equal in length to several of the separate groups. Thisnbsp;type of stele, in which the primary xylem consists in part ofnbsp;separate strands and in part of a continuous cylinder, forms anbsp;transition between that represented in fig. 200, A, and the stelesnbsp;of 8igillaria elegans (fig. 202, A) and most species of Lepidodendron. The tendency of the primary xylem strands to becomenbsp;united laterally, forming broader bands, was first described bynbsp;Solms-Laubach^ in a French specimen of 8igiUaria spinulosa innbsp;the Williamson collection. The leaf-traces arise from the middlenbsp;of the concave outer face of the primary xylem groups. The innernbsp;cortex is composed of small parenchymatous cells as in Lepido-

dendron, and it is noteworthy that traces of partially disorganised tissue, described as large canals, in the regionnbsp;external to the secondary wood, bear a resemblance* to thenbsp;secretory tissue of LepidodendTon.

Other interesting features are presented by the structure of the outer cortex and the parichnos. The outer cortex in thenbsp;leaf-scar region is composed of parenchyma, but for the mostnbsp;part it consists of radially elongated groups of thin-walled

parenchyma enclosed in a framework of thicker-walled and elongated elements (fig. 200, B, c�). This type of cortex, tonbsp;which Brongniart applied the name Dictyoxylon, would producenbsp;a cast in the case of a partially decorticated stem characterisednbsp;by a surface formed of irregularly oval and raised areas boundednbsp;by narrow grooves; the greater prominence of the former beingnbsp;due to the more rapid decay of the softer tissue, which would

produce depressions on the exposed face of the dead stem. Casts of this type are not uncommon in Carboniferous rocks,nbsp;and while some may belong to the Pteridosperm Lyginodendron,nbsp;others may be those of Sigillarian stems.

The large parichnos-strands, produced as in Lepidodendron, by the forking of a single strand arising in the middle corticalnbsp;region, consist in part of tissue containing secretory canals, anbsp;structure like that recently described by Miss Coward^ in thenbsp;large parichnos strands of Syringodendron stems.

An example of a decorticated specimen is described by Renault^ as Sigillaria xylina. This stem is presumably referrednbsp;to Sigillaria because the primary xylem consists of separatenbsp;strands. It is characterised by the unusually large development of secondary wood and by the relatively small size of thenbsp;pith. The xylem cylinder has a diameter of 4�5 cm. and thenbsp;pith is only 4�5 mm. in breadth.

Another example of a petrified Sigillaria stem has been described by Kidston� as S. elegans Brongn.^ (fig. 193, D), anbsp;species characterised by vertical rows of sub-hexagonal andnbsp;contiguous leaf-scars and by the presence of verticils of cone-scars. Fig. 193, D, represents Kidston�s specimen in surface-view ; one row of leaf-scars is shown, but most of the superficialnbsp;tissues have been destroyed. The crushed stele, 13 mm. in itsnbsp;longest diameter, has a continuous cylinder of primary xylem,nbsp;(fig. 202, A, x) characterised by a regularly crenulate outernbsp;margin with the smallest elements at the edge; the prominentnbsp;ridges separating the sinuses are rounded. The leaf-traces arisenbsp;from the bottom of each sinus; the leaf-bundles are mesarch,andnbsp;consist exclusively of primary elements. The secondary xylem,nbsp;x^, like that of the primary xylem, has a crenulate outer edge.nbsp;The most interesting feature of the outer cortex is afforded bynbsp;a tangential section which, in addition to the leaf-scars, cutsnbsp;through a cone-scar showing a solid primary stele surroundednbsp;by the cortex of the cone-peduncle.

Another type of Sigillaria, probably S. elongata Brongn. (fig. 202, B, C), which is very similar to S. scutellaia has been

briefly described by Prof. Bertrand^, to whom my thanks are due for the two photographs reproduced in fig. 202, B. C. Hisnbsp;specimen, from the Pas de Calais Coal-field, shows a ribbednbsp;Rhytidolepis form of surface (fig. 202, B). The stele (fig. 202, C)nbsp;agrees closely with that of S. elegans as described by Kidston,nbsp;but the ridges on the fluted surface of the primary xylem arenbsp;more pointed. � In the immediate neighbourhood of the originnbsp;of a leaf-trace, the spiral elements form a median band in thenbsp;middle of a sinus � and from this the leaf-traces are given off.nbsp;No secondary xylem was found in the leaf-traces at any part ofnbsp;their course.

Bertrand compares the stele of S. elongata with that of the type of Lepidodendron represented by the Burntisland speciesnbsp;named by Williamson L. hrevifolium (fig. 18G) and now usuallynbsp;referred to L. Veltheimianum; the chief distinguishing featuresnbsp;are the greater prominence in the French species of the surface-ridges or teeth of the primary xylem, a feature which occurs innbsp;L. W�nschianum, and the detachment of the leaf-traces fromnbsp;the bottom of each sinus (fig. 202, C, It) instead of from thenbsp;sides of the sinus. It is, however, not clear how far this latternbsp;distinction is a real one; in Lepidodendron W�nschianum thenbsp;leaf-traces appear to arise, as in Sigillaria, from the middle ofnbsp;each sinus.

Other types of ribbed Sigillaria stems have been briefly described by ScottKidston�, and more recently, by Arbernbsp;and Thomas ^

The specimen described by Scott agrees in the main with S. elegans of Kidston and with S. elongata of Bertrand.

Kidston�s sections of S. scutellata show a continuous primary xylem cylinder with a slightly and irregularly crenulate outernbsp;margin. It would seem that one important diagnostic characternbsp;in Sigillarian stems is afforded by the degree and form of thenbsp;crenulations on the outer surface of the primary xylem. S. scutellata has been described also by Arber and Thomas; thesenbsp;authors were the first to demonstrate the presence of a ligulenbsp;and ligular pit on the leaf-base in a petrified stem, and they

also contribute the important fact that the leaf-traces in passing through the phelloderm bifurcate and enter the leaf as twonbsp;distinct vascular strands. This double bundle has been referrednbsp;to in the description of Sigillaria leaves. (Page 214.)

Although our knowledge of the anatomy of Sigillaria has been considerably extended since Williamson^ drew attention to our comparative ignorance of the subject, there arenbsp;several points on which information is either lacking or verynbsp;meagre. As regards the stele, it is in all types so far investigated, of the medullated type and constructed on thenbsp;same plan as that of Lepidodendron W�nschianum, L. Velt-heimianum, and other species. Secondary xylem was developednbsp;at an early stage of growth, and its relation to the primarynbsp;xylem, from which as Kidston points out in his description ofnbsp;S. elegans, it may be separated by a few parenchymatous elements, is like that in Lepidodendron. The tendency of thenbsp;outer face of the secondary xylem to present a crenulatenbsp;appearance in transverse sections may, as Scott thinks ^ be anbsp;feature of some diagnostic importance, but this is not a constantnbsp;character in the genus. In origin and in their mesarch structure,nbsp;the leaf-traces closely resemble those of Lepidodendron. Thenbsp;earlier account of the structure of the leaf-traces of Sigillaria,nbsp;which were described as possessing both centrifugal and centripetal wood, led Mettenius� to draw attention to an importantnbsp;anatomical resemblance between this genus and modern Cycads.nbsp;This comparison was, however, based on a misconception; thenbsp;Cycadean leaf-trace, consisting solely of primary wood, is notnbsp;strictly comparable with those of some species of Sigillaria, innbsp;which one part of the xylem is primary and another secondary.nbsp;The occasional presence of secondary xylem in Sigillarian leaf-traces is matched in some Lepidodendra^, and cannot benbsp;accepted as a distinguishing feature.

The origin of the leaf-traces from the middle of the sinuses on the edge of the primary xylem is regarded as a difference; innbsp;Lepidodendron the leaf-traces are said to arise in some speciesnbsp;from the sides of the crenulations; but, as already pointed out.

this is a distinction of doubtful value. The division of the primary xylem into separate strands in some stems of Sigillarianbsp;of the Clathrarian and Leiodermarian forms is a characteristicnbsp;peculiarity; but 8. spinulosa forms a connecting link betweennbsp;this type and the continuous arrangement of the xylem in 8. elon-gata and 8. elegans. Kidston^ has shown that the discontinuousnbsp;primary xylem occurs in Lower Permian species, a fact consistent with the view that the greater abundance of thenbsp;centripetally developed wood, characteristic of the older species,nbsp;represents a more primitive feature. This is not merely anbsp;conclusion drawn from a consideration of geological age, but itnbsp;is in harmony with the view expressed by Scott^ that as plantsnbsp;achieved greater success in producing secondary centrifugalnbsp;wood, the retention of any considerable quantity of primarynbsp;xylem became superfluous. As yet we know very little ofnbsp;the structure of the perixylic tissues of 8igillaria, but therenbsp;is no sufficient reason for supposing that these differ innbsp;essentials from those in Lepidodendron. The middle and outernbsp;cortical tissues are practically identical in the two genera. Thenbsp;parichnos is of the same type, except that in 8igillaria itnbsp;reached greater dimensions in the outer part of its course.

1822. Clathraria Brardi, Brongniart, Classif. Veg. foss., PI. xil. fig. 5. 1828. Sigillaria Brardi, Brongniart, Hist. Veg. foss. p. 430, PI. CLVlii.nbsp;fig. 4.

1839. S. elegans, Brongniart, Arch. Mus. Nat. Hist. Paris, Yol. l. p. 406, PJ. XXV.

3 For fuller synonymy,[see Kidston (86) A. p. 179; and Zeiller (06) p. 160; Koehne (04) p. 62.

The aerial shoots of this species are occasionally branched dichotomously^, the apical portions bearing short crowded leaves'-*;nbsp;the surface of the bark is either completely covered with contiguous leaf-scars without definite leaf-cushions orwith projectingnbsp;cushions forming a narrow sloping surface surrounding each

leaf-scar. Other parts of the plant may possess cushions similar in their kite-shaped form to those of Lepidodendron, but withoutnbsp;a median vertical groove, or the leaf-scars may be spirally disposednbsp;at varying distances apart on a comparatively smooth and longitudinally wrinkled bark. The species exhibits striking instancesnbsp;of a transition between the Favularian, Clathrarian, and Leioder-marian forms of stems. The leaf-scars, which are hexagonal innbsp;^ Renault (96) A. PI. xxxv.; Zeiller (06) PI. xlii. � Grand�Eury (90) A.

outline,�the lateral angles pointed and transversely elongated, the upper and lower angles rounded,�bear three scars, thenbsp;central leaf-trace and two straight or curved lateral parichnosnbsp;scars; a ligular pit occurs immediately above the centre of thenbsp;upper edge of the leaf-scar and occasionally circular elevationsnbsp;with a central pit occur singly or in pairs below a leaf-scarnbsp;(fig. 196, A). The linear leaves, which may persist on shootsnbsp;having a fairly large diameter^ have a single median vein andnbsp;two stomatal grooves on the lower surface^ (fig. 200, D).

Partially decorticated and younger shoots are characterised by the occurrence of pairs of elliptical parichnos areas and anbsp;smaller median leaf-trace scar. The surface of older stems,nbsp;which may show signs of longitudinal splitting {Syringodendronnbsp;state), bears pairs of parichnos scars reaching a length ofnbsp;2�2�5 cm. and a breadth of 10�13 mm. The regularity of thenbsp;leaf-scar arrangement is interrupted at intervals by the occurrence of more or less regular verticils of scars marking thenbsp;position of deciduous shoots. Grand�Eury=* has figured conesnbsp;which he believes to be those of this species, and Zeiller refersnbsp;the large strobili, Sigillariostrobus major, to Sigillaria Brardi*.

The subterranean axes were characterised by spirally disposed rootlet-scars like those of Stigmaria ficoides (figs. 204, 205) andnbsp;by a cortical surface with the features of Stigmaria rimosa Gold.�

The anatomy of the stele and leaves has already been described (p. 219). The stele of the Stigmai�ian portion of thenbsp;plant consists of a band of centripetal primary xylem and anbsp;cylinder of centrifugally formed secondary wood with medullarynbsp;rays containing vascular bundles passing out to the rootlets�.

Sigillaria Brardi occurs not uncommonly in Permian rocks; it is recorded from Franceh Germany�, Pennsylvania�, andnbsp;elsewhere. It is found in the Upper, Middle, and Lowernbsp;Coal-Measures of England^ and in Permo-Carboniferous stratanbsp;in Africaand BraziP�.

CHAPTER XVIL

Stigmaria.

Stigmaria ficoides is the name given to cylindrical casts met with in Palaeozoic rocks, from the Devonian* to the Permiannbsp;characterised by a smooth or irregularly wrinkled surface bearing

spirally disposed circular scars bounded by a raised rim and containing a small central pit. It is not uncommon to findnbsp;evidence of a partial collapse of the substance of the plant as

seen in fig. 204; this is doubtless the expression of a shrinkage of the middle cortical region, which was composed of a delicatenbsp;and lacunar system of cells. There can be no reasonable doubtnbsp;that Stigmaria grew in water or in swampy ground. Specimensnbsp;are occasionally met with in which the cast terminates in anbsp;bluntly rounded apex; such are, perhaps, young branches whichnbsp;have not grown far from the base of the aerial stem from whichnbsp;they arose (cf. fig. 207, B, C). Other examples occur, such asnbsp;Goeppert^ figured and Gresley^ has more recently described,

Fig. 205. Stigmaria Jicoides. From a specimen in the York Museum, from Bishop Auckland, a, base of rootlet showing vascular bundle scar.

which are twisted and distorted as though obstacles had been encountered in the ground in which they grew.

The circular scars mark the bases of long single and occasionally forked appendages (rootlets) which spread on allnbsp;sides into the surrounding medium (figs. 205, 208). Thenbsp;occurrence of rootlets radiating through the shale or sandstonenbsp;affords proof that the Stigmarias are often preserved in theirnbsp;position of growth. This was recognised by Steinhauer^ andnbsp;LoganS and has been more recently emphasised by Potoni� as

Steinhauer (18) A. nbsp;nbsp;nbsp;* Logan (42).nbsp;nbsp;nbsp;nbsp;^ Potoni� (93!gt;).

an argument in favour of the view that the beds containing such specimens are old surface-soils.

Stigmaria usually shows regular dichotomous branching, the arms spreading horizontally or slightly downwards and alwaysnbsp;arising from four main branches in the form of a cross (fig. 207).nbsp;The most remarkable specimens found in England are describednbsp;by Williamson^ in his monograph of Stigmaria. One of twonbsp;large casts found near Bradford in Yorkshire, and now in thenbsp;Manchester Museum, shows four large primary arms radiatingnbsp;from the base of an erect stump 4 feet in diameter. Eachnbsp;arm divides a short distance from its base into two, and thenbsp;smaller branches extend almost horizontally for several feet^.

An illustration published by Martin in 1809� shows a characteristic feature of Stigmarian casts, namely the presencenbsp;of a smaller axis, usually occupying an eccentric positionnbsp;inside the larger. This represents the cast of the fairly broadnbsp;parenchymatous pith which, on decay, left a space subsequentlynbsp;filled by sand or mud: at a later stage the surrounding woodnbsp;and cortex were removed and the cavity so formed wasnbsp;similarly filled. A thin layer of coal formed by the carbonisation of some of the tissues frequently surrounds the medullarynbsp;cast, and Steinhauer, whose account of the genus is muchnbsp;fuller and more scientific than those of other earlier andnbsp;many later writers, recognised the true nature of this internalnbsp;cast. Artis^ regarded it as the remains of a young plant, whichnbsp;he described as � perforating its parent,� at length bursting itnbsp;and assuming its place, a gratuitously drastic interpretation.

In 1838� Bindley and Hutton figured a partially petrified specimen of Stigmaria obtained by Prestwich from Carboniferous rock of Shropshire. This example showed a fairlynbsp;broad cylinder of secondary wood penetrated by medullarynbsp;rays. The medullated stele consisted of a pith surrounded bynbsp;a small amount of primary xylem and by a cylinder of

^ A similar example, now in the Bergakademie of Berlin, has been described by Potoni� (90) A. ; see also a note on the German specimen by Seward (91).

secondary scalariform tracheae. The preservation of the tissues abutting on the edge of the wood is usually very imperfect, andnbsp;the middle cortex of lacunar parenchyma has practically in everynbsp;case eluded the action of mineralising agents; the outer cortex,nbsp;on the other hand, consists of more resistant elements and isnbsp;frequently well preserved. As in Lepidodendron and Sigillarianbsp;stems, meristematic activity produced a broad band of secondarynbsp;cortex; and beyond this were attached to cushion-like pads thenbsp;numerous appendages, each supplied with a single vascularnbsp;bundle which arose from the primary xylem and passed outwards through a medullary ray. There is abundant evidencenbsp;that the appendages were hollow, a fact in striking accord withnbsp;the aquatic and semi-aquatic habitat (cf. Isoetes root, fig. 133, G).

The piece of dried rhizome of Cyperus papyrus shown in fig. 206 is an almost exact counterpart of Stigmaria ficoides; thenbsp;wrinkled and shrivelled surface and the circular root-scarsnbsp;containing the remains of a vascular bundle are strikingnbsp;features in common and, it may be added, the two plants, thoughnbsp;very different in structure and in systematic position, illustratenbsp;anatomical adaptations to a similar environment-

1809. Phytolithus verrucoms, Martin, Petrifaot. DeA. H America 1818. PhytoUthm ve^-rncosus, Steinhauer, Trans. Ph�. Soc. America,

The first figure of Stigmaria is said to be by Petver in � Volkmann published illustrations of this common fossi innbsp;and Paikinson in 1804^. Binney, whose researches may enbsp;said to have inaugurated a new era in the investigation onbsp;plants, wrote in 1844; �Probably no fossil plant has excitednbsp;more discussion among botanists than the Stigmaria. It is t enbsp;most common of the whole number of plants found in the oanbsp;Measures, but there has hitherto been the patest uncertaintynbsp;as to its real nature^.� This uncertainty still exists, at least innbsp;the minds of some who know enough of the available data tonbsp;realise that our knowledge is imperfect.

To pass to the questions of the affinity and nature o Stigmaria: Brongniart^ at first compared his genus with recennbsp;Aroideae, but he afterwards� spoke of it as probably the root onbsp;Sigillaria. Other writers regarded Stigmaria as a dicotyledonousnbsp;plant comparable with Cacti and succulent Euphorbias. oi�nbsp;many years opinion was divided as to whether Stigmaiianbsp;represents an independent and complete plant or the under ^nbsp;ground system of Sigillaria.

Artis�, Bindley and Hutton^, as well as Goldenberg�, believe it to be a prostrate plant unconnected with any erect aerial stem.nbsp;Goldenberg figured one of the slender rootlets terminating innbsp;an oval body described as a reproductive organ. This seed-li enbsp;impression is either some extraneous body or an abnorma

2 nbsp;nbsp;nbsp;Goldenberg (55) p. 6.nbsp;nbsp;nbsp;nbsp;� Binney (44) p. 165.

�gt; Brongniart (22) A. p. 228. nbsp;nbsp;nbsp;� �bid. (49) A. p. 456.nbsp;nbsp;nbsp;nbsp;� Artis (2o) A. FI. x.

development at the end of a rootlet. In 1842 Logan drew attention to the almost complete monopolisation by Stigmarianbsp;of the underclays, the rock which as a general rule occurs belownbsp;a seam of coal. He wrote: �The grand distinguishing featurenbsp;of the underclays is the peculiar character of the vegetablenbsp;organic remains; they are always of one kind {Stigmarianbsp;ficoides) and are so diffused throughout every part of the bed,nbsp;that by their uniform effect alone the clay is readily recognisednbsp;by the eye of the miner h� This fact, which has played a verynbsp;conspicuous part in the perennial discussions on the origin ofnbsp;coal, led to the almost general recognition of the underclays asnbsp;surface-soils of the Coal period forests.

The next step was the discovery of Stigmaria in the Coal-Measures of Lancashire and in the Carboniferous rocks of Cape Breton, Nova Scotia, forming the basal branches of erect stemsnbsp;identified by Binney^, Bowman� and Richard Brown^ as undoubted Sigillariae. In one case Brown found what henbsp;considered to be convincing evidence of the continuity betweennbsp;Stigmaria and Lepidodendron.

In 1842 Hawkshaw� described certain fossil trees, the largest of which had a circumference at the base of 15 ft., discovered, innbsp;the course of excavations for a railway in Lancashire, in soft shalenbsp;at right angles to the bedding. The surface features were notnbsp;sufficiently clear to enable him to decide with certainty betweennbsp;Sigillaria and Lepidodendron, but while inclining to the former,nbsp;it is interesting to note that the occurrence of numerous Lepido-strobi near the root led him to recognise the possibility of anbsp;connexion between the Stigmarian roots and Lepidodendronnbsp;stems. In 1846 Binney gave an account of similar trees foundnbsp;at Dukinfield near Manchester : he spoke of one stem as unquestionably a Sigillaria with vertical ribs, furrows, and scars,nbsp;about 15 inches high and 4 ft. 10 inches in circumference. Henbsp;expressed his conviction that �Sigillaria was a plant of annbsp;aquatic nature�.� Similar descriptions of rooted stems in thenbsp;Coal-Measures of Nova Scotia were published by Brown in

1845, 1846 and 1849 ; in the last paper he figured a specimen, which has become famous, showing a Syringodendron stemnbsp;terminating in branching Stigmarian (or possibly Stigmariopsis)nbsp;roots bearing on the lower surface a series of quot;what he callednbsp;conical tap rootsh A similar specimen discovered in Centralnbsp;France nearly fifty years later demonstrated the accuracy ofnbsp;Brown�s description.

Despite these discoveries the root-like nature of Stigmaria was not universally accepted. It was, however, generally agreednbsp;that Stigmana formed the roots of SigillaTia; it Nvas, moreover,nbsp;held by some that Lepidodendron stems also possessed thisnbsp;type of root, an opinion based on Brown�s record and on thenbsp;occurrence of Stigmaria in beds containing Lepidodendron butnbsp;no Sigillana stems, as in the volcanic beds of Arran and elsewhere, and on observations of Geinitz and others'. There isnbsp;now general agreement that Lepidodendron and Sigillaria hadnbsp;the same type of � root,� though the connexion of Stigmarianbsp;with the former was not so readily admitted, and indeed thenbsp;evidence in support of it is still very meagre. Goeppert andnbsp;other authors were unable to believe that the numerous speciesnbsp;of Sigillaria possessed roots of so uniform a type, but Goeppert,nbsp;by his recognition of several varieties of Stigmaria, suppliednbsp;a partial answer to this objection.

Messrs Mellor and Leslie� have described and figured some large casts of roots exposed in Permo-Carboniferous rocks innbsp;the bed of the Vaal river at Vereeniging (Transvaal) whichnbsp;exhibit certain features suggesting comparison with Stigmaria.nbsp;Some of these reach a length of 40�50 feet and, whennbsp;complete, were probably not less than 100 feet long: in somenbsp;of them the centre of the cast from which forked arms spreadnbsp;almost horizontally shows a depression in the form of a crossnbsp;indicating a regular dichotomous branching like that of Stigmaria. The authors incline to the belief that the roots belongnbsp;to Noeggerathiopsis and not to a lycopodiaceous plant, thoughnbsp;Lepidodendroid stems are abundant in the sandstone a few feet

higher in the series. Despite the absence of any Stigmarian scars on the surface of the fossil it is probable that these finenbsp;specimens are the rhizomes of some lycopodiaceous plant,nbsp;possibly Bothrodendron, which is not uncommon in the Vereeni-ging beds.

Admitting that Stigmaria is part of Sigillaria, the next question is, is Stigmaria a root in the ordinary sense, the underground system formed on germination of the spore and of equalnbsp;age with the shoot, or did it bear a different relation to the Sigil-larian stems ? To this question different answers would still benbsp;given. Goeppert^ discussed evidence in favour of the view thatnbsp;aerial Sigillarian shoots were produced as vegetative budsnbsp;on pre-existing Stigmarian axes, like young moss plants onnbsp;a protonema. At a later date Renault^ developed a similarnbsp;view as regards Sigillaria; but we may pass on to consider thenbsp;more recent and complete observations of Grand'Enry* andnbsp;Solms-Laubacht

The recognition of two distinct types of Stigmariae in the Coal-Measures of Central France led Grand�Eury� to institute a new genus, Stigmariopsis. This type, which is characterised by a difference in habit as well as by other distinguishingnbsp;features, is represented by such specimens as those figured bynbsp;Goldenberg as Stigmaria abbreviata, bearing lenticular scarsnbsp;spirally disposed on a cortical surface characterised by irregularnbsp;longitudinal wrinklings. Stigmariopsis has frequently been foundnbsp;in direct continuity with Sigillarian stems of the Leiodermarian-Clathrarian type, spreading obliquely downwards in the form ofnbsp;rapidly narrowing arms clothed with slender and usually simplenbsp;appendages; and from the under surface of these arms shortnbsp;conical outgrowths are given off. It is probable, as Solms-Laubach believes, that Stigmariopsis was represented also bynbsp;long horizontally creeping rhizomes� of uniform breadth fromnbsp;which ribless Sigillarian aerial shoots arose as bud-like outgrowths. Grand�Eury, the author of the genus, confined thenbsp;term to the shorter and more rapidly tapered organs spreading

* nbsp;nbsp;nbsp;Goeppert (64) A. p. 197, Pis. 34�36.nbsp;nbsp;nbsp;nbsp;^ Renault (81).

* nbsp;nbsp;nbsp;Grand�Eury (77) A. p. 171.nbsp;nbsp;nbsp;nbsp;� For figures see Grand�Eury (87) A.; (90) A.

from the base of erect stems; the horizontal rhizomes of all Sigillarian stems he refers to Stigrnaria. The pith-casts ofnbsp;Sigillariopsis may be recognised by their long vertical ridgesnbsp;and grooves, a feature readily understood by reference to thenbsp;stem structure. The Stigmariopsis rhizomes though rare innbsp;England have been recognised by Dr Kidston�^ in the Middlenbsp;Coal-Measures of Yorkshire; he has figured a pith-cast verynbsp;like that illustrated in Solms-Laubach�s Memoir as Stigmariopsisnbsp;anglica.

The surface-features of a Stigmariopsis pith-cast are clearly shown on a specimen fi�om St Etienne in the Williamsonnbsp;collection^.

The most complete account of Grand�Eury�s views in regard to the anchoring and absorbing organs of Sigillaria is given innbsp;his monograph on the Coal-field of Gard�, St �tienne, and thesenbsp;are clearly stated also by Solms-Laubach^ who confirms thenbsp;conclusions of the French author as to the manner of development of the aerial shoots. Grand�Eury believes that bothnbsp;Stigrnaria and Stigmariopsis are rhizomes and not true roots.nbsp;The surface-features of Stigrnaria have already been described.nbsp;This type Grand�Eury speaks of as characterised by the uniform

cated axes; he thinks they grew both as floating rhizom on the ground; they may frequently be

aerial shoots, but occasionally they have been seen in _ union with Sigillarian stems. He believes that t ese rnbsp;were produced as the result of germination under wa er onbsp;spores of Sigillaria or Lepidodendron and develops as o gnbsp;branched aquatic rhizomes capable of independent exnbsp;Under certain conditions, as he thinks in shallowei wa enbsp;rhizomes produced bulb-like outgrowths whichnbsp;nbsp;nbsp;nbsp;^

of origin is practically the same as that describe y oep in 1865. The vascular medullated cylinder of these er

The next stage is that in which the undifferentiated bulb becomes swollen at the base and developes four primary rootsnbsp;(fig. 207 B, C) which grow obliquely downwards and producenbsp;numerous rootlets. Meanwhile the parent rhizome graduallynbsp;decays, finally setting free the aerial stems which are nownbsp;provided with spreading and forked roots (fig. 208) such as we

Fig. 208. Later stage in the development of Sigillaria; Syringodendron with Stigmariopsis. (After Grand�Eury.)

are familiar with in English specimens as Stigmaria ficoides, but which in the French specimens show the features of Stigmariopsis. At this later stage conical outgrowths are formednbsp;from the under surface of the Stigmariopsis arranged in anbsp;more or less regular series surrounding the centre of the forkednbsp;and spreading roots (fig. 209). These conical and positivelynbsp;geotropic organs were long ago described by Richard Brown as

tap-roots. Grand�Eury�s conclusions are briefly as follows: Sigillaria, and we may add Lepidodendron, had no true rootsnbsp;and in this respect are comparable with Psilotum (fig. 118);nbsp;the organs which are described by Grand�Eury as roots arenbsp;correctly so named in a physiological sense, but morphologicallynbsp;they do not strictly conform, either in origin or in the arrangement of their appendages, to true roots. The question as tonbsp;whether they are entitled to the designation root is one whichnbsp;it is needless and indeed futile to discuss in detail; it would benbsp;conceding too much to a formal academic standpoint to refrainnbsp;from applying to them the term root, as that best describesnbsp;their share in the life of the Sigillarian stems. The horizontalnbsp;Stigmarian axes are rhizomes in the ordinary sense of the termnbsp;and from these were developed Sigillarian shoots, characterised

in the lower portions by large parichnos stran s. , the base of the young bulbous shoots roots were orme .nbsp;roots being, in the French specimens, of the Stigmai iopsisnbsp;These conclusions require some modification w ren appnbsp;to British representatives of the arborescentnbsp;The long spreading and dichotomously branched root- r e organbsp;attached to the base of Sigillarian and Lepidoden ron �nbsp;are true examples of Stigmaria ficoides or other specie^ Wnbsp;mariopsis occurs but rarely. This marked difference etwenbsp;French and English specimens may be explained if we a opnbsp;the opinion of Solms-Laubach, who believes that t e ru

Lepidodendron, the Stigmariopsis form having the corresponding relation to the Leiodermarian-Clathrarian species.

The opinion expressed by Williamson^ in 1892 that Grand�-Eury�s hypothesis � appears to be identical with the vague and speculative guesses that were prevalent among us in the earlynbsp;years of the present [nineteenth] century � illustrates thenbsp;strength of conviction based on English specimens as to thenbsp;root-nature of Stigmaria.

There is undoubtedly considerable confusion, which can be cleared up only by further research, as to the precise relationnbsp;between Stigmaria and Stigmariopsis on the one hand and thenbsp;different types of Sigillariae on the other. The main contention,nbsp;and this is the most important point, of Renault, Grand�Eurynbsp;and Solms-Laubach as to the manner of formation of the aerialnbsp;shoots from rhizomes and the subsequent production of forkednbsp;roots and their ultimate separation from the parent rhizomenbsp;is, as I believe, correct. Williamson, held that Stigmaria mustnbsp;be regarded as a true root; he found no evidence to supportnbsp;the view that the large rooted stem discovered by Havvshaw,nbsp;Binney, and others had been originally produced from aquaticnbsp;rhizomes. It must, however, be remembered that Grand�Eur3?�snbsp;opinion is based on evidence afforded by the exceptionally wellnbsp;displayed Sigillarian forests of St Etienne, on a scale such asnbsp;English strata have not as yet afforded. Moreover, the absencenbsp;of any parent-rhizome in association with the rooted stumpsnbsp;described by Williamson and by others is not a serious argument against their rhizome origin.

The specimen represented in fig. 209, which was examined in situ by Solms-Laubach and Grand�Eury, shows a Sigillariannbsp;stem in the Syringodendron condition bearing rows of pairednbsp;parichnos scars; from the base forked and rapidly taperingnbsp;arms radiate through the surrounding rock and, as shown bynbsp;other specimens, these bear numerous appendages like those ofnbsp;the English Stigmarias. The surface-features of the arms arenbsp;those of Stigmariopsis and the centre of each, as seen on thenbsp;broken face, is occupied by a pith-cast characterised by parallelnbsp;longitudinal ridges resembling those on the medullary casts ofnbsp;1 Williamson (92).

The more important anatomical features of Stigmaria must be dealt with briefly. Williamson�s monograph, published innbsp;18871, is considerably in advance of the work of that of any ofnbsp;the numerous writers who had previously dealt with thenbsp;subject. The diagrammatic transverse section reproduced in flg.nbsp;210, H, illustrates the general arrangement of the tissues. Thenbsp;medullated stele was described by Williamson as consisijingnbsp;entirely of centrifugally developed secondary xylem andnbsp;distinguished, therefore, from the stele of a Lepidodendron ornbsp;Sigillaria by the absence of a centripetally produced primarynbsp;xylem zone. The secondary xylem tracheae are characterisednbsp;by scalariform pits on both radial and tangential walls and, asnbsp;shown in a flgure given by Solms-Laubach^, the spaces betweennbsp;the transverse bars are bridged across by fine threads, as innbsp;the tracheae of Lepidodendron.

One of the largest specimens of a petrified Stigmaria which I have seen is one lent to me by Mr Lomax from the Coal-Measures of Halifax in which the flattened transverse sectionnbsp;measures 18 cm. x 3�5 cm., the cylinder of wood being IT cm. xnbsp;7 mm. in diameter.

In French examples of Stigmaria or Stigmariopsis it has been demonstrated by Renault� that primary xylem strandsnbsp;occur very like those in the stem of some species of Sigillariaenbsp;(see p. 219). If a well-preserved section of an English Stigmarianbsp;is examined it will be seen that -the edge of the secondary woodnbsp;consists of a few narrower elements which do not exhibit thenbsp;radial seriation characteristic of secondary elements.

A type of Stigmaria characterised by centripetal primary wood has been described by Weiss^ and referred by him tonbsp;Bothrodendron munduni] the main results of his observations arenbsp;stated in the account of Bothrodendron on a subsequent page.nbsp;This discovery is of considerable interest not only as renderingnbsp;our knowledge of Bothrodendron remarkably complete but asnbsp;confirmatory of Renault�s account of French Stigmarian axes in

which centripetal primary wood is well developed between the secondary xylem and the centre of the stele. The Stigniariannbsp;axis of Bothrodendron was originally figured by Williamsonnbsp;Rs Lepidodendron munduni^. The chief difference between

D. nbsp;nbsp;nbsp;Rootlets from the outer cortex of E. -nbsp;nbsp;nbsp;nbsp;a.j mnllet

E. nbsp;nbsp;nbsp;Part of a large Stigmaria : St, stele ; s, �nbsp;nbsp;nbsp;nbsp;, towards the

G, nbsp;nbsp;nbsp;Outer cortex of Stigmaria.nbsp;nbsp;nbsp;nbsp;� r. rootlets.

H. nbsp;nbsp;nbsp;Diagrammatic section of Stigmaria : p, Pnbsp;nbsp;nbsp;nbsp;�

An interesting agreement between the French and English specimens is the occurrence in the cortex of groups of reticulatenbsp;elements: in Weiss�s section these are short and wide andnbsp;occur in the middle cortex; in Eenault�s plant they are morenbsp;fusiform and occur in the secondary cortical tissue. Thesenbsp;elements appear to have been arranged as an interlacingnbsp;network in the middle cortex and were in close connexion withnbsp;the rootlet-bundles, comparable, as Weiss points out, with thenbsp;transfusion tracheids accompanying Lepidodendron leaf-traces.

It is probable that these short and wide tracheal elements served for water-storage and thus afford another indication of thenbsp;xerophilous character of the Carboniferous Lycopods, a featurenbsp;possibly connected with a salt-marsh habitat.

The presence of conspicuous medullary rays gives the secondary xylem of Stigmaria the appearance of being dividednbsp;into several more or less distinct groups (fig. 210, E, St). Innbsp;tangential longitudinal section the xylem assumes the form ofnbsp;a broad reticulum with lenticular meshes filled with medullary-ray tissue through which strands of xylem are cut across in anbsp;transverse direction as they pass outwards from the inner edgenbsp;of the wood to supply the rootlets. In addition to these broadernbsp;or primary medullary rays, there were numerous secondarynbsp;rays composed of narrow plates of parenchymatous cells onenbsp;or several elements in depth. As Williamson pointed out, thenbsp;medullary-ray tissue consists in part of radially elongatednbsp;tracheal elements with spiral or scalar!form thickening bands likenbsp;those described in the same position in Lepidodendron stems.

Our knowledge of the minute structure of the tissues abutting on the secondary xylem is far from complete.

The xylem is succeeded by a zone of delicate cells which was the seat of meristematic activity. It is noteworthy that in anbsp;section figured by Williamson^ there is the same disparity innbsp;size between the outermost elements of the xylem and thenbsp;adjacent cells of the meristematic zone as in Lepidodendronnbsp;stems. Beyond this region an imperfectly preserved lacunar

tissue occurs like that which I have called the secretory zone in Lepidodendron stems; but information as to the structure ofnbsp;this part of Stigmaria is much more incomplete than in thenbsp;case of the aerial shoots. The middle cortex was of the samenbsp;lacunar type as in the stems, and the fact that it is never wellnbsp;preserved in large Stigmarian axes suggests that it may havenbsp;been even more richly supplied than in the aerial stems with annbsp;aerating system of spaces. The outer cortex, consisting in youngnbsp;examples of large-celled parenchyma, became at an early stagenbsp;of growth the seat of cambial activity which resulted in thenbsp;production of radially placed series of secondary elements (fig.nbsp;210, H, p). The outer and older elements of this secondarynbsp;cortex are more tangentially stretched than the inner cells, anbsp;necessary result of the position of the phellogen on the internalnbsp;edge of the tissue and of the increasing girth of the axis.

In comparatively young Stigmarian axes the outer cortex already possesses a band of secondary radially disposed cellsnbsp;characterised by the greater tangential extension of the morenbsp;external elements; usually this tissue terminates abruptly onnbsp;the inner edge and the line of separation no doubt marks thenbsp;position of the phellogen. Occasionally some delicate secondarynbsp;elements are preserved internal to the phellogen, and these innbsp;young specimens form a narrow cylinder composed in part ofnbsp;radially elongated cells showing signs of recent tangentialnbsp;divisions. In its earlier stage of activity the phellogen seems tonbsp;form a greater amount of secondary tissue on the outside, hutnbsp;this is clearly not of the nature of cork, the tissue which occupiesnbsp;a corresponding position in recent plants. The primary cortexnbsp;shows no signs of shrinkage or collapse as would be the casenbsp;were it cut off from the vascular system by a zone of impermeable cork.

Fig. 210, G, represents a piece of the external tissue of a specimen in which the slightly flattened xylem cylindernbsp;measures 1-4x1 cm.; the inner cortex has disappeared andnbsp;fragments only of the middle cortex are preserved. The outernbsp;cortex, with an average breadth of 2 mm., consists superficially ofnbsp;primary parenchyma with a somewhat uneven surface and withnbsp;a rootlet attached here and there; a short distance below the

surface is a band of conspicuous cells, b, characterised by dark contents suggesting very imperfectly preserved fungal hyphae,nbsp;but the nature of the substance filling the cells cannot be madenbsp;out -with certainty. It is, however, interesting to find that thisnbsp;dark band constitutes an obvious feature (fig. H, b); its position isnbsp;comparable with that of the dark-walled cells in the outernbsp;cortex of rootlets. A short distance internal to this dark bandnbsp;tangentially elongated cells form the outermost elements of thenbsp;secondary cortex; these become gradually narrower towards thenbsp;interior and pass into radial series of smaller cells of uniform size,nbsp;as seen on the inner edge of fig. 210, G. At the inner boundarynbsp;of this tissue, just below the region shown at the bottom of thenbsp;drawing, was situated the phellogen. Such traces of tissue asnbsp;occur on the inner side of the line where splitting has usuallynbsp;occurred, consist of thinner elements with recently formednbsp;tangential walls and probably represent an early stage in thenbsp;development of phelloderm.

A much older section is shown in part in fig. 210, E. The secondary xylem cylinder, St, is shown in the lower part of thenbsp;section ; beyond this is a band of secondary tissue which reachesnbsp;in some places a breadth of 6 cm. The greater part of thisnbsp;tissue consists of phelloderm of very uniform structure made upnbsp;of radial series of cells ; this is interrupted in most parts of thenbsp;section by a gap crowded with intruded rootlets (a portion ofnbsp;this is enlarged in fig. 210, D). Beyond this gap the secondarynbsp;tissue consists of radial series of cells characterised by thenbsp;considerable tangential elongation of many of the elements,nbsp;precisely like the tissue figured by Williamson. In all probability the gap represents a line of weakness due to thenbsp;phellogen, and if this is the case it is clear that in an oldnbsp;Stigmaria the phelloderm exceeded in amount the tissue formednbsp;external to the phellogen. The secondary tissue on the innernbsp;side of the phellogen is characterised by numerous irregularnbsp;concentric lines superfically resembling rings of growth in thenbsp;wood of a Conifer: these are, however, not the result of anynbsp;periodic change in external conditions, but are apparently duenbsp;to crushing of the tissue and are possibly, to some extent, thenbsp;result of the presence of secretory strands like those in the

phelloderm of Lepidodendron. The surface of this older rhizome retains patches of primary tissue, and an occasional rootlet, as atnbsp;r, fig. 210, E, is seen in connexion with the cortex ; the cortexnbsp;has been vertically fissured as the result of secondary growthnbsp;and presents an appearance like that shown in Lepidodendronnbsp;Wilnschianum and L. Veltheimianum (figs. 181, A, and 186, A).

The form in which a Stigmarian rootlet is usually preserved is shown in fig. 210, D; the single vascular bundle strand withnbsp;its endarch protoxylem (fig. 210, B, px) is enclosed by a ring ofnbsp;inner cortical parenchyma (fig. 210, F c^); the cells in immediatenbsp;contact with the xylem having usually disappeared. Beyondnbsp;the middle cortical space a second cylinder of parenchymanbsp;represents the outer cortex (F, (f) in which a layer of dark-walled cells (6, fig. 210, F) may be compared with the hypo-dermal band in the main Stigmarian axis (G, b). Thesenbsp;Stigmarian rootlets, usually less than 1 cm. in diameter, are thenbsp;commonest objects in sections of the calcareous nodules fromnbsp;English coal-seams. A good example of their abundance isnbsp;shown in fig. 210, D and E ; here they have invaded the spacenbsp;formed by the splitting of the secondary cortical tissues alongnbsp;the line of the phellogen and a few are seen here and there in thenbsp;deeper layers of the phelloderm (s, fig. 210, E). Not infrequentlynbsp;the close contact of these ubiquitous rootlets with the tissues ofnbsp;the plant which they have invaded leads to confusion betweennbsp;invader and invaded. Partially decayed tissues lying, probably,nbsp;Under water were penetrated by Stigmarian rootlets in exactlynbsp;the same way as the roots of recent plants bore through vegetable substances which happen to be in their path. The rootletnbsp;bundles are in the first instance composed of the primary tracheaenbsp;which line the inner edge of the secondary xylem ; these receivenbsp;additions from the meristematic zone, and thus, when seen innbsp;the cortex outside the stelar region, are found to consist in partnbsp;of primary and in part of a fan-shaped group of secondarynbsp;tracheae. On the other hand, the monarch bundle as itnbsp;appears in a free rootlet is xisually composed entirely ofnbsp;primary elements (fig. 210, A^�C, F). It has been shown bynbsp;Weiss^^ that in the Stigmarian rhizome of what is probablynbsp;1 Weiss, F. E. (02).

Lepidodendron fuliginosum, the rootlet bundle is accompanied by a parichnos strand, but this has not been detected in thenbsp;ordinary Stigmaria ficoides. When free from the parent axis anbsp;rootlet usually consists of an outer cylinder of cortex enclosingnbsp;a broad space in which remnants of lacunar tissue are sometimes seen. The relation of the external features of a well-preserved Stigmarian rootlet-scar to the internal structure of anbsp;petrified rootlet is very clearly seen on comparing such sectionsnbsp;as those represented in fig. 210, D, with the form of the scar onnbsp;a Stigmarian cast. A specimen figured by Hooker^ in 1848nbsp;affords a good illustration of the structure of a rootlet-base asnbsp;seen in an imusually complete cast; this correlation of anatomical and surface features is clearly described also bynbsp;Williamson^ and by Solms-Laubach^ It is probable thatnbsp;even during life the rootlets were hollow for a part at leastnbsp;of their length as are the roots of Isoetes (fig. 133, G).

An interesting discovery was made a few years ago which confirmed a statement by Renault which Williamson wasnbsp;unable to accept, namely that the xylem bundle of a rootletnbsp;occasionally gives off a delicate tracheal strand at right anglesnbsp;to the long axis of a rootlet. In some rootlets Weiss^ foiindnbsp;obliquely running delicate strands of xylem, surrounded by anbsp;layer of parenchymatous tissue, in the space between thenbsp;vascular bundle and the outer cortical cylinder. It is clearnbsp;that a few spiral tracheids are occasionally given off from thenbsp;protoxylem of a rootlet bundle: these follow an oblique coursenbsp;to the outer cortex, where in some cases they have been tracednbsp;into connexion with short and spirally marked cells resemblingnbsp;transfusion tracheae (fig. 210, A). This arrangement may servenbsp;as a means of facilitating the passage of water absorbed by thenbsp;superficial cells into the xylem strand. It should be noticednbsp;that, like roots of recent water-plants, the rootlets of Stigmarianbsp;had no root-hairs. Fig. 210, F, shows a transverse section ofnbsp;part of a rootlet in which the outer cortical cylinder, c^, isnbsp;connected, as in the roots of Isoetes, with the sheath surround-

� Hooker (48^ Pis. i. ii. The sections of Stigmaria figured by Hooker are in the British Museum (V. 8754).

CHAPTEE XVIII.

Although in many respects the genus Bothrodendron agrees very closely in habit and in its anatomical features withnbsp;Lepidodendron, there are reasons for referring it to a distinctnbsp;family of Palaeozoic Lycopods. As the following descriptionnbsp;shows, the external features do not differ in any essential pointsnbsp;from those of certain types of the genus Sigillaria, particularlynbsp;such a species as S. rimosa, Gold.^, which has recently beennbsp;refigured and described by Nathorst^ from Goldenberg�s type-specimen in the Stockholm Museum. The small size ofnbsp;the leaf-scars is, however, a characteristic feature of Bothrodendron (fig. 212, F); but a more important point is thenbsp;fact that in a recently described� English example of a cone ofnbsp;Bothrodendron (fig. 216), the sporangia are very like those ofnbsp;recent Lycopods, and differ from the radially elongated sporangia of Lepidostrobus. On the other hand, a French conenbsp;described by Zeiller'* as Lepidostrobus Olryi, which is probal^nbsp;a strobilus of Bothrodendron, has the radially elongated typenbsp;of sporangium (fig. 212, E). The comparative abundance ofnbsp;Bothrodendron in Lower Carboniferous and Devonian rocksnbsp;points to the greater antiquity of this member of the Lycopo-diales as compared with Lepidodendron.

Measures, characterised by two opposite rows o pressions like those shown in fig. 211 an , in onnbsp;specimens, by � a considerable number of minn e o s, anbsp;in a quincuncial manner.� The minute dots werenbsp;nbsp;nbsp;nbsp;,

as leaf-scars and the cup-like cavities were descri e as pr connected with the occurrence of large cones. n �nbsp;evidence this Palaeozoic plant, which was named Bothrodmd^onnbsp;punctatum, was considered by these authors as pronbsp;member of the Coniferales. The large stem rom enbsp;Measures in the neighbourhood of Mens, Belgium, s ownbsp;fig. 211, affords a good illustration of Bothrodendron in a pa �nbsp;nbsp;nbsp;nbsp;y

decorticated condition, exhibiting a row of depressions sim^ those on the Ulodendron form of Lepidodendrmi e t eimiannbsp;(fig. 157), but distinguished by the eccentricnbsp;nbsp;nbsp;nbsp;.

scar at the bottom of each cup-shaped cavity; in t e eg specimen,which is partially decorticated and shows the eanbsp;as small dots, the depressions have a diameter of 9 cm. isnbsp;believed by some authors that these Ulodendron s oo s onbsp;Bothrodendron and Lepidodendron owe their c aracnbsp;appearance to the pressure of large cones, but, as I have a r^ ynbsp;stated, there are reasons for preferring the view t at esenbsp;crater-like hollows are the scars of deciduous branc es.nbsp;knowledge of the strobili borne by Bothrodendron stems inbsp;still meagre, but we have no reason to assume the existence ^nbsp;any cones large enough to produce by the pressure onbsp;bases such depressions as those shown in fig. 211. In (me speciesnbsp;at least the strobili were borne terminally on slen er s oonbsp;(fig. 213). The Ulodendron condition has so far been recognisenbsp;in one species only, B. punctatwn.nbsp;nbsp;nbsp;nbsp;o � i u rl

In his catalogue of Palaeozoic plants, Kidstiin me u Bothrodendron punctatum as a synonym of Sigillaria discop lornbsp;Konig, a mistake which he afterwards rectified . thenbsp;name Bothrodendron was generally ignored by authors innbsp;belief that the specimens described by Bindley and Hutton

were not generically distinct from the fossils origina y gu by Rhode as Vlodendron. It was Prof. ZeiWernbsp;nbsp;nbsp;nbsp;.

demonstrated that the English authors were justi � � . choice of a new designation for stems with largenbsp;nbsp;nbsp;nbsp;^

in association with minute leaf-scars. In 1859 proposed a new family name Cyclostigmaceae for some ppnbsp;Devonian plants from County Kilkenny, Ireland. he escrinbsp;three species of his new genus Cyclostigma, yc os igmnbsp;kiltorkense C. minutum, and C. Griffithsi', these are now genCT ynbsp;recognised as a single species of Bothrodendron, t oug ,nbsp;Nathorst suggests, the Irish plant should perhaps be separat.nbsp;as a sub-genus Bothrodendron {Cyclostigma) by reason o cer ainbsp;minor differences which distinguish it from other specienbsp;the genus.nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp; u Pv

Boulay in 1876 for stems characterised by a finey wrin bark and small spirally disposed leaf-scars. A short escnpnbsp;of this type, which occurs in the Middle and Lowernbsp;Measures, may serve to illustrate the external eaturnbsp;the commonest British example of the genus.

Prance, p. 39, PL ill. fig. 1. nbsp;nbsp;nbsp;� � cnbsp;nbsp;nbsp;nbsp;France

1888. nbsp;nbsp;nbsp;Lepidostrobm Olryi, Zeiller, Flor. Valenciennes, p. 50^, ri.

In habit a plant of Bothrodendron recalls Lepidodendron and recent species of Lycopodium; the slender dichotomous y

fit

A. nbsp;nbsp;nbsp;Bothrodendron minutifolium, var. rotundata Weiss. After Weissnbsp;and Sterzel.

been mistaken for shoots of Lycopodium, and ragi branches might well be identified as impressions onbsp;The leaf-scars on the smaller shoots occur on elongated cusmnbsp;(fig. 212, C, D) with a transversely wrinkled surface, onnbsp;older branches the leaf-scars are separated by fair y argenbsp;of bark characterised by sinuous transverse grooves annbsp;ridges bearing numerous small pits, as shown on an en gnbsp;scale in fig. 212, A. The original surface-features are s ownbsp;on the left of the drawing, and a slightly deeper leve innbsp;cortex is represented on the right-hand side. Tim a senenbsp;leaf-enshions on the older shoots is probably t e resunbsp;secondary thickening, which also alters the size an s pnbsp;of the leaf-scars. Each scar has three pits on its sur a ,nbsp;in Lepidodendron', a central leaf-trace scar and latera painbsp;scars. The circular pit above the leaf-scars, whic occursnbsp;most species, marks the position of the ligule. The renbsp;the short leaves, 5 mm. long, to the leaf-cushions is snbsp;fig. 212, D. The absence of leaves, except in impressionsnbsp;slender twigs, may be interpreted as an indication tianbsp;were shed at an early stage and did not persist many yenbsp;The leatoMhions of the emaller ehoote ofnbsp;minutifolium closely resemble those figured bynbsp;Devonian plant, Lepidodendron Losseni^.

One of the few examples so far discovered of a o dendron cone is shown in fig. 213 ; this specimen, at ^nbsp;long, was found by Mr Hemingway in the Middle Coal- easu�nbsp;of Yorkshire and described by Dr Kidston. hsumerousnbsp;phylls are attached at right angles to the axis, the sur ace onbsp;which is protected by their upturned distal portions, the arrangenbsp;ment of the parts appears to be the same as in Lepidostio usnbsp;A specimen figured by Zeiller as Lepidostrobus Obyi,nbsp;Kidston is probably correct in identifying with Bothro en i onbsp;minutifolium, shows that each sporophyll carries a horizonta ynbsp;elongated sporangium (fig. 212, E).

Fig. 213. Bothrodendron minutifolium Cone. From a specimen in Dr Kidston�s Collection. (Slightly reduced. Kidston (02) PI. lix.)

in British Coal-Measures, has been described by several authors as Ulodendron on account of the occurrence of large depressions,nbsp;like those shown in fig. 211, on certain branches of the plant.

1 Lindley and Hutton (35) A. Pis. 80, 81. For synonymy, see Kidston (93) p. 344.

At the suggestion of Dr Kidston, Prof. Zeiller�^ figured English specimen of this species, presented to the Parisnbsp;Museum by Mr Hutton, in which the leaf-scars are preservednbsp;on the bark of a stem with Ulodendron scars. The surface ofnbsp;the bark is characterised by numerous small pits and discontinuous vertical lines in contrast to the transverse lines ofnbsp;B. minutifolium (cf. fig. 212, A and F). The leaf-scars on thenbsp;Smaller shoots may have a diameter of only 0'3�0'5 mm., whilenbsp;on the larger branches they reach a breadth of 1 mm. Thenbsp;ligule-pit may be in contact with the upper edge (fig. 212, F)nbsp;of the leaf-scar or separated from it by a short distance.

1859. Cyclostigma kiltorkense, Haughton, Journ. R- Soo. Dublin, Vol. II. p. 418, Pis. xiv.-xvii.

C. Griffithsi, Haughton, Journ. R. Soc. Dublin, Vol. ii. p. 418, Pis, XIV.-XVII.

1870. Zepidodendron Veltheimianwm, Heer {ex parte), K. Svensk. Vet. Akad. Hand!. Vol. ix. Pi. ix. figs. 2-4.

1889. Bothrodendron kiltorkense, Kidston, Ann. Mag. In at. Hist, [vi.], Vol. IV. p. 66.

1894. Bothrodendron kiltorkense, Nathorst, K. Svensk, Vet. Akad. Hand!. Vol. xxvi. No, 4, p. 65, Pis, xiv, xv.

1902. Bothrodendron {Cyclostigma) kiltorkense, ibid. Vol. xxxvi. No. 3, p. 31, Pis. X.�XIV.

The specimens from the Upper Devonian rocks of Co. Kilkenny on which Haughton founded this and two othernbsp;species may be regarded as representing one specific type. Henbsp;described the circular leaf-scars as arranged in alternatingnbsp;whorls. In habit the Irish species agrees with Bothrodendronnbsp;rninutifolium, but the leaf-scars are more elliptical (fig. 212, H)

and the ligule-pit is usually absent. The leaf-scar shown in tig. H is 1'2 ram. broad and 1'4 mm. in height. The largenbsp;collection obtained during the visit of a Swedish expedition tonbsp;Bear Island in 1898 under the leadership of Dr Nathorst hasnbsp;materially increased our knowledge of this ancient type. Thenbsp;form of the leaf-scars varies according to the age of the branchnbsp;and their disposition is far from constant even on the samenbsp;specimen; in some cases the scars are in fairly regularnbsp;whorls (tig. 212, G; an Irish specimen) while in others they arenbsp;in regular spirals. This irregularity of arrangement, which isnbsp;well illustrated by Nathorst�s figures of Bear Island and Irishnbsp;specimens, finds its counterpart, though in a less marked form,nbsp;in recent species of Lycopodium, e.g. Z. Selago. Partiallynbsp;decorticated stems may present a superficial resemblance tonbsp;Calamites, the fissured bark simulating the ribs of a Calamiteannbsp;cast. Such stems, as Nathorst has pointed out, were mistakennbsp;by Heer for Calamites radiatus. The smaller branches arenbsp;characterised by a smooth surface, and older shoots resemblenbsp;Bothrodendron minutifolium in the presence of fine verticalnbsp;lines. The preservation of only one pit on the leaf-scars ofnbsp;many examples led authors to conclude that the species isnbsp;peculiar in this respect, but Nathorst has shown that in morenbsp;perfectly preserved specimens each leaf-scar bears three smallnbsp;dots. A specimen from Ireland in the British Museum^ illustrates the dichotomous branching and the longitudinal wrinklingnbsp;of the bark; the leaf-scars are 2 mm. broad and 2'5 mm. deep.

Nathorst^ has described some examples in which the leaf-scars occur on the lower instead of on the upper end of the leaf-cushions; these and other specimens with obscure surface-features he suggests may be underground axes, comparablenbsp;in habit with Stigmaria though not identical as regardsnbsp;details. It is pointed out that the absence or scarcity ofnbsp;Stigmaria in the Bear Island beds renders it unlikely thatnbsp;Bothrodendron bore typical Stigmaria branches. F. E. Weiss'*nbsp;has recently described root-bearing organs possessing primarynbsp;xylem identical with that of Bothrodendron mundum; whilenbsp;closely resembling Stigmaria ficoides in certain anatomicalnbsp;1 No. 52524.nbsp;nbsp;nbsp;nbsp;2 Nathorst (02) PI. x. figs. 4, 5.nbsp;nbsp;nbsp;nbsp;^ Weiss, F, B. (08).

characters, they clearly represent a distinct type. This discovery of a Stigmaria-like axis almost certainly belonging to Bothrodendron is consistent with Nathorst�s views on some ofnbsp;the Bothrodendron impressions from Bear Island.

Information as to the cones of this species is restricted to a description by Schimper' of a specimen in the Dublin Museumnbsp;as Lepidostrobus Bailyanus] this has sporophylls with a sub-triangular base bearing several megaspores and terminatingnbsp;distally in a slender lamina 12 cm. in length.

An example of a Bothrodendron with more prominent leaf-cushions than those already mentioned is afforded by a species from Bear Island described by Heer^ as Lepidodendronnbsp;W�kianum and afterwards referred by Nathorst^ to Bothrodendron. The same type is recorded also by Schmalhausen^nbsp;from Lower Carboniferous or Devonian strata of Siberia.nbsp;Certain Scotch specimens from the Calciferous Sandstone, whichnbsp;Kidston� referred to Heer�s species, are regarded by Nathorst and,nbsp;in part at least, by Weiss� and Sterzelas representing a distinctnbsp;species which these authors designate Bothrodendron Kidstoni'^.

Without attempting the hopeless task of discriminating between the various Carboniferous and Devonian specimensnbsp;described under the names Cyclostigma or Bothrodendron,nbsp;reference may be made to the following records as illustratingnbsp;the wide distribution of the genus. Schmalhausen� recordsnbsp;Cyclostigma kiltorkense from Siberian rocks assigned to thenbsp;Ursa stage (Devonian or Lower Carboniferous). The fossilnbsp;described by Dawson� from the Devonian of Gaspe as Cyclostigma densifolium probably represents a badly preservednbsp;example of Bothrodendron: Weiss�s species Cyclostigma hercy-nium'^� from Lower Devonian rocks of the Hartz district may benbsp;identical with Bothrodendron kiltorkense. The supposed identitynbsp;of the latter species with Dechenia Roemeriana Goepp., as de-

* Sohimper (70) A. p. 71. nbsp;nbsp;nbsp;^ Heer (71) P]. vi. fig. 11; PI. ix. fig. 1.

scribed by Potoni�^ appears to require confirmation�, but if this author is correct the connexion demonstrates the continuity ofnbsp;Bothrodendron shoots and Stigmaria-like subterranean organs.nbsp;The specimens described from South Africa, from strata whichnbsp;may be correlated with the Upper or possibly with the Lowernbsp;Carboniferous series of Europe, as Bothrodendron Leslei^ in allnbsp;probability represents a species closely allied to the Irish andnbsp;Bear Island type. Bothrodendron Leslei named after Mr Leslienbsp;whose discoveries in the Carboniferous Sandstone of Vereenig-

H oi

f oT�

^ o

ing (Transvaal) have added considerably to our knowledge of the South African Palaeozoic types, is represented by imperfectly preserved casts characterised by more or less circularnbsp;scars displaying the same irregularity of arrangement asnbsp;in Bothrodendron kiltorkense. The leaf-scars appear to

have only one small pit, but this may not be an original feature. The identification of this plant as Bothrodendronnbsp;receives support from the discovery of rather more satisfactorynbsp;specimens at Witteberg sent to me for examination bynbsp;Dr Schwarz^ These fossils bear a striking resemblance tonbsp;B. Mltorkense. Gyclostigma australe^ Feist, described fromnbsp;the Lower Carboniferous rocks of New South Wales, thoughnbsp;too imperfectly preserved to refer with confidence to B. kiltor-kense, is no doubt a closely allied type.

Reference was made in Volume I. (p. 133) to the so-called paper coal of Carboniferous age from Central Russia, which consists of masses of thin strips of cuticle of Bothrodendron stems.nbsp;The figures published by Zeiller* show that the plant possessednbsp;RB epidermis consisting of polygonal cells interrupted by spirallynbsp;disposed gaps marking the position of leaves; the gaps measurenbsp;fi'5�1'5 mm. in breadth and agree, therefore, with the size ofnbsp;^ Seward (09).

the leaf-scars of the smaller forms of Bothrodendron. The specimens from the Eussian mines were first figured bynbsp;Trautschold and Auerbach ^ as Lepidodendron tenerrimum andnbsp;afterwards referred by Zeiller to Bothrodendron punctatumKnbsp;Nathorst�, however, states that an examination of the Eussiannbsp;material leads him to retain the name originally proposed; henbsp;records the same type from Upper Devonian rocks of Spitz-bergen. The chief interest of these Eussian specimens is theirnbsp;manner of preservation, which Eenault has described as thenbsp;result of bacterial action ; he claims to have recognised thenbsp;actual bacteria associated with the cuticular membranest

In 1889 Williamson* described several specimens of petrified shoots from the Coal-Measures of Halifax which he namednbsp;Lepidodendron mundwn: these are now known to be branchesnbsp;of a Bothrodendron. The discovery was made by Mr Lomax�nbsp;who found specimens showing the external characters of Bothrodendron and the anatomical characters of Lepidodendronnbsp;mundum. In some of the smaller twigs, the stele consists of anbsp;solid core of xylem with external protoxylem; but in thenbsp;majority of specimens the centre of the xylem is replaced bynbsp;parenchymatous tissue, either as a small axial strand or, as innbsp;the specimen shown in fig. 215, D, a wide pith, the elements ofnbsp;which are arranged in regular vertical series. A diagrammaticnbsp;section of a small axis is represented in fig. 215, A: this branch,nbsp;2 mm. in diameter, is composed of a broad outer cortex consisting exclusively of primary tissue the outer cells of whichnbsp;are smaller and have thicker walls than the more internalnbsp;elements. The leaf-traces. It, are accompanied by a strand ofnbsp;delicate tissue, the parichnos. The stele is almost solid; thenbsp;tissues in contact with the xylem have not been preserved butnbsp;the inner cortex is represented by a few layers of small paren-

� I am indebted to Mr Lomax for photographs of his specimens. For former references to Mr Lomax�s discovery, see Kidston (05); Weiss, F. E. (08); Scott,nbsp;D. H. (08) p. 200.

chymatous cells, The larger section shown in fig. 215, D, was cut from a specimen from Dulesgate of which the smoothnbsp;surface exhibits the characteristic leaf-scars of Bothrodendron.nbsp;The section measures 3 cm. in its longest diameter and thenbsp;stele has a breadth of 3 mm. The outer cortex has a smoothnbsp;surface and is composed of rather thick-walled cells succeedednbsp;by a zone of secondary elements. The middle cortex hasnbsp;disappeared and the space is partially occupied by Stigmariannbsp;rootlets, s, and crushed patches of cortical tissue. The positionnbsp;of a leaf-scar is seen at a; this is more clearly shown in thenbsp;enlarged drawing fig. E.

In his account of Lepidodendroti mundum, Williamson^ described a section in which the primary wood is surrounded by a considerable thickness of secondary xylem; a diagram ofnbsp;this is shown in fig. 215, C. An examination of the section lednbsp;me to compare the structure of the outer cortical cells, characterised by radial rows of tangentially elongated elements, withnbsp;the outer cortex of Stigmaria. It has recently been shown bynbsp;Weiss ^ that this and other similar sections present severalnbsp;points of agreement with Stigmaria, particularly with Stigmarianbsp;Brardi as described by Renault. At s in fig. 215, C, a vascularnbsp;strand is seen passing through the outer cortex; this is almostnbsp;certainly the bundle of a rootlet: in the sections described bynbsp;Weiss rootlets are shown in a similar position. The chiefnbsp;anatomical features of the Stigmaria-like organs of Bothrodendron are:�the considerable development of secondary xylem,nbsp;the structure of the outer cortex, which is practically identicalnbsp;with that of Stigmaria ficoides, and the association of groups ofnbsp;short transfusion tracheids with the bundles of the rootlets. Itnbsp;is very probable that the absence of secondary xylem in thenbsp;vegetative shoots of Bothrodendron is merely an accident andnbsp;not a real distinction between the aerial and subterraneannbsp;branches of the plant; a supposition rendered probable by thenbsp;occurrence of secondary xylem in the axis of the cone describednbsp;by Watson. As Weiss points out, there are certain differencesnbsp;between the true Stigmaria and the corresponding organ ofnbsp;Bothrodendron; the secondary xylem in Bothrodendron is notnbsp;^ Williamson (89) A.nbsp;nbsp;nbsp;nbsp;^ Weiss, F. E. (08).

broken up by broad medullary rays as in the common Stigmaria, and in Bothrodendron the occurrence of a ring of primary xylemnbsp;is another peculiarity.

In the vegetative shoots of Bothrodendron mundum the stele differs from those of Lepidodendron in the narrowernbsp;primary xylem ring and in the large size of the metaxylemnbsp;tracheae; from Lepidodendron Harcourtii and L. fuliginosumnbsp;the xylem is distinguished by its smoother outer face whichnbsp;consists of numerous narrow xylem elements.

The long and narrow cones referred to Bothrodendron minutifolium from English and French Coal-Measures are knownnbsp;only as impressions and it is not possible to say whether theynbsp;were heterosporous or homosporous; the drawing given bynbsp;Zeiller (Fig. 212, E) shows that the sporangia were of the same

form as those in Lepidostrobus, but we have no more exact information as to their morphology. A recently publishednbsp;description of a petrified strobilus by Mr Watson affords anbsp;welcome addition to our knowledge. There is little doubt thatnbsp;this cone was borne by a species of Bothrodendron; the evidencenbsp;for this conclusion is supplied by the agreement of the anatomical characters of the stele with that of the vegetative shootsnbsp;originally described by Williamson as Lepidodendron mundumnbsp;and by the constant association of the cones and vegetativenbsp;shoots. In 1880 Williamson described a crushed cone containingnbsp;both megaspores and microspores which he spoke of as � a diminutive organism, reminding us more of the dwarfed fruits ofnbsp;many living Selaginellas than of the large Lepidostrobi^.��nbsp;Watson�s specimens enable us to give a more complete accountnbsp;of this type. The axis of the strobilus bears short sporophyllsnbsp;bent upwards into a distal limb with a conspicuous ligule in anbsp;deep pit beyond the shortly stalked sporangium. The length ofnbsp;the strobilus is estimated at 10 mm.; the stele is of the samenbsp;type as that of Bothrodendron mundum, but it differs from thenbsp;specimens of the vegetative shoots so far found in having somenbsp;secondary xylem. As shown in the sketch reproduced in fig. 216nbsp;each sporophyll is characterised by two tangentially placednbsp;grooves, g, on the lower fiice, and by numerous transfusionnbsp;tracheids, tr, above the vascular bundle, vb, immediately belownbsp;the ligule, 1. Megasporangia and microsporangia occur on thenbsp;same cone, the megasporangia being on the lower sporophyllsnbsp;and containing a single tetrad of megaspores. Fig. 219, E, showsnbsp;a radial longitudinal section of a microsporophyll bearing anbsp;sporangium on the adaxial side of the ligule, I, below which isnbsp;the single vascular bundle and a group of short tracheids at t.nbsp;The sporangia closely resemble those of species of Selaginellanbsp;and Lycopodium and, as pointed out by Watson^, they alsonbsp;recall the sporangia of the Palaeozoic genus Spencerites. Both-rostrobus is distinguished from Spencerites by the presence ofnbsp;a ligule, by the structure of the axis, and by the different formnbsp;of the sporophylls. The occurrence of four spores only in thenbsp;megasporangia is another character in which the extinct typenbsp;1 Williamson (80) A. p. 500, PI. xv. 8.nbsp;nbsp;nbsp;nbsp;^ Watson (08^) p. 12.

resembles recent Lycopods. It is impossible to decide whether Watson�s cone represents a more or a less primitive type thannbsp;Lepidostrohus: if we accept Professor Bower�s views in regardnbsp;to the evolution of vegetative organs by the sterilisation ofnbsp;sporogenous tissue, we should probably place Lepidostrohus lowernbsp;in the series than Bothrostrobus \ but the greater resemblancenbsp;between the fertile and vegetative shoots of Bothrodendroii, asnbsp;compared with the more pronounced difference in the case ofnbsp;Lepidodendron, may be regarded as an argument in favour ofnbsp;recognising Bothrodendron as the more primitive type.

Another possible example of a Bothrodendron cone has been described by Nathorst from Spitzbergen as Lepidostrohusnbsp;Zeilleri^; this appears to consist of an axis bearing spirallynbsp;disposed sporangia without any indication of sporophylls. Thisnbsp;strobilus may belong to Bothrodendron, tenerrimum.

The name Pinakodendron^ was instituted by the late Prof Weiss for a type of stem closely resembling Bothrodendron butnbsp;differing in the presence of a fine reticulation on the outer barknbsp;and in the form of the leaf-scars. Weiss�s genus has been recognised by Kidston in Dumfriesshire but our knowledge of thenbsp;plant is as yet based solely on a few small specimens.

This generic name was instituted by White� for certain specimens of large stems originally described by Lesquereuxnbsp;from the Coal-Measures of North America as Lepidodendronnbsp;mammillatum and L. cyclostigma. The photograph reproducednbsp;in fig. 193, C, for which I am indebted to Dr Kidston representsnbsp;a specimen described by him from the Upper Coal-Measuresnbsp;of Somerset as Omphalophloios anglicus, and identified withnbsp;Lepidodendron anglicum of Sternberg.

^ Nathorst (94) A. p. 42, PI. xii. figs. 8�10. � White (98); (99) p. 218, Pis. lxv.�lxviii.

characterised by clearly defined rhomboidal areas or cushions (fig. 217, E) like those of Lepidodendron, except in the absence of anbsp;median keel, and similar to those on some forms of Sigillarianbsp;Brardi. A short distance above the centre of each cushion isnbsp;an oval or subcordate region bounded by a rim-like margin andnbsp;containing a small oval scar, presumably that of a vascular strand.nbsp;A triangular elevation which also shows a small pit (Fig. 217,nbsp;E, a) occurs below the oval area. The appearance of thenbsp;surface-features varies considerably on different parts of a singlenbsp;specimen. Fig. 217, D, represents one of the numerous figuresnbsp;published by White in his detailed account of the Americannbsp;material. Each cushion bears a widely open V-shaped ridge,nbsp;which is described as a leaf-scar; above this is an oval area (2'5

min. X 1�75 mm.), the surface of which is bounded by a narrow rim. Within the rim is a smaller concave oval region with anbsp;small pit near its upper end.

We cannot, in the absence of petrified material, arrive at any satisfactory conclusion as to the meaning of these surface-features. White considers that OmpJialophloios is probably anbsp;rhizome of one of the arborescent Lycopods, but whether ornbsp;not this is its true nature must be left for future discoveries.nbsp;The fact that the rootlet bundles of some Stigmarian axes arenbsp;accompanied by a parichnos strand, as Weiss has shown, maynbsp;prepare us for the discovery of surface-features on Stigmariaenbsp;not unlike those of Omphalophloios. (Fig. 193, C.)

A possible comparison may be suggested also with Sigillaria Brardi as figured by Germar (fig. 196, A) in which circularnbsp;scars, which may be the scars of rootlets, occur below the leaf-

base areas. It is not impossible that in the surface-features of Omphalophloios we have both leaf and rootlet scars represented.

The solid xylem core characteristic of the stele of some species of Palaeozoic Lycopodiales (e.g. Lepidodendron esnostensenbsp;and L. rhodumnense) may probably, as Tansley and Chick^ pointnbsp;out, be regarded as the lineal descendant of a primitive axialnbsp;strand of water-conducting elements. In the course of evolution the centre of the tracheal column became partially convertednbsp;into parenchymatous tissue, as in Lepidodendron vasculare.nbsp;The arrangement of the short cells in regular vertical series isnbsp;reminiscent of an early stage in the development of tracheae:nbsp;instead of forming tubular conducting elements the centralnbsp;part of the stelar meristem acquired the short-celled form;nbsp;some of the cells became lignified as isodiametric storage tracheaenbsp;while others persisted as thin-walled parenchyma.

The production of secondary xylem and an increase in the girth of the whole stem led to reduction in the amount of centri-petally developed conducting channels. Some of these assumed anbsp;new r�le and a shape in harmony with their functions. A laternbsp;stage is represented by a further encroachment of the centralnbsp;parenchyma on the cylinder of centripetal xylem, as seen innbsp;Lepidodendron Harcourtii and other species. The next stagenbsp;is afforded by ribless species of Sigillaria in which thenbsp;primary xylem is broken up into separate conducting strands.nbsp;As Kidston^ reminds us, it is in the geologically more recentnbsp;species of Sigillaria, such as 8. Brardi, which persist into thenbsp;Permian era, that this more extreme case of reduction occurs.nbsp;The older genus Lepidodendron seems to have retained tonbsp;the last the complete cylinder of primary xylem. In the stelenbsp;of Stigmaria, the rhizome of Sigillaria and of Lepidodendron,nbsp;reduction of the centripetal xylem has passed beyond the stagenbsp;represented by the broken cylinder of the ribless Sigillarias.nbsp;With the exception of the examples described by Kenault^ andnbsp;by Weissk Stigmaria is characterised by little or no centripetal

primary xylem. It is, however, noteivorthy that Eenault�s Stigmaria, in which centripetal xylem forms a prominentnbsp;feature, is attributed to Sigillaria Brardi, a species in whichnbsp;the vascular cylinder of the aerial stem illustrates a later andnbsp;not an earlier phase in the replacement of centripetal bynbsp;centrifugal wood.

It would seem, as Lady Isabel Browne^ says, that most Stigmarian axes had reached a more advanced stage innbsp;specialisation than is shown in the stelar structure of the aerialnbsp;shoots. The relatively greater and probably the more precociousnbsp;development of secondary xylem in Stigmaria than in Lepido-dendron or Sigillaria may have some significance in relationnbsp;to the smaller amount of � old wood^ � (in a phylogenetic sense)nbsp;in their steles.

As is pointed out in a later chapter, recent researches into the anatomy of extinct members of the Osmundaceae bynbsp;Kidston and Gwynne-Vaughan have brought to light a strikingnbsp;parallelism in evolutionary sequence between the Lepidodendreaenbsp;and the ancestors of Osmunda and Todea, the two survivingnbsp;genera of one of the most ancient families of ferns.

There can be little doubt as to a very close relationship between Sigillaria, Lepidodendron, and Bothrodendron. Sigillaria seems to have outlived Lepidodendron and Bothrodendron.nbsp;The two latter genera are recorded from Upper Devonian rocksnbsp;in several localities, Bothrodendron being particularly abundantnbsp;in the pre-Carboniferous floras of Bear Island and other parts ofnbsp;the world. A. remarkable stem described by Dr White ^ asnbsp;�^rchaeosigillaria primaeva from Upper Devonian shales ofnbsp;New York is spoken of by him as �one of the most highlynbsp;developed representatives of a fairly distinct archaic groupnbsp;foreshadowing the later genera Bothrodendron, Sigillaria,nbsp;Lepidodendron and Lepidophloios.� The type-specimen, whennbsp;first discovered, consisted of an apparently unbranched stemnbsp;reaching a length of 5 metres. From the swollen basal part

Seott (02) uses the terms old and new wood in discussing the evolutionary sequence in plant steles.

Stigmaria-like rootlets spread into the surrounding shale. At a higher level the fissured bark shows indistinctly definednbsp;leaf-cushions which pass gradually upwards into cushionsnbsp;and scars arranged in closer order on regular vertical ribs.nbsp;The surface-features in this region are practically those of anbsp;ribbed Sigillaria. Traced farther upwards the vertical ribsnbsp;die out and cushions of the Lepidodendroid form cover thenbsp;surface of the bark. The leaf-scars, with a supraposed ligularnbsp;pit and two vertically elongated parichnos-scars, are said to bearnbsp;a closer resemblance to those of Sigillaria and Bothrodendronnbsp;than to the leaf-areas of Lepidodendron. Nothing is known asnbsp;to the anatomy of this stem, nor have fertile shoots beennbsp;discovered. In the absence of more trustworthy evidence thannbsp;is available conclusions of a phylogenetic nature must benbsp;accepted at their true value. It is however legitimate tonbsp;describe Archaeosigillaria primaeva as one of the oldestnbsp;examples of a lycopodiaceous plant which shows well-preservednbsp;external features, and these are of exceptional interest asnbsp;indicating a combination of generic characters. This Devoniannbsp;type lends support to the view that Lepidodendron and Sigillarianbsp;are offshoots, differing from one another in comparativelynbsp;unimportant points, from a common ancestral type.

The generally accepted statement that arborescent Palaeozoic Lycopodiales bore their sporangia on specially modified leaves (sporophylls)grouped in cones which were usuallyproduced at thenbsp;tip of slender branches, has recently shared the fate of most rules.nbsp;Prof. Bower in his Origin of a Land Flora mentions a Belgiannbsp;specimen oi Pinakodendron musivumWeiss from the Westphaliannbsp;series (Middle Coal-Measures), to be described by Dr Kidston,nbsp;which bore its sporangia � associated with the leaves of certainnbsp;portions of the stem, without any cone-formation. The fertilenbsp;and sterile portions are distinguished only by the presence ornbsp;absence of sporangia�.�

Lepidodendron and Sigillaria can hardly be claimed as the direct ancestors of any existing type of Lycopodiales, but whilenbsp;exhibiting points of contact with Lycopodium, Selaginella, andnbsp;Psilotum they are perhaps more closely allied to Isoetes.

Lady Isabel Browne', who has recently published an excellent summary of the evidence on the relation of the Lepidodendreaenbsp;to Isoetes, concludes her examination of the arguments bynbsp;expressing the opinion that there is a strong probability of thenbsp;correctness of the view that Isoetes may be derived � from thenbsp;Lepidodendraceae in the widest sense of the word.� Thisnbsp;decision seems to me to accord best with the facts.

The further question as to the relation of these Palaeozoic, genera to plants higher in the scale must be reserved fornbsp;fuller consideration in another volume. An attempt will alsonbsp;be made to consider how far anatomical structure may be usednbsp;as a guide to the conditions under which Lepidodendron andnbsp;Sigillaria as well as other members of the Permo-Carboniferousnbsp;floras passed their lives. The secondary xylem of Lepidodendronnbsp;and Sigillaria affords a striking example of water-conductingnbsp;tissue of homogeneous structure comparable with the wood ofnbsp;Conifers rather than with that of Angiosperms. It was presumably formed, for the most part, under uniform climatic conditions : the absence of rings of growth points to uninterruptednbsp;supply to evergreen shoots exposed to no alternation of activitynbsp;and arrested growth. Attention has already been called to thenbsp;absence of any tissue corresponding to secondary phloem. Even innbsp;young shoots of Lepidodendron, no tissue has been found externalnbsp;to the meristematic zone agreeing in the form of its elements withnbsp;the channels through which the elaborated food is conveyed fromnbsp;the leaves of recent plants to the regions of cell-building. Thatnbsp;the � secretory zone � may have served this purpose, at least innbsp;young stems, is not improbable. On the other hand, it isnbsp;difScnlt to understand why older Lepidodendron stems shownbsp;no indication of additions to the secretory zone. If this tissuenbsp;served for the transport of proteids we should expect to findnbsp;provision made for its constant renewal pari passu with thenbsp;secondary growth of the xylem. The conclusion seems to menbsp;inevitable that the supply of building-material was otherwisenbsp;provided for than in recent vascular plants. The physiologicalnbsp;division of labour may have been less complete in the tissue-systems of the Palaeozoic Lycopods than in the more highlynbsp;' Browne (09) p. 37.

specialised organs of such an extinct genus as Lyginodendron or than in recent plants. Our knowledge of the anatomical structurenbsp;of many extinct types has already reached a stage when wenbsp;should take greater heed of the modus operandi of the complexnbsp;machinery revealed by a study of petrified stems. From thenbsp;known we proceed to interpret the unknown; but there is anbsp;danger of neglecting the possibilities of evolution during thenbsp;countless ages which separate the forests of the Coal period fromnbsp;those of the present era. We may easily allow preconceivednbsp;ideas to warp our judgment in attempting to distribute thenbsp;manifold activities which made up the life of a Lepidodendronnbsp;among the structural units of the plant-body.

CHAPTER XIX.

In 1877 Williamson^ published an account of some fossil seeds which he referred to Brongniart�s genus Cardiocar-pon^, a generic title for certain Gymnospermous seeds. Somenbsp;of these he identified, on the authority of the author of thenbsp;species, with Gardiocarpon anomalum Carruthers�. Severalnbsp;years later Wild and Lomax described a new type of strobilusnbsp;from the Lower Coal-Measures of Lancashire^. The resultnbsp;of this discovery and of the subsequent examination by Scottnbsp;of additional material, was to establish the fact that thenbsp;seeds described by Williamson and generally accepted asnbsp;Gymnospermous, are in reality sporangia belonging to anbsp;Lycopodiaceous cone. The seeds to which Carruthers gave thenbsp;name Gardiocarpon anomalum are, however, distinct from thosenbsp;described under the same name by Williamson and are those ofnbsp;a true Gymnosperm. For this seed-bearing strobilus Scott�nbsp;instituted the generic name Lepidocarpon, which he thusnbsp;defined: � Strobili, with the characters of Lepidostrohus, butnbsp;each megasporangium was inclosed, when mature, in an integument, growing up from the superior face of the sporophyll-pedicel. Integument, together with the lamina of the sporo-

phyll, completely enveloping the megasporangium, or nucellus, leaving only an elongated, slit-like micropyle above. A singlenbsp;functional megaspore or embryo-sac developed in each megasporangium, occupying almost the whole of its cavity. Megasporenbsp;ultimately filled by the prothallus or endosperm. Sporophyll,nbsp;together with the integumented megasporangium and itsnbsp;contents, detached entire from the axis of the strobilus, thenbsp;whole forming a closed, seed-like, reproductive body. Seed-likenbsp;organ horizontally elongated, in the direction of the sporophyll-pedicel, to which the micropylar crevice is parallel.�

An immature cone of L. Lomaxi is practically identical with a Lepidostrobus; its sporangia are naked and only acquire theirnbsp;integuments at a later stage. A mature strobilus has a diameternbsp;of at least 3 cm. and is about 4 cm. in length. As in typicalnbsp;Lepidostrobi, the axis bears spirally disposed sporophylls, andnbsp;each sporophyll has a long narrow pedicel approximately atnbsp;right angles to the cone axis with its distal end expanded intonbsp;a broad and thick lamina (fig. 218, B).

At the distal end the pedicel has a thin marginal wing (fig. 218, C, right-hand half) continuous with the upturnednbsp;protective lamina. To the upper face of each sporophyll isnbsp;attached along the whole length as far as the ligule, a singlenbsp;large sporangium; on each side of the base of the sporangiumnbsp;the sporophyll forms a supporting cushiop. The relation ofnbsp;the sporangium to the ligule, I, is shown in fig. 218, B, and innbsp;the tangential section, C, which illustrates the triangular formnbsp;of the sporangium near its distal end.

In mature cones, the sporangia assumed the form of seeds, the change being due to the growth of an investing integumentnbsp;from the upper face of the sporophylls on each side of thenbsp;sporangia. Fig. 218, A, illustrates the form of a sporangiumnbsp;as shown in tangential sections; the vascular bundle is seennbsp;below the base of the sporangium and the gaps right and left ofnbsp;it probably mark the position of parichnos strands. On eachnbsp;side of the sporangium, b, a fairly thick wall of tissue has grown

up from the sporophyll, forming an integument which overtops the apical ridge of the sporangium, leaving a narrow micropyle innbsp;the form of a long crevice (m, fig. 218, B). At the proximal endnbsp;of the sporangium the integument forms an enclosing wall; atnbsp;the distal end it abuts on and is continuous with the upturnednbsp;end of the sporophyll. It is clearly established by Scott that the

tissue which invests the sporangia is not the upturned margins of the sporophyll, but a new formation fully entitled to thenbsp;designation integument. It is noteworthy that the integument isnbsp;not developed until a late stage in the ontogeny of the strobilus;nbsp;it is not formed until after the production of the prothallush

The diagrammatic sketch, fig. 218, B, shows the relation of the integument to the sporophyll and sporangium, the outline ofnbsp;the latter being indicated by a broken line. The columnar wallnbsp;of the sporangium (fig. 218, A, h) forms a closed beak withinnbsp;the micropylar crevice, and in the interior of the sporangialnbsp;cavity the slightly shrivelled membrane, a, represents the singlenbsp;megaspore; traces of the aborted sister-cells of the megaspore arenbsp;occasionally met with. Scott describes a specimen in which thenbsp;megaspore is filled with tissue agreeing in appearance with thenbsp;prothallus in a megaspore of Isoetes or Selaginella', no undoubted archegonia or female organs have been discovered, nornbsp;has any spore been found containing an embryo.

The axis of L. Lomasci has a medullated stele constructed on the same plan as that of some species of Lepidodendron andnbsp;Lepidostrobus; the vascular bundles supplying the sporophyllsnbsp;pass obliquely upwards and outwards from the stele, St, fig.nbsp;218, B, and bend slightly downward just before entering thenbsp;pedicel of a sporophyll.

Dr Scott has also described a strobilus containing microsporangia partially enclosed by a rudimentary integument. It is, however, of considerable interest to find a partial development in the case of a male flower of an integumentary outgrowth,nbsp;which it would seem could only be of real functional importancenbsp;in the female shoot.

It is important to notice that specimens of a second species of Lepidocarpon, L. Wildianum, are recorded from Lowernbsp;Carboniferous beds of Scotland, a fact which points to a considerable antiquity for this seed-bearing Lycopodiaceous type^.

The most important question to consider in regard to Lepidocarpon is�are we justified in applying to the integu-mented sporangia the term seed ? The megaspore was notnbsp;set free as it is in recent Pteridophytes, such as Azolla andnbsp;other genera with which Lepidocarpon may be compared; itnbsp;was on the other hand retained in the sporangium, as maynbsp;sometimes happen even in recent species of Selaginella (cf.nbsp;fig. 131, D). Moreover, the megaspore is characterised by a thin

enclosing membrane in contrast to the thick coat of a spore which is destined to be shed. The peculiar slit-like form ofnbsp;the micropyle is a distinguishing feature, but this may benbsp;readily explained as a convenient form in the case of a radiallynbsp;elongated sporangium. The absence of an embryo, though anbsp;distinguishing feature of Lepidocarpon, cannot be held to benbsp;a serious obstacle to the use of the term seed; in recent Cycadsnbsp;the embryo, as Scott points out, may not begin to developenbsp;until the seed has been shed. It is possible that the seeds ofnbsp;Lepidocarpon were not pollinated on the parent plant.

The lesson which this extinct type teaches, is that certain Lycopodiaceous plants of the Palaeozoic era had reached annbsp;important stage in the evolution of a seed. The morphologicalnbsp;essentials of true seeds had been acquired; but we do not knownbsp;the biological conditions under which pollination and fertilisation were effected. Another point of considerable interest isnbsp;the value of this discovery as an argument in favour of thenbsp;view that some Gymnosperms are derived from Lycopod ancestors. Leaving the general question until later, it maynbsp;at any rate be stated that in Lepidocarpon we have a demonstration of the fact that the Lycopodiales were not alwaysnbsp;distinguished from Gymnosperms by the absence of seeds.nbsp;There are certain features in Lepidocarpon shared by the seedsnbsp;of Araucarieae^ which may well mean something more thannbsp;mere parallel development in two distinct phyla of the plant-kingdom^.

In 1894 Prof Bertrand^ published an account of certain fragments of petrified leaves and twigs of a small herbaceousnbsp;Lycopodiaceous plant, under the name Miadesmia membranacea,nbsp;which he discovered in English material in association withnbsp;Lepidodendron Harcourtii. Subsequently Scott recognisednbsp;the megasporophylls of the same plant, and microsporophyllsnbsp;have also been discovered. The most complete account of

Miadesmia so far published we owe to Dr Benson h whose description is based on specimens from several sources.

The slender stem, characterised by unequal dichotomy, has a single protostele composed of scalariform tracheids with 3�6nbsp;peripheral protoxylem groups. A zone of delicate tissue sur-

...-m

A. nbsp;nbsp;nbsp;Eadial section of megasporophyll: s, sporangium; m, megaspore ; I, ligule. (From a drawing kindly lent by Mrs D. H.nbsp;Scott.)

E. nbsp;nbsp;nbsp;Eadial section of microsporophyll of Bothrodendron. (From anbsp;section in the Manchester Museum ; Hick Collection E. 406.)

rounds the xylem; this is described as phloem, but it is not clear whether the designation is based on histological charactersnbsp;or primarily on its position. The cortex consists of an innernbsp;lacunar tissue and an outer region limited by a small-cellednbsp;superficial layer sharply contrasted with the underlying layers

of larger cells. The stem of Miadesmia is not uncommon in sections of the Lancashire calcareous nodules, and may benbsp;recognised by the delicate crushed tissue of which it mainlynbsp;consists and by large hjrpodermal parenchyma. The spirallynbsp;disposed leaves bear a conspicuous and relatively large ligule,nbsp;3 mm. long, in a deep pit (fig. 219, B and C) roofed over by anbsp;few layers of tissue corresponding to the velum in Isoetesnbsp;(cf. fig. 133, E, v). The fairly thick central region of the laminanbsp;is expanded laterally into thin wings, which in the livingnbsp;state probably bore delicate hairs. These delicate leaves,nbsp;apparently without stomata, were attached to the stem at annbsp;acute angle, and Miss Benson suggests that their form ananbsp;arrangement may have enabled them to hold water by surface-tension. As seen in fig. 219, B, C, which represents part ofnbsp;a transverse section near the leaf-base, the ligule is a verynbsp;characteristic feature, and the size of the single vein is innbsp;keeping with the almost filmy nature of the lamina.

In addition to the sections in British collections, I have been enabled by the kindness of Prof Bertrand to see photomicrographs of the sections on which he founded the genus.nbsp;One of these sections, transverse to the stem and leaves,nbsp;illustrates in a striking manner the relatively large size ofnbsp;the leaves and ligules in proportion to the delicate axis of thenbsp;shoot.

The megasporangiate cone has an axis which agrees in its structure with that of the vegetative stem and bears severalnbsp;megasporophylls approximately at right-angles. As in thenbsp;foliage leaves, the ligule is prominent and large, and lies in anbsp;groove which contains also the megasporangium; both ligule, I,nbsp;and sporangium, s, as seen in the transverse section representednbsp;in fig. 219, D, are covered by an integument or velum whichnbsp;arises in the proximal part of the leaf and leaves a circularnbsp;micropylar opening at the beak-like apex of the sporangium.nbsp;The circular micropyle is surrounded by numerous hairs bornenbsp;on the integument and which presumably played the part of anbsp;feathery stigma. A single megaspore with a thin membrane,nbsp;TO, abuts on the fairly strong sporangial wall, s; in some casesnbsp;the sporangium and megaspore walls may be indistinguishable.

a feature suggesting comparison with seed-structure. Some megaspores have been found filled with a prothallus. Thenbsp;longitudinal section shown in fig. 219, A, illustrates the characteristic horizontal position of the megasporophyll, as also thenbsp;relation of the ligule, I, to the sporophyll with its single vascularnbsp;bundle, and to the hairy integument, which overarches bothnbsp;sporangium and ligule; the line m shows the position of thenbsp;megaspore-membrane, detached from the sporangial wall on thenbsp;upper side but in contact with it below. The microsporophyllnbsp;shown in 219, E, was originally referred to Miadesmia but hasnbsp;since been recognised by Watson' as that of a Bothrostrobus.

Miadesmia affords an example of a Palaeozoic plant comparable with Isoetes and Selaginella; it agrees also with Lepidocarpon in possessing true seeds, and with Watson�snbsp;Bothrodendron cone in the shape of the sporangia, which arenbsp;more like those of Selaginella than the radially elongatednbsp;sporangia of Lepidostrobus. Miadesmia agrees with Selaginella,nbsp;e.g. S. spinosa, in its stelar structure, in the form of thenbsp;sporangia, and in the presence of a ligule. It is distinguishednbsp;by having only one instead of four megaspores in a sporangium,nbsp;in the possession of an integument which formed a close investment to the spore and served as a stigma (comparable withnbsp;the stigma-like integument of the male flower of Welwitscliia),nbsp;and in the shedding of the megasporophylls, which have beennbsp;aptly compared with winged seeds.

On the ground of their general anatomical features Lepidocarpon and Miadesmia are clearly entitled to be included among extinct representatives of the Pteridophyta. These plants had,nbsp;however, crossed what it has been customary to regard as thenbsp;boundary between Pteridophytes and Phanerogams: theynbsp;possessed megasporangia with the attributes of seeds. It hasnbsp;been suggested by Lester Ward^ that Pteridophytic seed-bearingnbsp;plants shall be recognised as a distinct phylum for which henbsp;proposes the name Pteridospermaphyta, a designation implyingnbsp;exclusion from the Spermatophyta as usually understood. Fornbsp;seed-bearing Lycopodiaceous genera he suggests the namenbsp;Lepidospermae. As knowledge of the Palaeozoic seed-plantsnbsp;' Watson (08-) p. 12.nbsp;nbsp;nbsp;nbsp;^ Ward (04).

increases revision of existing classifications and group names will become necessary, but as yet we are hardly in a position tonbsp;draw up a satisfactory scheme of grouping; we know little ofnbsp;Lepidocarpon as a whole and it would be premature to commitnbsp;ourselves, even provisionally, to a classification which is basednbsp;on such meagre evidence as we possess. Moreover the value tonbsp;be attached to the seed-habit as a basis of classification cannbsp;hardly be estimated until fuller information is obtained.

CHAPTER XX.

This division of the Pteridophyta includes both the true ferns (Filicineae) and the less familiar water-ferns or Hydro-pterideae. The almost complete absence of satisfactory evidencenbsp;in regard to the geological history of the latter renders thisnbsp;group of secondary importance from a palaeobotanical standpoint, but, on the other hand, we possess a wealth of materialnbsp;bearing on the past history and relative antiquity of the truenbsp;ferns.

The study of extinct types has so far rendered no substantial help towards bridging the wide gap between the Filicales and the lower plants. As Mr Tansley^ says in hisnbsp;admirable lectures on The Evolution of the Filicinean Vascula7-System, � The biggest gap in the plant kingdom at the presentnbsp;time is undoubtedly that which separates the Pteridophytesnbsp;from the plants definitely below them in organisation, andnbsp;directly we try to step behind the ferns we tumble into thisnbsp;abyss.� Kesemblances long ago recognised between certainnbsp;ferns and the cycads, a section of the Gymnosperms, werenbsp;regarded by a few botanists as indications of blood-relationship, and the results of recent researches into the morphologicalnbsp;characters of extinct Palaeozoic types are generally held tonbsp;confirm these surmises. Prof Chodat^ of Geneva has recentlynbsp;challenged the validity of the arguments on which thenbsp;affinity of cycads and ferns has been accepted by the greatnbsp;majority of botanists. Whether or not his criticisms stand the

test of unbiassed examination, they must at least lead us to substitute a critical consideration of the facts for a merenbsp;repetition of conclusions which appeal to our imagination.nbsp;Despite Prof. Chodat�s warning, we may still quote with confidence a phrase used in another connexion�ferns �are linksnbsp;in a chain and branches on the tree of life, with their roots innbsp;a past inconceivably remoteh�

Transitional forms which are regarded as pointing to a common origin for ferns and cycads are known in abundance;nbsp;other types have also been discovered which lead some authorsnbsp;to go so far as to derive the whole of the seed-bearing plantsnbsp;from an ancestry the descendants of which are represented bynbsp;existing ferns. While hesitating to allow the ferns or fern-likenbsp;plants the peculiar position of universal ancestors, we mustnbsp;admit that there is no group of plants with a history of greaternbsp;importance from an evolutionary standpoint than that withnbsp;which we are now concerned.

There are, however, some difficulties to face in attempting to decipher the history of the Filicineae as recorded in thenbsp;earth�s crust. Few fossil plants are so familiar as the well-preserved carbonaceous impressions of compound leaves on thenbsp;shales of our Coal-Measures, which were referred by older authorsnbsp;to recent genera and species of ferns and accepted by laternbsp;writers as undoubted examples of Palaeozoic ferns. The commonnbsp;belief in the dominance of ferns in Palaeozoic floras is reflectednbsp;in the novelist�s description of the Carboniferous period, �whennbsp;the forms of plants were few and often of the fern kind^,� Wenbsp;now know that very many of these Carboniferous leaves belonged to plants differing widely in morphological charactersnbsp;from the modern genera to which they exhibit so deceptivenbsp;a resemblance. These pseudo-ferns, recently christened Pteri-dosperms or seed-bearing fern-like plants, are dealt with in anbsp;later chapter. The discovery of this extinct group has addednbsp;enormously to our knowledge of plant-evolution and at thenbsp;same time has rendered much more difficult the task of unravelling the past history of the true ferns. As soon as it wasnbsp;demonstrated that many familiar Palaeozoic �ferns� are not

ferns, some authors went far towards concluding that however close might be the agreement between fossil and recent leavesnbsp;suspicion of close relationship must be set aside. Like thenbsp;earlier writers who described fossils as lusus naturae fashionednbsp;by devilish agency to deceive too credulous man, the discoverynbsp;of seed-bearing plants with the foliage of ferns threatened tonbsp;disturb the mental balance of palaeobotanists. The fact is,nbsp;we cannot in some cases determine from leaf-form alone whethernbsp;or not a fossil is a true fern; we may, as Professor Bower^nbsp;suggests, regard all fern-like fossils as ferns until they arenbsp;proved to be Pteridosperms, or in a spirit of scientific scepticism, we may at once admit that many Palaeozoic fern-likenbsp;leaves must await further evidence before their true positionnbsp;can be determined. It is impossible, as Zeiller^ says, in thenbsp;present state of our knowledge to range fern-like Palaeozoicnbsp;plants in two groups, one referred to Filicineae and the othernbsp;to the Pteridosperms.

The following classification of the Filicales is based on that adopted by Prof Engler in the latest edition of hisnbsp;Syllabus^ and on the results of Bower�s^ excellent work on thenbsp;spore-bearing members of recent ferns.

The members of the Filicales are characterised by the same well-marked physiological division of labour in their vegetativenbsp;parts as are the Lycopods; the plant is the asexual generationnbsp;(sporophyte), while the sexual generation (gametophyte) isnbsp;small and inconspicuous, either an independent green prothallusnbsp;or a tissue more or less completely enclosed in the spore. Thenbsp;large size of the leaves, which in the young state are usuallynbsp;coiled like a crozier (fig. 220, A), is a striking characteristic ofnbsp;the ferns; they are megaphyllous in contrast to the microphyllynbsp;of the Lycopods.

In these homosporous and heterosporous plants the sporangia are developed from single epidermal cells.

(a) Eufilicineae. The sporangia bear spores of one kind only; the wall of a sporangium consists of one layer of cells. In

Fiq. 220. Young fronds of (A) Angiopteris evecta and (B) Cyeas revoluta. (Eeduced.)

the great majority of cases the sporangia are characterised by the possession of a conspicuous row of thick-walled brownnbsp;cells, the annulush which serves as a mechanism for dehiscencenbsp;and spore-dispersal. The fertile leaves, identical in form with

1 For an account of the mechanism of spore-dispersal, see Goebel (05) p. 587; Atkinson (94); Leclerc du Sablon (85); and Bower (00).

the sterile, or more or less sharply contrasted, usually hear the sporangia on the under surface of the lamina in definite groupsnbsp;or sori, and not on the upper surface or grouped in strobili asnbsp;in the Lycopodiales. The stem is dorsiventral or radial innbsp;structure, creeping or erect, frequently clothed with chaffynbsp;scales (ramenta) and less often with multicellular hairs. Thenbsp;sexual generation is represented by a small green prothallusnbsp;which lives for a short period only and dies after nursing thenbsp;fern-plant through its earliest stages.

(b) Hydropterideae. Heterosporous water-ferns differing considerably in habit from the true ferns. Each megasporangium contains a single megaspore and several microspores arenbsp;produced in each microsporangium. The gametophyte is represented by tissue more or less enclosed in the spore. [Generanbsp;Salvinia, Azolla, Marsilia, Regnellidium, Pilidaria. Seenbsp;Chapter xxvi.]

(a) Eufilicineae. The classification of the true ferns in common use is based almost exclusively on the structure of thenbsp;sporangium, the form and position of the sori, and on thenbsp;presence or absence of an indusium (the tissue which in somenbsp;ferns partially or completely covers each sorus). In recentnbsp;years there has been considerable activity in the investigationnbsp;of fern anatomy with a view to elucidating the natural relationship between recent families or genera. The results of thesenbsp;researches are on the whole consistent with the scheme andnbsp;grouping adopted in the Synopsis Filicum of Hooker and Bakernbsp;and in general harmony with the main conclusions arrived atnbsp;by Bower from an intensive study of the development of fernnbsp;sporangia. The following classification is based on that ofnbsp;Bower who takes as a basis (i) the relative time of appearancenbsp;of the sporangia in a single sorus, (ii) the structure of thenbsp;sporangia and their orientation relative to the whole sorus,nbsp;(iii) the productiveness of sporangia (spore-output).

Simplices (Bower). The sporangia are relatively large and all the sporangia in a sorus have anbsp;simultaneous origin : the annulus is oblique.

(Gradatae (Bower). Sporangia arise in basipetal succession on a more or less elongated receptaclenbsp;(portion of the leaf lamina which projects as anbsp;cushion or column on which the sporangia arenbsp;home); annulus oblique ; indusium, if present,nbsp;in the form of a cup or flap of tissue arising fromnbsp;V the base of the sorus.

'Mixtae (Bower). This division includes the Poly-podiaceae, by far the largest family of ferns. The sporangia are characterised by their relatively small size, the presence of a slender stalk,nbsp;the absence of regular orientation or sequencenbsp;in development, and by the presence of a verticalnbsp;annulus.

I The Dipteridinae include species with the characters \ of the Mixtae, and one species in which thenbsp;I sporangia develope simultaneously {Simplices).

Sporangia large and rather stouter than those of other Leptosporangiate ferns, borne in small groups (filmy species ofnbsp;Todea) in linear and frequently confluent sori {Todea barbam;nbsp;fig. 221, D) or clustered round the axis of modified fertilenbsp;pinnae with much reduced lamina {Osmunda). The annulus isnbsp;represented by a group of thicker-walled cells a short distancenbsp;below the apex (fig. 221, C). This family stands apart amongnbsp;the ferns; in some respects, e.g. in the more robust sporangianbsp;occasionally forming synangia, and in the presence of stipularnbsp;wings, it forms a transitional series between the Leptosporangiatenbsp;and Eusporangiate ferns. The only European species of Osmunda,nbsp;0. regalis, is almost cosmopolitan in range; other species occurnbsp;in North and South America, in the Far East, the Malaynbsp;Peninsula, and in other regions, more especially in thenbsp;temperate zones. Todea is represented by (i) the Southnbsp;African and Australian species, T. barbara, a fern with a stem,nbsp;which may reach a height of several feet, thickly covered withnbsp;adventitious roots and bearing large and somewhat leathery

* For a fuller account of recent ferns, see Engler and Prantl (02), Christ (97), Hooker and Baker (68), and Bower (00) (08).

fronds; (ii) filmy species in New Zealand, New South Wales, New Caledonia, and elsewhere. A plant of the small tree-fernnbsp;Todea Wilkesiana (Fiji, Samoa, and other islands) in the filmy-fern house at Kew, to which my attention was drawn by mynbsp;friend Mr A. W. Hill, has a slender stem with the characteristicnbsp;leaf-scars exposed; it presents a striking similarity to some ofnbsp;the fossil species of Osmundaceae described in a later chapter.

recognised by the complete transverse apical annulus usually one layer of cells deep, but occasionally two layers in depth onnbsp;the side opposite the line of dehiscence* (fig. 224, B). Schizaeanbsp;(fig. 222) with the exception of one species in North America

(8. pusilla) is characteristic of Northern India, the Malay region, Australia, New Caledonia, S. Africa, and elsewhere southnbsp;of the Equator. Aneimia (figs. 223, 224, A, B), characterised bynbsp;the fertile segments with reduced lamina, is chiefly American:nbsp;the monotypic genus Mohria, resembling in habit the Poly-

Fig. 222. Schizaea elegans. (Slightly reduced.) A few of the segments terminate in narrow fertile lobes.

podiaceous genus Cheilanthes, occurs in S. Africa and Madagascar, while species of Lygodium are widely spread tropical ferns, with one species in temperate North America. Thisnbsp;family has disappeared from Europe.

Sporangia form circular naked sori composed of a variable number of sporangia, usually not more than ten and frequently

fewer, characterised by an obliquely horizontal and almost complete annulus (fig. 224, I). In some species of Gleichenianbsp;(sect. Eugleichenia) the ultimate segments are very small and

semi-ciruular in form (fig. 226, C), in others (sect. Mertensia^) the segments are linear (fig. 226, D), and in many species thenbsp;fronds are distinguished by the regular dichotomous branching

' Underwood (07), p. *243, has adopted Bernhardi�s genus Dicranopteris in place of Mertensia on the ground that the latter was used as early as 1793 for anbsp;Boraginaceous plant.

(fig. 225), frequently showing an arrested rachis bud in the forks' protected by modified pinnules (fig. 226, D, E). Innbsp;Platyzonia the leaves are simple, reaching a length ofnbsp;20�30 cm., and bear small revolute oval segments.

Gleichenia is represented by several species in the tropics and extends to south temperate and Antarctic latitudes. Thenbsp;species G. dichotoma (= G. linearis) is one of the more successfulnbsp;tropical ferns, while G. moniliformis (by some authors recognised as a distinct genus, Stromatopteris) is peculiar to Newnbsp;Caledonia. The monotypic genus Platyzoma is a xerophilousnbsp;Australian fern. The Gleicheniaceae are unrepresented innbsp;existing north temperate floras.

The genus Matonia, placed in the Cyatheaceae by Sir William Hooker and compared by other authors also withnbsp;the Gleicheniaceae, is now included in a special family. Thenbsp;sori are circular and consist of 5�11 large sporangia (fig. 224,nbsp;E, G) sessile on a Central columnar receptacle which spreadsnbsp;out into an umbrella-like indusium (D, i) with its incurvednbsp;margin tucked in below the ring of sporangia. The indusiumnbsp;is detached when the sporangia are ripe. The annuhisnbsp;is oblique and incomplete and often slightly sinuous; itnbsp;agrees in the main with that of Gleichenia. The speciesnbsp;Matonia pectinata is characterised by dichotomously branchednbsp;fronds (figs. 227, 228) with long and slender petioles; thenbsp;pinnae bear linear pinnules with forked lateral veins andnbsp;occasional lateral anastomoses (fig. 224, F). The only othernbsp;living representative is M. sarmentosa, discovered by Mrnbsp;Charles Hose at Niah, Sarawak^: this species has longnbsp;pendulous leaves apparently very different from those ofnbsp;M. pectinata, but the branching of the frond may be regarded asnbsp;a modification of a primitive form of dichotomy^. A small budnbsp;occurs in the angle between the forked linear segments and thenbsp;rachis, as in some species of Gleichenia*^. Matonia is confined to

the Malay region; M. pectinata grows in Western Borneo and in various localities in the Malay peninsula, while M. sarvientosa.

has been found in one locality only; the latter species has recently been transferred to a new genus Phanerosorus, butnbsp;in view of the practical identity in anatomical structure and

the close agreement as regards the sori of the two species there would seem to be no justification for this change of name'.

The New Zealand genus Loxsoma has marginal sori with a cup-like indusium surrounding an elongated receptacle bearing

pear-shaped sporangia provided with a complete oblique annulus. The genus is chiefly interesting because of its isolated position; itnbsp;agrees with Trichomanes (Hymenophyllaceae) in the structure ofnbsp;the sorus and with species of Bicksonid and Davallia in habit; itnbsp;shows some resemblance also to Gleicheniaceae and Schizaea-A new type of fern described by Christ� from Costa Rica

The sporangia, which are attached to a columnar receptacle or prolongation of a vein beyond the margin of the lamina, arenbsp;characterised by an obliquely transverse annulus (fig. 224, C). Anbsp;cup-like indusium surrounds the lower portion of the receptaclenbsp;which is two-lipped in Trichomanes and entire in Hymenophyllum (fig. 270, C, D). These two filmy ferns have a widenbsp;distribution both in tropical and extra-tropical regions; they

are represented in the British Isles by Hymenophyllum tun-hridgense, H. Wilsoni, and Trichomanes radicans.

The sporangia occur in indusiate or naked sori and have an obliquely vertical and incomplete annulus (fig. 229, E). In thenbsp;great majority of cases the fronds are large and highly com-

pound, but Gyathea sinuata Hook, a rare Ceylon species, bears simple narrow linear leaves. This family includes, with fewnbsp;exceptions, all the tree ferns The sori of Dicksonia arenbsp;enclosed in a two-valved indusium (fig. 229, F, G); in thenbsp;species represented in fig. 230 the fertile segments, whichnbsp;terminate in cup-like indusia, are characterised by the absence ofnbsp;a lamina and closely resemble those of Thyrsopteris (fig. 229, A).

In Gyathea the indusium has the form of a cup which is at first closed and afterwards opens at the apex (fig. 229, B); in He-mitelia the indusium is much reduced and in Alsophila the sorinbsp;are naked. Thyrsopteris is characterised by the reduced fertilenbsp;pinnules bearing stalked sori in deep cups (fig. 229, A). Thenbsp;appearance of this fern �is very remarkable, for the cup-shapednbsp;1 Scott, J. (74); Hannig (98).

sori hang down from the fronds in masses, looking just like masses of millet seed^.� The sporangia are described by Bower^nbsp;as large and of rather peculiar form. As seen in fig. 224, H, thenbsp;annulus is continuous; it forms a twisted loop of cells whichnbsp;vary in shape and in the thickness of the walls. The Cyatheaceaenbsp;are for the most part tropical ferns with a wide geographicalnbsp;range, usually in moist regions; they are, however, able tonbsp;flourish under widely different temperature conditions. Innbsp;Tasmania, as Diels* points out, tree ferns may occasionally benbsp;seen laden with snow, and on the west coast of New Zealandnbsp;they overhang the edge of a glacier^. The monotypic genusnbsp;Thyrsopteris is confined to Juan Fernandez. The Cyatheaceaenbsp;no longer exist in Europe.

This sub-tribe, instituted by Prantl, has been revived by Bower on the ground that the sori present features intermediate between those of Cyatheaceae and the Polypodia-ceous genus Davallia. The sporangia have a slightly obliquenbsp;annulus.

This section of the Leptosporangiate ferns, including several sub-tribes, comprises the great majority of recent genera. Thenbsp;sporangia form naked or indusiate sori and have a vertical incomplete annulus. In Plagiogyria^ the oblique annulus andnbsp;soral features suggest comparison with the Cyatheaceae. Anbsp;more intimate acquaintance with Polypodiaceous ferns willnbsp;undoubtedly demonstrate the existence of other generalisednbsp;types�.

From the point of view of the identification of fossil ferns it is important to bear in mind the very close resemblancenbsp;presented by some Polypodiaceous species, e.g. species ofnbsp;Davallia (fig. 229, C), to Cyatheaceous ferns (cf fig. 229, D).

� Prof. Bower informs me that he is now at work on Plagiogyria and other Polypodiaceae.

The almost spherical and scattered sporangia are characterised by the peculiar form of the vertical annulus, which is composed of numerous cells differing in their greater breadth

and smaller depth from those of a typical annulus. Exannulate sporangia have been described, while others occur showingnbsp;different stages between a rudimentary and a complete ring..nbsp;The single species of Ceratopteris, C. thalictroides, is an annualnbsp;aquatic fern widely spread in tropical countries^.

The genus Dipteris, formerly included in the Polypodiaceae, has been assigned to a separate family partly on account of thenbsp;slight obliquity of the vertical annulus (fig. 231, G) and on othernbsp;grounds^. The four species Dipteris conjugata, D. Wallichii,nbsp;D. Lobbiana {=D. bifurcata), and D. quinquefurcata (fig. 231)nbsp;are characterised by a creeping rhizome bearing fronds reachingnbsp;a length of 50 cm.; in D. conjugata and D. Wallichii the laminanbsp;is divided by a median sinus into two symmetrical halves, whilenbsp;in other species the leaf is dissected into narrow linear segments.nbsp;The main dichotomously branched ribs are connected by lateralnbsp;branches and these by tertiary veins, the delicate branches ofnbsp;which end freely within the square or polygonal areolae (fig.nbsp;231, A', E). The naked sori are composed of numerous sporangianbsp;and filamentous hairs: while in some species the soral development conforms to that characteristic of the Mixtae, it has beennbsp;shown that in one species, D. Lobbiana {�D. bifurcata'^), thenbsp;sporangia develope simultaneously as in the Simplices. Dipterisnbsp;occurs in company with Matonia on Mt Ophir and elsewhere innbsp;the Malay peninsula; it extends to the Philippines, Samoa,nbsp;New Caledonia, China, New Guinea, and the sub-tropical regionsnbsp;of Northern India.

The impossibility of drawing a hard and fast line between the divisions adopted in any system of classification is wellnbsp;illustrated by the ferns. In the main, the three-fold groupingnbsp;suggested by Bower is probably consistent with the order ofnbsp;evolution of the true ferns. The Polypodiaceae, which are nownbsp;the dominant group, are in all probability of comparativelynbsp;recent origin, while the Gradatae and Simplices represent smallernbsp;sub-divisions with representatives in remote geological epochs.nbsp;The genera Loxsoma, Matonia and Dipteris afford examples ofnbsp;ferns exhibiting points of contact with more than one of Bower�snbsp;sub-divisions: they are generalised types which, like many relicsnbsp;� of the past, are now characterised by a restricted geographicalnbsp;range.

It is noteworthy that while certain vegetative features may in some cases be cited as family-characters, such features arenbsp;not usually of much value from a taxonomic point of view.

While the typical tree ferns are practically all members of the Cyatheaceae, a few members of other families, e.g. Todea harbaranbsp;(Osmundaceae) and the monotypic Indian genus Brainea (Poly-

podiaceae), form erect stems several feet in height; but these differ in appearance from the Palm-like type of the Cyatheaceousnbsp;tree ferns. On the other hand, the thin, almost transparent,nbsp;leaf of Hymenophyllum tunbridgense and other filmy ferns isnbsp;a character shared by several species of Todea, Asplenium re-sectum, and Danaea trichomaiioides (Marattiaceae); the filmynbsp;habit is essentially a biological adaptation.

The form of frond represented by certain species of Gleichenia, characterised by a regular dichotomy of the axis andnbsp;by the occurrence of arrested buds, is on the whole a trustworthynbsp;character, though DavalUa aculeata (bearing spines on its rachis)nbsp;(fig. 232) and Matonia sarmentosa have fronds with a similarnbsp;m.ode of branching and also bear arrested radius-buds. A limitednbsp;acquaintance with ferns as a whole often leads us to regard anbsp;certain form of leaf as characteristic of a particular species, butnbsp;more extended enquiry usually exposes the fallacy of relyingnbsp;upon so capricious a feature. The form of leaf illustrated bynbsp;Trichomanes reniforme is met with also in Gymnogramme reni-forniis and is fairly closely matched by the leaf of Scolopendriumnbsp;nigripes. The fronds of Matonia pectinata (figs. 227, 228) bear anbsp;close resemblance to those of Gleichenia Gunninghami, Adiantumnbsp;pedatum, and Cheiropteris palmatopedata^.

The full accounts of the structure and life-history of the common Male Fern, given by Scott in his Structural Botany andnbsp;by Bower in the Origin of a Land Flora, render superfluousnbsp;more than a brief reference to certain general considerations innbsp;so far as they may facilitate a study of fossil types.

In size Ferns have a wide range: at the one extreme we have the filmy fern Trichomanes GoebelianunG, growing on treenbsp;stems in Venezuela, with leaves 2'5 to 3 mm. in diameter, andnbsp;at the other the tree ferns with tall columnar stems reaching anbsp;height of 40 to 50 feet and terminating in a crown of frondsnbsp;with a spread of several feet. A common form of stem isnbsp;represented by the subterranean or creeping rhizome covered

with ramental scales or hairs: the remains of old leaves may persist as ragged stumps, or, as in Oleandra, Polypodiiim vulgarenbsp;and several other species, the leaf may he cut off by the formation of an absciss-layer^ leaving a clean-cut peg projectingnbsp;from the stem. As a rule the branches hear no relation to thenbsp;leaves and are often given off from the lower part of a petiole,nbsp;but in a few cases, e.g. in the Hymenophyllaceae, it is noteworthynbsp;that true axillary branching is the rule^. In the typical tree-fern the surface resembles that of a Cycadean trunk coverednbsp;with persistent leaf-bases and a thick mass of roots. Amongnbsp;epiphytic ferns highly modified stems are occasionally met with,nbsp;as in the Malayan species Pulypodium {Lecanopteris) carnosmnnbsp;and P. sinuosunp.

The leaves of ferns are among the most protean of all plant organs; as Darwin wrote, �the variability of ferns passes allnbsp;bounds^.� The highly compound tri- or quadripinnate leavesnbsp;of such species as Pteris aquilina, DavalUa and other generanbsp;stand for the central type of fern frond; others exhibit a well-marked dichotomy, e.g. Lygodium, Gleichenia, Matonia, etc., anbsp;habit in all probability associated with the older rather thannbsp;with the more modern products of fern evolution. Beforenbsp;attempting to determine specifically fossil fern fronds, it isnbsp;important to familiarise ourselves with the' range of variabilitynbsp;among existing species and more especially in leaves of thenbsp;same plant. A striking example of heteromorph}^ is illustratednbsp;in fig. 233. Reinecke� has figured a plant of Asplenium multi-lineatum in which the segments of the compound fronds assumenbsp;various forms. In Teratophyllum acideatum var. inermis Mett.,nbsp;a tropical climbing fern believed by Karsten� to be identicalnbsp;with Acrostichum {Lornariopsis) sorhifolium�sn� identificationnbsp;which GoebeB questions,�the fronds which stand free of thenbsp;stem supporting the climber differ considerably from thenbsp;translucent and much more delicate filmy leaves pressed againstnbsp;the supporting tree. From this fern alone Fee is said to havenbsp;created 17 distinct species. In this, as in many other cases.

differences in leaf-form are the expression of a physiological division of labour connected with an epiphytic existence. Somenbsp;tropical species of Polypodium (sect. Drynaria), e.g. P. querci-folium (fig. 234 and fig. 231, D), produce two distinct types ofnbsp;leaf, the large green fronds, concerned with the assimilation ofnbsp;carbon and spore-production, being in sharp contrast to the small

slightly lobed brown leaves which act as stiff brackets (fig. 234, M) for collecting humus from which the roots absorb rawnbsp;material. Similarly in Platycerium the orbicular mantle-leavesnbsp;differ widely from the long pendulous or erect fronds fashionednbsp;like the spreading antlers of an elk. In Hemitelia capensis, anbsp;South African Cyatheaceous species, the basal pinnae assume

the form of finely divided leaves identified by earlier collectors as those of a parasitic Trichomanes (fig. 235). In a letter writtennbsp;by W. H. Harvey in 1837 accompanying the specimen shownnbsp;in fig. 235, he says, � Apropos of Hemitelia, be it known abroadnbsp;that supposed parasitical Trichomanes.. .is not a parasite, butnbsp;a part of the frond of Hemitelia.� The delicate reduced pinnaenbsp;remain on the stem and form a cluster at the base of thenbsp;fronds

sterile fronds by segments with little or no chlorophyllous tissue, as in Onoclea struthiopteri.s�� in which, each year, the plant produces a funnel-shaped group of sterile leaves followed later innbsp;the season by a cluster of sporophylls; or, as in many othernbsp;genera, the fertile leaves are distinguished also by longer petiolesnbsp;and thus serve as more efficient agents of spore-dissemination.nbsp;In Ceratopteris the narrow segments of the taller fertile leaves

^ A striking example of these so-called Aphlebiae of Hemitelia may be, seen at the Royal Gardens, Kew.

are in striking contrast to the broader pinnules of the submerged foliage leaves. Leaf-form is in many cases obviously the expression of environment; the xerophilous fern Jamesonia^nbsp;from the treeless paramos of the Andes ^ is characterised bynbsp;its minute leaflets with strong revolute margins and a thick feltnbsp;of hairs on the lower surface; in others, xerophilous features take

Fig. 235. Hemiteliacapensis'R.Bi-ovni. Nat. size, a, Pinna of normal frond. [From a specimen in the British Museum. M.S.]

the form of a covering of overlapping scales {Geterach), or a development of water-tissue as in the fleshy leaves of thenbsp;Himalayan fern Drymoglossum carnosum. In the Bracken fernnbsp;Boodle^ has shown how the fronds may be classed as shade andnbsp;1 Goebel (91) PI. xiii.nbsp;nbsp;nbsp;nbsp;� Spruce (08) ii. p. 232.nbsp;nbsp;nbsp;nbsp;* Boodle (01).

aceae and thalloid Liverworts^ is worthy of mention as one of the many possible pitfalls to be avoided by the palaeobotanicalnbsp;student. The long linear fronds of such genera as Vittaria andnbsp;Monogramme might well be identified in a fossil state as thenbsp;leaves of a grass-like Monocotyledon, or compared with the

foliage of Isoetes or Pilularia. The resemblance of some fern leaves with reticulate venation to those of Dicotyledons has lednbsp;astray experienced palaeobotanists; it is not only the anastomosing venation in the leaves of several ferns that simulatesnbsp;1 Goebel (06); Baker (67).

dicotyledonous foliage, but the compound leaves of many dicotyledons, e.g. Paullinia thalictrifolia (Sapindaceae) and species of Umbelliferae, may easily be mistaken for fronds of ferns.

The dichotomously lobed lamina of some Schizaeas, e.g. S. dichotonia and S. elegans (fig. 222), bears a close resemblancenbsp;to the leaves of Baiera or Ginkgo^. The original description bynbsp;Kunze^ of the South African Cycad Stangeria paradoxa as anbsp;Polypodiaceous fern illustrates the difficulty, or indeed impossibility, of distinguishing between a sterile simply pinnate fernnbsp;frond and the foliage of some Cycads. The deeply dividednbsp;segments of Gycas Micholitzii^ simulate the dichotomouslynbsp;branched pinnae of Lygodium dichotomum, and the leaves ofnbsp;Aneimia rotundifolia (fig. 223) and other species are almostnbsp;identical in form with the Jurassic species Otozamites Beani,nbsp;a member of the Cycadophyta.

There are certain facts in regard to the geographical distribution of ferns to which attention should be directed. Mr Baker in his paper on fern distribution writes: �With thenbsp;precision of an hygrometer, an increase in the fern-vegetationnbsp;marks the wooded humid regions^.� If in a collection of fossilnbsp;plants we find a preponderance of ferns we are tempted tonbsp;assume the existence of such conditions as are favourable to thenbsp;luxuriant development of ferns at the present day. On thenbsp;other hand, we must bear in mind the wonderful plasticity ofnbsp;many recent species and the fact that xerophilous ferns are bynbsp;no means unknown in present-day floras.

Ferns are admirably adapted to rapid dispersal over comparatively wide areas. Bower� estimates that in one season a Male Fern may produce about 5,000,000 spores: withnbsp;this enormous spore-output are coupled a thoroughly efficientnbsp;mechanism for scattering the germs and an unusual facilitynbsp;for wind-dispersal. When Treub� visited the devastated andnbsp;sterilised wreck of the Island of Krakatau in 1886, three yearsnbsp;after the volcanic outburst, he found that twelve ferns hadnbsp;already established themselves; the spores had probably been

carried by the wind at least 25 to 30 miles. It is not surprising, therefore, to find that many ferns have an almost world-widenbsp;distribution; and, it may be added, in view of their efficientnbsp;means of dispersal, wide range by no means implies greatnbsp;antiquity. Prof CampbelP has recently called attention to thenbsp;significance of the wide distribution of Hepaticae in its bearing onnbsp;their antiquity; the spores are incapable of retaining vitality fornbsp;more than a short period, and it is argued that a world-widenbsp;distribution can have been acquired only after an enormousnbsp;lapse of time. If we apply this reasoning to the Osmundaceaenbsp;among ferns, it may be legitimate to assume that their shortlived green spores render them much less efficient colonisersnbsp;than the great majority of ferns; if this is granted, the widenbsp;distribution of Osmundaceous ferns in the Mesozoic era carriesnbsp;their history back to a still more remote past, a conclusionnbsp;which receives support from the records of the rocks.

The Bracken fern which we regard as characteristically British is a cosmopolitan type; it was found by Treub amongnbsp;the pioneers of the New Flora of Krakatau; in British Centralnbsp;Africa, it greets one at every turn � like a messenger from thenbsp;homeland^�; it grows on the Swiss Alps, on the mountains ofnbsp;Abyssinia, in Tasmania, and on the slopes of the Himalayas.nbsp;The two genera Matonia (fig. 228) and Dipteris, which grownbsp;side by side on Mount Ophir in the Malay Peninsula, are examplesnbsp;of restricted geographical range and carry us back to the Jurassicnbsp;period when closely allied types flourished abundantly in northern latitudes. Similarly Thyrsopteris elegans, confined to Juannbsp;Fernandez, exhibits a remarkable likeness to Jurassic speciesnbsp;from England and the Arctic regions.

The proportion of ferns to flowering plants in recent floras is a question of some interest from a palaeobotanical point of view;nbsp;but we must bear in mind the fact that the evolution ofnbsp;angiosperms, effected at a late stage in the history of the earth,nbsp;seriously disturbed the balance of power among competitors fornbsp;earth and air. The abundance of ferns in a particular region is,nbsp;however, an unsafe guide to geographical or climatic conditions.nbsp;Many ferns are essentially social plants; the wide stretches of

moorland carpeted with Pteris aquilina afford an example of the monopolisation of the soil by a single species. In Sikkim Sirnbsp;Joseph Hooker speaks of extensive groves of tree ferns, and innbsp;the wet regions of the Amazon, Bates^ describes the wholenbsp;forest glade as forming a � vast fernery.� In a valley in Tahitinbsp;Alsophila tahitiensis is said to form �a sort of forest almost tonbsp;the exclusion of other ferns In the abundance of Glossopterisnbsp;(figs. 334, etc.) fronds spread over wide areas of Permo-Carboniferous rocks in S. Africa, Australia, and India, we havenbsp;a striking instance of a similar social habit in an extinct fernnbsp;or at least fern-like plant.

Acrostickwm aureum, with pinnate fronds several feet long, is an example of a recent fern covering immense tracts, but thisnbsp;species� is more especially interesting as a member of thenbsp;Filicineae characteristic of brackish marshes and the banks ofnbsp;tropical rivers in company with Mangrove plants and thenbsp;� Stemless Palm � Nipa. This species exhibits the anatomicalnbsp;characters of a water-plant and affords an interesting parallelnbsp;with some Palaeozoic ferns (species of Psaronius) which probablynbsp;grew under similar conditions.

The text-book accounts of fern-anatomy convey a very inadequate idea of the architectural characters displayed by the vascular systems of recent genera. When we are concernednbsp;with the study of extinct plants it is essential to be familiar notnbsp;only with the commoner recent types, but particularly withnbsp;exceptional or aberrant types. The vascular system of manynbsp;ferns consists of strands of xylem composed of scalariformnbsp;tracheae associated with a larger or smaller amount of parenchyma, surrounded either wholly or in part (that is concentricnbsp;or bicollateral) by phloem ; beyond this is a pericycle, one layernbsp;or frequently several layers in breadth, limited externally by annbsp;endodermis, which can usually be readily recognised. The vascular strands are embedded in the ground-tissue of the stem

consisting of thin-walled parenchyma and, in most ferns, a considerable quantity of hard and lignified mechanical tissue.nbsp;The narrow protoxylem elements are usually characterised by anbsp;spiral form of thickening, but in slow-growing stems the first-formed elements are frequently of the scalariform type.

A study of the anatomy of recent ferns both in the adult state and in successive stages of development from the embryonbsp;has on the whole revealed �a striking parallelism'� betweennbsp;vascular and sporangial characters in leptosporangiate ferns.nbsp;For a masterly treatment of our knowledge of fern anatomy

pp^

from a phylogenetic point of view reference should be made to Mr Tansley�s recently published lectures: within the limits ofnbsp;this volume all that is possible is a brief outline of the mainnbsp;types of vascular structure illustrated by recent genera.

to a type of stele consisting of a central core of xylem surrounded by phloem, pericycle, and endodermis. While admitting thatnbsp;steles of this type may sometimes be the result of the modification of less simple forms, we may confidently regard thenbsp;protostele as representing the most primitive form of vascularnbsp;system. The genus Lygodium affords an example of a proto-stelic fern; a solid column of xylem tracheae and parenchyma isnbsp;completely encircled by a cylinder of phloem succeeded by anbsp;multi-layered pericycle and an endodermis of a single layer ofnbsp;cells. In this genus the stele is characterised by marginalnbsp;groups of protoxylem ; it is exarch. An almost identical type is

represented by species of Gleichenia, but here the stele is mesarch, the protoxylem being slightly internal (fig. 237, C).nbsp;Trichomanes scandens (fig. 238) has an exarch protostele likenbsp;that of Lygodium; but, as Boodle* has suggested, the protostelicnbsp;form in this case is probably the result of modification of anbsp;collateral form of stele such as occurs in Trichomanes reniformenbsp;(fig. 237, E). A second type of stele has been described innbsp;species of Lindsaya'^ in which the xylem includes a small groupnbsp;of phloem near the dorsal surface. This Lindsaya type is oftennbsp;passed through in the development of � seedling � ferns and may

be regarded as a stage in a series leading to another well-marked type, the solenostele. The solenosteleh a hollow cyclinder ofnbsp;xylem lined within and without by phloem, pericycle, and en-dodermis, occurs in several genera belonging to different families,nbsp;e.g. Dipteris, species of Pteris, species of Lindsaya, Polypodium,nbsp;Jamesonia, Loxsoma, Gleichenia and other genera. In a smallernbsp;number of ferns the stele consists of what may be called anbsp;medullated protostele similar to the common form of stele innbsp;Lepidodendron: this type is found in species of Schizaea and innbsp;Platyzoma (fig. 239). It is important to notice that in the

l.t. ii

solenostele and as a rule in the medullated protostele when a leaf-trace passes out from the rhizome stele the vascular cylindernbsp;is interrupted by the formation of a foliar gap {Platyzoma^,nbsp;fig. 239, is an exception). This fact has been emphasizednbsp;by Jeffrey^ who draws a distinction between the Lycopodiaceousnbsp;type of stele, which is not broken by the exit of leaf-traces, andnbsp;the fern stele in which foliar gaps are produced: the former henbsp;speaks of as the cladosiphonic type (Lycopsida) and the latter asnbsp;the phyllosiphonic (Pteropsida).

may be described as the result of the replacement of some of the axial conducting tracheae by parenchyma or other non-vascularnbsp;tissue consequent on an increase in diameter of the whole stelenbsp;and the concentration of the true conducting elements towardsnbsp;the periphery^.

The occurrence of the internal cylinder of phloem, pericycle, and endodermis in a solenostele is rendered intelligible by anbsp;study of fern seedlings and by a comparative examination ofnbsp;transitional types connecting protosteles and solenosteles throughnbsp;medullated protosteles and steles of the Lindsaya type. A

further stage in stelar evolution is illustrated by what is termed the dictyostele, the arrangement of vascular tissue characteristicnbsp;of Nephrodium Filix-mas, Gyathea (fig. 240), Polypodiuni vulgarenbsp;and many other common ferns.

If a solenostele is interrupted by leaf-gaps at intervals sufficiently close to cause overlapping, a transverse section atnbsp;any part of the stele will show apparently separate curvednbsp;bands of concentrically arranged xylem and phloem, which onnbsp;dissection are seen to represent parts of a continuous lattice-

^ For an account of the probable methods by which this has been effected and of the factors concerned, see Tansley (08).

work or a cylinder with the wall pierced by large meshes. The manner of evolution of the dictyostele has been ably dealt withnbsp;by Gwynne-Vaughan^ and other authors. In a few ferns, e.g.nbsp;Matonia pectinata^, a transverse section of the stem (fig. 237, B)nbsp;reveals the presence of two or in some cases three concentricnbsp;solenosteles with a solid protostele in the centre: this polycylicnbsp;type may be regarded as the expression of the fact that innbsp;response to the need for an adequate water-supply to the largenbsp;fronds, ferns have increased the conducting channels by a methodnbsp;other than by the mere increase of the diameter of a single stele.nbsp;Fig. 237, A, shows the vascular tissue of a petiole of Matonia innbsp;transverse section.

The two genera of Osmundaceae, Todea and Osmunda, are peculiar among recent ferns in having a vascular cylinder composed of separate strands of xylem varying considerably innbsp;shape and size, from U-shaped strands with the concavitynbsp;facing the centre of the stem and with the protoxylem in thenbsp;hollow of the U, to oval or more or less circular strands withnbsp;a mesarch protoxylem or without any protoxylem elementsnbsp;(fig. 221, A, B). These different forms are the expression of thenbsp;change in contour or in structure which the parts of the lattice-work undergo at different levels in the stem�. Beyond thisnbsp;ring of xylem bundles is a continuous sheath of phloem ofnbsp;characteristic structure. A transverse section of a stem ofnbsp;Osmunda regalis may show 15 or more xylem strands; in 0.nbsp;Claytoniana there may be as many as 40. In Todea barbaranbsp;(fig. 221, B) the leaf-gaps are shorter, and in consequence of thenbsp;less amount of overlapping the xylem cylinder becomes annbsp;almost continuous tube. The recent researches of Kidston andnbsp;GWynne-Vaughan^ have resulted in the discovery of fossilnbsp;Osmundaceous stems with a complete xylem ring, the stelenbsp;being of the medullated protostele type; in another extinctnbsp;member of the family the stele consists of a solid xylem core.nbsp;The Osmundaceous type of stele is complicated in 0. cinnamomeanbsp;(fig. 221, A) by the occurrence of local internal phloem and by

an internal endodermis, a feature which leads Jeffrey to what I believe to be an incorrect conclusion that the vascular arrangement found in Osmunda regalis has been evolved by reductionnbsp;from a stele in which the xylem was enclosed within and withoutnbsp;by phloem. New facts recently brought to light enable us tonbsp;derive the ordinary Osmundaceous type from the protostelenbsp;and solenostele. It is worthy of remark that the Osmundaceaenbsp;occupy a somewhat isolated position among recent ferns; theirnbsp;anatomy represents a special tjrpe, their sporangia differ innbsp;several respects from those of other leptosporangiate ferns and innbsp;some features Osmunda and Todea agree with the Eusporangiatenbsp;ferns. The possession of such distinguishing characters as thesenbsp;suggests antiquity; and the facts of palaeobotany, as also thenbsp;present geographical range of the family, confirm the correctnessnbsp;of this deduction.

Before leaving the stelar structure of leptosporangiate fern stems, a word must be added in regard to a type of structurenbsp;met with in the Hymenophyllaceae. In this family Trichomanesnbsp;reniforme (fig. 237, E) may be regarded, as Boodle suggests, asnbsp;the central type: the stele consists of a ring of metaxylemnbsp;tracheae, the dorsal portion having the form of a flat archnbsp;and the ventral half that of a straight band. This flattened ringnbsp;of xylem encloses parenchymatous tissue containing scatterednbsp;tracheae some of which are protoxylem elements. In Trichomanes radicans the rhizome is stouter than in T. reniforme andnbsp;the stele consists of a greater number of tracheae. The stelenbsp;is cylindrical like that shown in fig. 238, but the centre is occupied by two groups of protoxylem and associated parenchyma.nbsp;In Hymenophyllum tunbridgense the stele is of the subcollateral type; the ventral plate of the xylem ring has disappearednbsp;leaving a single strand of xylem with endarch protoxylem andnbsp;completely surrounded by phloem. Trichomanes muscoidesnbsp;possesses a still simpler stele consisting of a slender xylemnbsp;strand with phloem on one side only. Reference has alreadynbsp;been made to the occurrence in this family of the protostelicnbsp;type. The Hymenophyllaceae afford a striking illustrationnbsp;of the modification in different directions of stelar structurenbsp;connected with differences in habit, and of the correlation of

demand and supply as shown in the varying amount of conducting tissue in the steles of different species.

The leaf-trace in a great number of ferns is characterised by its C-shaped form^ as seen in transverse section: this in somenbsp;genera, e.g. Matonia (fig. 237, A), is complicated by the spiralnbsp;infolding of the free edges of the C; in other ferns (e.g.nbsp;some Cyatheaceae) (fig. 278, C) the sides of the C are incurved,nbsp;while in some species the xylem is broken up into a largenbsp;number of separate strands.

An elaborate treatment of the leaf-traces of ferns was published a few years ago by MM. Bertrand and Cornaille^nbsp;in which the authors show how the various systems of vascularnbsp;tissue in the fronds of ferns may be derived from a common type.nbsp;As Prof. Chodat� justly remarks this important work has notnbsp;received the attention it deserves, the neglect being attributednbsp;to the strange notation which is adopted^.

The roots of ferns are characterised by a uniformity of plan in marked contrast to the wide range of structure met with innbsp;the stem and to a less extent in the leaves. The xylem maynbsp;consist of a plate of scalariform tracheae with a protoxylemnbsp;group at each end, or the stele may include six or more alternating strands of xylem and phloem.

The Marattiaceae, the single family of ferns included in the Marattiales, comprise the genera Angiopteris, Archangiopteris,nbsp;Marattia, Danaea, and Kaulfussia, which are for the most partnbsp;tropical in distribution. These genera are characterised bynbsp;eusporangiate sori or synangia, the presence of stipules at thenbsp;base of the petioles, and by the complex arrangement of thenbsp;vascular tissue. In view of the fact that many fossil fernsnbsp;show a close resemblance to the recent Marattiaceae, the surviving genera are briefly described. The prothallus is greennbsp;and relatively large.

2 nbsp;nbsp;nbsp;Bertrand and Cornaille (02).nbsp;nbsp;nbsp;nbsp;� Chodat (08) p. 15.

Angiopteris. This genus occurs in Polynesia, tropical Asia, and Madagascar ; it is characterised by a short and thick fleshy stem bearing large bipinnate leaves which occasionally show a forking of the rachis^, anbsp;feature reminiscent of some Palaeozoic fern-like fronds. One of thenbsp;large plants of Angioptens evecta in the Eoyal Gardens, Kew, bears leavesnbsp;12 feet in length with a stalk 6 inches in diameter at the base. Thenbsp;sessile or shortly stalked and rather leathery linear or broadly lanceolatenbsp;pinnules have a prominent midrib and dichotomously branched lateralnbsp;veins. The surface of an old stem is covered with the thick stumps ofnbsp;petioles enclosed by pairs of fleshy stipules (fig. 241, A) and bearsnbsp;numerous fleshy roots, which hang free in the air or penetrate the soil.nbsp;The young fronds (fig. 220, A) exhibit very clearly the characteristicnbsp;circinate vernation. The proximal part of each primary pinna is characterised by a pulvinus-like swelling. The sporangia, in short linear

elliptical sori near the edge of the pinnules, consist of free sporangia (fig. 242, A�D) provided with a peculiar type of �annulus in the formnbsp;of a narrow band of thicker-walled cells, which extends as a broad stripnbsp;on either side of the apex. An examination of sections through thenbsp;sporangia of Angiopteris in different planes^ illustrates the difficulty ofnbsp;determining the precise nature of the annulus in a petrified sporangiumnbsp;which is seen only in one or two planes. Many of the sporangia from thenbsp;English Coal-Measures, compared by authors with those of Leptosporangiatenbsp;ferns, are in all probability referable to the Marattiaceous type.

The vascular system^ of the stem constitutes a highly complex dictyo-stelic or polycylic type which may consist of as many as nine concentric series of strands of xylern surrounded by phloem, with large sieve-tubes

1 nbsp;nbsp;nbsp;Observed in plants in the Botanic Gardens of Brussels and Leipzig. A.C.S.

2 nbsp;nbsp;nbsp;For an account of the spore-producing members of the Marattiaceae, seenbsp;Bower (97).

and a pericycle which abuts on the parenchymatous ground-tissue without any definite endodermal layei�. A peculiarity in the vascular strands isnbsp;that the first-formed elements of the phloem lie close to the edge of thenbsp;xylem, the metaphloem being therefore centrifugal in its development.nbsp;The ground-tissue is devoid of mechanical tissue and is penetrated bynbsp;roots, a few of which arise from the outer vascular strands while othersnbsp;force their way to the surface from the more internal dictyosteles. Leaf-traces, consisting of several strands, are given off from the outermost

D. nbsp;nbsp;nbsp;Section of sporangium, showing the two lateral bands of thick-walled cells.

E. nbsp;nbsp;nbsp;Danaea: a, roof of synangium, with pores; b, sporangialnbsp;cavities; v, vascular bundle; i, indusium.

cylinder and a segment of the second dictyostele moves out to fill the gap formed in the outermost network, while the gap in the second cylindernbsp;receives compensating .strands from the third. A few layers below thenbsp;surface of the petiole there is a ring of thick-walled elements (s, fig. 243),nbsp;and in both petiole and stem numerous mucilage ducts and tannin-sacsnbsp;occur in the ground-tissue. It has been shown by Farmer and HilF thatnbsp;in some of the vascular strands in an Angiopteris stem a few secondary

tracheae are added to the primary xylem by the activity of the adjacent parenchyma. The vascular bundles in the petiole form more or less regularnbsp;concentric series ; they have no endodermis and are characterised also bynbsp;the large size of the sieve-tubes {st, fig. 243).

Fig. 243. Angiopteris evecta. Section of petiole (considerably reduced) and of a single vascular bundle (magnified): px, protoxylem; st, sieve-tubes.

larger number of xylem and phloem groups ; the stele is polyarch and not diarch, tetrach or hexarch as in most Leptosporangiate ferns.

Archangiopteris. This monotypic genus, discovered by Mr Henry in South Eastern Yunnan, was described by Christ and Giesenhagen innbsp;18991. The comparatively slender rhizome has a fairly simple vascularnbsp;system^. The simply-pinnate leaves bear pinnules like those of Danaea,nbsp;1 Christ and Geisenhagen (99).nbsp;nbsp;nbsp;nbsp;� Gwynne-Vaughan (05).

but the 8ori agree with those of Angwpteris except in their greater length and in the larger number of sporangia.

Marattia. This genus, which extends � all round the world within the tropics',� includes some species which closely resemble Angiopteris, whilenbsp;others are characterised by more finely divided leaves with smallernbsp;ultimate segments. The fleshy stipules occasionally have an irregularlynbsp;pinnatifid form (fig. 241, B). The sporangia are represented by ovalnbsp;synangia^ (fig. 245, A; the black patches at the ends of the lateral veins)

composed of two valves, which on ripening come apart and expose two rows of pores formed by the apical dehiscence of the sporangial compartments (fig. 245, A', B). In Marattia Kavlfussii the sori are attachednbsp;to the lamina by a short stalk (fig. 246, B, B') and the leaf bears a close

^ The term synangium is applied to sporangia more or less completely united with one another and producing spores in groups separated by walls ofnbsp;sterile cells. A synangium may be regarded as a spore-forming organ producednbsp;by partial sterilization of sporogenous tissue or as a group of ooalescent sporangia.

resemblance to those of the Umbelliferous genera Anthriscus and Chaero-phyllum. The vascular system is constructed on the same plan as that of Angiopteris but is of simpler form.

Danaea. Danaea, represented by about 14 species confined to tropical America, is characterised by simple or simply pinnate leaves with linearnbsp;segments bearing elongated sori extending from the midrib almost to thenbsp;margin of the lamina. Each sorus consists of numerous sporangia in twonbsp;parallel rows united into an oblong mass partially overarched by annbsp;indusium (fig. 242, E, i) which grows up from the leaf between the sori.nbsp;In the portion of a fertile segment shown in fig. 242, E, the apical poresnbsp;are seen at a; and at �, where the roof of the S3mangium has been removed,nbsp;the spore-bearing compartments are exposed. The vascular system' agreesnbsp;in general plan with that characteristic of the family.

Kaulfussia. The form of the leaf (Vol. I. p. 97, fig. 22) closely resembles that of the Horse Chestnut; the stem is a creeping dorsiventral rhizome with a vascular system in the form of a �much perforatednbsp;solenostele^.� The synangia are circular, with a median depression;nbsp;each sporangial compartment opens by an apical pore on the sloping sidesnbsp;of the synangial cup (fig. 245, C)^.

Copeland has recently described a Marattiaceous leaf which he makes the type of a new' genus, Maa-ogloaswm alidae. The sori are nearer thenbsp;margin than in Angiopteris and are said to consist of a greater number ofnbsp;sporangia. The photograph^ of a single pinna which accompanies thenbsp;brief description hardly affords satisfactory evidence in support of thenbsp;creation of a new genus. The structure of a petiole which I have hadnbsp;an opportunity of examining, through the kindness of Mr Hewitt ofnbsp;Sarawak, shows no distinctive features.

The three genera, Ophioglossum, Botrychiuni, and Helmintho-stachys, are characterised by the division of the leaves into a sterile and a fertile lobe. The. fertile lobe in Ophioglossumnbsp;bears two rows of spherical sporangia sunk in its tissue; innbsp;Botrychium and Helminthostachys the spores are contained innbsp;large sporangia with a stout walP. The prothallus is subterranean and without chlorophyll. In the British species ofnbsp;Ophioglossum, 0. vulgatum (the adder�s tongue fern), an almostnbsp;cosmopolitan species, the sterile part of the frond is of ovalnbsp;form and has reticulate venation. In 0. pendulum and 0.

' Brebner (02) ; Eudolph (05). nbsp;nbsp;nbsp;^ Tansley (08) p. 90 ; Kiihn (90).

� Pelourde (08) has recently dealt with the anatomy of recent and fossil Marattiaceous ferns.

palmatum the lamina is deeply lobed. In the genus Botry-chium, represented in Britain by B. Lunaria, both sterile and fertile branches of the frond are pinnately divided, while innbsp;Helminthostachys the sporangia are borne on sporangiophores

Fig. 246. Ophioglossum vulgatum. Transverse section of petiole and single bundle : p, phloem; px, endaroh protoxylem.

A, diagrammatic section of stem; B, portion of the stele and endodermis enlarged.

given off from the margin of the fertile branch of a frond similar in habit to a leaf of Hellehorus.

The stem of Ophioglossum is characterised by a dictyostele of collateral bundles with endarch protoxylem: the vascularnbsp;system of the leaf-stalk is also composed of several separate

strands (fig. 246). In Botrychium the stele is a cylinder of xylem surrounded externally by phloem. This genus affordsnbsp;the only instance among ferns of a plant in which the additionnbsp;of secondary tracheae occurs on a scale large enough to producenbsp;a well-defined cylinder of secondary xylem traversed by radialnbsp;rows of medullary-ray cells' (fig. 247). The unsatisfactorynbsp;nature of the evidence in regard to the past histor}' of thenbsp;Ophioglossales renders superfluous a fuller treatment of thenbsp;recent species.

' Jeffrey (98). For an account of the anatomy of Helminthostacliys, see Farmer and Freeman (99).

CHAPTEK XXI.

FOSSIL FERNS.

Osmundaceae.

From the Culm of Silesia, Stur^ described impressions of sterile fronds which he named Todea Lipoldi on the ground ofnbsp;the similarity of the finely divided pinnules to those of Todeanbsp;superha and other filmy species of the genus. The type-specimen of Stur (in the Geological Survey Museum, Vienna)nbsp;affords no information as to sporangial characters and cannot benbsp;accepted as an authentic record of a Lower Carboniferousnbsp;representative of the family. Another more satisfactory butnbsp;hardly convincing piece of evidence bearing on the presence ofnbsp;Osmundaceae in pre-Permian fioras has been adduced bynbsp;Renault^, who described petrified sporangia from the Culmnbsp;beds of Esnost in France as Todeopsis primaeva (fig. 256, F).nbsp;These pyriform sporangia are characterised by the presence ofnbsp;a plate of large cells comparable with the subapical group ofnbsp;� annulus � cells in the sporangia of the recent species (fig. 221).

Zeiller� has published a figure of some sporangia described by Renault from Aiitun resembling the Osmundaceous type innbsp;having a plate of thick-walled cells instead of a true annulus,nbsp;but the plate is larger than the group of cells in the recentnbsp;sporangia, and both sporangia and spores are smaller in thenbsp;fossil. The sporangia from Carboniferous rocks described bynbsp;Weiss as Sturiella ¹ bear some resemblance to those of recent

The generic name Pteridotheca is employed by Scott as a convenient designation for unassigned petrified sporangia ofnbsp;Palaeozoic age with an annulus and other characters indicatingnbsp;fern-affinity. In the species P. ButterworthP the sporangia arenbsp;characterised by a group of large cells suggesting comparisonnbsp;with the annulus, or what represents the annulus, in Osmundaceae and Marattiaceae. Scott has also described a sporangium from the Coal-Measures containing germinating spores^;nbsp;the structure is similar to that of recent Osmundaceous sporangia,nbsp;and it is interesting to note that germinating spores have beennbsp;observed in the recent species Todea hymenophylloides^.

Additional evidence of the same kind is afforded by fertile specimens of a quadripinnate fern with deeply dissected oval-lanceolate pinnules described by Zeiller from the Coal-Measuresnbsp;of Heraclea in Asia Minor as Kidstonia heracleensis * (fig.nbsp;256, E). Carbonised sporangia were found at the base ofnbsp;narrow lobes of the ultimate segments and, as seen in fig.nbsp;256, E, the sporangial wall is distinguished by a plate of largernbsp;cells occupying a position like that of the � annulus � of recentnbsp;Osmundaceae. Zeiller regards the sporangia as intermediatenbsp;between those of Osmundaceae and Schizaeaceae. From thenbsp;same locality Zeiller describes another frond bearing somewhatnbsp;similar sporangia as Sphenopteris (Discopteris) Rallii (fig.nbsp;256, D)�: the term Discopteris was instituted by Stur fornbsp;fertile fronds referred by him to the Marattiaceae�.

It is by no means safe to assume that these and such Upper Carboniferous sporangia as Bower�� compared with those ofnbsp;Todea were borne on plants possessing the anatomical charactersnbsp;of Osmundaceae rather than those of the extinct Palaeozoicnbsp;family Botrjmpterideae. This brings us to the important fact,nbsp;first pointed out by Renault, that the Botryopterideae arenbsp;essentially generalised ferns exhibiting many points of contactnbsp;with the Osmundaceae�. It is clear that whether or not we

are justified in tracing the Osmundaceae as far back as the Lower Carboniferous period, some of the characteristics of thenbsp;family were already foreshadowed in rocks of this age.

Through a fortunate accident of preservation, unequivocal evidence of the existence of Osmundaceae in the Palaeozoic eranbsp;is supplied by the Russian Upper Permian genera Zalesskyanbsp;and Thamnopteris.

This generic title has been instituted by Kidston and Gwynne-Vaughan�^ for two Russian stems of Upper Permiannbsp;age, one of which was named by Eichwald^ Chelepteris gracilis,nbsp;but the probability that the type of the genus Chelepteris isnbsp;generically distinct from Eichwald�s species necessitated a newnbsp;designation for the Permian fern.

In habit the stem of Zalesskya resembles that of an Osmunda or a Todea, but it differs in the possession of a stelenbsp;composed of a continuous cylinder or solid column of xylemnbsp;surrounded by phloem, and by the differentiation of the xylemnbsp;into two concentric zones. The leaves are represented bynbsp;petiole-bases only; the sporangia are unknown. The stem andnbsp;leaf-base anatomy fully justifies the inclusion of Zalesskya innbsp;the Osmundaceae.

The type-specimen is a partially decorticated stem, from Upper Permian beds in Russia, provided with a single stele,nbsp;13 mm. in diameter, surrounded by a broad thin-walled innernbsp;cortex containing numerous leaf-traces and occasional roots; thisnbsp;was doubtless succeeded by a sclerotic outer cortex. In itsnbsp;main features Zalesskya gracilis agrees closely with Z. diploxylonnbsp;represented in fig. 249. The stele consists of a continuousnbsp;cylinder of xylem exhibiting a fairly distinct differentiationnbsp;into two zones, (i) a broader outer zone of narrower scalariformnbsp;tracheae {x ii, fig. 248) in which 20 to 25 protoxylem strands (px)

rn��m

occur just within the edge, (ii) an inner zone of broader and shorter tracheae (fig. 248, x i). The protoxylem elementsnbsp;{px, fig. 248) are characterised by a single series of scalariformnbsp;pits, while the metaxylem elements have multiseriate pits like

ph, phloem; xi, x ii, xylem ; px, protoxylem. (After Kidston and Gwynne-Vaughan. x 20.)

those on the water-conducting elements of recent Osmundaceae. The tracheae show an interesting histological character in thenbsp;absence of the middle substance of their walls, a feature recognised by Gwynne-Vaughan' in many recent ferns. Exteimal tonbsp;1 Gwynne-Vaughan (08).

the xylem and separated from it by a parenchymatous sheath is a ring of phloem, ph, composed of large sieve-tubes andnbsp;parenchyma separated from the inner cortex by a pericyclenbsp;4 to 5 layers in breadth. The occurrence of a few scleroticnbsp;cells beyond the broad inner cortex points to the formernbsp;existence of a thick-walled outer cortex. The leaf-traces are

given off as mesarch strands from the edge of the xylem; they begin as prominences opposite the protoxylem and becomenbsp;gradually detached as xylem bundles, at first oblong in transverse section, then assuming a slightly crescentic and reniformnbsp;shape, while the mesarch protoxylem strand takes up annbsp;endarch position. As a trace passes further out the curvature

increases and the protoxylem strands undergo repeated bifurcation ; it assumes in fact the form and general type of structure met with in the leaf-traces of Todea and Osmunda. Numerousnbsp;diarch roots, given off from the stele at points just below thenbsp;out-going leaf-traces, pass outwards in a sinuous horizontalnbsp;course through the cortex of the stem.

In Zalesskya gracilis the xylem cylinder was probably wider in the living plant than in the petrified stem. In Zalesskyanbsp;diploxylon^, in all probability from the same Russian locality,nbsp;there can be little doubt that the xylem was originally solid tonbsp;the centre (fig. 249). In this species also the phloem formsnbsp;a continuous band {gh, fig. 249) consisting of four to six layersnbsp;of sieve-tubes.

In 1849 Brongniart^ proposed the name Thamnopgt;teris for a species of fern from the Upper Permian of Russia originally described by Eichwald as Anornopteris Schlechtendalii. Anbsp;new name was employed by Brongniart on the ground that thenbsp;fossil was not generically identical with the species previously-named by him Anornopteris Mougeotii^. Eichwald�s specimen hasnbsp;been thoroughly investigated by Kidston and G Wynne-Vaughan^.nbsp;The stem (Frontispiece) agrees in habit with those of Zalesskyanbsp;and recent Osmundaceae; on the exposed leaf-bases the action ofnbsp;the weather has etched out the horse-shoe form of the vascularnbsp;strands and laid bare numerous branched roots boring theirnbsp;way through the petiole stumps. The centre of the stem isnbsp;occupied by a protostele 13 mm. in diameter consisting of solidnbsp;xylem separated by^ a parenchymatous sheath from a cylindernbsp;of phloem. The xylem is composed mainly of an axial columnnbsp;of short and broad reticulately pitted tracheae (fig. 250 h andnbsp;Frontispiece), distinguished from the sharply contrasted peripheral zone of normal scalariform elements, a, by their thinner

walls and more irregular shape. The protoxylem, px, is represented by groups of narrower elements rather deeply immersed in the peripheral part of the metaxylem. A many-layerednbsp;pericycle, per, and traces of an endodermis, en, succeed thenbsp;phloem, ph, which is characterised by several rows of largenbsp;contiguous sieve-tubes; beyond the endodermis is a broad thin-walled inner cortex. The leaf-traces arise as in Zalesskya,nbsp;but the protoxylem in Thamnopteris is at first central; as the

trace passes outwards a group of parenchyma appears immediately internal to the protoxylem elements and gradually assumes the form of a bay of thin-walled tissue on the innernbsp;concave face of the curved xylem. The next stage is thenbsp;repeated division of the protoxylem strand until, in the scleroticnbsp;outer cortex, the traces acquire the Osmundaceous structurenbsp;(fig. 312, A, p. 453). The petiole bases have stipular wings asnbsp;in Todea and Osmunda.

The striking feature exhibited by these Permian plants is the structure of the protostele, which in Thamnopteris andnbsp;probably in Zalesskya diploxylon consists of solid xylem surrounded by phloem: this may be regarded as the primitivenbsp;form of the Osmundaceous stele. In Osmunda regalis and innbsp;other recent species of the genus the xylem cylinder has the formnbsp;of a lattice-work; in other words, the departure of each leaf-trace makes a gap in the xylem and the overlapping of thenbsp;foliar-gaps results in the separation of the xylem into a numbernbsp;of distinct bundles. In Zalesskya gracilis the continuity of

the xylem is not broken by overlapping gaps; in this it agrees with Lepidodendron. In Thamnopteris the centre of the stelenbsp;was occupied by a peculiar form of xylem obviously ill-adaptednbsp;for conduction, but probably serving for water-storage andnbsp;comparable with the short and broad tracheae in Megaloxylon'^.nbsp;There is clearly a well-marked difference in stelar anatomynbsp;between these two Permian genera and Todea and Osmunda;nbsp;this difference appears less when viewed in the light of thenbsp;facts revealed by a study of the Jurassic species Osmmiditesnbsp;Dunlopi.

As possible examples of Triassic Osmundaceae reference may be made to some species included in Stur�s genusnbsp;Speirocarpus^. 8- virginiensis was originally described bynbsp;Fontaine^ from the Upper Triassic rocks of Virginia asnbsp;Lonchopteris virginiensis {amp;g. 251) and has recently been figurednbsp;by Leuthardt^ from the Keuper of Basel. The sporangia, whichnbsp;are scattered over the lower surface of the pinnules, arenbsp;described as globose-elliptical and as having a rudimentarynbsp;apical annulus; no figures have been published. In habitnbsp;the frond agrees with Todites Williamsoni, but the lateralnbsp;veins form an anastomosing system like that in the Palaeozoicnbsp;genus Lonchopteris (fig. 290, B). There would seem to be an anbsp;priori probability of this species being a representative of thenbsp;Osmundaceae and not, as Stur believed, of the Marattiaceae.nbsp;Seeing that Lonchopteris is a designation of a purely provi.sionalnbsp;kind, it would be convenient to institute a new generic namenbsp;for Triassic species having the Lonchopteris venation, whichnbsp;there are good reasons for regarding as Osmundaceous ferns.

Similarly Speirocarpus tenuifolius (Emmons) (= Acrostichites tenuifolius Font.), which resembles Todites Williamsoni (seenbsp;p. 339) not only in habit and in the distribution of the sporangianbsp;but also in the venation, is probably an Osmundaceous species.

This species -was found in Jurassic rocks in the Otago district of New Zealand in association with Cladophlebis denticidata^nbsp;(fig. 257). The type-specimen forms part of a stem 17 mm. innbsp;diameter surrounded by a broad mass of crowded leaf-bases.nbsp;The stele consists of an almost continuous xylem ring (fig. 252)nbsp;enclosing a wide pith; the phloem and inner cortex are notnbsp;preserved but the peripheral region of the stem is occupied bynbsp;a sclerotic outer cortex. The mass of encasing leaf-basesnbsp;resolves itself on closer inspection into zones of foliage-leaf

petioles and the petioles of scale-leaves with an aborted lamina. A similar association of two forms of leaf is seen in the existingnbsp;American species Osmunda Claytoniaiia and 0. cinnamoniea. Thenbsp;cortex and armour of leaf-bases are penetrated by numerousnbsp;diarch roots. The xylem cylinder, six to seven tracheae broad,nbsp;is characterised by the narrower diameter of its innermost elements and�an important point�by the fact that the detachmentnbsp;of a leaf-trace does not break the continuity of the xylemnbsp;cylinder (fig. 252). Each leaf-trace is at first elliptical innbsp;section; it then becomes curved inwards and gradually assumesnbsp;the horse-shoe form as in Zalesskya and in the recent species.nbsp;The single endarch protoxylem becomes subdivided until innbsp;the petiole it is represented by 20 or more strands.

In the continuity of the xylem cylinder this species of Osmundites shows a closer approach to Todea barbara ornbsp;T. superba (fig. 221, B) than to species of Osmunda', it differsnbsp;from Zalesskya in having reached a further stage in the reduction of a solid protostele to one composed of a xylemnbsp;cylinder enclosing a pith. This difference is of the same kindnbsp;as that which distinguishes the stele of Lepidodendron rhodum-nense from L. Harcourtii. In Lepidodendron short tracheaenbsp;occasionally occur on the inner edge of the xylem cylinder, and

in recent species of Todea the same kind of reduced tracheae are met with on the inner edge of the xylem^. In both cases thenbsp;short tracheae are probably vestiges of an axial strand of conducting elements which in the course of evolution have beennbsp;converted into parenchymatous cells. In Lepidodendron vasculare the mixed parenchyma and short tracheae in the centre

of the stele represent an intermediate stage in xylem reduction, and the arrangement in vertical rows of the medullary parenchyma in Lepidodendron is precisely similar to that described bynbsp;Kidston and Gwynne-Vaughan in Thamnopteris. In both casesnbsp;' Seward and Ford (03).

the rows of superposed short cells have probably been produced by the transverse septation of cells which began by elongatingnbsp;as if to form conducting tubes and ended by assuming the formnbsp;of vertical series of parenchymatous elements.

In another Jurassic species, Osmundites Gibhiana^, the xylem is of the Osmunda type and consists of about 20 strands insteadnbsp;of a continuous or almost continuous cylinder.

This species was founded on a specimen obtained by Mr Kolbe from the Uitenhage series of Cape Colony^. The fossil

flora and fauna of this series point to its correlation with the Wealden or Neocomian strata of Europe�. The type-specimennbsp;consists of several pieces of a stem (fig. 253) which reachednbsp;a length of about 90 cm. On the weathered surface thenbsp;remains ot petiole-bases are clearly seen and on the reversenbsp;side of the smaller piece shown in the figure numerous sinuousnbsp;roots are present in association with the leaf-stalks. Thenbsp;depression c in the larger specimen may mark the positionnbsp;of a branch: at a fig. 253 (enlarged in fig. 254, a) thenbsp;vascular strand of a petiole is exposed as a broad U-shapednbsp;band and at 6 (fig. 254, 6) the form of the petiole-basesnbsp;is clearly shown^. With the stem were found imperfectlynbsp;preserved impressions of fronds referred to Gladophlebis denti-

culata, a common type of leaf which was found also in association with the slightly older New Zealand stem, Osmnndites Dunlopi.

An examination of the internal structure of the South African stem by Dr Kidston and Mr Gwynne-Vaughan hasnbsp;revealed many interesting features, which will be fully describednbsp;in Part IV. of their Monograph on fossil Osmundaceous stems.nbsp;I am greatly indebted to these authors for allowing me tonbsp;publish the following note contributed by Dr Kidston:�

� The section of Osmundites Kolbei Seward, shown in fig. 255, presents the usual appearance of an Osmundaceous stock. Thenbsp;parts contained in this section are the stele, inner and outernbsp;cortex and a portion of the surrounding mantle of concrescentnbsp;leaf-bases. The whole specimen has sufiered much from pressure,nbsp;but if restored to its original form the xylem ring must havenbsp;been about 19 mm. in diameter. The number of xylem strandsnbsp;is about fifty-six and several of them are more or less joinednbsp;as in the modern genus Todea. The tracheae are of the typicalnbsp;Osmundaceous type, that is to say, the pits are actual perforations and several series of them occur on each wall of the largernbsp;tracheae.

The most interesting structural characteristic of Osmundites Kolbei is not well seen in the figure owing to the compressionnbsp;of the xylem ring. This consists in the occurrence of tracheaenbsp;in the pith. In fact, we have here a mixed pith, composednbsp;of parenchyma and true tracheae, a condition which connectsnbsp;the Osmundaceae with a parenchymatous medulla with thosenbsp;possessing a solid xylem stele like Zalesskya and Thamnopterisnbsp;and so completes the series of transitions extending from thenbsp;older and solid-steled forms to the modern medullated membersnbsp;of the Osmundaceae.��

This lower Cretaceous Canadian species,first described by Pen-hallow^ and more recently by Kidston and Gwynne-Vaughan^, is remarkable for the large size of the stem, the stele alone havingnbsp;a diameter of 2'4 cm. Penhallow figures a fragment of a leafnbsp;' Penhallow (02).nbsp;nbsp;nbsp;nbsp;^ Kidston and Gwynne-Vaughan (07).

bearing a superficial resemblance to that of OsmundaClaytoniana, which may be the foliage borne by Osmundites skidegatensis.nbsp;The xylem cylinder is broken by the exit of leaf-traces into 50nbsp;or more strands varying in size and shape, and it is noteworthynbsp;that the phloem is also interrupted as each leaf-trace is givennbsp;off. In recent species the xylem cylinder is almost alwaysnbsp;interrupted, but the phloem retains its continuity. In thenbsp;Canadian fossil an internal band of phloem occurs between thenbsp;xylem and the pith, and this joins the external phloem at eachnbsp;leaf-gap. This internal phloem finds an interesting parallel innbsp;certain recent species^, but in these the internal and externalnbsp;phloem do not meet at the foliar gaps as they do in the extinctnbsp;type. In Osmunda cinnamomea the internal phloem occursnbsp;only at the regions of branching of the stem stele; in the fossilnbsp;it is always present.

It is clear that Osmundites skidegatensis represents the most complex type of stem so far recognised in the Osmundaceae; itnbsp;illustrates a stage in elaboration of the primitive protostele innbsp;advance of that reached by any existing species.

The primitive Osmundaceous stele was composed of solid xylem surrounded by phloem {Thamnopteris and Zalesskya)',nbsp;at a later stage the xylem cylinder lost its inner zone ofnbsp;wide and short tracheae and assumed the form seen innbsp;Osmundites Kolbei, in which the centre of the stele consists ofnbsp;parenchyma with some tracheae. Another type is representednbsp;by 0. Dowkeri in which the pith is composed wholly of parenchyma and the xylem ring is continuous. From this type,nbsp;by expansion of the xylem ring and by the formation of overlapping leaf-gaps, the form represented by Osmunda regalis wasnbsp;reached. Osmunda cinnamomea, with internal phloem in thenbsp;regions of stelar branching, probably represents a further stage,nbsp;as Kidston and Gwynne-Vaughan believe, in increasing complexity due to the introduction of phloem from without throughnbsp;gaps produced by the branching of the stele. In Osmunditesnbsp;skidegatensis the leaf-gaps became wider and the external phloemnbsp;projected deeper into the stele until a continuous internal

phloem zone was produced. This most elaborate type proved less successful than the simpler forms which still survive.

Impressions of fertile pinnae with narrow linear segments bearing exannulate sporangia described by Raciborski fromnbsp;Lower Jurassic rocks in Poland as Osmunda Starii^ may withnbsp;some hesitation be included in the list of Mesozoic Osmundaceae.

Under this name Carruthers^ described a petrified stem from Lower Eocene beds at Herne Bay, which in the structure of thenbsp;stele agrees closely with the Jurassic species 0. Gibbiana andnbsp;conforms to the normal Osmundaceous type. It is possible,nbsp;as Gardner and Ettingshausen^ suggested, that the foliage ofnbsp;this species may be represented by some sterile Osmunda-likenbsp;fragments recorded from the Middle Bagshot beds of Boveynbsp;Tracey and Bournemouth as Osmunda lignitum.

This generic name^ has been applied to fossil ferns exhibiting in the structure of the sporangia and in the general habit of thenbsp;fertile fronds a close resemblance to the recent species Todeanbsp;barbara (fig. 221, D, p. 286).

1828. nbsp;nbsp;nbsp;Pecopteris WiHiamsonis, Brongniart, Prodrome, p. 57 ; Hist. veg.

� nbsp;nbsp;nbsp;P. whitbiensis, Brongniart, Hist. v�g. foss. p. 321, PI. cix. figs.

1829. nbsp;nbsp;nbsp;Pecopteris recentior, Phillips, Geol. Yorks, p. 148, PL viii. fig. 15.

1833. Neuropteris recentior, Lindley and Hutton, Foss. Flora, Vol. i. PL LXVIII.

^ Carruthers (70) A.; Kidston and Gwynne-Vaughan (07) p. 768; see also Seward, Vol. i. p. 212.

1867. Acrostichites Goeppertianus, Schenk, Foss. Flor. Grenzsch. p. 44, PI. V. fig. 5, PI. VII. fig. 2.

[B, C, from specimens (13491; 39234) in the British Museum (B, very slightly reduced; C, J nat. size); �gt;, B, after Zeiller; E, after Eenault; G,nbsp;after Eaciborski.]

1883. A. linnaeaefolius, Fontaine, Older Mesoz. Flora Virginia, p. 25, Pis. VI.�IX.

1889. Gladophlehis virginiemis, Fontaine, Potomac Flora, p. 70, PI. iii. figs. 3�8; PI. IV. figs. 1, 4.

It is hopeless to attempt to arrive at satisfactory conclusions in regard to the applicability of the name Todites Williamsoninbsp;to the numerous fronds from Jurassic and Rhaetic rocks,nbsp;agreeing more or less closely with Brongniart�s type-specimen.nbsp;Specimens from the Rhaetic may not be specifically identicalnbsp;with those from the Jurassic; the main point is that, whethernbsp;actually identical or not, both sets of fossils clearly represent thenbsp;same general type of Osmundaceous fern^ and may for presentnbsp;purposes be included under the same designation. The abovenbsp;synonymy, though by no means complete^, serves to illustratenbsp;the confusion which has existed in regard to this widely spreadnbsp;type of Mesozoic fern.

Frond bipinnate; long linear pinnae (20�30 cm.) of uniform breadth arise at an acute angle, or in the lower part of a frond, almost at rightnbsp;angles, from a stout rachis. Closely set pinnules attached by a broadnbsp;base; slightly falcate, the side towards the rachis strongly convex and thenbsp;outer margin straight or concave and bulged outwards towards the base ofnbsp;each segment, margin usually entire, or it may be slightly lobed. Fertilenbsp;pinnules similar to the sterile; sporangia of the Osmimdaceous type andnbsp;often scattered over the whole lower suface of the lamina (fig. 256, B,nbsp;B', G). Venation of the Cladophlehis type (cf. fig. 256, A).

It is not always easy to distinguish Todites Williamsoni from Cladophlehis (fenfrcttZate,another common Jurassic fern,but in thenbsp;latter the pinnules are usually longer and relatively narrower andnbsp;the rachis is more slender (cf. fig. 256, B and 257). Schenk^ andnbsp;Haciborski^ have shown that the sporangia of Todites conformnbsp;in the absence of a true annulus to those of Todea (fig. 256, G)nbsp;and Osmunda. Nathorst� has recently figured a group of sporesnbsp;of Todites Williamsoni in illustration of the use of the treatment of carbonised impressions with nitric acid and potassiumnbsp;chlorate. This species, though widely distributed in Jurassicnbsp;rocks, is hardly distinguishable from the German Rhaetic frondsnbsp;figured by Schenk from Bayreuth as Acrostichites Goeppertianus^,

or from other fossils referred to an unnecessarily large number of species by Fontaine^ from Upper Triassic rocks of Virginia^.

It would seem from the paucity of later records of Os-mundaceae that the family reached its zenith in the Jurassic era. When we pass to the later Tertiary and more recent deposits

evidence is afforded in regard to the geographical range of Osmunda regalis. It has been shown to occur in the Pliocenenbsp;forest-bed of Norfolk^ as well as in Palaeolithic and Neolithicnbsp;deposits*.

^ The geographical distribution of Todites and other genera will be dealt with in Volume in.

Africa referred to Cladophlebis (Todites) Roesserti (Presl)* represents in all probability an Osmundaceous fern closely alliednbsp;to Todites Williamsoni. The same species is described bynbsp;Zeiller^ from Khaetic rocks of Tonkin and very similar typesnbsp;are figured by Leuthardt� from Upper Triassic rocks of Baselnbsp;as Pecopteris Rutimeyeri Heer, and by Fontaine^ from rocks ofnbsp;the same age in Virginia.

The generic name Cladophlebis was instituted by Brongniart for Mesozoic fern fronds characterised by ultimate segments ofnbsp;linear or more or less falcate form attached to the pinnae bynbsp;the whole of the base, as in the Palaeozoic genus Pecoptens,nbsp;possessing a midrib strongly marked at the base and dividingnbsp;towards the distal end of the lamina into finer branches andnbsp;giving off secondary forked and arched veins at an acutenbsp;angle. The term is generally restricted to Mesozoic fern frondsnbsp;which, on account of the absence or imperfection of fertile pinnae,nbsp;cannot be safely assigned to a particular family. In the case ofnbsp;the species described below, the evidence in regard to systematicnbsp;position, though not conclusive, is sufficiently strong to justifynbsp;its inclusion in the Osmundaceae.

This species is often confused� with Todites Williamsoni. The name Pecopteris whitbiensis has been used by differentnbsp;writers for Jurassic fronds which are undoubtedly specificallynbsp;distinct: specimens so named by Brongniart should be referrednbsp;to Todites Williamsoni, while P. whitbiensis of Bindley and

� E.g. by Yokoyama (06) who identifies specimens of Cladophlebis denticulata from Jurassic rooks of China as Todites Williamsoni. �

Hutton' is Brongniart�s Cladophlebis denticulata. It is impossible to determine with accuracy the numerous examples described as Pecopteris whitbiensis, Asplenium whitbiense,nbsp;Cladophlebis Albertsii (a Wealden species^), Asplenium, ornbsp;Cladophlebis, nebbense^, etc., from Jurassic and Ehaetic strata.nbsp;The Cladophlebis denticulata form of frond is one of thenbsp;commonest in recent ferns; it is represented by such species asnbsp;Onoclea Struthopteris, Pteris arguta, Sadleria sp., Gleichenianbsp;dubia, Alsophila lunulata, Cyathea dealbata, and species ofnbsp;Polypodium. It is, therefore, not surprising to find records ofnbsp;this Mesozoic species from many localities and horizons. Allnbsp;that we can do is to point out what appear to be the mostnbsp;probable cases of identity among the numerous examples ofnbsp;fronds of this type from Mesozoic rocks, particularly Ehaeticnbsp;and Jurassic, in different parts of the world. The name Cladophlebis denticulata may be employed in a comprehensive sensenbsp;for fronds showing the following characters:�

Leaf large, bipinnate, with long spreading pinnae borne on a comparatively slender rachis. Pinnules, in nearly all cases, sterile, reaching a length of 3�4 cm., acutely pointed, finely denticulate or entire, attachednbsp;by the whole of the base (fig. 257). In the apical region the pinnulesnbsp;become shorter and broader. Venation of the Cladophlebis type (fig. 256, A).nbsp;Fertile pinnules rather straighter than the sterile, characterised bynbsp;linear sori parallel to the lateral veins (fig. 258).

In endeavouring to distinguish specifically between fronds showing a general agreement in habit with C. denticulata,nbsp;special attention should he paid to venation characters, thenbsp;shape of the pinnules, the relation of the two edges of thenbsp;lamina to one another, and to the amount of curvature of thenbsp;whole pinnule. Unless the material is abundant, it is oftennbsp;impossible to distinguish between characters of specific valuenbsp;and others which are the expression of differences in age or ofnbsp;position on a large frond, to say nothing of the well-knownnbsp;variability which is amply illustrated by recent ferns. It isnbsp;remarkable that very few specimens are known which thrownbsp;any light on the nature of the fertile pinnae. Fig. 258 repre-

sents an impression from the Inferior Oolite rocks of the Yorkshire coast in which the exposed upper surface of thenbsp;pinnules shows a series of parallel ridges following the coursenbsp;of the lateral veins and no doubt formed by oblong sori on thenbsp;lower surface. There can be little doubt that the specimennbsp;figured by Lindley and Hutton and by others as Pecopterisnbsp;undans^ is, as Nathorst suggests, a portion of a fertile frond ofnbsp;C. dentioulata. A fertile specimen of a frond resembling innbsp;habit C. denticulata, which Fontaine has described from the

Fig. 258. Fertile pinnae of Cladophlebis denticulata. (From a Yorkshire specimen in the Sedgwick Museum, Cambridge.)

Jurassic rocks of Oregon as Danaeopsis StorrsiP, exhibits, as that author points out, a superficial resemblance to the specimennbsp;named by Lindley and Hutton Pecopteris undans. There is,nbsp;however, no adequate reason for referring the Americannbsp;fragment to the Marattiaceae. In the absence of sporangia wenbsp;cannot speak confidently as to the systematic position of thisnbsp;common type; but there are fairly good grounds for the assertionnbsp;that some at least of the fronds described under this namenbsp;are those of Osmundaceae, The English specimen shown in

fig. 258 is very similar to some Indian fossils figured by Feist-mantel as Asplemtes macrocarpus^, which are probably identical with Pecopteris australis Morris^, a fern that is indistinguishablenbsp;from Cladophlebis denticulata. Kenault^ figured a fertile specimen of the Australian fossil as Todea australis, which agreesnbsp;very closely with that shown in fig. 258, and the sporangianbsp;figured by the French author are of the Osmundaceous type.nbsp;Another example of a fertile specimen is afforded by a Rhaeticnbsp;fern from Franconia, Asplenites ottonis, which is probablynbsp;identical with A lethopteris Roesserti Presl [= Cladophlebisnbsp;(Todites) Roesserti], a plant closely resembling Cladophlebisnbsp;denticulata. Another argument in favour of including C. denticulata, in the Osmundaceae is supplied by the association ofnbsp;pinnae of this type with the petrified stem of Osmunditesnbsp;Dunlopi recorded by Kidston and Gwynne-Vaughan.

Evidence bearing on the existence of this family in Carboniferous floras is by no means decisive. The generic name Aneimites proposed by Dawson^ for some Devonian Canadiannbsp;plants resembling species of the recent genus Aneimia, andnbsp;adopted by White� for a species from the Potsville beds ofnbsp;Virginia, is misleading. The Canadian plants give no indication of the nature of the reproductive organs, and the frondsnbsp;described by White are, as he shows, those of a Pteridospermnbsp;and bore seeds.

An examination of the suspiciously diagrammatic drawings published by Corda� of the small fertile pinnules of a Carboniferous fern from Bohemia, which he named Senftenbergianbsp;elegans, leads us to conclude that the sporangia are almostnbsp;certainly those of a Schizaeaceous species. The small linearnbsp;pinnules bear two rows of sessile sporangia, singly as in recentnbsp;Schizaeaceae and not in sori, characterised by 4�5 rows ofnbsp;regular annular cells (fig. 270, A) surrounding the apex. Itnbsp;has already been pointed out that the apical annulus of recent

Schizaeaceae, though normally one row deep, may consist in part at least of two rows. Zeiller' examined specimens ofnbsp;Corda�s species and decided in favour of a Schizaeaceousnbsp;affinity; he describes the sporangia as 0'85�0'95 mm. in length,nbsp;with 3 to 5 and occasionally only two rows of cells in the apicalnbsp;annulus. Zeiller�s figures (fig. 270, A) confirm the impressionnbsp;that Corda�s drawings are more beautiful than accurate. Stur^,nbsp;on the other hand, who first pointed out that the type-specimensnbsp;of Senftenhergia came from the Radnitz beds of Bohemia andnbsp;not from the Coal-Measures, convinced himself that thenbsp;sporangia have no true annulus (fig. 270, E). He describes themnbsp;as characterised by a comparatively strong wall and by thenbsp;presence of a band of narrow vertical cells marking the line ofnbsp;dehiscence, features which lead him to assign the plant to thenbsp;Marattiales, a group which seems to have exercised a dominatingnbsp;influence over his judgment. In a later publication Zeiller^nbsp;replies to Stur�s criticism but adheres to his original opinion.nbsp;Solms-Laubach^, while expressing himself in favour of Marat-tiaceous affinity, recognises that Zeiller�s arguments cannot benbsp;set aside.

The question must remain open until further evidence is forthcoming; but it would seem that this Carboniferous type,nbsp;not as yet recognised in Britain, possessed sporangia having anbsp;distinct resemblance to those of the Schizaeaceae, though thisnbsp;similarity does not amount to proof of the existence of thenbsp;family in the Palaeozoic era.

Palaeozoic floras may be described as rich in generalised types, types foreshadowing lines of evolution, which in thenbsp;course of ages led to a sorting and a redistribution of characters.nbsp;It may be that Senftenhergia is one of these generalised types.

It is not until we ascend the geological series as far as the older Jurassic rocks that we meet with a type which can withnbsp;confidence be classed with the Schizaeaceae, as least so far asnbsp;sporangial characters are concerned. The species Klukia exilis

is selected as the best known and most widely-spread representative of Jurassic Schizaeaceae.

of the Yorkshire coast as Pecopteris exilis. Bunbury�s^ discovery (supplemented by additional evidence obtained by Raciborski)nbsp;of well-preserved sporangia justified the substitution of anbsp;distinctive designation for the provisional term Pecopteris.

Frond tripinnate, of the Cladophlebis type; pinnae linear, lanceolate, attached to the rachis at a wide angle. Ultimate segments short andnbsp;linear, entire or, in the lower part of a frond, crenulate, 5 mm. long ornbsp;occasionally longer. Sporangia 0-5 mm. in length, borne singly on the lowernbsp;surface of the lamina in a row on each side of the midrib.

A re-examination� of the specimen described by Bunbury confirmed his account of the structure of the sporangia. Thenbsp;pinna shown in fig. 259 is characterised by unusually smallnbsp;fertile pinnules some of which bear 10 sporangia in two rows;nbsp;the annulus includes about 14 cells. Fertile specimens of thisnbsp;and similar forms are figured by Raciborski� from Jurassic rocksnbsp;of Poland, and good examples of the English species may be seen

in the Leckenby collection, Cambridge, in the British Museum, the museums of Manchester, Scarborough, and other places.

It is possible that specimens referred to K. exilis by Yokoyama^ from Wealden strata in Japan may afford evidencenbsp;of the persistence of the species beyond the Jurassic era, but in

Specimens from the Wealden of Su.ssex; British Museum ; V. 2333, V. 2160, V. 2166.)

view of the close resemblance of the sterile fronds described from Wealden strata as Cladophlebis Brownii^ and C. Dunkeri^nbsp;to those of Klukia exilis, identity can be established only by annbsp;examination of fertile specimens. A Jurassic fern recentlynbsp;described by Yabe� from Korea as Cladophlebis koraiensis maynbsp;be identical with K. exilis and there is little doubt as to thenbsp;existence of the species in Jurassic Caucasian strata*.

' Yohoyama (89). nbsp;nbsp;nbsp;� Seward (94^) A.nbsp;nbsp;nbsp;nbsp;3 Yabe (05) PI. iii.

This Wealden fern^ has been doubtfully assigned to the Schizaeaceae on the ground of the resemblance of the sterilenbsp;fronds to those of some species of Aneimia, and because of thenbsp;difference between the sterile and fertile pinnae (Fig. 260).nbsp;Ruffordia cannot be regarded as a well authenticated membernbsp;of the Schizaeaceae,

Fragments of forked pinnules, agreeing very closely in venation and general appearance with recent species of Lygodium, have been identified by Gardner and Ettingshausen^ fromnbsp;English Eocene beds and by Knowlton from the Miocene beds ofnbsp;the Yellowstone Park� as Lygodium Kaulfussi Heer (fig. 261, B).nbsp;Despite the absence of sporangia it is probable that these

^ Gardner and Ettingshausen (82) p. 47, Pis. vii. x.; Heer (55) A. PI. iii. p. 41. 3 Knowlton (99) PI. Lxxx.

fragments are correctly referred to the Schizaeaceae. The sterile and fertile specimens figured by Heer^ from Tertiary bedsnbsp;of Switzerland agree very closely with recent examples ofnbsp;Lygodium. Similar though perhaps less convincing evidencenbsp;of the existence of this family in Europe is furnished bynbsp;Saporta^, who described two Eocene species from France.

The application by Goeppert^ and other earlier writers of the generic name Oleichenites to examples of Palaeozoic fernsnbsp;was not justified by any satisfactory evidence. One of Goeppert�snbsp;species, Gleichenites neuropteroides, is identical with Neuropterisnbsp;heterophylla^, a plant now included in the Pteridosperms.

The resemblance of sporangia and sori, whether preserved as carbonised impressions or as petrified material, from Carboniferous rocks, to those of recent species of Gleicheniaceaenbsp;is in many cases at least the result of misinterpretation ofnbsp;deceptive appearances. Williamson� drew attention to thenbsp;Gleichenia-like structure of some sections of sporangia fromnbsp;the English Coal-Measures, but he did not realise the ease withnbsp;which sections of Marattiaceous sporangia in different planesnbsp;may be mistaken for those of annulate (leptosporangiate)nbsp;sporangia. In the regular dichotomous habit of Carboniferousnbsp;fronds described as species of Biplothmema (Stur) andMariopterisnbsp;(Zeiller)� we have a close correspondence with the leaves ofnbsp;Gleichenia, but the common occurrence of dichotomous branchingnbsp;among ferns is sufficient reason for regarding this feature asnbsp;an untrustworthy criterion of relationship. It is, however,nbsp;interesting to find that in addition to the existence of somenbsp;Upper Carboniferous ferns with sori like those of recent Glei-chenias, the type of stelar anatomy illustrated by Gleichenianbsp;dicarpa (fig. 237, C, p. 310) and other species is characteristicnbsp;of the primary structure of the stem of the Pteridospermnbsp;Heterangium. We find in Carboniferous types undoubtednbsp;indications of anatomical and other features which in succeedingnbsp;ages became the marks of Gleicheniaceae.

Some Carboniferous fronds with short and small pinnules of the Pecopteris type, bearing sori composed of a small number ofnbsp;sporangia, have been assigned by Grand�Eury and other authorsnbsp;to the Gleicheniaceae; the same form of sorus is met withnbsp;also on fronds with Sphenopteroid segments. The former isnbsp;illustrated by Oligocarpia GutbierP and the latter by 0. Brong-niarti described by Stur and by Zeiller^. Zeiller has describednbsp;the circular sori of Oligocarpia (fig. 270, B) as consisting ofnbsp;three to ten pyriform sporangia borne at the ends of lateralnbsp;veins and possessing a complete transverse annulus, but Stur*nbsp;believes that the annulus-like appearance is due to the mannernbsp;of preservation of exannulate sporangia. In this opinion Sturnbsp;is supported by Solms-Laubachand by Schenk�. Despitenbsp;an agreement between Oligocarpia and Gleichenia, as regardsnbsp;the form of the sori and the number of sporangia, it is notnbsp;certain that the existence of a typical Gleicheniaceous annulusnbsp;has been proved to occur in any Palaeozoic sporangia�.

From Upper Triassic beds of Virginia, Fontaine has figured several fronds for which he instituted the genus Mertensides��.nbsp;The habit, as he points out, is not dichotomous, but the sori arenbsp;circular and are said to be composed in some species of four tonbsp;six sporangia. No satisfactory evidence is brought forward innbsp;support of the use of a designation implying a close relationshipnbsp;with recent Gleichenias (sect. Mertensia). One of the speciesnbsp;described by Fontaine was originally named by Bunburynbsp;Pecopteris bullatus^, the imperfect type-specimen of which isnbsp;now in the Museum of the Cambridge Botany School. In thenbsp;form of the frond, the thick rachis, and in the pinnules thisnbsp;Triassic species resembles Todites Williarnsoni, but the resemblance does not extend to the sori. Two of Fontaine�s speciesnbsp;are recorded by Stur from Austria�, but he places them in thenbsp;genus Oligocarpia and includes them in the Marattiaceae.

� Dr Scott tells me that an examination of Dr Zeiller�s specimens led him to agree with the latter�s description of the annulus of Oligocarpia. (A. C. S.)

� Bunbury (47) PI. n. fig. 1; Seward (94^) A. p. 189. nbsp;nbsp;nbsp;� Krasser (09) p. 16.

Leuthardt^ figures what appears to be a Gleicheniaceous fern from the Upper Triassic beds of Basel as Gleichenites gracilisnbsp;(Heer) showing sori composed of five sporangia (fig. 265, C)nbsp;with a horizontal annulus. A Rhaetic species Gleichenitesnbsp;microphyllus Schenk^ from Franconia agrees in the form of itsnbsp;small rounded pinnules with Gleichenia, but no sporangia havenbsp;so far been found.

An impression of a frond from Jurassic rocks of northern Italy figured bj^ Zigno as Gleichenites elegans^ closely resemblesnbsp;in habit recent species of Gleichenia] though no sporangia havenbsp;been found, the habit of the frond gives probability to Zigno�snbsp;determination.

A Jurassic species from Poland, Gleichenites Bostafinskii, referred by Raciborski^ to Gleichenia, exhibits a close agreement in habit and in the form of the soral impressions to somenbsp;recent species of Gleichenia.

As we pass upwards to Wealden and more recent rocks it becomes clear that the Gleicheniaceae were prominent membersnbsp;of late Mesozoic floras in north Europe and reached as far northnbsp;as Disco Island. In English Wealden beds portions of sterilenbsp;fronds have been found which were assigned to a new genusnbsp;Leckenhya^, but it is probable that these specimens would benbsp;more correctly referred to Gleichenites. Similarly fragments ofnbsp;Gleichenia-like pinnae with very small rounded pinnules occur innbsp;the Wealden rocks of Bemissart, Belgium�, in north Germany^,nbsp;and elsewhere. Conclusive evidence has been obtained bynbsp;Prof Bommer of the existence of Gleichenites in Wealden bedsnbsp;near Brussels, where many plant remains have been found in anbsp;wonderful state of preservation. The specimens, which I had annbsp;opportunity of seeing some years ago, might easily be mistakennbsp;for rather old and brown pieces of recent plants. Some of thenbsp;Belgian fragments, of which Prof. Bommer has kindly sentnbsp;me drawings and photographs, are chai-acterised by an arrange-

ment of vascular tissue identical with that in the petioles and rhizomes of some protostelic Gleichenias. The stele of one ofnbsp;the Belgian rhizomes appears to be identical with that ofnbsp;Oleichenia dicarpa (fig. 237, C. p. 310).

This species, originally described by Corda as Pecopteris ZippeP and afterwards figured by Heer^ as Oleichenia Zippeinbsp;1 Corda, in Eeuss (46) p. 95, PI. xi.ix. ^ Heer (75), p. 44, Pis. iv.�vii.

(fig. 262, D) from Urgonian rocks of Greenland, affords a striking example of a Mesozoic member of the Gleicheniaceae.nbsp;It is characterised by the dichotomous branching of the frondnbsp;and by the occurrence of arrested buds in the forks. The longnbsp;and slender pinnae, reaching a length of 9 cm. and a breadth ofnbsp;6.�8 mm., bear small crowded pinnules occasionally with circularnbsp;sori which are described by Heer as consisting of a small numbernbsp;of sporangia (cf fig. 262, C). Several other Lower Cretaceousnbsp;species are recorded by Heer from Greenland, some of whichnbsp;are probably unnecessarily separated from Gleicheiiites Zippei.nbsp;Examples of these are represented in fig. 262, A, B, C.

A Gleicheniaceous species described by Debey and Ettings-hausen from Lower Cretaceous rocks of Aix-la-Chapelle as Didymosoribs comptonifoliiis^ is very similar in habit to somenbsp;of Heer�s Greenland species; this should probably be referrednbsp;to the genus Gleichenites.

From the Eocene beds of Bournemouth, Gardner and Ettings-hausen^ have described under the name Gleichenia hantonensis what is in all probability a true Gleichenia (fig. 263). Thisnbsp;species, originally recorded by Wanklyn�, is characterised by anbsp;slender forked rachis showing what may be traces of arrestednbsp;buds between the arms of the branches, by circular sori ofnbsp;six or eight sporangia and by the presence of peculiar tendrillike appendages on the pinnae. If the description of thenbsp;tendrils is correct, this British species affords one of the fewnbsp;instances of ferns adapted for climbing and may be comparednbsp;with the recent species Davallia aculeata (fig- 232, p. 299).

The genera Laccopteris and Matonidium may be described as examples of Mesozoic ferns exhibiting a very close agreementnbsp;with Matonia.

Laccopteris. This genus, founded by Preslb may be described as follows:�

Frond pedate, in habit resembling Mat�nia pectinata, with pinnate or pinnatifid pinnae; ultimate segments linear, provided with a well-markednbsp;midrib giving off numerous dichotomously branched secondary veins whichnbsp;are in places connected by lateral anastomoses. Sori circular, forming anbsp;single row on each side of the midrib (fig. 278, B); sporangia 5�15 in eachnbsp;sorus, with an oblique annulus and tetrahedral spores. The presence ofnbsp;an indusiiim is not certainly established.

Schenk^, who described several specimens of Laccopteris from Rhaetic rocks of Germany, compared the genus withnbsp;Gleichenia but he also recognised the close resemblance to

Matonia pectinata. Zeiller^ first established the practical identity of the sori and sporangia of Laccopteris and Matonia.nbsp;The Uhaetic species, such as L. Muensteri, L. elegans, and L.nbsp;Goepperti, agree very closely with L. polypodioides and need notnbsp;be described in detail.

The Ehaetic species Laccopteris elegans, represented in fig. 264, illustrates the characteristic habit of the genus andnbsp;shows a feature usually overlooked^, namely the occurrence ofnbsp;anastomoses between the lateral veins. The form of the sorus

of another Rhaetic species is shown in fig. 265, E. Schenk figures an interesting series of fronds of L. Goepperti in differentnbsp;stages of growth'; one of the younger leaves is seen in fig. 265, D.nbsp;An examination of Rhaetic specimens of Laccopteris in thenbsp;Bergakademie of Berlin convinced me of the correctness of thenbsp;published descriptions of the sori.

Laccopteris polypodioides (Brongniart).

1828. Phlebopteris polypodioides'^, Brongniart, Hist. v�g. foss. p. 372, PL LXXXIII. fig. 1.

In habit this species closely resembles Matonia and Matoni-dium, the long petiole divides distally into several spreading

pinnatifid pinnae with linear ultimate segments (fig. 278, A). Circular sori (indusiate?) occur in a single row on each side ofnbsp;the midrib containing 12�14 large sporangia (fig. 266) characterised by an obliquely vertical annulus. The midrib of the

pinnules gives off secondary veins at a wide angle and these form a series of elongated meshes parallel to the median rib,nbsp;as in the recent genus Woodwardia; forked and anastomosingnbsp;branches are given off from these to the edge of the laminanbsp;(fig. 267).

A very similar, possibly a specifically identical plant, was described by Leckenby from English Jurassic rocks as Phlebo-pteris WoodwardP, the distinguishing features of which are thenbsp;greater number of lateral veins and the smaller sori (fig. 267, A).

differing slightly from those of Laccopteris in venation characters: he included Laccopteris Woodwardi in this genus, but suchnbsp;differences as are recognisable in the venation hardly justify thenbsp;use of a distinct generic title. Similarly, specimens described bynbsp;Debey and Ettingshausen� from Lower Cretaceous rocks of Aix-la-Chapelle as species of Carolopteris may also be included innbsp;Laccopteris.

1 nbsp;nbsp;nbsp;Leckenby (64) A. p. 81, PI. viii. fig. 6. (Type-specimen in the Sedgwicknbsp;Museum, Cambridge.)

2 nbsp;nbsp;nbsp;Saporta (73) A. p. 306.nbsp;nbsp;nbsp;nbsp;^ Debey and EttingsHausen (59) PI. iir.

This species is represented in several Wealden localities by fragments of fertile pinnae similar to those of L. polypodioides.nbsp;It is almost impossible to distinguish small specimens of thenbsp;Wealden fern from Heer�s genus Nathorstia (Marattiaceae)nbsp;unless the sori are well preserved. This species occurs innbsp;Wealden beds in England, Germany, Belgium, and elsewherenbsp;and has been discovered by Dr Marcus Gunn in Upper Jurassicnbsp;plant-beds of Sutherlandshire (N.E. Scotland).

Laccopteris is widely spread in Rhaetic, Jurassic and Lower Cretaceous floras. It affords evidence of the former abundancenbsp;in northern latitudes of a family now represented by the twonbsp;species of Matonia confined to a restricted area in the southernnbsp;hemisphere.

Schenk^ instituted this convenient term for fossil fern fronds agreeing in habit and in their sori with Matonia pectinata (figs.nbsp;227, 228, p. 292). Zeiller^ has drawn attention to the fact thatnbsp;the Mesozoic species differ from the surviving types in thenbsp;greater number of sporangia in each sorus, and, it may be added,nbsp;in Matonidium the fertile pinnules are more richly supplied withnbsp;sori than are those of Matonia. Unfortunately our knowledgenbsp;of the structure of the sporangia of Matonidium is less completenbsp;than in the case of Laccopteris, but such evidence as is availablenbsp;justifies the conclusion that Matonia is a direct descendant ofnbsp;ferns which formed a prominent feature in European Jurassicnbsp;and Wealden floras. It is interesting to find that in a Cretaceousnbsp;species, described by Krasser (fig. 265, A) since the publicationnbsp;of Zeiller�s paper, the sori appear to be identical in distributionnbsp;and in appearance with those of the recent species.

I am indebted to Prof Bommer for permission to reproduce the unpublished drawing represented in fig. 237 D (p. 310) of anbsp;section of the rhizome of Matonidium from the Belgian Wealden

beds of Hainaut (� Flore Bernissartienne �). The section shows an arrangement of vascular tissue identical with that in the recentnbsp;species: there may be two solenosteles and in addition a solidnbsp;axial strand. The form of the leaf-trace in the fossil appears tonbsp;be identical with that in Matorda pectinata (fig. 237, A, p. 310).

Oolite and Wealden strata in Britain and elsewhere. It is, however, not impossible that if more information were available,nbsp;we should find adequate reasons for recognising two specificnbsp;types. Fontaine^, adhering rigidly to the rules of priority,nbsp;speaks of this species as Matonidium Althausii (Dunker), butnbsp;Ettingshausen�s specific term is better known.

1 nbsp;nbsp;nbsp;Ettingshausen (52) p. 16, Pi. v. For synonymy, see Seward (94^) A; (00).

Fronds pedate and apparently identical in habit with those of Matonia pectinata; ultimate segments linear, slightly falcate and bluntly pointed.nbsp;Sori circular or oval, numerous, containing 15 to 20 sporangia with an obliquenbsp;annulus, in two rows on the lower surface of the pinnules; indusium as innbsp;Matonia.

The English examples have so far afforded no information in regard to sporangial structure, but Schenkhas recognised anbsp;distinct annulus in German material. In his description of fossilnbsp;plants from Lower Cretaceous rocks in California, Fontaine^nbsp;doubtfully identifies two very small fragments as Matonidiumnbsp;Althausii', the evidence is, however, wholly inadequate.

This Cenomanian (Cretaceous) species from Moravia appears to be identical in habit with the older type. The pinnules arenbsp;larger and bear fewer sori. Krasser�s figures of the sterile pinnulesnbsp;show no lateral anastomosing between the secondary veins, butnbsp;the small vascular network below each sorus (fig. 265, A) isnbsp;identical with that in Matonia pectinata. The indusiate sorinbsp;contain about six sporangia with an oblique afinulus.

The very wide geographical distribution of the Matonineae during the Mesozoic era affords a striking contrast to thenbsp;limited range of the Malayan survivals.

The frequent use of the generic name Hymenophyllites as a designation of Palaeozoic ferns, more particularly in the oldernbsp;literature, is another instance of the undue importance whichnbsp;palaeobotanists have always been prone to attach to externalnbsp;resemblances of vegetative organs. The fragment of laminanbsp;described by Stur for the Culm Measures of Austria as Hymeno-phyllum waldenhurgense^ has no claim to consideration asnbsp;evidence of Palaeozoic Hymenophyllaceae. On the other hand,

^ Schenk (71) p. 19. nbsp;nbsp;nbsp;^ Fontaine, loc. cit. PI. lxv. figs. 22, 23.

there are a few records of fertile fronds which, though not to be accepted without reserve, are worthy of more careful examination. Some petrified sporangia described by Eenault' fromnbsp;the Culm of Esnost are referred to Hymenophyllites on accountnbsp;of the position of the annulus, which appears to encircle aboutnbsp;two-thirds of the circumference; it is, however, not certain thatnbsp;the annulus is horizontal as in the recent genus.

The Culm species Bhodea patentissima described by Ettings-hausen^ as Hymenophyllites patentissima and subsequently

E. nbsp;nbsp;nbsp;G. Sphenopteris (Hymenophyllites) quadridactylites.nbsp;F, G, after Zeiller ; D, after Hooker ; B, after Stur.)

referred by Stur^ to Bhodea, is regarded by these authors as closely allied to Hymenophyllum simply on the ground of thenbsp;finely divided and delicate sterile fronds; another species, Bhodeanbsp;moravica (Ett.), which Ettingshausen referred to Trichomanes,nbsp;is compared with recent species of that genus. In neithernbsp;case do we know anything of sporangial characters.

A fertile sphenopteroid frond figured by Schimper as Hymenophyllum Weissi^ from the Coal-Measures of Saarbrilcken

bears some resemblance to recent Hymenophyllaceae, but the figures are by no means convincing: an examination of thenbsp;type-specimens in the Strassburg Museum led Solms-Laubach^nbsp;to express dissent from Schimper�s determination. A morenbsp;satisfactory example is that afforded by the fertile pieces of anbsp;frond described by Zeiller^ from French Coal-Measures asnbsp;Hymenophyllites quadridactylites (Gutbier). Some of thenbsp;ultimate segments with a truncated tip are preserved in closenbsp;association with a group of oval sporangia with a completenbsp;transverse annulus (fig. 270, F, G). The position of the sporangianbsp;is such as to suggest their separation from a terminal columnarnbsp;receptacle like that in Trichomanes and Hymenophyllum. Innbsp;his account of this species from the Coal-Measures of the Forestnbsp;of Wyre, Kidston� states that Zeiller informed him that he hadnbsp;noticed traces of what appeared to be a columnar receptacle innbsp;the French specimens.

The records of Hymenophyllaceae from the Mesozoic and Tertiary formations are not such as need detain us. The factsnbsp;bearing on the geological history of this family are singularlynbsp;meagre. There is no evidence which can be adduced innbsp;favour of regarding the Hymenophyllaceae as ferns of greatnbsp;antiquity, which played a prominent part in the floras ofnbsp;the past.

It is interesting to find that the genus Aiikyropteris^, one of the Botryopterideae (a group of Palaeozoic Ferns for which Inbsp;propose the name Coenopterideae), has a morphological characternbsp;in common with Trichomanes, namely the production of axillarynbsp;buds: there are also features in the stelar anatomy shared bynbsp;the Botryopterideae and Hymenophyllaceae�. These resemblances, though by no means amounting to proof of nearnbsp;relationship, point to a remote ancestry for certain featuresnbsp;retained by existing members of the Hymenophyllaceae.

1 Solms-Laubach (91) A. p. 153. nbsp;nbsp;nbsp;^ Zeiller (83) p. 155; (88) A. PI. viii. figs. 1-3.

3 Kidston (84�) p. 593. nbsp;nbsp;nbsp;* See p. 450.nbsp;nbsp;nbsp;nbsp;� Scott (08) p. 343.

Stur founded the species Thyrsopteris schistoruni^ are too imperfectly preserved to warrant the use of this generic name. Goeppert^ in 1836 instituted the genera Gyatheites, Hemitelites,nbsp;and Balantites for species of Carboniferous ferns believed to benbsp;closely allied to recent Cyatheaceae, but a fuller knowledgenbsp;of these types has clearly demonstrated that in all cases thenbsp;reference to this family had no justification.

The Upper Carboniferous species Dicksonites Pluckeneti, of which SterzeU described fertile specimens in 1886 as possessingnbsp;circular sori, has since been shown by Grand�Eury* to be anbsp;Pteridosperm bearing small seeds. In Sphenopteris (Discopteris)nbsp;cristata (Brongn.) Zeiller� has described sori very like those ofnbsp;Cyathea and Alsophila, but differing in the exannulate sporangia;nbsp;this species, like so many of the Palaeozoic ferns, is probably morenbsp;akin to the Marattiaceae than to the Cyatheaceae.

We have as yet no satisfactory evidence of the existence of the Cyatheaceae in Palaeozoic floras. It is not until we reachnbsp;the Jurassic period that trustworthy data are obtained. Raci-borski� has identified as Cyatheaceous fertile Jurassic frondsnbsp;from Poland, but his figures are inconclusive. In Alsophilanbsp;polonica it is not clear whether the annulus is vertical or oblique,nbsp;and in another supposed member of the family, Gonatosorusnbsp;Nathorsti, in which the indusium is described as bivalvate,nbsp;there is no proof of affinity to Cyatheaceae.

In attempting to decipher the past history of the Cyatheaceae it is important to remember the close resemblance between the fertile segments of some species of Bavallianbsp;(Polypodiaceae) and those of Dicksonia (fig. 229, C, D, p. 294).nbsp;Unless the sporangia are well enough preserved to show thenbsp;position of the annulus, it is frequently impossible to feel muchnbsp;confidence in the value of the grosser features, such as thenbsp;reduced lamina of the fertile segments and the form of thenbsp;sori. It is, however, probable that the widely-spread Jurassicnbsp;species Coniopteris hymenophylloides is correctly referred to the

Cyatheaceae, but even in the case of this species the evidence of external form needs confirmation by an examination ofnbsp;individual sporangia.

Coniopteris.

This genus was instituted by Brongniart^ for fossil fronds characterised by pinnules more or less intermediate betweennbsp;the Pecopteris and Sphenopteris type and agreeing in the formnbsp;of the sori with the leaves of recent species of Bicksonia. Itnbsp;should be noted that Stur included in this genus a species,nbsp;Coniopteris lunzensis�-� from the Upper Trias of Lunz, which henbsp;regarded as a Marattiaceous fern.

1828. nbsp;nbsp;nbsp;Sphenopteris hymenophylloides, Brongniart, Hist. veg. foss.nbsp;p. 189, PI. LVi. fig. 4.

1835. nbsp;nbsp;nbsp;Tympanophora simplex, Bindley and Hutton, Foss. Flor.nbsp;PL CLXX. A.

1851. Sphenopteris nephroearpa, Bunbury, Quart. Journ. Geol. Soc. Vol. VII. p. 129, PL XII. fig. 1.

1876. Thyrsopteris Murrayana, Heer, Flor. Foss. Arct. Vol. iv. (2) p. 30, Pis. I. ii. VIII.

The above list represents a small selection of the names applied to Jurassic ferns from different localities which therenbsp;are good grounds for regarding as referable to a single type�.

Frond tripinnate; pinnae linear acuminate, attached to the rachis at a wide angle ; the pinnules vary considerably in size and shape; in some thenbsp;lamina is divided into a few broad and rounded lobes (fig. 275, B) while innbsp;others the leaflets are dissected into narrow linear segments. The sori arenbsp;borne at the ends of veins; the fertile pinnules have a much reducednbsp;lamina and, in extreme oases, bear a close resemblance to those of

Thyrsopteris elegans (fig. 229, A, p. 294). The sori are partially enclosed in a cup-like indusium and the sporan_gia appear to have an oblique annulus.

The pinna shown in fig. 271 is the type-specimen of Sphenopteris arguta Lind, and Hutt. from the Yorkshirenbsp;Inferior Oolite and is indistinguishable from the Englishnbsp;examples on which Brongniart founded his species 8. hymeno-phylloides. Fig. 272 shows a specimen from the York Museumnbsp;illustrating the difference between the sterile and fertile pinnae.nbsp;The resemblance of some fertile pinnae of Goniopteris hymeno-

Fic. 271. Goniopttrishyvienophylloides (Biongn.). Nat. size. From a specimen in the Manchester Museum.

phylloides to those of Thyrsopteris elegans has led to a frequent use, without any solid justification, of the generic name of thenbsp;Juan Fernandez fern for Jurassic and Wealden plants. It is notnbsp;impossible that some of the fossils described by Heer fromnbsp;Jurassic rocks of Siberia^ as species of Thyrsopteris are Cya-theaceous ferns, but it is impossible to say with certainty thatnbsp;they are generically identical with the recent species. In hisnbsp;monograph of the Potomac fiora of Virginia^ and Maryland,nbsp;Fontaine has described as species of Thyrsopteris several speci-

mens of fronds which afford no evidence as to the nature of the sori or sporangia. Some of the fronds referred by this authornbsp;to Thyrsopteris rarinervis'^, which I examined in the Washingtonnbsp;Museum, are in all probability examples of Onychiopsis, a genusnbsp;included in the Polypodiaceae. The fragments .described bynbsp;Lester Ward^ as species of Thyrsopteris from the Lowernbsp;Cretaceous of the Black Hills of North America afford no

Fig. 272. Coniopteris hymeiiophylloides. Specimen from the Inferior Oolite, Scarborough; in the York Museum. [M.S.]

satisfactory evidence of relationship to the recent type. Similarly Velenovsky has described a Lower Cretaceous Onychiopsis fromnbsp;Bohemia^ as a species of Thyrsopteris, although the fertilenbsp;segments bear little or no resemblance to those of the Cyathe-aceous genus. Some fertile portions of fronds described bynbsp;Heer^ as Asplenium Johnstrupi and afterwards as Dicksonia

Johnstr upi^ from the Cretaceous beds (Kome series) of Greenland are very similar to Goniopteris hymenophylloides.

This species, originally described by Phillips^ as Sphenopteris quinqueloha, is very similar in habit to G. hymenophylloides,nbsp;differing chiefly in the smaller size of the leaf and in thenbsp;narrower ultimate segments. The specimen shown in fig. 273, B,nbsp;illustrates the form of the sorus and sporangia.

The sterile pinnae of this species bear pinnules of a type met with in various species of ferns from different horizons;nbsp;the smaller ones are entire and slightly falcate, while on thenbsp;lower part of a frond the ultimate segments are longer andnbsp;have a crenulate margin. The fertile pinnae bear pinnulesnbsp;reduced to a midrib with a narrow border, and terminatingnbsp;in a cup-like indusium (fig. 275, A). In habit the sterile leafnbsp;(fig. 274) of this species is similar to the Jurassic Schizaeaceousnbsp;fern Klukia exilis.

fern from Bohemia originally figured as Lepidodendron punc-tatum^ and assigned to a Palaeozoic horizon; it was afterwards

/�

Fig. 274. Coniopteris arguta. (Nat. size. From a specimen in the Sedgwick Museum, Cambridge.)

named by Corda^ Protopteris Sternhergii and referred by Brongniart� to Sigillaria. The genus Protopteris stands for

fossil fern-stems with the habit and, in the main, the structural features of recent tree-ferns. Persistent leaf-bases and sinuousnbsp;adventitious roots cover the surface of the stems: the vascularnbsp;system is of the dictyostelic type characteristic of Gyatheanbsp;(fig. 240, p. 313) and Alsophila. It is by the pattern formed bynbsp;the vascular tissue on the exposed surface of the leaf-bases thatnbsp;Protopteris is most readily recognised; the leaf-trace has a horseshoe form with the ends curled inwards and the sides more ornbsp;less indented (fig. 277). The generic name Gaiilopteris isnbsp;used by some authors in preference to Presl�s genus; butnbsp;Protopteris is more conveniently restricted to Mesozoic Cyathe-aceous stems and Caulopteris to Palaeozoic stems, with thenbsp;internal structure of Psaronius (see Chap, xxrii.). Stenzel

A, from the Inferior Oolite of Yorkshire (British Museum); B, from Jurassic rocks in Turkestan.

applies Caulopteris to Mesozoic stems in which the leaf-trace consists of several separate strands and not of a continuous band.

Lower Cretaceous casts of tree-fern stems in the Prague Museum have been described under the names Alsophilina andnbsp;Oncopteris; the figures of the latter (fig. 276) given by Feist-manteP and by Velenovsky� show the petiole-bases arrangednbsp;in vertical rows and characterised by leaf-traces consisting ofnbsp;two separate strands in the form of two Vs lying on their sides.

Tree-fern stems described under various generic names are not infrequently found in European Lower Cretaceous rocks: theirnbsp;comparative abundance affords an example of striking changesnbsp;in geographical distribution since the latter part of the Mesozoicnbsp;epoch. The Cyatheaceae no longer exist in Europe and thenbsp;1 Feistmantel (72).nbsp;nbsp;nbsp;nbsp;^ Velenovsky (88),

The earliest information in regard to the anatomy of this widely spread Lower Cretaceous fern we owe to Corda, whonbsp;showed that the species agrees in essentials with existing

tree-ferns. The English example described by Carruthers� from Upper Greensand beds in Dorsetshire (now in the Britishnbsp;Museum) shows only the external features. The sandstone cast

Fiq. 278. A. Laccopteris polypodioideSjBiongn. [From a specimen (39275) in the British Museum ; slightly reduced.]

(14 cm. in diameter), of which a portion is seen in fig. 277, was described by Heer from Disco Island (Greenland) as a Carboniferous species', but afterwards correctly assigned to the Cenomanian series'*. This species is recorded also from the Lowernbsp;Cretaceous of Bohemia by Fric and Bayer^. Among examplesnbsp;of petrified stems exhibiting a general agreement with Pro-topteris punctata are those described b}'^ StenzeD from Turoniannbsp;rocks in Germany. In one of these, Rhizodendron oppoliensenbsp;G�pp., attention is drawn to branches given off from the stemnbsp;stele which have a solenostelic structure in contrast to thenbsp;dictyostele of the stem; also to the minute structure of thenbsp;tracheae which appear to have their ends perforated, a featurenbsp;shown by Gwynne-Vaughan� to be characteristic of the xylemnbsp;elements of many ferns.

Protopteris Witteana Schenk� (fig. 278, G, H), a Wealden species recorded from Germany and England, represents a closelynbsp;allied or possibly an identical type. The section of the stemnbsp;(fig. H) shows the narrow vascular bands, x, of a dictyostelenbsp;similar to that of recent Cyatheaceous tree-ferns and a formnbsp;of meristele (fig. G, x) resembling that ^of P. punctata.nbsp;Adventitious roots are seen in section at R (figs. G and H).

Sections of petrified sporangia from the English Coal-Measures (Pteridotheca sp.) occasionally exhibit a striking resemblance to those of recent Polypodiaceae��, but in thenbsp;absence of material in which it is possible to recognise thenbsp;true orientation of the sporangia, the exact position of thenbsp;annulus is almost impossible to determine. We have as yetnbsp;no satisfactory evidence of the existence of true Polypodiaceaenbsp;in the Palaeozoic era. It is noteworthy that apart from thenbsp;absence of ferns which can reasonably be included in this family,nbsp;the anatomical features of the Botryopterideae (Coenopterideae)nbsp;and of the Cycadofilices or Pteridosperms do not foreshadow those

of Polypodiaceous ferns. On the other hand, as we have already noticed, anatomical characters of such families as the Gleicheni-aceae, Hymenophyllaceae, and Schizaeaceae are met with innbsp;certain generalised Palaeozoic types. These facts are perhapsnbsp;of some importance as supplying collateral evidence in favour ofnbsp;the relatively more recent origin of the dominant family ofnbsp;ferns in modern floras.

The use of the generic name Adiantites for fern-like fronds of Lower Carboniferous age characterised by cuneate

pinnules like those of species of Adiantum, suggests an affinity which is in all probability non-existent. It has been pointednbsp;out that this generic name was applied in the first instance tonbsp;the leaves of the Jurassic plant Ginkgo digitata^ and should,nbsp;therefore, be discarded. Schimper^ used the designationnbsp;Adiantides, and Ettingshausen^ more rashly than wisely, preferred Adiantum. The specimens described by Kidston^ as

Adiantides antiquus (Ett.) (fig. 279, A) from the Carboniferous limestone of Flintshire are portions of tripinnate fronds bearingnbsp;cuneate segments with numerous forked veins radiating from thenbsp;contracted base of the lamina. It is not improbable, in view ofnbsp;Dr White�s^ discovery of seeds on a very similar plant from thenbsp;Pottsville beds of North America, that this characteristic Lowernbsp;Carboniferous genus is a Pteridosperm.

From Jurassic rocks in various parts of the world numerous fossils have been described under the generic names Aspidiuni,nbsp;Asplenium, Davallia, Polypodium, and Pteris. In the greatnbsp;majority of cases such records leave much to be desired fromnbsp;the point of view of students who appreciate the dangers ofnbsp;relying on external similarity between vegetative organs, andnbsp;on resemblances founded on obscure impressions of sori. Thenbsp;generic term Woodwardites^, which suggests affinity with thenbsp;recent genus Woodwardia, has been used for Rhaetic plantsnbsp;belonging to the Dipteridinae.

A plant described as Adiantides Lindsayoides from Jurassic rocks of Victoria^ characterised by marginal sori which appearnbsp;to be protected by the folded-over edge of the^ leaflets, and bynbsp;the resemblance of the pinnules to those of recent species ofnbsp;Lindsaya, may be a true Polypodiaceous fern ; but in this case,nbsp;as in many similar instances, nothing is known of the structurenbsp;of the sporangia. Some sterile pinnae described by Yabe fromnbsp;Jurassic rocks of Korea as Adiantites SewardP may perhaps benbsp;identical with the A ustralian species.

In such a species as Polypodimn oregonense Font., from Jurassic rocks of Oregon, the generic name is chosen becausenbsp;the � fructification seems near enough to that of Polypodium tonbsp;justify the placing of the plant in that genus�.� But the factnbsp;that no sporangia have been found is a fatal objection to thisnbsp;identification.

gt; White (04). nbsp;nbsp;nbsp;^ Schenk (67) A. PI. xiii. ; Zeiller (03) p. 91, PI. xvii.

3 Seward (Ol�) p. 162, PL viii. fig. 5. nbsp;nbsp;nbsp;* Yabe (05) p. 39,,PI. i. figs. 1-8.

Japanese Wealden species, previously described by Geyler* as Thyrsopteris elongata, on the ground that, in addition to anbsp;similarity in habit of the sterile fronds, the fertile pinnaenbsp;present a close agreement to those of the recent genusnbsp;Onychium.

The Japanese species Onychiopsis elongata may perhaps be identical with this common Wealden fern which, as Fontainenbsp;points out, should be called 0. psilotoides if the rule of prioritynbsp;is to be observed irrespective of long usage.

1824. Hymenopteris psilotoides, Stokes and Webb, Trans. Geol. Soc. [ii.], Vol. I. p. 423, PI. XLVi. fig. 7.

1828. Sphenopteris Mantelli, Brongniart, Hist. v�g. foss. p. 170, PI. XLV. figs. 3�7.

Frond bipinnate, ovate lanceolate, rachis winged; pinnae approximate, given off' at an acute angle; pinnules narrow, acuminate, with a singlenbsp;vein; the larger segments serrate and gradually passing into pinnae withnbsp;narrow ultimate segments. Fertile segments sessile or shortly stalked,nbsp;linear ovate, sometimes terminating in a short awn-like prolongation.

The fertile segments (fig. 278, D) bear so close a resemblance to those of species of Onychium that it would seem justifiablenbsp;to regard the plant as a member of the Polypodiaceae. Thisnbsp;fern is one of the most characteristic members of the Wealdennbsp;floras; it occurs in abundance in the English Wealden, innbsp;Portugal, Germany, Belgium, Japan, Bohemia, South Africa,nbsp;and elsewhere. A piece of rhizome figured from the Englishnbsp;W�ealden^ is very similar to the creeping rhizomes of recentnbsp;species of Polypodiaceae. The English Wealden specimens

^ For synonymy, see Fontaine, in Ward (05) p. 155; Richter (06) p. 6 ; Seward (94) A. p. 41; (03) p. 5.

shown in fig. 280, A and B, illustrate the difference in form presented by leaves of this species; the smaller pinnae reproduced in fig. A are more characteristic of the species than arenbsp;those of the slightly enlarged example represented in fig. 280, B.

Fig. 280. Onychiopsis Mantelli. (Prom Wealden specimens in the British Museum; No. 13495 and No. V. 2615. A, natural size; B, verynbsp;slightly enlarged.)

Among British Tertiary species referred to Polypodiaceae, it is interesting to find what may well be an authentic recordnbsp;of a fern closely allied to the recent tropical species Acrostichiininbsp;(Chrysodium) aureum. This Eocene species from Bournemouthnbsp;is described as Chrysodium lanzaeanunC. The frond is simplynbsp;pinnate and apparently coriaceous in texture, with lanceolate ornbsp;oblong lanceolate pinnules (fig. 261, A, A', p. 3.50), differing fromnbsp;those of Acrostichum aureum in being sessile. A prominentnbsp;1 Gardner and Ettingshausen (82) Pis. i. ii.

midrib gives off numerous anastomosing veins. No fertile pinnules have been found.

Specimens described by Forbes from the Eocene beds of the Island of Mull as Onoclea hebraidica^ bear a strong likenessnbsp;to the North American and Japanese recent species Onocleanbsp;sensihilis. Fertile specimens referred to the latter species arenbsp;recorded by Knowlton^ from Tertiary beds of Montana.

A species described by Saporta� from the Eocene of S�zanne as Adiantum apalophyllum is recorded by Gardnernbsp;and Ettingshausen from Bournemouth; an identification whichnbsp;is based on somewhat meagre evidence.

The following remarks by Gardner and Ettingshausen are worthy of repetition as calling attention to circumstances oftennbsp;overlooked in analyses of fossil floras. They speak of ferns asnbsp;relatively rare in British Eocene rocks and add,��the florasnbsp;consist principally of deciduous dicotyledonous leaves, which...nbsp;fell into the water and were tranquilly silted over. Ferns, onnbsp;the other hand, would require some violence to remove themnbsp;from the place of their growth, and their preservation wouldnbsp;consequently be exceptional, and they would be mutilated andnbsp;fragmentary. This may account for their rarity. Few as thenbsp;British ferns are in the number of species, they neverthelessnbsp;form the largest and most impoi'tant series of Eocene ferns,nbsp;even of Tertiary ferns, yet described from one group of beds�.�

This genus was founded by Bindley and Hutton for a pinnatifid leaf from the Jurassic rocks of Yorkshire which theynbsp;regarded as probably dicotyledonous and named D. rugosum^.nbsp;Several ferns of this genus have since been found with well-preserved sori which demonstrate a close similarity to the recentnbsp;fern Diptej�is. Dictyophyllum may be defined as follows:�

Fronds large and palmate, characterised by the equal dichotomy of the main rachis into two arms which curve outwards and then bend inwardsnbsp;(fig. 281); from the surface of each arm are given off' numerous spreadingnbsp;pinnae with a lamina more or less deeply dissected into lobes varying innbsp;breadth and in the form of the apex. Each lobe has a median vein, fromnbsp;which branches are given off approximately at right angles and then subdivide into a reticulum, in the meshes of which the veinlets end blindlynbsp;(fig. 282, A and E). Sori composed of annulate sporangia are crowded on thenbsp;lower surface of the lamina. In habit and in sporangial characters thenbsp;genus closely resembles Dipteris, and in the branching of the frondnbsp;suggests comparison with Matonia. The rhizome {Rhizomopteris) isnbsp;creeping and dichotomously branched, bearing leaf-soars with a horseshoe form of vascular strand.

Dictyophyllum is represented by several types to which various specific names have been assigned, the distinguishingnbsp;features being the form of the pinna lobes, the degree of concrescence between the basal portions of the pinnae, and similarnbsp;features which in some cases can only be safely used as criterianbsp;when large specimens are available for comparison.

The restoration, after Nathorsth shown in fig. 281 illustrates the habit of this striking fern, examples of which or of closelynbsp;allied species are recorded from Khaetic rocks of Germany, Scania,nbsp;Persia, Bornholm, Tonkin, China, and elsewhere^. The petiole,nbsp;reaching a length of 60 cm., forks at the apex into two equalnbsp;arms leaving between them an oval space and occasionallynbsp;crossing one another. The axes of these branches are twistednbsp;so that the pinnae, which may be as many as 24 on each arm,

and arise from the inner side, by torsion of the axes assume an external position. An interesting analogy as regards thenbsp;twisted rachis of Dictyophyllum exile and Camptopteris isnbsp;afforded by the leaves of the Cycads, Macrozamia Fawcettiae andnbsp;M. corallipes, which are also characterised by the torsion ofnbsp;the rachis. The habit, justly compared by Nathorst with

that of Matonia pectinata, affords another illustration of the common occurrence in older ferns of a dichotomous systemnbsp;of branching. The pinnae, characterised by circinate vernation,nbsp;reach a length of 60 cm. and are divided into linear lobesnbsp;inclined obliquely or at right angles to the pinna axis. Thenbsp;whole of the under surface of the lamina may be covered withnbsp;sporangia, 4�7 sporangia in each sorus; the annulus isnbsp;incomplete and approximately vertical (fig. 282, D). Thenbsp;rhizome is probably represented by the dichotomously branchednbsp;axis described by Nathorst from Scania as Rhizomopterisnbsp;major; the leaf-scars show a horse-shoe leaf-trace.

This type, represented by a splendid series of specimens from the Rhaetic beds of Tonkin, agrees very closely withnbsp;D. exile. It differs, however, in the basal parts of the pinnaenbsp;which are concrescent for a length of 5 to 8 cm. instead of free asnbsp;in D. exile; and, to a slight degree, in the form of the ultimatenbsp;segments. In habit and in soral characters the two species arenbsp;practically identical. Each sorus contains 5 to 8 sporangia,nbsp;which are rather larger than those of Dipteris.

1828. nbsp;nbsp;nbsp;Phlehopteris Phillipsii, Brongniart, Hist. veg. foss. p. 377,nbsp;PL oxxxii. fig. 3; PI. cxxxiii. fig. 1.

1829. nbsp;nbsp;nbsp;Phyllites nervulosis, Phillips, Geol. Yorks, p. 148, PI. viii.nbsp;fig. 9.

This species, which is characteristic of Jurassic rocks, is less completely known than the two types described above,nbsp;but in the form and venation of the pinnae there is littlenbsp;difference between the Rhaetic and Jurassic plants. The leavesnbsp;of the Jurassic species appear to have been smaller and morenbsp;1 Zeiller (03) p. 109, Pis. xxiii.�xxviii.

like those of Dipteris conjugata (fig. 231); there are no indications of the existence of the two curved arms at the summit of the petiole which form so striking a feature in D. exile and

D. Nathorsti. No sporangia have been found on English specimens, hut it is safe to assume their agreement with thosenbsp;of other species. A more complete list of records of D. rugosumnbsp;is given in the first volume of the British Museum Cataloguenbsp;of Jurassic plants h

Nathorst^ has recently drawn attention to certain differences between Dictyophyllum and Dipteris. The pinnate division ofnbsp;the pinnae is not represented in the fronds of the recent species,nbsp;but this method of lobing, which is a marked characteristic ofnbsp;Dictyophyllum, is less prominent in Clathropteris; and in Camp-topteris Ivnzensis Stur�, an Austrian Upper Triassic species, thenbsp;pinnae are entire. In Dictyophyllum the sori cover the wholenbsp;lower surface of the leaf; in Dipteris they are more widelynbsp;separated and the sporangia have a diameter of 0'02 mm., butnbsp;in Dictyophyllum the diameter is 0'4�0'6 mm. Moreover innbsp;Dictyophyllum the sori contain 5 to 8 sporangia, whereas innbsp;Dipteris they are much more numerous. Despite these differences it is clear, as Nathorst says, that Dictyophyllum,nbsp;Glathropteris, and Gamptopteris are existing types very closelynbsp;allied to Dipteris. It is a matter of Secondary importancenbsp;whether we include all in the Dipteridinae or follow Nathorst�snbsp;suggestion and refer the fossil genera to the separate familynbsp;Camptopteridinae.

This genus, founded by Goeppert^ for a Rhaetic plant from Bayreuth, is by some authors� regarded as identicalnbsp;with Dictyophyllum, but it has recently been resuscitated bynbsp;Nathorst� for specimens which he names T. Schenki, formerlynbsp;included by Schenk in his species T. By-auniana''. It bearsnbsp;a close resemblance, in the long linear pinnules with an entirenbsp;or crenulate margin, to Dictyophyllum Fuchsi described bynbsp;Zeiller� from Tonkin, and it would seem hardly necessary tonbsp;adopt a distinctive generic designation. The sporangia have

a vertical or slightly oblique annulus and the rhizome is similar to that of Dictyophyllum exile. The habit of thenbsp;genus is shown in fig. 284, which represents one of thenbsp;German Rhaetic species.

Clathropteris, founded by Brongniart^ for Rhaetic specimens from Scania, agrees very closely with some species of Dictyophyllum, but in view of the more rectangular form of thenbsp;venation-meshes it is convenient to retain both names. Thenbsp;type-species was originally named Filicites meniscoides^ andnbsp;afterwards transferred to Clathropteris. An examination of

Brongniart�s specimens has convinced Nathorst of the specific identity of C. meniscoides and G. platyphylla. The Tonkinnbsp;leaves described by Zeiller^ under the latter name should,nbsp;therefore, be included in G. meniscoides, which may be thusnbsp;defined:

The petiolate frond is characterised by an equal dichotomy of the rachis, as in Dietyophyllum; each branch bore 5�15 pinnae, disposednbsp;en �venta�, reaching a length of '20�30 cm. and fused basally as innbsp;D. Nathorati Zeill. Pinnae linear lanceolate, slightly contracted at the

lower end and gradually tapered distally. The lamina, 3�14 cm. broad, is characterised by obtusely pointed marginal lobes. From the midribnbsp;of each pinna lateral veins are given off at a wide angle, and adjacentnbsp;veins are connected by a series of branches which divide the laminanbsp;into a regular reticulum of rectangular and polygonal meshes (fig. 285).nbsp;The sori are abundant and contain 5�12 sporangia like those of Dictyo-phyllum.

What is probably the rhizome of this species has been described by Nathorst {Rhizomopteris cruciata); it is similarnbsp;to that of Dictyophyllum, but the leaf-scars are more widelynbsp;separated. This species occurs in Upper Triassic, Rhaetic ornbsp;Lower Jurassic rocks of Scania, France, Germany, Switzerland,nbsp;Bornholm, North America, China, Tonkin, and Persia and isnbsp;represented by fragments in the Rhaetic beds of BristoP.

The specimen on which this species was founded was discovered in the Nubian Sandstone east of Edfu; the age of the beds is uncertain, but the presence of Clathropteris suggests anbsp;Lower Jurassic or Ehaetic horizon 2. Seven strong ribs radiatenbsp;through the lamina from the summit of the petiole; at a and hnbsp;small pieces of the projecting ribs are shown in the grooves.

From the main veins slender branches are given off at right angles and, as seen in the enlarged drawing, these again subdivide into a delicate reticulum with free-ending veinlets.

� The evidence of the shells is stated by Mr E. B. Newton (09) to be in favour of the Cretaceous age of the Nubian Sandstone.

resembling those of Glathropteris and Dictyophyllum, but differing in the form of the pinnae and in habit. The habit of the type-species, 0. spiralis, is shown in fig. 287. An examinationnbsp;of the specimens in the Stockholm Museum convinced me ofnbsp;the correctness of Nathorst�s restorationh Each of the forkednbsp;arms of the rachis bore as many as 150�160 long and narrownbsp;pinnae characterised by an anastomising venation (fig. 282, C)nbsp;and by a spiral disposition due to the torsion of the axes. Thenbsp;sporangia agree in essentials with those of Dictyophyllum.

A critical and exhaustive account of this genus has been given by Prof. Von Richter^ based on an examination ofnbsp;specimens found in the Lower Cretaceous rocks of Quedlinburgnbsp;in Germany. The name was proposed by Dunker� for leavesnbsp;from the Wealden of Germany characterised by a deeply dissectednbsp;dichotomously branched lamina. Andrae subsequently institutednbsp;the genus Protorhipis'^ for suborbicular leaves with dichotomously branched ribs from the Lias of Steierdorf. A similarnbsp;but smaller type of leaf was afterwards described by Zigno�nbsp;from Jurassic beds of Italy as P. asarifolius, and Nathorst�nbsp;figured a closely allied form from Rhaetic rocks of Sweden.nbsp;While some authors regarded Hausmannia and Protorhipis asnbsp;ferns, others compared them with the leaves of Baiera (Gink-goales); Saporta suggested a dicotyledonous affinity fornbsp;leaves of the Protorhipis type. The true nature of thenbsp;fossils was recognised by Zeiller�, who called attention to thenbsp;very close resemblance in habit and in soral characters to thenbsp;recent genus Dipteris. A comparison of the different speciesnbsp;of Dipteris, including young leaves (fig. 231, p. 297), with thosenbsp;of the fossil species reveals a very striking agreement�. Therenbsp;can be no doubt, as Richter points out, that the names Hausmannia and Protorhipis stand for one generic type.

Hmosmannia may be defined as follows:

Rhizome creeping, slender, diohotomously branched; leaf-stalks slender (2�25 cm. long), bearing a leathery lamina (1�12 cm. long and broad),nbsp;wedge-shaped below, occasionally cordate or reniform, entire or more or

less deeply lobed into broad linear segments. The leaf is characterised by dichotomously branched main ribs which arise from the summit of thenbsp;rachis as two divergent arms and radiate in a palmate manner, withnbsp;repeated forking, through the lamina. Lateral veins are given off at anbsp;wide angle, and, by subdivision, form a fairly regular network similar tonbsp;that in Dictyophyllum, Clathropteris, and Dipteris.

This Wealden species, represented in the North German flora and in beds of approximately the same age at Quedlin-burg, has been discovered by Dr Marcus Gunn in Uppernbsp;Jurassic rocks on the north-east coast of Scotland. The laminanbsp;(12 cm. or more in length) is divided into five to seven linearnbsp;segments and bears a close superflcial resemblance to leavesnbsp;of Baiera and to recent species of Schizaea (fig. 222, p. 287).nbsp;Each segment contains one or two main ribs (fig. 288, A). Anbsp;similar form is described by Bartholin^ and by Moeller^ asnbsp;H. Forchatnmeri from Jurassic rocks of Bornholm.

In this species, instituted by Richter from material obtained from the Lower Cretaceous beds of Strohberg^ the comparatively slender rhizome bears fronds with petioles reaching anbsp;length in extreme cases of 25 cm. but usually of aboutnbsp;10 cm. The lamina (1�7 cm. long and 1�10 cm. broad) isnbsp;described as leathery, obcordate, and divided into two symmetrical halves by a median sinus which, though occasionallynbsp;extending more than half-way through the lamina, is usuallynbsp;shallow. The venation consists of two main branches whichnbsp;diverge from the summit of the petiole (fig. 278, F) and subdivide into dichotomously branched ribs; finer veins (not shownnbsp;in the drawing) are given off from these at right angles andnbsp;form more or less rectangular meshes as in other members ofnbsp;the Dipteridinae and in such recent ferns as Polypodiumnbsp;quercifolium (fig. 231, D, p. 297).

Hausmannia Richteri or may be a distinct species; it shows some of the finer veins connecting the shorter forked ribs,nbsp;which formed part of the reticulate ramifying system in thenbsp;mesophyll. This specimen was obtained from the plant-beds ofnbsp;Culgower on the Sutherlandshire coast, which have been placednbsp;by some geologists in the Kimmeridgian series.

The smaller type represented in fig. 278, E, is referred by Richter to a distinct species, Hausmannia Sewardi'^, foundednbsp;on a few specimens from the Lower Cretaceous strata ofnbsp;Strohberg. This species is characterised by a stouter rhizome

bearing smaller leaves consisting of a short petiole (3�4 cm. long) and an obovate lamina (1�2 cm. long and broad). Therenbsp;are usually two opposite leaflets on each leaf-stalk, and thesenbsp;may be equivalent to the two halves of a single deeply dissectednbsp;lamina.

It is interesting to compare these different forms of Hausmannia with the fronds of recent species of Dipteris represented in fig. 231. The more deeply dissected type, such as H. dicho-toma, closely resembles H. Lohbiana or B. quinquefurcata,nbsp;while the more or less entire fossil leaves (fig. 278, E, F andnbsp;fig. 289) are very like the somewhat unusual form of Dipterisnbsp;conjugata shown in fig. 231, B, p. 29..

Other species of the genus are recorded from Liassic rocks of Steierdorf' (Hungary) and of Bornholm^. Nathorst� hasnbsp;described a small Rhaetic species from Scania: a Frenchnbsp;Permian plant described by Zeiller^ and compared by him withnbsp;H. dichotoma, may be a Palaeozoic example of this Dipteris-like genus.

Some segments of leaves from the Eocene beds (Middle Bagshot) of Bournemouth, and now in the British Museum,nbsp;described by Gardner and Ettingshausen� as Podoloma poly-podioides, bear a close resemblance in the venation to thenbsp;lamina of Dipteris conjugata.

CHAPTER XXII.

The discovery of Pteridosperms has necessarily led to a considerable modification of the views formerly held that existing genera of Marattiaceae represent survivors of a group whichnbsp;occupied a dominant position in the forests of the Coal age.nbsp;Mr Arber writes:�� The evidence, formerly regarded as beyondnbsp;suspicion, that the eusporangiate ferns formed a dominantnbsp;feature of the vegetation of the Palaeozoic period, has beennbsp;undermined, more especially by the r�taarkable discovery ofnbsp;the male organs of Lyginodendron by Mr Kidston. At bestnbsp;we can only now regard them as a subsidiary group in thatnbsp;epoch in the past history of the vegetable kingdom^.� Dr Scottnbsp;expresses himself in terms slightly more favourable to the viewnbsp;that the Marattiaceae represent the aristocracy among thenbsp;Filicales. He says :�� We now have to seek laboriously fornbsp;evidence, which formerly seemed to lie open to us on all hands.nbsp;I believe, however, that such careful investigation will result innbsp;the resuscitation of the Palaeozoic ferns as a considerable, thoughnbsp;not as a dominant group2.� Zeiller�s faith� in the prospect ofnbsp;Marattiaceous ferns retaining their position as prominentnbsp;members of Palaeozoic floras, though shaken, is not extinguished : he recognises that they played a subordinate part.

Reference has already been made to the impossibility of determining whether Palaeozoic fern-like fronds may be legitimately retained in the Filicales, or whether they must benbsp;removed into the ever widening territory of the Pteridosperms.

The difficulty is that the evidence of reproductive organs is very far from decisive. In the absence of the female reproductive organs, the seeds, we cannot in most cases be certainnbsp;whether the small sporangium-like bodies on fertile pinnulesnbsp;are true fern sporangia or the microsporangia of a heterosporousnbsp;pteridosperm. What is usually called an exanniilate fernnbsp;sporangium, such as we have in Angiopteris and in manynbsp;Palaeozoic plants, has no distinguishing features which cannbsp;be used as a decisive test. The microsporophylls of thenbsp;Mesozoic Bennettitales produced their spores in sporangialnbsp;compartments grouped in synangia like those of recent Ma-rattiaceae; and in the case of Grossotheca, a type of frondnbsp;always regarded as Marattiaceous until Kidston� proved it tonbsp;be the microsporophyll of Lyginodendron, we have a strikingnbsp;instance of the futility of making dogmatic assertions as tonbsp;the filicinean nature of what look like true fern sporangia. Innbsp;all probability Dr Kidston�s surmise that the supposed fernnbsp;sporangia known as Dactylotheca, Renaidtia, Urnatopteris arenbsp;the inicrosporangia of Pteridosperms will be proved correct^.nbsp;The question is how many of the supposed Marattiaceous sporangia must be assigned to Pteridosperms ? Thei'e is, however,nbsp;no reasonable doubt that true Marattiaceae formed a part ofnbsp;the Upper Carboniferous flora. All that can be attempted innbsp;the following pages is to describe briefly some of the numerousnbsp;types of sporangia recognised on Palaeozoic fern-like foliage,nbsp;leaving to the future the task of deciding how many of themnbsp;can be accepted as those of ferns. It is impossible to avoidnbsp;overlapping and some repetition in the sections dealing withnbsp;true Ferns and with Pteridosperms. The filicinean nature ofnbsp;the stem known as Psaroniua (see page 415) has not as yetnbsp;been questioned.

The nomenclature of supposed Marattiaceous species from Carboniferous and Permian rocks is in a state of some confusionnbsp;owing to a lack of satisfactory distinguishing features betweennbsp;certain types to which different generic names have beennbsp;assigned. As we have already seen in the case of supposed

leptosporangiate sporangia, the interpretation of structural features in petrified or carbonised sporangia does not affordnbsp;an example of unanimity among palaeobotanical experts.

This generic name, proposed by the late Professor Weissh is applied to a type of fructification illustrated by the plantnbsp;which Brongniart named Pecopteris unita, a species common innbsp;the Upper Coal-Measures of England^. It is adopted by Kidstonnbsp;for fertile specimens from Radstock which he describes asnbsp;Ptychocarpus oblongus^, but the precise nature of the fertilenbsp;pinnules of this species cannot be determined.

Ptychocarpus miito(Brongn.^). Fig. 291, A, B. (= Goniopteris unita, Grand'Eury.)

This species has tripinnate fronds with linear pinnae bearing contiguous pinnules of the Pecopteris type (fig. 291, B), 4�5 mm.nbsp;long, confluent at the base or for the greater part of their length.nbsp;On the under surface of the fertile segments, which are identical with the sterile, occur circular synangia (flg. 291, A) consisting of seven sporangia embedded in a common parenchymatousnbsp;tissue and radially disposed round a receptacle supplied withnbsp;vascular tissue. The synangium is described as shortly stalkednbsp;like those of Marattia Kaulfussii (flg. 24.5, B', p. 320). In shape,nbsp;in the complete union of the sporangia, and presumably innbsp;the apical dehiscence, Ptychocarpus agrees very closely withnbsp;Kaulfussia (flg. 245); but we cannot be certain that we havenbsp;not a collection of microsporangia simulating a fern synangium.

A synangium closely resembling Ptychocarpus has been described by Mr Watson� from the Lower Coal-Measures of

1 nbsp;nbsp;nbsp;Weiss, C.E. (69) p. 94, PI. xi. fig. 2. The specimens figured by Weiss bearnbsp;a somewhat remote resemblance to that described by Renault (96) A, under thenbsp;same generic name.

^ Renault (96) A. p. 9; Zeiller (88) A. p. 162; Grand�Eury (77) A. PI. viii. fig. 13.

Lancashire as Gyathotrachus altus, but there is no convincing evidence as to the nature of the plant on which it was home.

This generic name, instituted by Goeppertb has been used by authors without due regard to the nature of the evidencenbsp;of affinity to Danaea. The type named by Stur Banaeitesnbsp;sarepontanusquot;^ (fig. 291, E) bears small pecopteroid pinnulesnbsp;with ovoid sporangia in groups of 8�16 in two contiguousnbsp;series on the lower face of the lamina. The sporangia dehiscenbsp;by an apical pore and are more or less embedded in thenbsp;mesophyll of the segments. No figures have been publishednbsp;showing any detailed sporangial structure, and such evidencenbsp;as we have is insufficient to warrant the conclusion that thenbsp;resemblance to Banaea is more than an analogy.

The plant described by Grand�Eury� from the Coal-fields of Gard and St Etienne, and made the type of a new genus, isnbsp;characterised by pinnules intermediate between those of Pecop-teris and Neuropteris^ and by the presence of two rows of unitednbsp;sporangia along the lateral veins, as in Banaea and Banaeites.

Certain species of Pecopteris fronds from Carboniferous strata are characterised by circular sori or synangia consisting of anbsp;small number (3�8) of exannulate sporangia attached to anbsp;central receptacle and free only at their apices. Strasburger�nbsp;suggested a Marattiaceous affinity for Asterotheca and Stur�nbsp;describes the species Asterotheca Sternbergii Goepp. (fig. 291,nbsp;C, D) as an example of a Marattiaceous fern. The latternbsp;author retains Corda�s genus Hawleaquot;^ for the fertile fronds

� Strasburger (74). nbsp;nbsp;nbsp;� Stur (85) A. p. 183.nbsp;nbsp;nbsp;nbsp;�� Corda (45) A. PI. lvii.

of the common Coal-Measures species Pecopteris Miltoni, while on the other hand Kidston^ includes this type in Asterotheca.

Pecopteris {Asterotheca) Miltoni (Artis).

1828. Pecopteris abbreviata, Brongniart, Hist. v�g. loss. p. 337, PI. cxv. figs. 1�4; Bindley and Hutton, Poss. Flor. Vol. in.nbsp;PL 184.

1877�1888. Hawlea Miltoni, Stur, Culm Bdora, p. 293; Fame Carbon. Flora, p. 108, Pis. lix. lx.

1888. Pecopteris {Asterotheca) abbreviata, Zeiller, Flor. Yaleno. p. 186, PL XXIV. figs. 1�4.

E. nbsp;nbsp;nbsp;Pecopteris (Dactylotheca) plumosa [ = P. (Pactylotheca) dentata

The fronds of this species reached a length of more than 3 metres and a breadth of 2 metres. They are characterisednbsp;by the presence of aphlebiae^ appressed to the rachis and bynbsp;circular sori composed of a small number (3�6) of sporangia.

In habit and in the form of the pinnules this type is similar to Dactylotheca plumosa.

Stur^ retains this generic name for sori in which the 1 See Chap, xxvii.nbsp;nbsp;nbsp;nbsp;� Stur (85) A. p. 106.

sporangia are free and united only by the proximal end to a central receptacle (fig. 291, F, G). He describes the individual sporangia as possessing a rudimentary annulus, anbsp;comparatively strong wall, and terminating in a pointed distalnbsp;end. He emphasises the greater degree of cohesion betweennbsp;the sporangia of Asterotheca as the distinguishing feature ofnbsp;that genus ; but this is a character difficult to recognise in somenbsp;cases, and from the analogy of recent ferns one is disposed tonbsp;attach little importance to the greater or less extent to whichnbsp;sporangia are united, at least in such cases as Asterotheca andnbsp;Hawlea when the cohesion is never complete.

Zenker^ gave this name to detached fertile pinnules from the Lower Permian of Saxony, which he described as Scolecopteris elegans. He recognised the fern nature of the sorinbsp;and suggested that the pinnules might belong to the frondsnbsp;of one of the � Staarsteinen � (Psaronius), ^ view which subsequent investigations render far from improbable. The sori,nbsp;which occur in two rows on the lower surface of the smallnbsp;pecopteroid segments with strongly revolute margins (fig. 291,nbsp;H�K), contain 4�5 sporangia attached to a stalked receptaclenbsp;comparable with that of Marattia Kaulfussii. These pedicellatenbsp;synangia were fully described by Strasburger^, who decided innbsp;favour of a Marattiaceous alliance. The lower portions of thenbsp;distally tapered sporangia are concrescent, the distal ends beingnbsp;free (fig. 291, H). Stur includes in Scolecopteris the commonnbsp;species Pecopteris arhorescens (fig. 376), but Kidston^ statesnbsp;that the British example of Scolecopteris is S. polymorphanbsp;Brongn. from the Upper Coal-Measures.

Scolecopteris elegans Zenk. furnishes an example of a plant, or plant fragment, which has been assigned to the animalnbsp;kingdom. Geinitz* described silicified pinnules as Palaeojidusnbsp;dyadicus, the generic name being chosen because of the re-

^ Zenker (37). nbsp;nbsp;nbsp;^ Strasburger (74).nbsp;nbsp;nbsp;nbsp;^ Kidston (91^) p. 20.

^ Geinitz (72). See Solms-Laubacb (83), who gives in full the early history of the genus Scolecopteris.

semblance to Millipedes such as the genus Julus. The mistake is not surprising to anyone who has seen a block of siliceousnbsp;rock from Chemnitz crowded with the small pinnules withnbsp;their concave surfaces formed by the infolding of the edges.nbsp;Sterzeb, who pointed out the confusion between Myriapodsnbsp;and Filices, has published figures which illustrate the deceptivenbsp;resemblance of the pinnules, with their curved lamina dividednbsp;by lateral veins into segments, to the body of a Millipede (fig.nbsp;291, K). He points out that Geinitz searched in vain for thenbsp;head and legs of Palaeojulus and expressed the hope thatnbsp;further examination would lead to fresh discoveries: the examination of sections revealed the presence of sporangia andnbsp;demonstrated the identity of Palaeojulus and Scolecopteris.

Stur^ instituted this genus for fertile fronds from the Upper Carboniferous Schatzlarer beds, including two species Discopterisnbsp;karwinensis and D. Schumanni. He described the smallnbsp;Sphenopteroid pinnules as characterised by disc-shaped sorinbsp;made up of 70�100 sporangia attached to a hemisphericalnbsp;receptacle; the absence of a true annulus led him to refer thenbsp;genus to the Marattiaceae. In his memoir on the coal-basin ofnbsp;Heraclea (Asia Minor), Zeiller^ instituted the species Sphenopterisnbsp;{Discopteris) Rallii and figured sporangia resembling thosenbsp;described by Stur in the possession of a rudimentary �apicalnbsp;�annulus.� He compared the sporangia with those of recentnbsp;Osmundaceae and Marattiaceae. In the later memoir on thenbsp;Upper Carboniferous and Permian plants of Blanzy and Creusot,nbsp;�Zeiller^ gives a very full and careful description of fertilenbsp;specimens of Sphenopteris {Discopteris) cristata, a fern originally described by Brongniart as Pecopteris cristata�'. Many ofnbsp;the Sphenopteroid pinnules of this quadripinnate fern frondnbsp;show the form and structure of the sori with remarkablenbsp;clearness in the admirable photographs reproduced in Platesnbsp;I.�III. of Zeiller�s Blanzy memoir. The lobed pinnules of this

1 Sterzel (78); (80). nbsp;nbsp;nbsp;� Stur (85) p. 140.nbsp;nbsp;nbsp;nbsp;* Zeiller (99) p. 17.

species are of oval-triangular form, 5�15 mm. long and 2�5�6 mm. broad'. An examination of the type-specimensnbsp;of Discopteris from Vienna enabled Zeiller to correct Stur�snbsp;original description of the sori; he found that the Austriannbsp;and French specimens, though specifically distinct, undoubtedlynbsp;belong to one genus. The sori in Discopteris cristata arenbsp;globular, as in the recent genera Cyathea and Alsophila, andnbsp;frequently cover the whole face of the lamina. The individualnbsp;sporangia are 0'4�0'5 mm. long and 0T5�0'2 mm. in diameter;nbsp;they are exannulate, but for the annulus is substituted a groupnbsp;of thicker-walled and larger cells in the apical and dorsal region.nbsp;The description by Stur of a hemispherical receptacle seemednbsp;to indicate an important difference between the Austrian andnbsp;French species; but Zeiller found that this feature does notnbsp;actually exist and that it was so described as the result ofnbsp;misinterpretation. Zeiller succeeded in isolating spores, 40�50 pnbsp;in diameter, from some of the sporangia of D. cristata andnbsp;found that they exhibited the three-rayed pattern characteristicnbsp;of fern-spores and which is indicative ef their formation innbsp;tetrads. The conclusion arrived at is that the genus Discopteris,nbsp;as represented by D. karwinensis, D. cristata etc., may benbsp;regarded as a true fern and included in the Marattiaceae. Asnbsp;Zeiller points out, the sori of Discopteris differ from those ofnbsp;recent Marattiaceae in their pluriseriate construction andnbsp;agree in this respect with those of the Cyatheaceae. Thenbsp;comparison already made^ between the sporangia of D. Ralliinbsp;and those of recent Osmundaceae holds good: the genusnbsp;affords another example of a generalised type, in this casenbsp;probably a fern, combining features which are now distributednbsp;among the Marattiaceae, Osmundaceae and Cyatheaceae.

In addition to genera founded on true synangia or groups of free or partially united sporangia, the literature of Palaeozoicnbsp;ferns contains several generic names applied to sporangia whichnbsp;occur singly on Sphenopteroid or Pecopteroid pinnules. Thenbsp;following may serve as examples; but it should be stated thatnbsp;these will probably be transferred eventually to the Pterido-1 Kenault and Zeiller (88) A. PI. xxiv.nbsp;nbsp;nbsp;nbsp;^ Page 325

sperms. It is, however, immaterial whether they are dealt with here or in the chapter devoted to the seed-bearing � ferns.�

Dactylotheca.

Zeiller' created this genus for fertile fronds of Pecopteris dentata Brongn. (= P. plumosa Artis^), a common British speciesnbsp;in the Upper and Middle Coal-Measures. Stur� included P.nbsp;dentata in his list of species of Senftenbergia, the genus tonbsp;which reference was made under the Schizaeaceae.

1828. Pecopteris plumosa, Brongniart, Hist. v�g. foss. p. 348, Pis. cxxi. CXXII.

1832. Sphenopteris cavdata, Bindley and Hutton, Foss. Flor. Vol. i. PI. XLVIII.; Vol. II. PI. cxxxviii.

1834. Pecopteris serra, Bindley and Hutton, ibid. Vol. II. PI. evil. 1834. Schizopteris (idnascens, Bindley and Hutton, ibid. Vol. l.nbsp;Pis. c. or.

1869. Pecopteris silesiacus, Schimper, Trait, pal. v�g. Vol. l. p. 517. � Cyathocarpus dentatus, Weiss, Flora der jiingst. Stk. undnbsp;Roth. p. 86.

1886. Dactylotheca plumosa, Kidston, Cat. Palaeozoic Plants, p. 128. 1888. Dactylotheca dentata, Zeiller, Flor. Valenc. Pis. xxvi.�xxviii.

For a fuller synonymy reference should be made to Kidston�s account of this species^, from which the above list is compiled.nbsp;The large fronds of this species are tri- or quadripinnate. Thenbsp;pinnules vary much in shape and size and in degree of lobing,nbsp;according to their position on the frond (fig. 293). The primary

pinnae are subtended by two Aphlebiae (fig. 293, A) appressed to the rachis, like the delicate leaves of the recent fern Tera-tophyllum aculeatum (see page 301). The sporangia (0'5�0'65)nbsp;are oval and exannulate and are attached parallel to the lateralnbsp;veins; they may occupy the whole of the space between thenbsp;midrib and the edge of the pinnules. This species occurs in thenbsp;Upper, Middle, and Lower Coal-Measures of Britain, reaching

its maximum in the Upper Coal-Measures. The aphlebiae undoubtedly served to protect the young fronds, as shown by anbsp;specimen figured by Kidston (fig. 293, B); they may also havenbsp;served other purposes, as suggested by the above comparisonnbsp;with Teratophyllum, in the mature frond. Bindley and Huttonnbsp;regarded the aphlebiae as leaves of a fern climbing up the

rachis; which they named Schizopteris adnascens, a confusion similar to that already mentioned in the description of Hemitelianbsp;capensis (see p. 304).

This name was proposed by Zeiller' for Upper Carboniferous fertile pinnae of the Sphenopteroid type, bearing ovoid sporangianbsp;either singly or in marginal groups of 2 to 5 at the ends of thenbsp;veins. The appearance of the apical cells occasionally suggestsnbsp;the presence of a rudimentary annulus. Kidston has recordednbsp;this type of fructification in Britain^. Stur describes fertilenbsp;pinnules of the same type under the generic name Hapalopteris^-

This genus was founded by Kidston^ for fertile pinnae of a very delicate fern, Zeilleria delicatula (Sternb.) characterisednbsp;by filiform ultimate segments bearing an indusium-like body,nbsp;spherical when immature and splitting at maturity into fournbsp;small valves. Kidston, in his earlier paper, compared thenbsp;species with recent Hymenophyllaceae. In the same genus henbsp;includes.^.a!;oWensfs^(Stur) assigned by Stur to Galymmatotheca,nbsp;a genus described by some authors as characterised by groups ofnbsp;radially elongated sporangia at the tips of the pinnules; thesenbsp;supposed sporangia are now known to be the valves of annbsp;indusium-like organ or cupule, as Stur asserted. There can benbsp;little doubt that the fertile fronds placed in Galymmatothecanbsp;and in Zeilleria were borne by Pteridosperms.

The Upper Carboniferous fronds of a delicate Sphenopteris habit, to which this name was assigned by Kidston�, werenbsp;described by him as Eusphenopteris tenella (Brongn.)^ andnbsp;compared with Hymenophyllaceae; subsequently Kidston expressed the opinion that Urnatopteris may be a Marattiaceousnbsp;fern, as Williamson� believed; he has since suggested thatnbsp;the sporangia are the mierosporangia of a Pteridosperm�. Thenbsp;sterile and fertile pinnae differ in the absence of a lamina innbsp;the latter. The sporangia (or microsporangia) are characterisednbsp;by a poricidal dehiscence.

The records from strata higher in the geological series than the Permian, disregarding many of doubtful value, afford amplenbsp;testimony to the existence of Marattiaceae in Upper Triassicnbsp;and Rhaetic floras.

The generic name Danaeopsis was applied by Heer� to an Upper Triassic fern, previously described by Presl as Taeni-

opteris marantacea. A splendid specimen from the Keuper of Stuttgart is figured in Schimpers Atlas' showing thenbsp;pinnate habit of the frond and the broadly linear segments,nbsp;25 cm. X 3'5 cm., bearing rows of contiguous sporangia. Thenbsp;large pinnules have a strong midrib giving off curved andnbsp;forked lateral veins. Presl�s species may most appropriately benbsp;included in the genus Marattiopsis. A specimen of M. marantacea described by Leuthardt^ as Danaeopsis marantacea fromnbsp;the Upper Trias of Basel shows a peculiarity in the venation ;nbsp;the lateral veins often fork near their origin, as noticed bynbsp;other authors, but each vein forks a second time near the edgenbsp;of the lamina and the two arms converge, forming a series ofnbsp;intramarginal loops (fig. 265, B).

This widely spread Rhaetic plant affords the best example of a post-Permian species which may be accepted as an authenticnbsp;record of fossil Marattiaceae. Various generic names have beennbsp;used for this species; Goeppert originally described the plantnbsp;as Taeniopteris Muensteri^; Schimper^ proposed the namenbsp;Marattiopsis, and Schenk� substituted Angiopteris on thenbsp;ground that the fertile pinnules resemble that genus rathernbsp;than Marattia. Marattiopsis, if interpreted as indicating anbsp;family resemblance rather than quot;special affinity to the genusnbsp;Marattia, would seem to be the more appropriate designation.

This species has been figured by several authors and in many instances with fertile pinnules; the best illustrationsnbsp;are those published by Zeiller� in his monograph of Tonkinnbsp;plants.

The pinnate fronds are characterised by a broad rachis bearing sessile broadly linear pinnules rounded at the base, obtusely pointed at the apex,nbsp;reaching a length of 15�20 cm. and a breadth of 12�35 wm. From a

well-marked midrib are given off secondary veins dichotomously branched close to their origin. The linear synangia near the ends of the veinsnbsp;contain two rows of sporangial compartments and open as two valves asnbsp;in Marattia. (Cf, fig. 245, A, p. 320.)

This species occurs in the Ehaetic beds of Scania, Franconia, and Tonkin. A similar type is figured by Fontaine from Jurassic beds in California as Angiopteridium californicimA,nbsp;and Bartholin^ and Moeller* record M. Muensteri from the Lias ofnbsp;Bornholm. Schenk�s species from China^, Angiopteris Richt-hofeni, is a closely allied species, and a similar form is recordednbsp;from Jurassic and Caucasian strata*. The microscopical examination by Nathorst� of a group of spores from a synangiumnbsp;of M. Muensteri shows that they resemble those of recentnbsp;Marattiaceae.

From the Upper Triassic plant beds of Lunz, Stur has included several species of ferns in the Marattiaceae, and ofnbsp;these Krasser�� has recently published full diagnoses but unfortunately without illustrations. In addition to Marattiopsisnbsp;marantacea (Presl) the list includes species referred to Goni-opteris, to Speirocarpus, a genus founded by Stur, to Oligocarpia,nbsp;Asterotheca, and Bernouillia (Heer).

As already pointed out, some at least of these Austrian ferns are more probably Osmundaceous than Marattiaceous.

The pinnate fronds described by Feistmantel� from the Middle Gondwana rocks of India and recorded from Rhaeticnbsp;strata in South Africa�, China��, and Tonkin�, may belong tonbsp;a member of the Marattiaceae, but no fertile specimens havenbsp;been described. The close agreement between the sterile leavesnbsp;from India and South Africa and the fertile fronds of Marattiopsisnbsp;marantacea suggests generic identity.

The Upper Triassic ferns described by Heer, Krasser^, and Leuthardt^ as Bernouillia have been referred to the Marattiaceae,nbsp;but without trustworthy evidence in favour of this affinity.

The large leaves, 70 cm. long and 7 cm. broad, described by Zigno'* from the Jurassic of Italy as Danaeites Heeri, arenbsp;probably Cycadean. The Polish Jurassic species Danaea micro-phylla^ is a more satisfactory record.

This name was instituted by Heer� for pieces of pinnate fronds from Lower Cretaceous rocks of Greenland. The resemblance of the long pinnules to the fertile segments of

Laccopteris is so close that generic identity might well be assumed, but it has recently been shown by Nathorst' thatnbsp;the soral characters justify Heer�s use of a distinctive namenbsp;for the Arctic fern. The circular sori arranged in two rowsnbsp;(fig. 294, A, B) are superficially identical with those of Laccopteris, but consist of concrescent sporangia forming a circularnbsp;synangium (fig. 294, C, D) like those of Kaulfussia andnbsp;Ptychocarpus. The lighter areas in fig. 294, D, represent thenbsp;sporangia: fig. C shows the radial disposition of the numerousnbsp;sporangial compartments round a central receptacle. From anbsp;stout midrib lateral veins arise at right angles, but their distalnbsp;terminations are not preserved. It is probable, as Nathorstnbsp;suggests, that Bayer�s^ species Drynaria fascia from the Lowernbsp;Cretaceous rocks of Bohemia should be referred to Heer�s genus.nbsp;In the absence of well-preserved sori it would be exceedinglynbsp;difficult, or even impossible, to distinguish between pinnules ofnbsp;Laccopteris and Nathorstia.

A Tertiary species, Marattia Hookeri (fig. 261, C, p. 350), described by Gardner and Ettingshausen� firom the Eocene bedsnbsp;of the Isle of Wight is referred by them to the Marattiaceaenbsp;because of a resemblance of the sterile pinnae to those ofnbsp;M. Kaulfussii; but this is insufficient evidence of relationship.

* Bayer (99). nbsp;nbsp;nbsp;* Gardner and Ettingshausen (82) PI. xii. figs. 1�7.

CHAPTER XXIII.

This family name, first suggested by Unger, may be conveniently adopted for the numerous species of petrified tree-fern stems characteristic of the Lower Permian and Upper Carboniferous strata. In his monograph Uher die Staarsteinenbsp;published in 1854, StenzeP referred to the Psaronieae as anbsp;special sub-division of the Filices most nearly allied to thenbsp;Polypodiaceae. There is now a consensus of opinion in favournbsp;of including Psaronius in the Marattiales, or at least of regardingnbsp;the genus as more closely allied to the Marattiaceae than to anynbsp;other family. While admitting that the balance of evidence isnbsp;in favour of this view, it is probably wiser to retain the distinctive term Psaronieae on the ground that species of Psaroniusnbsp;differ in several respects from any recent ferns, and becausenbsp;of our comparative ignorance in regard to the nature of thenbsp;fructification.

This generic name was proposed by Cotta in his classic work Die Dendrolithen^. The stems so named, formerly includednbsp;by SprengeP in the genus Endogenites, had long been familiarnbsp;as petrified fossils. Most of the specimens described by thenbsp;earlier writers were obtained from Lower Permian rocks in thenbsp;neighbourhood of Chemnitz, Saxony. The mottled appear-

ance presented by their polished surfaces is said to have given rise to the appellation Staarsteine (starling stones), anbsp;term expressing a resemblance, more or less remote, to anbsp;starling�s breast. It has been suggested that this word is anbsp;corruption of Stern Steine or star stones h a descriptive termnbsp;suggested by the stellate arrangement of the vascular strandsnbsp;in transverse sections of the roots. Parkinson^, in his Organicnbsp;Remains of a former World, speaks of these stems as starrynbsp;stones. The history of our knowledge prior to 1854 is summarised by Stenzel. At first compared with corals or thenbsp;stems of sea-lilies, Psaronii were recognised by Sprengel, whonbsp;first used a lens in the examination of the fossils, as fern stemsnbsp;most nearly allied to those of recent Cyatheaceae. By othernbsp;authors, e.g. Schlotheim and Sternberg, they were referred tonbsp;Palms, and by Brongniart considered to be the lower portionsnbsp;of Lycopodiaceous {Lepidodendron) stems. Corda and manynbsp;subsequent authors selected the Marattiaceae as the mostnbsp;closely allied family among existing plants.

Psaronius is represented by specimens obtained from the Lower Permian of Saxony and Upper Carboniferous rocks innbsp;Central France, also from Bohemia, Brazil and North America.nbsp;As yet a few fragments only have been found in the Englishnbsp;Coal-Measures. The genus was recognised by Williamson* whonbsp;described the roots and a small piece of the vascular tissue of anbsp;stem which he called P. Renaulti, and this type has since beennbsp;more fully described by Scott^. The roots of another speciesnbsp;have been described by Butterworth� as P. Cromptonensis.

It was pointed out in the account of Lepidodendron that several generic names have been used for the same type ofnbsp;stem in different states of preservation; in Psaronius accidentsnbsp;of fossilisation have been responsible for a similar confusion innbsp;nomenclature. The name Psaronius is applied to petrifiednbsp;specimens which, as a rule, lack external features. Casts ornbsp;impressions of Palaeozoic tree-fern stems provided with leaf-scars are described as species of Gaulopteris, Megaphyton, andnbsp;less commonly as Ptychopteris (figs. 297�299). The first name

is applied to stems exhibiting spirally disposed leaf-scars like those of recent tree-ferns; in Megaphyton the scars are dis-tichously arranged, in two rows, while Ptychopteris is appliednbsp;to decorticated stems. These terms are used for stems belongingnbsp;to one generic type and possessing the structure of Psaroniusnbsp;stems.

The researches of Grand�Eury^ led to the discovery that certain Psaronius stems bore fronds of the Pecopteris typenbsp;some of which bore sori of the Asterotheca or Scolecopteris tj^e.nbsp;The same author^ has also contributed many interesting facts.

obtained by an examination of the relation of Psaronius stems to the sediments of French Coal-fields in which they occur,nbsp;in regard to habitat and manner of growth. The specimennbsp;represented in fig. 295 shows a portion of a Psaronius stem, thenbsp;upper part of which illustrates the Caulopteris state of preservation, while the lower part is covered by a mass of roots. Itnbsp;is probable, as Rudolph� suggests, that this rich development ofnbsp;roots, which gives to an old Psaronius stem the appearance of an

elongated cone, may have served an important mechanical purpose analogous to the secondary thickening in a Dicotyledonnbsp;or a Conifer. A specimen of Psaronius Cottai in the Hofnbsp;Museum, Vienna, is cited in illustration of the enormous breadthnbsp;of the root-system: the radii of the stem proper and of thenbsp;encasing cylinder of roots bear the ratio 17 to 165. Thenbsp;comparatively frequent occurrence of a lacunar cortex in thenbsp;roots points to the growth of the stems in swampy ground, anbsp;conclusion in harmony with the evidence afforded by the anatomical features of many other Palaeozoic genera.

Tree-fern stems, oobasionally reaching a height of 50 feet or more, closely resembling in habit recent tree ferns, but exhibiting in the structurenbsp;and arrangement of the vascular system a close agreement with recentnbsp;Marattiaceae. Leaves, in such cases where a connexion between fronds andnbsp;stems is known, large and highly compound and of the Pecopteria type,nbsp;borne in more or less crowded spirals {Psaronius polystichi), in four rowsnbsp;(P. tetrastichi), or in two opposite rows {P. distichi). Leaves deciduous,nbsp;leaving a clearly defined oval scar containing the impression of the leaf-trace in the form of an open U, or a closed ovJ^l with a small invertednbsp;V-shaped band a short distance below the upper end of the long axis ofnbsp;the oval (figs. 297, 298) ; in Mega.phyton the alternate soars of the twonbsp;opposite series are larger and characterised by a different form of meri-stele. The surface of the cortex below the leaf-scars occasionally showsnbsp;impressions of pits similar to the lenticel-like organs on recent Tree-fernnbsp;stems. The central region of the .stem is occupied by a complex system ofnbsp;concentrically disposed steles (dictyosteles), which in transverse sectionnbsp;present the appearance of flat or curved bands varying in extent and innbsp;degree of curvature. The vascular bands consist of xylem surrounded bynbsp;a narrow zone of phloem; the xylem is composed either exclusively ofnbsp;tracheae or of tracheae and parenchyma; the protoxylem in the onenbsp;instance in which it has been clearly recognised is eudarch'. The stelesnbsp;are embedded in parenchymatous tissue and in some species are associatednbsp;with mechanical tissue (e.g. P. infarctus, fig. 296, A, B). The central ornbsp;vascular region of the stem may be surrounded externally by a cylinder ofnbsp;, mechanical tissue interrupted by outgoing leaf-traces and adventitiousnbsp;roots. The leaf-traces arise as single bundles from an internal stelarnbsp;band and pursue an obliquely radial course towards the outside, eventually anastomosing with peripheral cauline steles, which in some speciesnbsp;form with the leaf-traces the outermost zone of the vascular region.nbsp;The leaf-traces have the form of loops which pass into the petioles as

V-shaped meristeles or closed oval cylinders. As a leaf-trace passes out compensating strands occupy the foliar gap.

The vascular region is surrounded by a parenchymatous cortex, which in younger plants, or in the apical region of an older plant, forms thenbsp;surface of the stem to which the leaf-stalks are attached. From thenbsp;peripheral steles, or from the more external bands of the vascular network,nbsp;roots are given off which pass in a sinuous vertical course through thenbsp;cortex, appearing on the surface between the leaf-bases. In older stems,nbsp;after leaf-fall, the tissue immediately external to the vascular regionnbsp;produces secondary parenchyma with which the roots become intimatelynbsp;associated by their outermost cells. As a result of the secondary corticalnbsp;development and the gradual increase in the number of roots invadingnbsp;the cortical tissue from above, the stem is enclosed by a cylinder of rootsnbsp;and associated parenchymatous tissue of secondary origin. In still oldernbsp;portions of a stem the more external roots are free from the stem-cortexnbsp;and form a thick felted mantle, which increases in thickness towardsnbsp;the base of the tree.

The roots (fig. 296, E) are polyarch, 5�10 groups of xylem alternating with strands of phloem, and similar in structure to those of recent speciesnbsp;of Marattia and Angiopteris-, the stele is enclosed by an inner cortex ofnbsp;compact or lacunar tissue containing secretory sacs, and this is surroundednbsp;by a cylinder of mechanical tissue. In one or two instances secondarynbsp;xylem has been observed wholly or partially enclosing the root-stele i.

Our knowledge of the anatomy of Psaronius is based largely on the investigations of Stenzel considerably extended bynbsp;Zeiller�s more intensive studies and, more recently, by the laternbsp;work of StenzeP and that of Rudolph. A striking fact, whichnbsp;has led to various suggestions, is that in a transverse section ofnbsp;a Psaronius stem with its encasing cylinder of roots no signs ofnbsp;leaf-traces are met with in the root-region. If the roots simplynbsp;penetrated the cortex, as in some recent species of Lycopodiumnbsp;(fig. 125, A) or as in Angiopteris, we should expect to findnbsp;leaf-traces in the outer region (root-cylinder) of Psaroniusnbsp;stems. An explanation of the absence of leaf-traces which wasnbsp;suggested by Stenzel, is that the cortical zone formed anbsp;comparatively narrow band in the young leaf-covered stem;nbsp;after leaf-fall it became the seat of active growth in its innernbsp;layers and so produced a constantly widening zone of secondarynbsp;parenchyma, which pushed the superficial cortical tissue with

^ Butterworth (00). Pelourde (08^) has recently described the structure of the roots of several species of Psaronius.

the leaf-bases or leaf-scars farther out until it was exfoliated. Farmer and HilF find it difficult to accept this explanation; but,nbsp;as Rudolph shows, the radial arrangement of the cortical cellsnbsp;between the adventitious roots and their elongation in a radialnbsp;direction are arguments in support of the secondary nature ofnbsp;the cortical zone.

In sections of the adventitious roots of Psaronius Renanlti figured by Williamson^, the spaces between the cylindrical rootsnbsp;are partially occupied by cell-filaments which, at first sight,nbsp;suggest root-hairs; it may well be, as Rudolph suggests, thatnbsp;these felted hairs represent the outermost and looser part ofnbsp;the growing secondary cortex which gradually passes into thenbsp;covering mass of free extra-cortical roots.

As StenzeP has shown, slender stems of Zygopteris (= An-kyropteris) are occasionally met with growing through the web of Psaronius roots.

This species, which Zeillerquot;* has investigated from sections of Unger�s material, illustrates a type in which the vascular tissuenbsp;is very richly developed and forms crowded concentric series ofnbsp;curved plates associated, in the more peripheral series, withnbsp;bands of mechanical tissue. The outermost part of the vascularnbsp;region consists of (i) a series of loops or variously curved bandsnbsp;of conducting tissue representing leaf-traces at different stagesnbsp;in their outward course, (ii) a series of similar vascular strandsnbsp;(peripheral steles of Zeiller) confined to the stem (cauline) andnbsp;from which roots are given off, and (iii) bands of mechanicalnbsp;tissue associated with the leaf-traces and peripheral steles.nbsp;The peripheral steles (fig. 296, A, B, p) form anastomoses withnbsp;the leaf-traces and contribute to their formation.

The form of some of the vascular bands in the section of Psaronius infarctus shown,in fig. 296, A, illustrates thenbsp;occasional anastomosing of one dictyostele with another: the

different degrees of looping of other bands represent stages in the giving off of leaf-traces which eventually pass outnbsp;as V-shaped meristeles. Beyond the leaf-traces and scleren-chymatous bands the section consists of transverse sections ofnbsp;adventitious roots.

The surface-features of Psaronius infarctus are probably represented, as Zeiller points out, by the cast described bynbsp;Lesquereux as Gaulopteris peltigera (fig. 298, A).

The Psaronius shown in fig. 297 is one of the few examples illustrating the connexion between fronds and stem. The leafnbsp;{Pecopteris Sterzeli Zeill. and Ren.^) is quadripinnate and isnbsp;described as reaching a length of at least 3 metres; thenbsp;ultimate segments are entire or lobed. The stem is characterised by elliptical scars, 6�8 cm. x 3�5�4 cm., with leaf-traces like those in Gaulopteris peltigera. The fronds ofnbsp;Pecopteris Pluckeneti, a Pteridosperm, bear a very close

resemblance to those of P. Sterzeli, which are as yet known only in a sterile state.

Psaronius hrasiliensis Unger, a species founded by Unger on a piece of silicified stem acquired by Martius in Brazil andnbsp;now in the Rio Museum, is a good example of a tetrastichousnbsp;species. Solms-Laubach^ has recently told the history of thisnbsp;type, which is represented by sections, cut from the Rio stem,nbsp;in several European collections. A well-preserved section innbsp;the British Museum is figured by Arber^ in his cataloguenbsp;of the Glossopteris flora and by other authors. Scott givesnbsp;a concise description of the species in his Studies in Fossilnbsp;Botany*. The roots of P. hrasiliensis are stated by Pelourde*nbsp;to have a lacunar cortex.

This species from the Lower Permian of Chemnitz and the Coal-Measures of Bohemia affords an example of the distichousnbsp;type in which the leaves are borne in two rows. The vascularnbsp;bands, as seen in a section of the dictyosteles, occur in regularnbsp;parallel series. The stelar region is separated from the cylindernbsp;of encasing roots by a sclerenchymatous sheath, broken atnbsp;intervals where roots pass out from the vascular region.

Psaronius coalescens^ (flg, 296, C) illustrates a somewhat different arrangement of vascular tissue which approaches morenbsp;closely to the polycyclic structure characteristic of such recentnbsp;ferns as Matonia and Saccoloma. A still closer resemblance tonbsp;the solenostelic type is seen in Psaronius Renaulti from thenbsp;Lower Coal-Measures of England which Scott^ describes asnbsp;characterised by a single annular stele, interrupted only bynbsp;the exit of leaf-traces. As he points out, it is noteworthy thatnbsp;this species is distinguished by the simplest form of stele metnbsp;with in the genus; it is the oldest species and may be regarded

�* Pelourde (08^). nbsp;nbsp;nbsp;� Stenzel (06) PI. vi.; Goeppert (64) A.; Stenzel (06).

Psaronius stems preserved as casts showing surf ace-features, or in a decorticated state.

This generic name was instituted by Lindley and Hutton^ for tree-fern stems from the English Coal-Measures showingnbsp;circular or oval scars arranged quincuncially. The vascular tissuenbsp;of the petiole is represented by a U-shaped impression on the

scar, the ends of the U being incurved, or by a closed oval ring with a wide-open and inverted V near its upper end. Thenbsp;surface between the leaf-scars bears the impression of adventitious roots. Caulopteris represented, in the Upper Coal-Measures of England, by G. anglica^ Kidst. The speciesnbsp;C. peltigera (fig. 298, A), originally described by Brongniart asnbsp;Sigillaria, illustrates the closed form of leaf-trace and, as Zeillernbsp;suggests, it is the cast of a Psaronius stem which possessednbsp;a vascular system on the same plan as that of P. infarctus.nbsp;C. Saportae^ illustrates the open U-shaped type of petiole stele.

Gaulopteris peltigera has scars measuring 6�9 by 4�6 cm.; it occurs in the Commentry Coal-field of France in association with the fronds known as Pecopteris cyathea, a speciesnbsp;which Kidston regards as identical with P. arborescens^.

The first use of this name was by Artis�, who gave it to a long flattened cast, Megaphyton frondosum, found in Carboniferousnbsp;strata in Yorkshire, characterised by two vertical rows of largenbsp;scars and by impressions of sinuous roots. Kidston records thenbsp;genus from the Middle and Upper Coal-Measures of Britain.nbsp;A good example of this type of cast is afforded by M. McLayinbsp;Lesq.� from the Coal-Measures of North America, which hasnbsp;been recognised in European Carboniferous rocks. The leaf-scars are rounded or oval, broader than high; the vascularnbsp;impression has the form of a closed ring (5�8 x 3�6 cm.),nbsp;more or less circular and with a tendency to a rectangularnbsp;outline, characterised by a deep inverted U-shaped sinus in thenbsp;middle of the lower surface and by a W-shaped impressionnbsp;of an internal strand (fig. 298, B)^.

This generic name, instituted by Corda�, is applied to decorticated stems of Psaronius, the surface of which is thatnbsp;of the vascular region on which the form of the leaf-scars isnbsp;more or less clearly defined. The scar-areas are limited by annbsp;impression of the sclerenchymatous sheath enclosing the leaf-meristele, and internal to this is the impression of the leaf-trace.nbsp;In some specimens a layer.of coaly material which representsnbsp;the carbonised cortex and adventitious roots covers the Ptychopteris cast. The Ptychopteris cast represented in fig. 299nbsp;shows the decorticated surface of part of a long stem on whichnbsp;the leaf-scars are arranged as in Megaphyton. An example of

Ptychopteris is figured by Fontaine and White^ from Virginia as Caulopteris gigantea.

A comparison of Psaronius with the Marattiaceae and other recent ferns leads to the conclusion that, on the whole, thenbsp;evidence is in favour of the view usually held, namely thatnbsp;this genus is more closely related to the Marattiaceae than tonbsp;any other recent ferns. It is, however, important not to overlook the differences between Psaronius and recent genera ofnbsp;Marattiaceae, or the resemblances between the extinct genusnbsp;and the Cyatheaceae. In habit Psaronius agrees closely withnbsp;recent tree-ferns; in the vascular system and in the sequence

Dicksonia antarctica (half of stem in transverse section): st, stele; s, sclerenchyma.

of events connected with the production of leaf-traces, there are striking resemblances between Psaronius and the Cyatheaceousnbsp;fern Saccoloma adiantoides (= Dichsonia Plum.ieri Hook.) asnbsp;described by Mettenius^. The piece of stem of Dicksonianbsp;antarctica represented in fig. 300 exhibits a fairly close agreement with species of Psaronius, e.g. P. infarctus (fig. 296, A, B).nbsp;Moreover, the peripheral steles, which Zeiller has shown arenbsp;confined to the stem and play an important part in the production of the roots and in the anastomoses with loaf-traces,

are not represented in any Marattiaceous fern; on the other hand, they are comparable with the accessory strands met withnbsp;in stems of recent Cyatheaceoiis tree-ferns^ (c� fig. 240). Thenbsp;complex system of concentric dictyosteles is a feature more closelynbsp;matched in Angiopteris (Marattiaceae) than in any Cyathe-aceous genus, the chief difference being in the more band-likenbsp;form of the steles in Psaronius, though in a stem of Angiopterisnbsp;figured by Mettenius we see a close approach to the extinctnbsp;type. The position of the protoxylem has unfortunately notnbsp;been clearly defined in Psaronius stems, but in P. Renaulti itnbsp;is stated by Scott^ to be endarch, a position which some of thenbsp;protoxylem strands occupy in Angiopteris^. The occurrence ofnbsp;large sieve-tubes described by Scott in P. Renaultii is anothernbsp;feature shared by recent Marattiaceae. In many of the continental species of Psaronius the phloem has not been preserved,nbsp;and our knowledge of this tissue is comparatively meagre. Innbsp;the Marattiaceae the roots arise mainly from the inner portionsnbsp;of the stele, while in Psaronius they are usually formed fromnbsp;the external vascular bands. The formatioft of secondary corticalnbsp;tissue is a peculiarity of Psaronius', on the other hand, ifnbsp;Butterworth^ is correct in referring to that genus the rootsnbsp;with secondary xylem, which he describes as P. Cromptonensis,nbsp;a comparison may be made with the occurrence of secondarynbsp;tracheae in the stem steles of Angiopteris^.

The absence of mechanical tissue in the stem of Angiopteris is in contrast with its occurrence in the fossil stems and innbsp;recent tree-ferns; but this is a character of secondary importance and one which can be readily explained by the differencenbsp;in habit between Angiopteris and Psaronius.

The roots of Psaronius, more especially as regards the stelar structure, are in close agreement with those of Marattiaceae.

The reference to Marattiaceae of the great majority of fertile fern-like fronds from Permian and Carboniferous rocks constituted a strong a priori argument in favour of includingnbsp;Psaronius stems in the same family, especially when it wasnbsp;known that leaves with Marattiaceous synangia were borne by

1 Rudolph (05). nbsp;nbsp;nbsp;2 Scott, D. H. (08).nbsp;nbsp;nbsp;nbsp;3 Shove (00).

species of this genus. It is, however, well to remember the change in our views as to the dominance of Marattiaceae innbsp;Palaeozoic floras consequent on the discovery of the Pterido-sperms. The association of fronds bearing Asterotheca andnbsp;Scolecopteris types of fructification with Psaronws stems recordednbsp;by Grand�Eury 1 is a point in favour of the Marattiaceous affinitynbsp;of this extinct genus, but it is not impossible that Fsaroniusnbsp;stems bore fronds which produced Pteridosperm organs of reproduction. In this connexion the specimen represented innbsp;fig. 297 is of interest, as the fronds (Pecopteris Sterzeli) bornenbsp;on the Fsaronius stems are hardly distinguishable from thenbsp;seed-bearing leaves known as Pecopteris Pluckeneti.

The position of Fsaronius may be best expressed by assigning it to a separate family, the Psaronieae, as advocated by Stenzel,nbsp;and by regarding it as one of the many instances of a generalisednbsp;type which in the sum of its characters approaches most nearlynbsp;to the Marattiaceae.

CHAPTER XXIV.

The fossils hitherto classed with the Ophioglossales are not such as afford any satisfactory evidence in regard to the pastnbsp;history or phylogeny of the group. In the generalised class ofnbsp;Palaeozoic ferns, the Botryopterideae, we find certain charactersnbsp;suggesting comparison with recent members of the Ophioglos-

Fig. 301. Rhacopteris sp., Ballycastle, Ireland. From a specimen in the Manchester Museum. [M.S.]

saceae, but no trustworthy records of these eusporangiate ferns are furnished by the older plant-hearing strata.

The genus Rhacopteris (fig. 301), characteristic of the Culm flora, has been compared with Botrychium, but on grounds which

are wholly inadequate. The species R. paniculifera Stur^ is characterised by a stout rachis bearing two rows of laterallynbsp;attached rhomboidal or subtriangular segments with a more ornbsp;less deeply lobed margin and spreading veins. The rachisnbsp;branches distally into two arms, and these are again symmetrically subdivided into fertile axes bearing clusters of smallnbsp;spherical bodies 1 mm. broad, which Stur speaks of as ex-annulate sporangia similar to those of Botrychium. He includesnbsp;the species in the Ophioglossaceae. As Zeiller^ pertinentlynbsp;remarks, Rhacopteris differs essentially in habit from any recentnbsp;member of this family. Rhacopteris also includes species characterised by leaflets deeply dissected into linear segments; annbsp;example of this form is represented by Rhacopteris jlabellatanbsp;(Tate) recorded by Kidston* from rocks of Calciferous Sandstone age in Flintshire.

The specimen described by Renault^ from the Carboniferous rocks of Autun as Ophioglossites antiqua is equally unconvincing:nbsp;it consists of a carbonised fragment, 7 cm. x 1�5 cm., regarded asnbsp;part of a fertile lamina characterised by a vertical series ofnbsp;transversely elongated slits, 7 mm. wide, some of which, onnbsp;slight magnification, are seen to contain a mass of small orange-yellow granulations. The slits are compared with the surface-openings of the sunken sporangia of Ophioglossum, and thenbsp;yellow bodies are identified as spores. The material is toonbsp;imperfect to justify the use of the name Ophioglossites.

This genus of uncertain position may be briefly described here, though it has little claim to recognition as a representative of the Ophioglossales. It is characteristic of Lowernbsp;Carboniferous rocks and is compared by Stur� with recentnbsp;Ophioglossaceae. Noeggerathia foliosa Sternb. (fig. 302) maynbsp;be cited as a typical example of the genus. It consists of an

� Kidston (89�), Pis. i. ii. For other figures of Rhacopteris see also Stur (75) A.

axis bearing ovate leaves with numerous spreading veins. The upper part of the axis forms a spike composed of fertile leavesnbsp;in the form of transversely oval bracts 2 cm. broad with a serrate

edge bearing on the upper face several sporangia (3x4 mm.) in some of which spores have been seen (fig. 302, B, C). Innbsp;another form described by Weiss^ the bracts bear a greaternbsp;1 Weiss, C. E. (79).

number of sporangia characterised by the presence of an arillus-like basal ring.

Geinitzh who first described fertile specimens of Noeggera-placed the genus in the Gymnosperms, and 0. FeistmanteP was in favour of this view. C. FeistmanteP, who describednbsp;the small bodies in the sporangia, suggested comparison withnbsp;Schizaeaceae, and Weiss^ discussed various possibilities, askingnbsp;but not answering the question, are the so-called sporangia

rightly so named or are they fruits ? Potoni� places the genus in the Cycadofilices. An important feature is thenbsp;occurrence of the sporangia on the upper face of the bractsnbsp;as in Lycopodiales and 8phenophyllum, but in other respectsnbsp;Noeggerathia bears no resemblance to these two groups. Sterilenbsp;examples of the genus are similar in habit to Rhacopteris, butnbsp;in the latter genus the leaves or leaflets are laterally attachednbsp;and not obliquely inserted. Further, we may assume that in

* C. Feistmantel (79). nbsp;nbsp;nbsp;C. E. Weiss (79).nbsp;nbsp;nbsp;nbsp;� Potoni� (99) p. 167.

Rhacopteris the segments are leaflets of a compound leaf, whereas in Noeggerathia they are probably single leaves. Wenbsp;must leave the position of this Lower Carboniferous genusnbsp;undecided, merely expressing the opinion that it is perhapsnbsp;more nearly allied to the Cycads than to any other group.

The plant figured by Lindley and Hutton from the English Coal-Measures as Noeggerathia flahellata, which some authorsnbsp;quote as a species of Noeggerathia, is generally recognised asnbsp;a Psygmophyllum and placed with some hesitation in thenbsp;Ginkgoales.

This genus was founded by Kurr on a leaf characterised by anastomosing venation from Keuper beds near Stuttgart.nbsp;A resemblance in form and venation to the leaves ofnbsp;recent species of Ophioglossum led authors to suggest thenbsp;inclusion of Kurr�s specimen in the Ophioglossaceae. Wenbsp;have, however, no justification for considering Chiropteris as anbsp;member of this family; it may be a fern,'and that is all thatnbsp;can be said. The leaf represented in fig. 303 is the type-specimen of a South African Rhaetic species Chiropterisnbsp;Zeilleri^. The genus is recorded also from Rhaetic rocks innbsp;Queensland. ^

Newberry� describes some leaves from the Lower Cretaceous of Montana as species of Chiropteris: one of his types,nbsp;C. spatulata, is almost certainly a Sagenopteris, similar tonbsp;S. Phillipsi (figs. 327, 328) or S. Mantelli. A second species,nbsp;C. Williamsii, is probably not generically identical with thenbsp;specimen represented in fig. 303.

CHAPTER XXV.

The term Botryopterideae, first used by Eenault, has been applied to a group of Palaeozoic ferns ranging from the Lowernbsp;Carboniferous to the Permian and containing several genera,nbsp;the distinguishing features of which are supplied by thenbsp;anatomical structure of the stems or, in many cases, by thatnbsp;of the petiolar vascular strand. Scott ^ subdivides the Botryopterideae into the Botryopteris and the Zygopteris sections. Innbsp;an admirable monograph recently published by Paul Bertrand^nbsp;considerable changes are proposed in current nomenclature; henbsp;substitutes the name Inversicatenales for Botryopterideae, anbsp;designation, which as Scott remarks, is � probably too technicalnbsp;to command general acceptance.� A more serious criticism isnbsp;that the name Inversicatenales has reference to a characternbsp;(the inverse curvature of the leaf-trace in relation to the axisnbsp;of the stem) which is by no means universal in the group�.

In the following account, necessarily incomplete, the generic terminology of Bertrand is adopted, but this decision does notnbsp;carry with it any obligation to accept the name Inversicatenales.nbsp;We may speak of the types of Palaeozoic ferns dealt with innbsp;the following pages as members of a group differing in manynbsp;respects from any existing genera of the Filicales, and exhibiting the characteristics associated with generalised plants.nbsp;Williamson, as early as 1883, spoke of Renault�s Botryopte-

rideae as comprising � altogether extinct and generalised � types For these generalised Palaeozoic ferns I propose to usenbsp;the name Coenopterideae^. This term may be adopted in anbsp;wider sense than Kenault�s name Botryopterideae. The namenbsp;Primofilices proposed by Arber* might be employed, but thenbsp;implication which it carries is an argument against itsnbsp;adoption. We have not yet reached a stage in the investigation of extinct types at which we are able to recognisenbsp;what are actually primary or primitive ferns. The search fornbsp;origins will continue; as new discoveries are made our pointnbsp;of view shifts and the primitive type of to-day may to-morrownbsp;have to take a higher place. The epithet primitive or primarynbsp;is in reality provisional: to adopt such a name as Primofilices suggests a finality which has not been, or is likely tonbsp;be, achieved. The true ancestral type�the Urform�whichnbsp;we strive to discover eludes the pursuer like a will-o�-the-wisp.

Seeing that the number of true ferns of Palaeozoic age has been recently considerably reduced and is likely to suffer furthernbsp;reduction, the consideration of such undowbted Carboniferousnbsp;and Permian examples of the Filicales as are left acquires anbsp;special importance. In the first place it is natural to asknbsp;whether the Palaeozoic ferns include any types which, if notnbsp;themselves ancestral forms, may serve to indicate the probablenbsp;lines of evolution of existing families. It is probable that innbsp;the near future our knowledge of the Coenopterideae will benbsp;considerably extended; as yet we possess meagre informationnbsp;in regard to those characters on which most stress has generallynbsp;been laid in the classification of recent ferns, namely thenbsp;structure of the spore-bearing organs. The sporangia ofnbsp;Diplolabis and Staiiropteris (figs. 309, A; 322) are exannulate;nbsp;in the former genus they occur in sori or synangia consistingnbsp;of a small number of sporangia, while in the latter they arenbsp;home singly at the tips of ultimate ramifications of a highly

- KOigt;'�s=Lat. communis, common or general. I am indebted to my friend Mr L. H. G. Greenwood, Fellow of Emmanuel College, for supplying me withnbsp;a name to express the idea of the generalized nature of these Palaeozoic ferns. ^

compound leaf. The resemblance of the synangium of Diplo-labis to that of Kaiilfussia (fig. 245, C) is not shared in an equal degree by the sporangia of Stauropteris, which are innbsp;some respects comparable with those of the Ophioglossaceae.nbsp;In the Zygoptereae, or at least in the case of such fertile frondsnbsp;as are known, and in Botryopteris (fig. 319), the sporangia occurnbsp;in groups, and the pedicel of each sporangium is supplied withnbsp;vascular tissue as in Uelmintliostachys. Another characteristicnbsp;of the sporangia of the extinct types is the possession of annbsp;annulus several cells in breadth, a peculiarity which supplies anbsp;point of contact with the Osmundaceae. In the sporangia ofnbsp;Kidstonia we have a similar though not an identical type (fig. 256,nbsp;E, p. 340). So far, then, as the evidence afforded by sporangialnbsp;characters is concerned, it points to comparison with thenbsp;Ophioglossaceae, the Osmundaceae, and the Marattiaceae.nbsp;When we compare the steles of the stems we find a widenbsp;range of structure. All the genera agree in being monostelic;nbsp;in Tiihicaulis and Grammatopteris the protoxylem is exarch,nbsp;in Botryopteris it is internal, while the foliar strand ofnbsp;Stauropteris and the stele of Ankyropteris corrugata arenbsp;mesarch. The axillary branching of species of Ankyropterisnbsp;suggests comparison with the Hymenophyllaceae.

The investigation of the vascular system of the petioles has afforded results which in the hands of P. Bertrand have led tonbsp;conclusions in regard to inter-relationships. We must, however,nbsp;not overlook the danger of attributing an excessive importancenbsp;to this single criterion and of neglecting the facts of stemnbsp;anatomy.

Renault instituted this genus for petrified stems from the Permo-Carboniferous beds of Autun. Grammatopteris RigollotA,nbsp;the type-species, is represented by a fragment, 12�15 mm. innbsp;diameter, surrounded by crowded petioles characterised by anbsp;vascular strand in the form of a short and comparatively broadnbsp;^ Eenault (96) A. p. 46, Pis. xxx. xxxi. See also Tansle3' (08) fig. 2, p. 13.

plate with the smallest tracheae at each end. The solid xylem of the stem stele (protostele) has peripheral groups of proto-xylem. Nothing is known as to the form of the leaves, butnbsp;sporangia similar to those of Etapteris (Zygopteris) were foundnbsp;in association with the stem. It is possible, as P. Bertrandnbsp;suggests, that Eenault�s species may, be the stem of anbsp;Tubicaulis.

This species from the Lower Permian of Saxony has been fully described by StenzeP. It is characterised by a very slendernbsp;-erect stem bearing numerous spirally disposed leaves associatednbsp;with adventitious roots; the single stele (protostele) consistsnbsp;exclusively of tracheae, described as intermediate between thenbsp;scalariform and reticulate type, surrounded by phloem. Leaf-traces are given off from the periphery of the stele where groups

Pio. 304. Tubicaulis solenites. (From Tansley, after Stenzel.) Stem and petioles: the latter numbered in the order of their age.

of smaller elements occur; these have the form of a wide-open U-shaped strand with the base of the U facing the axis of thenbsp;stem. As the trace passes out towards the leaves, the ends ofnbsp;the U become more or less incurved. The stem is said to reach

a metre in length and to bear compound fronds a metre long. The orientation of the leaf-trace with its concavity turnednbsp;outwards is in striking contrast to the relation between leaf-trace and stem in recent ferns.

In this species the vascular axis, 2 mm. in diameter, is almost cylindrical and of the protostelic type with the proto-xylem � near to or at the edge � ; the tracheae are scalariformnbsp;or reticulate. The leaf-traces, when first separated from thenbsp;edge of the stele, are oval and gradually assume the curvednbsp;form seen in T. solenites (fig. 304) with the convex side towardsnbsp;the axis of the stem. The transition fi�om the scalariform tonbsp;the reticulate type of pitting on the tracheal walls referred tonbsp;by Miss Stopes has also been noticed in some recent fernsnbsp;(e.g. Helminthostachys) and in Sigillaria (fig. 200, C, p. 212).nbsp;The fact that the scalariform type of pitting is practicallynbsp;universal in the xylem of recent ferns, would seem to shownbsp;that this character has been acquired in the course of evolutionnbsp;and retained in preference to the reticulate form characteristicnbsp;of several Palaeozoic species. The distinction between the twonbsp;methods of pitting is one of little phylogenetic importance.

This genus, founded by Renault on a specimen from Autun, is represented in the Lower Coal-Measures of England bynbsp;Botryopteris hirsuta {= Rachiopteris hirsuta Will), B. ramosanbsp;(= R. ramosa Will.^) (fig. 306) and B. cylindrica (fig. 305), alsonbsp;by B. antiqua (fig. 307) from the Culm of Pettycur, Scotland.

An important characteristic of the genus is the solid stele of the stem which agrees with that of Tuhicaulis and Gram-matopteris, except in the central or peripheral position of thenbsp;smallest tracheae.

2 nbsp;nbsp;nbsp;Williamson (89) A. p. 162. The term Bachiopteris was adopted bynbsp;Williamson for petrified petioles from the Coal-Measures which he believednbsp;to be filicinean.

The stem of this species from the Upper Carboniferous of St Etienne is 1'7 om. x 7'5 mm. in diameter. The solid stelenbsp;consists of reticulate tracheae with the smallest elements onnbsp;the outer edge. The comparatively broad cortex of the type-specimen is traversed by a leaf-trace in an almost verticalnbsp;course and by vascular strands passing horizontally to roots.nbsp;The petioles are circular in section and their vascular strandnbsp;has the form of an to in transverse section (fig. 319, G), thenbsp;three projecting arms pointing to the axis of the stem. Bothnbsp;stem and leaves bore large multicellular hairs, spoken of bynbsp;Renault as equisetiform because of the finely toothed sheathsnbsp;of which they are composed. The compound fronds had fleshynbsp;lobed pinnules with dichotomously branched veins (fig. 309, B);nbsp;stomata are said to be confined to the upper surface, annbsp;observation which leads Renault to describe the plant asnbsp;aquatic on evidence which is hardly convincing.

The pyriform and pedicellate sporangia are borne in groups of two to six on the ultimate divisions of the frond; thenbsp;wall is composed of two layers of cells and on one side of thenbsp;sporangium is an annulus several cells in breadth (fig. 319,nbsp;I), F). An interesting type of sporangium described by Oliver^nbsp;from Grand�Croix in France may, as he suggests, belong tonbsp;Botryopteris forensis; the differences between Renault�s andnbsp;Oliver�s specimens being the result of the more perfect preservation of the tissues in the latter. The sporangium describednbsp;by the English author is circular in section and measuresnbsp;0-65 X 0-53 mm.; the wall is in part composed of a single layernbsp;of cells and in part of two to three layers, a character recallingnbsp;the � annulate � sporangia of Botryopteris. Between the spore-mass and the wall is an interrupted ring of short trachealnbsp;elements similar to the xylem-mantle which occurs at thenbsp;periphery of the nucellus of certain Palaeozoic gymnospermousnbsp;seeds. In the absence of proof of a connexion between thisnbsp;sporangium and Botryopteris it is convenient to use thenbsp;generic name Tracheotheca subsequently proposed by Oliver�.

In the recent ferns Helminihostachys and Botrychium, and, as Oliver notices, in the microsporangia of the Australian Cycadnbsp;Bowenia spectabilis, vascular strands extend almost to thenbsp;sporogenous tissue, but the fossil sporangium is unique innbsp;having a tracheal layer in immediate contact with the spores.nbsp;These xylem elements may, as Oliver suggests, have served thenbsp;purpose of conveying water to the ripening spores.

This English species has a slender axis bearing numerous leaves with petioles equal in diameter to the stem. The surfacenbsp;of the vegetative organs bears large multicellular hairs. Thenbsp;leaf-traces resemble those of B. forensis, but the projecting teethnbsp;which terminate in protoxylem elements are less prominentnbsp;than in the French species; the petioles were named by Felixnbsp;Rachiopteris tridentata^. As a leaf-trace passes into the stelenbsp;of the stem the three protoxylem strands unite and take up annbsp;internal position in the solid stele. The stele may, therefore,nbsp;be described as endarch. The small tracheae at the edge ofnbsp;the stele supply the xylem strands of adventitious roots.

Sporangia similar to those of B. forensis have been found in association with the English species.

A plant originally described by Williamson� from the Lower Coal-Measures of England as Rachiopteris cylindrica (fig. 305)nbsp;and afterwards more fully dealt with by Hick*, has a slendernbsp;stem with a cylindrical stele characterised by well-definednbsp;central protoxylem elements in one or two groups. The leaf-traces are semi-lunar in section with the protoxylem on thenbsp;fiatter side. The stele of Botryopteris cylindrica (fig. 305, A)nbsp;is more cylindrical in section than that of B. ramosa (fig. 306)nbsp;and shows more clearly the differentiation into smaller centralnbsp;and larger peripheral tracheae. In the section reproduced innbsp;fig. 305, B the stele is giving off a branch almost identical in

structure with the main vascular axis. Scott�, in referring to the inclusion of this type in the genus Botryopteris, expressesnbsp;the opinion that its habit must have been very different from

This species, which bears a close resemblance to Botryopteris hirsuta, was originally described by Williamson from the Lowernbsp;Coal-Measures of England as Rachiopieris ramosa^, the specificnbsp;name being chosen on account of the numerous and crowdednbsp;branches given off from the main axis. The section shown innbsp;fig. 306, A, illustrates Williamson�s description of the stem asnbsp;being �always surrounded [when seen in transverse sections]nbsp;by a swarm of similar sections of the large and small branches,nbsp;though of varying shapes and sizes.� The stele is composed ofnbsp;a solid and more or less cylindrical rod of xylem tracheae ofnbsp;the reticulate type surrounded by phloem (figs. A and D); onenbsp;or more internal groups of smaller protoxylem elements occurnbsp;in an approximately central position (fig. A, px). The stele isnbsp;in fact endarch like those of Selaginella spinosa and Tricho-manes reniforme, a feature which, as Tansley^ believes, probablynbsp;entitles the vascular axis to be considered a primitive form ofnbsp;protostele. In the specimens represented in fig. 306 the phloemnbsp;and inner cortical tissues were almost completely destroyednbsp;before petrifaction. The thick-walled outer cortex bears at itsnbsp;periphery numerous multicellular hairs. Some of the xylemnbsp;strands given off from the stele no doubt supplied adventitiousnbsp;roots, but in most cases the outgoing branches are leaf-tracesnbsp;and the numerous sections of axes of different sizes seen innbsp;fig. A point to a repeated subdivision of the crowded fronds.nbsp;The structure of a petiole is shown in figs. C and D. As seennbsp;in fig. C, the oval vascular strand has three protoxylem groups,nbsp;px, on its flatter side; a well-defined epidermal layer is shownnbsp;at e in fig. C.

Fig. B shows at a a section of a leaf-axis in the act of branching and the row of branchlets at b represents a further

1 nbsp;nbsp;nbsp;Williamson (91^) A. p. 261. The two species describ^icl by Williamsonnbsp;as Rachiopteris hirsuta and R. ramosa were first identified as Botryopteris bynbsp;Scott in 1898 {British Assoc, Report, Bristol Meeting, p 1050).

stage in subdivision. At sp in fig. A the section has cut through a single sporangium characterised by a group of largernbsp;(� annulus �) cells on one side of the wall.

This slender fern with its numerous repeatedly branched leaves may perhaps have lived epiphytically on more robustnbsp;plants.

This species, recently described by Kidston^ from the Culm of Pettycur near Burntisland, is represented by sections of anbsp;small stem with a cylindrical stele 0'40 mm. in diameter composed entirely of scalariform tracheae without any recognisablenbsp;protoxylem. The petioles are larger than the stem; thenbsp;meristele (fig. 307) is oval with protoxylem elements on thenbsp;slightly more rounded adaxial face. As Kidston suggests, this

stem may belong to a scrambling plant which required support to bear its relatively large leaves. An interesting feature isnbsp;the absence of projecting teeth in the leaf-trace, a character innbsp;marked contrast to the co form assumed by the petioles ofnbsp;Botryopteris forensis (fig. 319, G) and B. hirsuta. This leadsnbsp;Kidston to suggest that the vascular strand of the petiolenbsp;tends �to become more simple...as traced back in geologicalnbsp;time.� The greater similarity in this species between the st�le

of the stem and that of the petiole is probably another mark of a more primitive type.

In these three types, Orammatopteris, Tubicaulis, and Botryo-pteris, we have monostelic plants, for the most part of very small size, with leaf-traces varying in shape from the oblong band-formnbsp;in Grammatopteris, and the oval form of Botryopteris antiqua, tonbsp;the �) type represented in its most pronounced form by B. forensis.nbsp;In several species the stem stele is endarch. Our knowledgenbsp;of the leaves is very meagre: in B. forensis they were repeatedlynbsp;branched and apparently bore small fleshy pinnules; the sporangia, though differing from those of recent ferns, may benbsp;compared with the spore-capsules of Osmundaceae as regardsnbsp;the structure of the annulus. The abundance of hairs on thenbsp;stems and leaves of some species, the tracheal sheath in thenbsp;sporangium described by Oliver' as Tracheotli,eca (=Botryopterisl),nbsp;and the apparent absence of a large well-developed lamina,nbsp;may perhaps be regarded as evidence of xerophilous conditions.

Corda� proposed the generic name Zygopteris for petrified petioles from the Permian of Saxony, included by Cotta in hisnbsp;genus Tubicaulis, which he named T. primarius. Corda�s genusnbsp;has been generally used for petioles of Palaeozoic ferns characterised by a vascular strand having the form of an H innbsp;transverse section (fig. 308, D). Since the generic name wasnbsp;instituted, information has been obtained in regard to thenbsp;nature of the stems which bore some of the petioles of thenbsp;Zygopteris type; and for other species of Zygopteris, the stemsnbsp;of which are still unknown, new generic names have beennbsp;proposed. P. Bertrand^ retains Zygopteris for one species only,nbsp;Z. primaria. Fig. 308, D, shows the character of the petiolarnbsp;vascular strand; the chief points are the comparatively longnbsp;cross-pieces (antennae of P. Bertrand) inclined at an angle ofnbsp;45� to the plane of symmetry of the petiole axis, and the groups

of protoxylem elements shown by the white patches in fig. D. In this as in other members of the Zygoptereae the main rachisnbsp;of the leaf gives off four sets of branches in pairs alternately

The white patches in the xylem in figs. B�G mark the position of protoxylem elements.

from the right and left side of the primary vascular axis. This method of branching of the stele in the primary rachis ofnbsp;several members of the Coenopterideae shows that the fronds

bore pinnae laterally disposed, in some cases in one row and in others in two rows on each side of the rachis. In a typical fernnbsp;frond, as represented by recent and most fossil species, branching

of the rachis occurs in the plane of the frond, that is in the plane represented by the horizontal arm of xylem in Zygopterisnbsp;primaria connecting the two antennae or cross-pieces. In thenbsp;Zygoptereae the branches from the petiole vascular axis lie innbsp;a plane at right angles to that of the frond; they lie in thenbsp;transverse and not in the horizontal plane. The two strandsnbsp;shown in fig. 308, B, 4, have been formed by the division of anbsp;single strand, 3, in the transverse plane (i.e. in the plane of thenbsp;paper). As Tansley� points out, a type of branching superficially similar to, though not identical with this, is seen innbsp;some recent species of Oleichenia and Lygodium. In thisnbsp;connexion it is worthy of note that a fern figured by Ungernbsp;from Thuringia as Sphenopteris petiolata Goepp. ^ bears pinnaenbsp;in two rows on the rachis which are characterised by repeatednbsp;branching and by a very narrow lamina or by slender nakednbsp;axes; the occurrence of this form of frond in rocks containing Clepsydropsis antiqua (fig. 308, A) suggests a possiblenbsp;connexion between the petrified rachis and the impressions ofnbsp;the leaves.

The vascular strand of the rachis of Zygopteris primaria (fig. 308, D) is simpler than that of most of the Zygoptereaenbsp;and exhibits a close resemblance to the type of strand describednbsp;by Renault as Diplolabis (fig. 308, C).

Renault� instituted this genus for two species from the Culm beds and Coal-Measures of France based on the structurenbsp;of the petioles. The stems are unknown. The main rachisnbsp;has a stele similar to that of Zygopteris primaria, but distinguished by its greater similarity, in transverse section, tonbsp;an X rather than to the letter H : the long transverse bar innbsp;Zygopteris is here much reduced in size. The petiole ofnbsp;Diplolabis forensis Ren. (fig. 308, C) has a diameter of

�* The Diplolabis type of strand is very similar in the form of the inetaxylem to the conducting strand of a lateral vein in Scolopendrium officinarum [cf.nbsp;Pelourde (09) fig. 3, p. 117].

1'5�2 cm. From the antennae a pair of small bundles is given off alternately from the right and left side, as innbsp;Zygopteris] the members of each pair coalesce after leavingnbsp;the antennae and then separate to pass into the lateralnbsp;branches of the frond. The position of the protoxylem andnbsp;the formation of the lateral xylem strands previous to theirnbsp;separation are shown in fig. 308, C. On the side of thenbsp;vascular strand shown in fig. C, 2, the two lateral extensionsnbsp;of the antennae are converging towards one another previousnbsp;to their separation and subsequent union. The ovoid sporangianbsp;occur in groups of three to six and are coalescent below with anbsp;central receptacle; they have no annulus, but the cells on thenbsp;side next the receptacle are smaller than those on the externalnbsp;wall (fig. 309, A). The synangial form of the sorus suggestsnbsp;comparison with Marattiaceae.

The species described by Renault from the Culm of Esnost is regarded by P. Bertrand as identical with that described bynbsp;Solms, from the Culm of Falkenberg, as Zygopteris Roemeri^.nbsp;Diplolabis is compared by P. Bertrand with Metaclepsydropsis,nbsp;the generic name given to the Lower Carboniferous petiolenbsp;described by Williamson as Rachiopteris duplex^.

Mr Gordon has recently described in a preliminary note a new type of stem stele under the name Zygopteris pettycurensisnbsp;from the Lower Carboniferous plant bed of Pettycur^: he regardsnbsp;the petioles attached to the stem as identical with Zygopterisnbsp;Roemeri Solms-Laubachk This species, founded by Solms-Laubach on petioles only, is placed by Bertrand� in the genusnbsp;Diplolabis and regarded as identical with D. esnostensis Ren.nbsp;The stele found by Mr Gordon may therefore be assigned to thenbsp;genus Diplolabis: it includes two regions composed exclusivelynbsp;of tracheae and is cylindrical in transverse section. The innernbsp;xylem zone consists of short, square-ended, reticulately pittednbsp;elements and the outer zone is composed of long and pointednbsp;conducting tracheae. The scalariform protoxylem elements are

� Gordon (09). Mr Gordon�s more complete account of this plant will shortly be published. I am indebted to him for furnishing me with thenbsp;main facts in regard to the anatomical features.

�* Solms-Laubach (92) PI. ii. fig. 13. nbsp;nbsp;nbsp;� Bertrand, P. (09) p. 211.

situated between the two metaxylem zones. As Mr Gordon says : this type of stem occupies a position � in the Zygopteroidnbsp;alliance � corresponding to that which Thamnopteris Schlech-tendalii (p. 329) occupies in the Osmundaceous series. Thenbsp;discovery of this stem supplies another link between the twonbsp;fern groups, Osmundaceae and Coenopterideae. Pelourde^ hasnbsp;described an imperfectly preserved vascular strand from anbsp;locality near Autun as the type of a new genus Flicheia esnostensis.nbsp;Mr Gordon has pointed out to me that this is a partially rottednbsp;petiole of Diplolabis esnostensis (= Zygopteris Roemeri).

In their recent account of fossil Osmundaceous genera, Kid-ston and Gwynne-Vaughan^ speak of the central parenchyma of the existing medullated stele as being derived from trachealnbsp;tissue. They add that if the Zygopteroid line of descent is atnbsp;all close to the Osmundaceous, we must be prepared for thenbsp;existence of a Zygopteris with a solid xylem like that ofnbsp;Thamnopteris: � such a discovery, in fact, we hopefully anticipate^.� The new Pettycur stem amply justifies this prophecy.nbsp;It is noteworthy that Mr Gordon�s stem affords an instance ofnbsp;the occurrence of a type of stele, similar in its cylindrical formnbsp;and in the absence of parenchyma to that of Botryopteris, in anbsp;plant bearing leaves characterised by the Zygopteris type ofnbsp;vascular strand.

Metaclepsydropsis duplex (Will.) fig. 310, At [= Rachiopteris duplex, Williamson 1874. Asterochlaena (Clepsydropsis)nbsp;duplex, Stenzel 1889. Clepsydropsis, Renault 1896.]

The vascular axis of the main axis of the frond is characterised by the hour-glass shape of the xylem which consists entirely of tracheae, most of which are reticulately pitted. Innbsp;a transverse section (fig. 310, A) the two ends of the stele arenbsp;dissimilar; at one end of the long axis is a small bay of thin-walled tissue (phloem) enclosed by a narrow band of xylem, and

** A Culm species Machiopteris aphyllus (Unger) is closely allied to Metaclepsydropsis duplex. [See Solms-Laubach (96) p. 30.]

at the other the bay is open and has two protoxylem groups. The latter represents the earliest stage in the production ofnbsp;secondary bundles: at a later stage the bay is closed by thenbsp;elongation of the edges, the enclosed group of phloem isnbsp;vertically extended, and the protoxylem strands are morenbsp;widely separated. The curved band of xylem becomes detached as a curved arc and divides into two (fig. 310, A). Innbsp;a single section of this species one often sees several strands ofnbsp;xylem enclosed in a common cortex with the main vascularnbsp;axis; these are the xylem bundles of lateral pinnae. Meta-clepsydropsis duplex shows the method of branching of thenbsp;petiole vascular axis which has already been noticed in Diplo-lahis and Zygopteris. In reference to this feature, Williamsonnbsp;wrote in 1812�� I know of no recent fern in which the secondarynbsp;branches of the petiole are thus given off in pairs, which pairsnbsp;are distichously arranged on the primary axis, and each ofnbsp;which secondary petioles sustains ternary ones arranged distichously.� By slightly altering the primary stele of this typenbsp;of frond, by narrowing of the constricted portion of the hourglass and extending the lateral groups of xylem obliquelynbsp;upwards, the form of stele shown in fig. 810, A, would benbsp;converted into the Diplolahis type (fig. 308, C).

linger^ instituted this genus as a subdivision of Corda�s family Ehaciopterideae the name having reference to thenbsp;hour-glass form of the vascular axis^. The type-speciesnbsp;C. antiqua (fig. 308, A) is spoken of as the commonest fossilnbsp;plant in the Devonian rocks of Thuringia. In some sectionsnbsp;the xylem has the form seen in fig. 308, A, in which annbsp;invagination of thin-walled tissue occurs at each end; innbsp;other sections (fig. 308, A') the bays become islands in thenbsp;xylem. Solms-Laubach speaks of Unger�s species as Rachio-pteris (Clepsydropsis) antiqua. P. Bertrand^, who has recentlynbsp;described Unger�s plant, while recognising that G. antiqua and

Stenzel adopted this name for a subdivision of Corda�s genus Zygopteris, applying it to a species described by Renault asnbsp;Z. Brongniarti, to a Permian species described by himself asnbsp;Z. {Ankyropteris) scandens, and to Z. Lacattii Ren.; Rachio-pteris Orayi Will, and Rachiopteris corrugata Will, are alsonbsp;included in this genus. The characters emphasised by StenzePnbsp;' Stenzel (89) p. 25.

are (i) the double anchor-like form of the H-shaped petiole strand in which the lateral arms (antennae) are curved like thenbsp;flukes of an anchor, and (ii) the emission of four rows of branchesnbsp;instead of two. The latter distinguishing feature no longernbsp;holds good, as Z. primuria also |pves off four rows of bundlesnbsp;and not two as Stenzel supposed. P. Bertrand has adoptednbsp;Stenzel�s genus in a narrower sense�-.

This Lower Permian species is very similar to or perhaps identical with Ankyropteris Grayi (Williamson). The stem of

^ Dr Scott points out to me that recent observations, which have not yet been published, both by Dr Kidston and himself show that Bertrand�snbsp;terminology requires modification. There are many points to be cleared upnbsp;before we can hope to obtain a satisfactory classification of the Zygoptereae.

A. scandens was found in association with the roots of a Psaronius stem evidently petrified in situ as it burrowed,nbsp;like Tmesipteris, tropical aroids, and other recent plants, amongnbsp;the living roots of the tree-fern. The stem, 10�11 mm.nbsp;in diameter, bore fronds with an H-shaped vascular strand,nbsp;small scale-leaves, and adventitious roots. The stele consistsnbsp;of a five-angled cylinder of scalariform tracheae surrounding annbsp;axial strand of parenchyma containing scattered tracheae ofnbsp;smaller diameter. This axial tissue extends as a narrow stripnbsp;into each of the short and obtusely truncated arms (cf fig. 311).nbsp;A striking feature is the production of a shoot in the axil of thenbsp;foliage-leaves (fig. 310, D), a manner of branching characteristicnbsp;of Trichomanes (see page 365).

In describing this species, Williamson wrote��That no classification of these fossil ferns based solely upon transversenbsp;sections of the petiolar bundles is or can be of much value, isnbsp;clearly shown when tested amongst those living ferns thenbsp;classification of which is chiefly based upon the sporangialnbsp;reproductive organs h� This is a view entirely opposed to thatnbsp;which inspires P. Bertrand�s recent monograph. Whether thenbsp;value attached to the vascular structure of petioles as a basisnbsp;of classification is upheld or not, it is noteworthy that sincenbsp;Williamson expressed his opinion, our knowledge of the anatomynbsp;of ferns and of the value of anatomical evidence has enormouslynbsp;increased. The slender stem^ of this Lower Coal-Measuresnbsp;species agrees closely with that of A. scandens] it bore spirallynbsp;disposed fronds, scale-leaves, and roots. The stele has the formnbsp;of an irregular five-rayed star (fig. 311) in which the relativenbsp;length of the arms varies in different sections owing to thenbsp;separation of the distal ends to form leaf-traces. The axialnbsp;region is composed of parenchyma and associated narrownbsp;tracheae, as in A. scandens. The xylem, with protoxylemnbsp;elements at the ends and especially at the angles �f the arms,nbsp;is completely surrounded by phloem. The cortex consists

internally of parenchyma which becomes thicker-walled towards the periphery and bears multicellular epidermal hairs. A leaf-trace is detached in the form of a triangular strand and isnbsp;formed by the tangential extension of the distal end of an armnbsp;of the stele. The trace, on its way through the cortex, dividesnbsp;into two; the outer branch gradually changes from a slightlynbsp;curved band to an H-shaped meristele; the inner branch,nbsp;which supplied an axillary shoot, is similar to the stele of the

stem, but smaller. Scott ^ has recently recorded the occurrence of scale-leaves (aphlebiae) in this species like those described bynbsp;Stenzel in A. scandens.

Bertrand includes in Ankyropteris Kenault�s species Zy-gopteris bibractensis- and Williamson�s species Rachiopteris corrugata^-. the former he names A. bibractensis var. west-

1 nbsp;nbsp;nbsp;Scott (07) p. 180.nbsp;nbsp;nbsp;nbsp;2 Renault (69); Williamson (74) A. p. 697.

phaliensis. The fossil described by Williamson as R. irregularis or inaequalis^ are the secondary branches of A. hihractensis.

The rachis stele of this species, which is represented by portions of fronds only, has the form of a double anchornbsp;(fig. 313); the antennae are continued at the outer edge ofnbsp;their distal ends into a narrow band (� filament � of P. Bertrand)nbsp;(fig. 312, C, and 313, a) composed of smaller tracheae andnbsp;separated from the xylem of the antennae by a strip of thin-walled tissue (phloem ?). A group of protoxylem occurs at the

junction of the filament and antennae. The whole of the xylem is surrounded by phloem.

The section reproduced in fig. 313 shows the characteristic form of the petiolar vascular axis, consisting of a horizontalnbsp;band of metaxylem with groups of much smaller tracheae onnbsp;both the upper and lower margins. At the junction betweennbsp;the antennae, curved like the flukes of an anchor, and thenbsp;horizontal band of xylem, the latter is only one^ trachea innbsp;breadth. The narrow loops of smaller xylem elements are

shown oh the outer edge, a (fig. 313), of the antennae separated from the arcs of larger tracheae by a dark line which representsnbsp;a crushed band of delicate tissue. The spaces enclosed by thenbsp;incurved antermae are largely occupied by parenchymatousnbsp;ground-tissue. The cylinder of outer cortex consists internally ofnbsp;comparatively thin-walled parenchyma succeeded externally bynbsp;a zone of dark and thicker-walled cells characterised by a fairlynbsp;regular arrangement in radial series, as if formed by a secondarynbsp;meristem; there is, however, no indication of a meristematicnbsp;layer. Below the small-celled epidermis are a few layers ofnbsp;thinner-walled cells which are not arranged in radial series.nbsp;The structure of the outer part of the cortex is similar to thatnbsp;in the petiole of recent species of Angiopteris (fig. 243, p. 319)nbsp;and Marattia, in which a more delicate hypoderm is succeedednbsp;by a band of mechanical tissue.

The rachis of this type of frond gives off two rows of lateral branches from the vascular axis, the plane of symmetry beingnbsp;at right angles to the primary rachis. Estch pinna bore at itsnbsp;base two aphlebiae supplied with vascular strands from thenbsp;leaf-traces.

We have no certain information in regard to the sporangia of this species, but Scott points out that �pear-shaped sporangia,nbsp;with a very broad and extensive annulus, are commonly foundnbsp;associated with Zygopteris bibractensis and Z. corrugata in thenbsp;petrifactions of the English Lower Coal-Measuresh�

The stem of this type of Zygoptereae was described by Williamson from the Lower Coal-Measures of Lancashire asnbsp;Rachiopteris corrugata and included by him in the sub-groupnbsp;Anachoropteroides. The stele (fig. 314, B) is oval in transversenbsp;section; it consists of a cylinder of xylem tracheae enclosing anbsp;central region occupied by parenchymatous tissue and scatterednbsp;narrow scalariform tracheae. The central tissue extends radiallynbsp;in the form of narrow arms which reach almost to the outernbsp;edge of the tracheal tissue and divide it up into 5�7 groups.

A cylinder of thin-walled tissue encloses the xylem and in this occur groups of large sieve-tubes (fig. 314, D, Sv).

In a section of this species in the Williamson Collection^ the long axis of the stele has a length of 5 mm. and thenbsp;diameter of the stem as a whole is 2'5 cm. The greater partnbsp;of the extra-stelar tissue consists of large parenchymatous cellsnbsp;passing near the periphery into a band of darker and thicker-walled tissue.

Reniform vascular strands traverse the cortex in an obliquely ascending course on their way to the leaves, also smaller bundles,nbsp;some of which are given off directly from the stele, while othersnbsp;are branches of the petiole vascular strands. The petiolesnbsp;described by Williamson as Rachiopteris insignis'^ were afterwards recognised by him as those of Ankyropteris corrugata,nbsp;though this conclusion was not published�. Williamson�snbsp;species R. insignis must not be confused with Unger�s Culmnbsp;species Arctopodium insigne, which Solms-Laubach^ refers to asnbsp;Rachiopteris insignis. The leaf-bundle of Ankyropteris corrugata is at first reniform in contour (fig. 314, C, P), but as itnbsp;becomes free from the stem it gradually assumes the H-shapednbsp;form (figs. 315�317). This petiolar strand differs from that ofnbsp;Ankyropteris bibractensis (fig. 313) in the shorter and lessnbsp;strongly curved antennae; and, as Williamson first noticed, thenbsp;tracheae are frequently filled with thin-walled parenchymanbsp;(fig. 312, B). The existence of scale-leaves or aphlebiae likenbsp;those of Ankyropteris scandens and A. Qrayi has been recordednbsp;by Scott in A. corrugata^.

The section represented in fig. 314, C, shows the relatively small size of the stele 8 in the stem of Ankyropteris corrugata.nbsp;The main mass of the cortex consists of uniform parenchymanbsp;passing near the surface into darker and stronger tissue: twonbsp;vascular bundles are shown in the cortex, one of which formsnbsp;the conducting strand of a petiole, P, which has nearly freednbsp;itself from the stem: the other bundle, as shown by thenbsp;examination of a series of sections, eventually passes into

another leaf-stalk. A root of another plant has invaded the cortex at R, fig. 314, C.

The form and structure of the stele is diagrammatically represented in fig. 314, B. The outer portion (black) consists

of a cylinder of scalariform tracheae in which the position o groups of smaller elements (protoxylem) is shown by the whit enbsp;patches. Tbe xylem is thus seen to be mesarch. The pro mi.nbsp;nent group of xylem on the lower right-hand side of the section

consists of tracheae, cut across in an oblique direction, which are about to pass out as a separate strand. The centre of the stelenbsp;is occupied by parenchymatous tissue in which are includednbsp;scattered tracheae, either singly or in small groups. Thesenbsp;medullary tracheae are rather narrower than those of the mainnbsp;xylera cylinder. A characteristic feature is the radial outwardnbsp;extension into the xylem of the medullary parenchyma, whichnbsp;tends partially to divide the tracheal cylinder in^o broad groups.

Fig. 315. Ankyropteris corrugata (Will.). Petiolar vascular strand. [From a section in the University College (London) Collection; after Tansleynbsp;x36.]

Fig. 314, A, enlarged from fig. B, a, shows the mesarch position of a protoxylem group, and a few of the parenchymatous cells of one of the narrow arms of the axial tissue. Atnbsp;Sv in fig. D a group of large sieve-tubes is seen separated fromnbsp;the xylem by a few parenchymatous cells, and beyond thenbsp;sieve-tubes are some tangentially elongated cells. Both the

sieve-tubes, 8v, and the flattened cells resemble tissues in a corresponding position in the steles of modern Osmundaceae.

In a section of Ankyropteris corrugata in the Williamson Collection the radial arrangement of the more external meta-xylem elements suggests the addition of secondary tracheae^.nbsp;This suggestion of secondary thickening, a point which requiresnbsp;much more thorough investigation, is interesting in relation tonbsp;a new type of stem named by Scott Botrychioxylon'^, but notnbsp;yet fully described. This generic name has been given to

a stem stele which closely resembles that of Ankyropteris corrugata except in the regular radial arrangement of the peripheral xylem elements. The name Botrychioxylon was chosen by Scottnbsp;because of the secondary xylem characteristic of the recentnbsp;genus Botrychium (fig. 247, p. 322).

In the petiolar vascular strand represented in fig. 315 the narrow band of tracheae which forms a loop external to thenbsp;antennae is clearly seen, also the small-celled parenchymanbsp;between the loops and the larger metaxylem elements of the

1 nbsp;nbsp;nbsp;British Museum, section No. 245. Cf. figures by Williamson and Bertrand:nbsp;Williamson (77) PI. v. fig. 19; Bertrand, P. (09) PI. xii. fig. 87.

antennae. The crushed tissue lying on the outer face of each of the loops probably represents the phloem and pericycle; thenbsp;thin-walled elements above and below the horizontal band ofnbsp;metaxylem are probably sieve-tubes.

Fig. 316 shows a transverse section of a petiole of this species: the loops, a, of small tracheae are seen bending roundnbsp;the outer edge of the antennae. The inner and more delicatenbsp;cortical tissue is partially preserved and spaces, h, have beennbsp;formed in it as the result of contraction previous to petrifaction.

In the petiole represented in fig. 317 the tracheae of the horizontal band are considerably crushed; the section is, however, of interest because of the presence of Lyginodendronnbsp;roots, I, in the space originally occupied by the inner cortex.

In a paper on the tyloses of Rachiopteris corrugata, Weiss^ draws attention to the fact that similar inclusions have notnbsp;been found in the tracheae of recent ferns. The occurrence ofnbsp;thin-walled parenchymatous cells in the large tracheae ofnbsp;Ankyropteris corrugata petioles and of other species is anbsp;striking feature. Williamson^ compared these cells with thenbsp;tyloses in the vessels of recent fiowering plants, and in a laternbsp;paper� he suggested that the included cells may belong tonbsp;saprophytic or parasitic fungi. It is, as Weiss points out,

^ Weiss, F. E. (06). nbsp;nbsp;nbsp;^ Williamson (77).nbsp;nbsp;nbsp;nbsp;� Williamson (88) A.

difficult to explain the occurrence of tyloses in tracheae not immediately in contact with living parenchyma. It may benbsp;that the pits in the tracheae of Anki/ropteris were open spacesnbsp;as in the xylem of recent ferns described by Gwynne-Vaughan,nbsp;and if so this would facilitate the invasion of the conductingnbsp;elements by growing cells. A comparison is made by Weissnbsp;between certain cell-groups found by him in the tracheae ofnbsp;Ankyropteris and by Miss Jordan* in the vessels of the recent dicotyledon Cucumis sativa. In a more recent paper onnbsp;tyloses Miss McNicoP expresses the opinion that pseudoparenchyma in the tracheae of the fossil petioles owes its originnbsp;to fungal hyphae.

Williamson compared the petiole bundles of Ankyropteris corrugata with those of recent Osmundaceae, a comparisonnbsp;based on the structure of the leaf-trace before its separationnbsp;from the stem and its assumption of the H-form. It isnbsp;noteworthy, however, that this comparison has acquired anbsp;greater significance as the result of receijt work. The stelenbsp;of Ankyropteris bears a fairly close resemblance to that ofnbsp;Zalessky a described by Kidston and Gwynne-Vaughan; innbsp;both types the xylem is represented by two kinds of trachealnbsp;tissue. In the Permian Osmundaceous genus the centre ofnbsp;the stele consists of short storage tracheids, while in Ankyropteris we may regard the central parenchyma and scatterednbsp;tracheae as derivatives of the solid xylem core of some ancestralnbsp;type. Moreover, the appearance and arrangement of the phloemnbsp;and the tangentially elongated elements external to it (fig. 314)nbsp;remind one of the extra-xylem zone in recent Osmundaceae.nbsp;That the Osmundaceae and Zygoptereae are closely relatednbsp;groups there can be little doubt; of this affinity and commonnbsp;origin� Ankyropteris corrugata affords striking evidence.

The difference between the steles of Ankyropteris Grayi and A. scandens (figs. 310, D; 311) and that of Ankyropterisnbsp;corrugata is comparatively small. In the two former speciesnbsp;the cylindrical form has become stellate owing to the radialnbsp;extension of the xylem arms. It may be that this morenbsp;elaborate style of vascular construction is connected with thenbsp;1 Jordan (03).nbsp;nbsp;nbsp;nbsp;^ McNiool (08).nbsp;nbsp;nbsp;nbsp;� Gwynne-Vaughan (09).

climbing habit of A. scandens and possibly A. Grayi. The radial extension of the xylem and the consequent alternation ofnbsp;the yielding parenchymatous cortex and the more rigid trachealnbsp;arms would probably render the water-conducting elements lessnbsp;liable to injury in a twisting axis^. In Artachoropteris De-caisnii^, described by Renault, and more especially in Astero-chlaena laxa^ Stenzel, a Lower Permian type from Saxonynbsp;(fig. 324), the xylem of the stele is much more deeply lobednbsp;than in Ankyropteris Grayi or A. scandens.

P. Bertrand has proposed this name for a species of petiole from the Lower Coal-Measures of England referred by Binney^nbsp;to Zygopteris Lacattii Ren., and included by Williamson� innbsp;his comprehensive genus Rachioj^teris. Bertrand� regards thenbsp;English species, which is recorded also from Germany^', asnbsp;distinct from Renault�s type� and therefore proposes a newnbsp;name. The petiole stele has the H-form, but its structurenbsp;is simpler than that of the Ankyropteris petiole.

The horizontal band of xylem has at each end two oval groups of tracheae connected with it by a single row of xylem

' Compare figures of the vascular cylinders of climbing Dicotyledons given by Sohenok (93).

Binney (72). nbsp;nbsp;nbsp;5 Williamson (74) A.nbsp;nbsp;nbsp;nbsp;� Bertrand, P. (09).

elements (fig. 318). From the lower part of each oval group a small strand is detached; the two strands from one side ofnbsp;the stele coalesce and then separate to pass into two pinnae.nbsp;Fig. 308, B, shows four stages in the giving-off of the secondarynbsp;branches. This species, therefore, produces four rows of branchesnbsp;in alternate pairs from the right and left sides of the petiole.

The first stage is shown at 0, 0, fig. 308, B; the two projecting groups of protoxylem mark the points of departurenbsp;of a pair of small strands. At 1, the projections are morenbsp;prominent, and at 2 a pair of strands has become detached : atnbsp;a later stage, 3, these two strands unite to divide later (4) intonbsp;two slightly curved bundles.

Our knowledge of the fructification of Etapteris is based on Renault�s account of sporangia, which he regarded as belongingnbsp;to Zygopteris {Etapteris) Lacattii. They have the form of

sporangium has a pedicel, and three to eight sporangia are attached to a common peduncle; the walls of the sporangia arenbsp;at least two cell-layers in thickness and the aimulus consists ofnbsp;a band of thick-walled cells passing from the crest down eachnbsp;side (figs. B and C), thus differing from the sporangia of Botryo-ptems (fig. 319, D, F) in which the broad annulus is confined tonbsp;one side.

It is practically certain that the fronds described by Grand�Eury^ as Schizopteris pinnata (fig. 309, E) and Schizo-stachys frondosus represent respectively the sterile and fertile

leaves of Etapteris. Zeiller^ gives expression to this by substituting the generic name Zygopteris for Schizopteris, and we may now speak of the leaves as Etapteris. Dr White�* has referrednbsp;to a new genus, Brittsia, some impressions of pinnate frondsnbsp;from the Coal-Measures of Missouri which, as he points out,nbsp;bear a close resemblance to Schizopteris pinnata Grand�Eurynbsp;(fig. 309, E). No sporangia have been found; it is, however,nbsp;probable that Brittsia prohlematica represents fragments of a

1 nbsp;nbsp;nbsp;Grand�Eury (77) A. PI. xvii.nbsp;nbsp;nbsp;nbsp;2 Renault and Zeiller (88) A.

leaf borne by a plant closely allied to Etapteris {Zygopteris). The broad rachis bears crowded pinnae given off at a widenbsp;angle; the small pinnules are rather deeply lobed or pin-natifid (3�10 mm. long by 1�5�3 mm. broad). The laminanbsp;is traversed by irregularly lobed and occasionally anastomosingnbsp;veins. In the fertile pinnae the segments have no lamina butnbsp;bear bundles of pedicellate sporangia.

It should be noticed that the sporangia described by Renault and by other authors as those of Zygopteris (fig.nbsp;319, A�C) have not been found in organic continuity withnbsp;a frond showing a well-preserved vascular strand. It is, however,nbsp;certain that this characteristic annulate sporangium, borne onnbsp;branched and slender pedicels, was produced on fronds withnbsp;a much reduced lamina belonging to some species of thenbsp;Zygoptereae, Etapteris and probably also Ankyropteris.

This genus was instituted by Binney for petioles from the Lower Coal-Measures of Oldham (Lancashire).

Stauropteris oldhamia Binney^ is characterised by a stele (figs. 308, E�G; 310, C; 320; 321) composed of four groupsnbsp;of xylem which Bertrand regards as homologous with thenbsp;antennae of Diplolahis, Ankyropteris, and Etapteris, the horizontal cross-piece of these genera being absent in Stauropteris.nbsp;Williamson spoke of this species as � one of the most beautifulnbsp;and also one of the most perplexing of the plants of the Coal-Measures � ; he discussed its possible affinity with both Lycopodsnbsp;and ferns, deciding in favour of the latter groups. In transversenbsp;section the petiolar vascular axis is approximately square, thenbsp;xylem groups forming the ends of the diagonals; the trachealnbsp;groups are separated by phloem and the centre of the stele innbsp;the primary rachis is also occupied by that tissue, which isnbsp;connected by four narrow strips with the external phloem. Thenbsp;structure of the petiolar vascular axis is very clearly shown innbsp;the drawing by Mrs Tansley reproduced in fig. 320. Proto-

xylem elements occur close to the surface of each of the four arms of the xylem; the bays between the two lateral and thenbsp;two lower xylem groups contain large sieve-tubes. Portions ofnbsp;the inner cortex are seen in places abutting on the small-cellednbsp;pericyclic tissue.

The right and left halves of the stele are not absolutely identical (fig. 820; fig. 308, E); this is due to the fact thatnbsp;secondary branches are given off in four rows, two alternatelynbsp;from the right and left sides. The preparation for the departurenbsp;of the lateral strands alters the configuration of the stelar xylemnbsp;groups. The protoxylem groups are not external but separatednbsp;from the surface by one or two layers of metaxylem. In fig. 808, E,nbsp;the occurrence of two protoxylem strands in the right-handnbsp;groups of metaxylem marks an early stage in the detachmentnbsp;of branches. These two protoxylems are the result of divisionnbsp;of single protoxylem strands like those in the left-hand half ofnbsp;the stele. At a later stage the petiolar stele assumes the formnbsp;shown in fig. 308 F, and two small bundles are detached to supplynbsp;aphlebiae; this is followed by the stage shown in fig. G, wherenbsp;two four-armed strands are passing out to a pair of branches ofnbsp;the leaf axis. The separation of these two meristeles leavesnbsp;the right-hand half of the stele in the condition seen on thenbsp;left-hand side of fig. E. The diagrammatic sketch representednbsp;in fig. 310, C, shows one pair of branches in organic connexionnbsp;with the rachis, and each of these arms contains an obliquelynbsp;cut vascular strand like those in fig. 308 G.

The cortex consists for the most part of fairly thick-walled parenchyma (fig. 321) which in the hypodermal region is replacednbsp;by a zone of thin-walled lacunar tissue. A few stomata havenbsp;been recognised in the epidermisf The lower left-hand branchnbsp;fseen in fig. 310, C, has been shaved by the cutting wheel sonbsp;that the aerenchymatous tissue, I, is shown in surface-view; anbsp;portion of this tissue is enlarged in fig. C'. The same delicatenbsp;chlorophyllous tissue forms a folded and shrivelled layer withnbsp;an uneven margin on the surface of the rachis and lateralnbsp;branches. This hypha-like tissue, which was discovered by

Scott^ and figured by Bertrand^, doubtless represents the much reduced lamina of the highly compound leaves; it maynbsp;be compared with the green outer cortex of Psilotum shootsnbsp;and with the lacunar tissue in the capsule of the commonnbsp;moss, Funaria hygrometrica.

The rachis reproduced in fig. 321 is surrounded by an enormous number of sections, some transverse, others more ornbsp;less vertical, of branchlets of various sizes. Fig. 310, B, showsnbsp;the three-rayed vascular axis of a branch of a lower order than

those seen in fig. C, and the single vascular strands of still finer ramifications of the leaf The extraordinary abundance of axesnbsp;of dilferent sizes, many of which are cut in the plane ofnbsp;branching, in close association with the rachises of Stauropterisnbsp;affords a striking demonstration of the extent to which the subdivision of the frond was carried in a small space. The leavesnbsp;must have presented the appearance of a feathery plexus ofnbsp;delicate green branchlets devoid of a lamina, some of whichnbsp;bore terminal sporangia. It may be that the delicate frondsnbsp;were borne on a slender rhizome which lived epiphytically in anbsp;moist atmosphere on the stouter stems of a supporting plant.

The sporangia^ of Stauropteris oldhamia are exannulate and nearly spherical, with a wall of more than a single row of cells;nbsp;they occur at the tips of slender and doubtless pendulousnbsp;branchlets. The discovery by Scott ^ of germinating sporesnbsp;(fig. 323) in a sporangium of this type supplies an interestingnbsp;piece of evidence in favour of the fern nature of these reproductive organs. Similar germinating spores have been describednbsp;by Boodle^ in sporangia of Todea.

This Lower Carboniferous plant identified by Williamson with the Oldham plant from the Lower Coal-Measures isnbsp;referred by Bertrand to a distinct species. In the structurenbsp;of the rachis stele it agrees closely with Stauropteris oldhamia;nbsp;the main vascular strand gives off four rows of branches, twonbsp;from each side, and aphlebiae were present at the commonnbsp;1 Scott (04); (053).nbsp;nbsp;nbsp;nbsp;2 Scott (062).nbsp;nbsp;nbsp;nbsp;3 Boodle (08).

base of each pair of pinnae. Mrs Scotty who has recently described the sporangia of this species, speaks of one specimennbsp;in which germinating spores were found. The same authornbsp;gives an account of some curious spindle-shaped bodies whichnbsp;she found in association with S. burntislandica. The naturenbsp;of these organs is uncertain; Mrs Scott inclines to regardnbsp;them as glands borne in pairs on lateral pedicels of thenbsp;frond: she adopts for these the name Bensonites fusiformisnbsp;proposed by Dr Scott. If there is a reasonable probability, asnbsp;there certainly seems to be, in favour of connecting these organsnbsp;with Stauropteris, it is legitimate to question the desirabilitynbsp;of adding to the long list of names included in the groupnbsp;Coenopterideae.

Pig. 323. Germinating spores from a sporangium of Stauropteris. Tausley, after D. H. Scott.)

This genus was founded by Daily ^ on fragments of a fern from Carboniferous rocks in County Limerick, Ireland, characterised by a peculiar type of fructification which he namednbsp;Corynepteris stellata. More complete examples of the samenbsp;genus have been described by Zeiller^ from the Coal-field of

2 nbsp;nbsp;nbsp;Baily (60) PI. xxi. Kopivri, a club or mace.nbsp;nbsp;nbsp;nbsp;^ Zeiller (83),

Valenciennes. The sporangia are large, ovoid, and sessile; the annulus (fig. 309, D) has the form of a complete vertical bandnbsp;several cells in breadth: five to ten sporangia are groupednbsp;round a receptacle. Zeiller describes two species as Spheno-pteris {Corynepteris) coralloides Gutb. and 8. {Corynepteris)nbsp;Essinghii And.; in both the fronds are quadripinnate and bearnbsp;aphlebiae at the base of the pinnae. The former species isnbsp;recorded by Kidston^ from the South Wales Coal-field. Anbsp;single pinnule of C. coralloides is shown in fig. 309, C. Potonie^nbsp;refers this frond to his genus Alloiopteris: the portion of anbsp;pinna represented in fig. 354 G show�s the characteristic modifiednbsp;pinnule next the rachis. Zeiller draws attention to the occurrence of two parallel lines on the rachis of a specimen ofnbsp;Corynepteris coralloides which he figures^, and suggests thatnbsp;these may indicate the existence of an H-shaped form ofnbsp;vascular strand like that of Etapteris and Ankyropteris. Thenbsp;sorus of Corynepteris is comparable with that of the Marattia-ceae, but the broad annulus is a difference which suggestsnbsp;affinity to Etapteris. The sorus is similar to that in Diplolabisnbsp;(fig. 309, A), but in that genus the sporangia are exannulate.

The vascular axis in the stems of diff'erent members of the Coenopterideae assumes a variety of types. In Botryopterisnbsp;antiqiia the xylem forms a solid protostele in which no proto-xylem strands have been recognised; in other species, e.g.nbsp;B. ramosa, the cylindrical stele is similar to that of Tricho-manes radicans (Hymenophyllaceae) in the more or less centralnbsp;position of the protoxylem. In Botryopteris forensis thenbsp;protostele is said to be exarch. The probability is that thenbsp;central Botryopteris type is the endarch protostele, a form ofnbsp;vascular axis which may be regarded as primitive. The leaf-tracesnbsp;of the Lower Carboniferous Botryopteris antiqua are simplenbsp;oval strands differing but slightly from the cylindrical stelenbsp;of the stem. In the Upper Carboniferous British species thenbsp;petiolar vascular strand has become more specialised andnbsp;farther removed from that of the stem; in B. forensis the

distinction between leaf and stem steles is still more pronounced. It is perhaps legitimate to regard these types as representingnbsp;an ascending series, the more primitive of which are distinguished by the greater similarity between leaf and stem,nbsp;organs differentiated from a primitive thallush that is from anbsp;vegetative body. Portions of this ultimately became specialisednbsp;as lateral members or leaves, while a portion acquired thenbsp;character of a radially constructed supporting axis or stem.

The vascular strand characteristic of the Zygoptereae is represented by the H-shaped form as seen in Ankyropterisnbsp;corrugata or in a more complex form in A. hihractensis. Thisnbsp;style of strand may be regarded as a development from thenbsp;simple strands of Grammatopteris and Tubicaulis or Botryo-pteris antiqua along other lines than those followed bynbsp;B. forensis. The extension of the xylem in two symmetricallynbsp;placed arms at the ends of the cross-piece of the H is correlatednbsp;with the habit of branching of the leaf-system which forms onenbsp;of the striking peculiarities of many of the Zygoptereae. Thenbsp;solid type of stele characteristic of the Botryoptereae is closelynbsp;matched by that in the Lower Carboniferous stem discoverednbsp;by Mr Gordon^. By the partial transformation of the centralnbsp;xylem region into parenchymatous tissue and the concentration of water-conducting elements in the peripheral region thenbsp;style of Ankyropteris corrugata was developed. The vascularnbsp;strand of the older plant, which is of the Diplolabis type, maynbsp;be regarded as a more primitive style than that of the H-formnbsp;of petiole strand represented by Ankyropteris corrugata. Anbsp;further stage in evolution is seen in the stem stele of Ankyropteris Grayi and A. scandens, both of which have the H-formnbsp;of meristele. This step in increasing complexity of stem stele,nbsp;though probably connected with the increasing specialisation ofnbsp;the leaf-traces, as held by Mr Tansley, may also be associatednbsp;with the development of a climbing habit. In Asterochlaenanbsp;laxa Stenzel (fig. 324) and A. raniosa (Cotta)^ the tendencynbsp;towards a stellate expansion of the originally cylindrical form

of stele reaches a higher degree, with the result that a style is evolved which agrees closely with that of the conducting tissuenbsp;of some existing Dicotyledonous Lianes.

Attention has already been drawn to the generalised features exhibited by the Coenopterideae both in the anatomy of thenbsp;steles and in the structure of the sporangia. The conclusionnbsp;arrived at is that while the Coenopterideae foreshadow in some

I7c7) nbsp;nbsp;nbsp;(i)

of their characters more than one group of more recent ferns, some at least of their members afford convincing evidence ofnbsp;the correctness of the view�which is also that of Dr Kidstonnbsp;and Mr GWynne-Vaughan�that the Osmundaceae and thenbsp;Coenopterideae are offshoots of a common stock.

CHAPTER XXVI.

Marsiliaceae.

The unsatisfactory and meagre records in regard to the past history of these heterosporous Filicales render superfluous morenbsp;than a brief reference to the recent species.

Marsiliaceae.

This family is usually spoken of as including the two genera Marsilia and Pilularia. Lindman^ has however founded anbsp;third genus, Regnellidium, on a Brazilian plant which is distinguished by some well-defined characters from all species ofnbsp;Marsilia. The members of the Marsiliaceae live for the mostnbsp;part in swampy situations. Marsilia is represented in Europenbsp;by M. quadrifoliata L. which occurs in Portugal, France,nbsp;Germany and other parts of the Continent, extending als'onbsp;to Kashmir, Northern China, and Japan. Of the other 53nbsp;species, 17 are recorded from different regions in Africa, whilenbsp;others occur in South America, Asia^, Australia, and elsewhere.

Pilularia glohulifera L. is the only British representative of the Hydropterideae. The remaining four species of the genusnbsp;occur in South America, California, New Zealand, Australia,nbsp;and P. minuta Dur. is met with in the South of France, Algeria,nbsp;and Asia Minor in subtropical or warm temperate regions.

Schizaeaceae than to any other familj'^ of homosporous ferns^. Their heterospory, the production of sporangia in closed fruitlike sporocarps, and the anatomical features associated withnbsp;existence in marshy habitats, tend to obscure the resemblancesnbsp;to the true ferns.

The genus Marsilidium proposed by Schenk ^ for a piece of an axis, bearing apparently a whorl of six leaflets, from thenbsp;Wealden of Osterwald, cannot be regarded as satisfactorynbsp;evidence of the existence of the Marsiliaceae in the Wealdennbsp;flora of North Germany.

The six leaflets of Marsilidium speciosam, having a length of 5 cm., are similar in shape to the four leaflets of recentnbsp;species of Marsilia, but they differ in the repeated dichotomynbsp;of the veins from the reticulate venation of the recent forms.nbsp;It is worthy of note, however, that in Lindman�s Brazilian typenbsp;Regnellidium diphyllum (fig. 326, A), the leaflets are characterisednbsp;by dichotomous and not by anastomosing veins.

Hollick� has described some impressions of imperfect orbicular leaves with a �finely flabellate obscurely reticulated (?) venation�nbsp;from Cretaceous rocks of Long Island as Marsilia Andersoni,nbsp;but these are too fragmentary to be accorded this genericnbsp;designation. My friend Dr Krasser informs me that he isnbsp;describing some well-preserved leaves from Cretaceous beds ofnbsp;Gr�nbach in Lower Austria as Marsilia Nathorsti*. He compares these with the recent form Marsilia elata, a variety ofnbsp;M. Drummondi.

Another Lower Cretaceous species Marsilia perucensis has been figured by Fric and Bayer� as a stalked fruit-like bodynbsp;from Bohemia. This was originally described by Velenovsky asnbsp;M. cretacea, but under this name Heer� had previously recordednbsp;a supposed sporocarp from Greenland. These fossils have littlenbsp;claim to recognition as examples of Marsiliaceous plants.

5 Holliek (94) PI. lxxi. * Mentioned by Krasser (06) in a preliminary note. � Fri5 and Bayer (01) p. 86, fig. 34.nbsp;nbsp;nbsp;nbsp;� Heer (82) PI. xvi.

Oeningen as Pilularia pedunculata is too small to determine with reasonable accuracy. Other supposed representatives ofnbsp;the family mentioned in palaeobotanical literature are not ofnbsp;sufficient importance to describe.

The two genera of Salviniaceae, Salvinia and Azolla, are water plants, and are usually described as annuals which survivenbsp;the less favourable season in the form of detached sporocarps.nbsp;GoebeP states that all the tropical species of Salvinia known tonbsp;him have an unlimited existence.

Salvinia nadans, Hoffm., the only European species, extends from the South of France to Northern China and the plains ofnbsp;India; the other twelve species are mostly tropical. Azolla,nbsp;represented by four species, occurs in Western and Southernnbsp;North America, South America, Madagascar, Australia, Newnbsp;Zealand, and is widely spread in tropical Asia and Africa.

Species of Azolla frequently form a considerable proportion of the floating carpet of vegetation on inland waters^ growingnbsp;under conditions which might be supposed favourable for preservation in a fossil state.

The Salviniaceae, though probably rather farther removed than the Marsiliaceae from the homosporous Filicineae, are considered by Bower� to be related to the Gradatae, but modifiednbsp;in consequence of their aquatic habit and the assumption ofnbsp;heterospory.

No undoubted examples of fossil species of Azolla have been described. Salvinia, on the other hand, is represented bynbsp;several Tertiary species, for the most part founded on leavesnbsp;only, and Hollick^, who published a list of fossil Salvinias, hasnbsp;described detached leaves as Salvinia elliptica Newb. from whatnbsp;may be Upper Cretaceous rocks from Carbonado, Washington.

^ See Seward (94) p. 441, for a description of the floating plants on the lagunas of Gran Chaco (S. America) by Prof. Graham Kerr.

Some of the leaves figured as Tertiary Salvinias are of no value as evidence of the former distribution of the genus^.

From the Coal-beds of Yen-Bai (Tonkin), probably of Miocene age, Zeiller^ has figured some well-preserved impressions ofnbsp;oval or orbicular leaves, 15 mm. long and 10�20 mm. broad,nbsp;characterised by reticulate venation and by cordate bases,nbsp;which he refers to Heer�s Swiss species Salvinia formosa^.

Dr Zeiller^ in the most recently published part of his series of valuable r�sum�s of palaeobotanical literature refers to anbsp;description by Brabenec of specimens of this species fromnbsp;Bohemian Tertiary beds showing both microspores and megaspores.

One of the most complete specimens so far discovered has recently been described by Fritel� from Eocene beds of the Parisnbsp;Basin as Salvinia Zeilleri. This species, founded on portions ofnbsp;stems bearing floating leaves, submerged root-like leaves, andnbsp;sporocarps, is compared with a recent tropical American speciesnbsp;8. auriculata.

It is noteworthy that no authentic records of Hydropterideae have been discovered in Palaeozoic rocks�. Comparisons havenbsp;been made in the case of the genera Traquairia Carr, andnbsp;Sporocarpon Will, with the reproductive organs of Azolla'^,nbsp;but these rest on a wholly insufficient basis.

Dawson� proposed the generic name Protosalvinia for some spores of Devonian age, which he regarded on inadequatenbsp;grounds as evidence of Palaeozoic Hydropterideae.

Zeiller�, in discussing the possible relationships of the problematical type Ghorionopteris gleichenioides Cord., suggestsnbsp;a possible alliance with the Hydropterideae. Corda foundednbsp;the genus Ghorionopteris'^�^ on some small fragments of pinnules,nbsp;6�7 mm. long, found in the Carboniferous rocks of Eadnitz innbsp;Bohemia.

' E.g. Lesquereux (78) PI. lxiv. fig. 14; PI. v. fig. 10. Staub (87) PI. xix. fig. 2.

� See also Arber (06) p. 228. nbsp;nbsp;nbsp;�� Solms-Laubach (91) A. p. 183.

The lobes of the pinnules are incurved distally to form a capsule, containing four sporangia, which apparently opened onnbsp;dehiscence into four valves; the spores are of one size. Thenbsp;material is however insufficient for accurate determination.

There is no evidence contributed by fossil records which indicates a high antiquity for the Hydropterideae. It is unsafenbsp;to base any conclusion on the absence of undoubted Palaeozoicnbsp;representatives of this group; but the almost complete absencenbsp;of records in pre-Tertiary strata is a fact which may be allowednbsp;some weight in regard to the possible evolution of the hetero-sporous filicales at a comparatively late period in the earth�snbsp;history.

A description of the Mesozoic genus Sagenopteris may be conveniently included in this chapter, though as in many othernbsp;instances the inclusion of a genus under the heading of a recentnbsp;family name does not by any means imply that the position ofnbsp;the extinct type is regarded as settled.

This generic name was applied bj^ PresP to small fronds composed of four or rarely two palmately disposed leaflets withnbsp;a more or less distinct midrib and anastomosing secondarynbsp;veins. Schimper^ compared Sagenopteris with Marsilia, butnbsp;did not regard the resemblance as evidence of relationship.nbsp;Nathorst^ expressed the opinion that certain fruit-like bodiesnbsp;obtained from the Rhaetic beds of Scania are of the nature ofnbsp;sporocarps and were borne by Sagenopteris, with the leaves ofnbsp;which they were associated. He published a drawing of part ofnbsp;a fruit showing on its partially flattened surface some raisednbsp;oval bodies which are considered to be spores. Dr Nathorstnbsp;kindly placed at my disposal the drawings reproduced innbsp;fig. 325 made from some of his specimens found at Bjuf innbsp;Scania.

In contour and superficial features, e.g. the veining on the wall, these bodies bear a fairly close resemblance to the

sporocarps of recent species of Mars�ia. They were found in association with the leaves of Sagenopteris undulata Nath.,nbsp;an abundant Scania type similar in form to the English Jurassicnbsp;species (S, Phillipsi (figs. 327, 328). Heer was independentlynbsp;led by an examination of some examples of the Swedish �fruits�nbsp;to compare them with the sporocarps of Marsilia. A smallnbsp;spherical body is figured by Zigno^ close to a leaf of hisnbsp;species S. angustifolia, which may be a sporocarp. In a recentnbsp;paper, Salfeld^ says that he found fructification on the lowernbsp;face of the leaflets of 8. Nilssoniana Brongn. from Germannbsp;Jurassic rocks, but he brings forward no evidence in support of

this statement. The systematic position of Sagenopteris is by no means settled. In a previous account of the genus Inbsp;expressed the view that it is probably a member of the truenbsp;ferns^, but the resemblance of Dr Nathorst�s drawings to thenbsp;Marsilian sporocarps influences me in favour of his opinion thatnbsp;Sagenopteris may belong to the Hydropterideae. The evidence,

� In a footnote to Pontaine�s description of Jurassic plants of Oregon, Lester Ward writes;��Seward treats Sagenopteris as a fern, classing it now (Jur. FI.nbsp;Yorkshire Coast, 1900, p. 161) in the family Polypodiaceae, although in hisnbsp;Wealden Flora, 1894, p. 129, he placed it in the Schizaeaceae.� [Ward (05)nbsp;p. 83, note t.] My words are � I am disposed to regard Sagenopteris as probablynbsp;a genus of ferns � (loc. cit. 1900, p. 161). I have never referred this plant tonbsp;the Polypodiaceae or Schizaeaceae or to any other family.

as Solms-Laubach^ states, is not wholly satisfactory: Schenk points out that the frequent occurrence of detached Sagenopterisnbsp;leaflets suggests that they easily fell off the petiole, whereas innbsp;Marsilia the leaflets do not fall off independently. The discovery of a new type of Marsiliaceae in Brazil, which Lindmannbsp;has described as Regnellidiiim diphyllum^ (fig. 326, A), affordsnbsp;an additional piece of evidence bearing on the comparison

of Sagenopteris with members of this family. In Regnellidium the leaves differ from those of Marsilia in bearing two insteadnbsp;of four leaflets, and in the former the veins are repeatedly forked,nbsp;and do not anastomose as in Marsilia. In the possessionnbsp;of only two leaflets Regnellidium agrees with some forms ofnbsp;Sagenopteris (fig. 328).

^ I am indebted to my friend Dr Nathorst for calling my attention to Lindman�s paper.'

1828. Glossopteris Phillipsi, Brongniart, Hist. v�g. foss. p. 225, Pis. LXI. his, LXIII.

1838. Sagenopteris Phillipsi, Presl, in Sternberg�s Flor. Vorwelt, vii. p. 69.

The fronds of this common Jurassic species, which is recorded from many European localities, from North America,nbsp;Australia, the Antarctic regions^, and elsewhere, are very variablenbsp;as regards the form, size, and number of the leaflets.

1 For a fuller synonymy, see Seward (00) p. 162. nbsp;nbsp;nbsp;^ Nathorst (04^).

Frond petiolate, in some forms the petiole bears four linear or oval-lanceolate leaflets having a distinct midrib and oblique anastomosing veins. In others a shorter winged petiole bears one or two shorter andnbsp;broader, somewhat obouneate, leaflets without a midrib.

It is probable that Bunbury' was correct in his opinion that the specimen figured by Bindley and Hutton^ as Otopterisnbsp;cuneata, characterised by two leaflets (fig. 328), is not specificallynbsp;distinct from the normal form with four leaflets (fig. 327).

Similarly, such specimens as that represented in PI. XVIII., fig. 3 of the first part of my Jurassic Flora, in which a shortnbsp;stalk bears only one leaflet may, provisionally at least, benbsp;included in Brongniart�s species. Yabe� describes a form withnbsp;two leaflets from Jurassic rocks of Korea as Sagenopteris

Fig. 328. Sagenopteris PhiUipsi. From a specimen in the Manchester University Museum. Nat. size.

bilobata which resembles S. PhiUipsi; and Moeller^ records a specimen similar to that represented in fig. 328 fromnbsp;Bornholm as 8. cuneata (Lind, and Hutt.).

The leaf shown in fig. 327, A, in which the longest segments are 4�5 cm. in length, represents the most abundant form andnbsp;illustrates the very close agreement between 8. PhiUipsi andnbsp;the Bhaetic species gt;S. rhoifolia. Fig. 327, B, which is drawnnbsp;from a specimen figured by Bindley and Hutton**, shows a leafnbsp;with longer (6'5 cm.) and much narrower segments. Broadernbsp;leaflets are occasionally met with in which the lamina reachesnbsp;a length of 11 cm.�

� Yabe (05) PI. iii. fig. 16. nbsp;nbsp;nbsp;�* Moeller (02) PI. vi. fig. 10.

� Bindley and Hutton (33) A. PI. lxiii. fig. 2. nbsp;nbsp;nbsp;� Seward (00) p. 169, fig. 26.

represented in fig. 327, B, are described by Zigno^ from Jurassic beds of Italy as S. angustifolia and by Moeller^ from the Jurassicnbsp;of Bornholm as S. Phillipsi f pusilla. A coarser type ofnbsp;venation than that of S. Phillipsi is occasionally found innbsp;Jurassic examples, as in S. grandifolia Font.� from Oregon andnbsp;8. Nathorsti Barth, from Bornholm^.

Sagenopteris is recorded also from several Rhaetic fioras. The best known species, 8. rhoifolia PresP, is hardly distinguishable from some forms of 8. Phillipsi or from the Italiannbsp;Jurassic species described byZigno as 8. Goeppertiana^, thoughnbsp;the leaflets are usually rather larger. This species was firstnbsp;described by Brongniart as Filicites Nilssoniana�, and a, fewnbsp;authors^ have adopted this specific name because of its prioritynbsp;over Presl�s designation. As Nathorst remarks, to give upnbsp;the well-known name 8. rhoifolia for 8. Nilssoniana is � merenbsp;pedantry.� The epidermis of 8. rhoifolia as figured by Schenk�nbsp;consists of cells with straight and not undulating walls:nbsp;stomata occur on the lo^er surface (fig. 326, B).

Rhaetic leaves of the type represented by 8. rhoifolia have a wide geographical distribution.

The specimens described by Feistmantel from the Damuda series of India as 8agenopte7'is longifolia are no doubt frondsnbsp;of Glossopteris longifolia^quot;.

The Wealden species Sagenopteris Mantelli (Dunk.)'^ agrees closely in habit and in the form of the leaflets with 8. Phillipsinbsp;and 8. rhoifolia. It is probable that some of the leavesnbsp;described by Velenovsky'^� from Lower Cretaceous rocks innbsp;Bohemia as Thinnfeldia variahilis are portions of Sagenopterisnbsp;fronds. 8. Mantelli is recorded from several European localities,nbsp;from California'�, and elsewhere.

� Fontaine, in Ward (05); Salfeld (09) PI. i. nbsp;nbsp;nbsp;� Schenk (67) A. PI. xiii.

'� Fontaine, in Ward (05) PL lxv. Newberry�s Chiropteris spatulata from Montana may be founded on leaflets of Sagenopteris Mantelli. Newberry (91).

Sagenopteris appears to have been widely distributed during the Rhaetic, Jurassic and Lower Cretaceous floras. The verynbsp;great similarity between the specimens recorded from these threenbsp;formations renders the genus an uncertain guide in regard tonbsp;geological age. Decisive evidence as to its position in thenbsp;plant kingdom is at present lacking: the inclusion of the genusnbsp;as a possible member of the Hydropterideae has still to benbsp;justified.

CHAPTER XXVII.

The genera and species described in this Chapter are founded on sterile leaves or portions of leaves, and in the greatnbsp;majority of cases the reproductive organs are either imperfectlynbsp;known or have still to be discovered. Some of the genera, thenbsp;smaller number, are no doubt true ferns, while most of themnbsp;may safely be regarded as plants which will ultimately be shownnbsp;to belong to some other group, in most cases that of thenbsp;Pteridosperms. It is possible that a few of the types may benbsp;members of the Cycadophyta rather than of the Pteridospermeae,nbsp;but evidence as to systematic position is for the most part of anbsp;negative kind or too incomplete to lead to any definite expressionnbsp;of opinion as to the cycadean or pteridosperm nature of the imperfectly known Palaeozoic or Mesozoic species. Many of thenbsp;genera are of little botanical interest, though even the most problematical are of importance as criteria of geological age. Generanbsp;which there is good reason for including in the Pteridospermsnbsp;are dealt with in this section, in order that the Chapter innbsp;Volume III. devoted to this important group may be limitednbsp;to more completely known types.

In most text-books it is customary to employ family names for sterile fern-like fronds which possess similar venationnbsp;features or have in common certain vegetative characters, thenbsp;value of which it is impossible to estimate. In the followingnbsp;account family or group names are not adopted, on the groundnbsp;that such slight utility as they may have is more than counter-

balanced by the risk attending a grouping under jne name of plants which may agree only in unessential characters. Thenbsp;practice of classifying fossil plants has been carried to excess.nbsp;Grouping together genera as a matter of convenience unavoidablynbsp;creates a prejudice in favour of actual relationship, which maynbsp;or may not exist.

This generic name was instituted by Brongniart^ for simple linear or broadly linear leaves with a prominent midrib fromnbsp;which secondary veins, simple or dichotomously branched, arenbsp;given off at right angles or obliquely. The frond of the type-species Taeniopteris vittata (fig. 332), characteristic of Jurassicnbsp;floras, was compared by Brongniart with the pinnules of Danaeanbsp;and Angiopteris. Among recent ferns the Taeniopteris formnbsp;of frond and venation is represented by Oleandra neriiformis,nbsp;Asplenium nidus, and many other species. Though usuallynbsp;applied to fronds which there is good reasqn for regarding asnbsp;simple leaves, the generic designation Taeniopteris has beennbsp;extended to include pinnate fronds, e.g. the Upper Palaeozoicnbsp;species T. jejunata Grand�Eury, and T. Carnoti Ren. and Zeill.nbsp;(fig. 330, A). The compound fronds from the Lower Coal-Measures of Missouri described by Dr White ^ as T. missouriensisnbsp;are characterised by decurrent and confluent Taeniopteroidnbsp;pinnules. In a later reference� to this plant White pertinentlynbsp;adds, � perhaps it belongs more properly in A lethopteris.�

Leaves of the Taeniopteris type are described by several authors as species of Oleandridium, Angiopteridium, Danaeites,nbsp;Marattia, and other genera. In such species of Taeniopteroidnbsp;leaves as have been dealt with in a former Chapter, thenbsp;occurrence of sori justifies the substitution of a name denotingnbsp;a close relationship to existing members of the Marattiaceae,nbsp;but in the absence of fertile specimens the provisional designation Taeniopteris should be retained. It is often difficult tonbsp;decide between Taeniopteris and Nilssonia as the more suitable

name to apply to fragments of fossil leaves of Mesozoic age. Taeniopteris is, however, distinguished from the Cycadean genusnbsp;by the greater prominence of the rachis, also by the dichotomousnbsp;branching of the secondary veins, usually close to their originnbsp;and at varying distances between the axis of the frond and thenbsp;edge of the lamina. The genus Taeniopteris, though mostnbsp;abundant in Ehaetic and Jurassic strata, occurs also in Uppernbsp;Carboniferous and Lower Permian rocks. The generic namenbsp;Macrotaeniopteris instituted by Schimper�^ has been used fornbsp;leaves differing only in size from the usual type of Taeniopteris,nbsp;but there is no adequate reason for its retention.

The species included in Taeniopteris afford no satisfactory evidence as to their systematic position. It is obviously unwisenbsp;to adopt such generic titles as Oleandridium, Marattiopsis, etc.,nbsp;merely because of resemblance in the venation of sterilenbsp;fragments to Oleandra or Marattiaceous ferns.

Some specimens of Taeniopteris fronds described by Mr Sellards^ from Permian rocks of Kansas, which are referrednbsp;to later, have furnished unconvincing evidence of reproductivenbsp;organs.

The late Dr Weiss^ instituted this species (which he designated Taeniopteris multinervia, though the specific name midti-nervis is constantly used) for a fragment of a leaf from the Lower Permian of Lebach characterised by numerous forked veinsnbsp;given off at right angles from a prominent rachis (fig. 329, B).nbsp;This type of frond is recorded from the Permian of Trienbachnbsp;(Alsace) by Zeiller^, by Renault� and Zeiller� from the Uppernbsp;Carboniferous of Autun, and from other localities. The laminanbsp;of the simple leaf reaches a breadth of 6 cm. and a lengthnbsp;of 40 cm. (fig. 329, A); the numerous secondary veins (25�36nbsp;per cm. of laminal are either at right angles to the rachisnbsp;or given off at an acute angle. The mesophyll consists of

3 Weiss, C. E. (69) p. 98, PI. vi. fig. 13. nbsp;nbsp;nbsp;* Zeiller (94) p. 169.

polygonal cells some of which are elongated at right angles to the surface of the lamina. A very similar form is describednbsp;by Fontaine and White from the Permian of Virginia as T.nbsp;Lescuriana^.

Taeniopteris leaves into well-defined species: all we can do is to group the specimens under different names, using as artificial distinctions such characters as the shape of the leaf, thenbsp;number of veins per centimetre, and the prominence of the rachis.nbsp;Another Virginian species of Permian age described by Fontaine

and White T. Newherriana, is said to bear sori, but no satisfactory information is given as to the nature of thesenbsp;organs. Specimens referred with some hesitation to thisnbsp;species and to a similar species, T. coriacea, have been describednbsp;by Sellards^ from material obtained from Permian beds innbsp;Kansas. The lamina of the simple linear fronds is characterisednbsp;by the occurrence of small oval bodies half immersed in thenbsp;substance of the leaf between the secondary veins (figs. 330, D, E).nbsp;One of these bodies is represented in an apparently dehiscednbsp;condition in fig. 330, D. Sellards suggests the possibility thatnbsp;these bodies are sporangia, but, as he points out, they afford nonbsp;indication of cellular structure nor are they in direct connexionnbsp;with the veins.

This species differs from T. multinervis in its bipinnate fronds; the linear or oval-linear pinnae are attached by a short stalk tonbsp;the primary rachis and reach a length of 25 cm.; the veins arenbsp;less crowded, 12�15 per centimetre.

T. jejunata is recorded from the Coal-fields of the Loire and Commentry* in France, from the Lower Permian of Thuringia�nbsp;and elsewhere.

This species, founded on portions of pinnate fronds from the Coal-field of Commentry, is characterised by rather broadernbsp;(25�30 mm.) pinnules, with short pedicels and a cordate base,nbsp;reaching a length of 25�30 cm. The secondary forked veins arenbsp;more numerous than in T. jejunata. In T. multinervis thenbsp;pinnules are still broader and have a stronger midrib.

Several species of Taeniopteris have been described from Triasso-Rhaetic rocks in Europe, India, Tonkin and elsewhere.

^ Fontaine and White (80) PI. xxxiv. figs. 1�8. nbsp;nbsp;nbsp;^ gellar^s (01).

^ Grand�Eury (77) A. p. 171. nbsp;nbsp;nbsp;�� Renault and Zeiller (88) A.

In some cases it is practically impossible to recognise clear specific distinctions between Rhaetic and Jurassic types.

From the Damuda and Panchet series of India (Triasso-Rhaetic) Feistmantel has described large sterile fronds as Macrotaeniopteris Feddeni^ which reach a breadth of 20 cm.:nbsp;these may be compared with the Indian species Taeniopteris latanbsp;Oldham^, and to T. gigantea from the Rhaetic of Franconia�nbsp;and Scania. A specimen of this species figured by Nathorst^nbsp;from Scania has a lamina 33 cm. broad. Other examples arenbsp;afforded by M. Wianamattae Feist.� from rocks of the same agenbsp;in Australia and by Taeniopteris superha. Sap.� from Lowernbsp;Rhaetic rocks near Autun.

From the Rhaetic of Tonkin, Zeiller records several species, among which may be mentioned T. Jourdyi Zeill.^ andnbsp;T. spatulata MacClelland (fig. 330, B, C). Both have simplenbsp;fronds. Those of T. Jourdyi reach a length of 10�40 cm.nbsp;and a breadth of 10�70 mm.; the rachis is characterised bynbsp;crowded and discontinuous transverse folds, and the secondarynbsp;veins (35�50 per cm.) are usually at right afigles to the rachis.nbsp;This Tonkin species is compared by Zeiller with the Europeannbsp;Rhaetic species T. tenuinervis Brauns.

The polymorphism of the fronds is a striking feature: in one case described by Zeiller the lamina appears to be divided intonbsp;segments like those characteristic of the leaf of the Cycadeannbsp;genus Anomozamites. It is obviously difficult in many instances to distinguish between detached Taeniopteroid pinnaenbsp;of a compound frond and complete simple leaves. In somenbsp;compound fern fronds, as in the recent Polypodiaceous genusnbsp;Didymochlaena, the pinnules are deciduous, and the same featurenbsp;undoubtedly characterised the fronds of many extinct species.nbsp;A specimen figured by Zeiller which shows several petioles ofnbsp;T. Jourdyi attached to a thick stem� demonstrates the simplenbsp;nature of the leaves. In other cases, e.g. T. vittata, specimensnbsp;occur in which the slightly enlarged petiole-base has a clean-cutnbsp;surface indicating abscission from a rhizome (fig. 332).

� Feistmantel (90) A. PI. xxvii. nbsp;nbsp;nbsp;� Saporta (73) A. Pis. lxi. lxii.

^ Zeiller (02) Pis. x.�xiv. p. 66. nbsp;nbsp;nbsp;� Zeiller (02) PI. xi. fig. 4.

The fronds described by Zeiller as T. spatulata^ (fig. 330, B, C) closely resemble Jurassic leaves from Victoria referred tonbsp;Taeniopteris Daintreei McCoy

Supposed sporangium of T. coriacea. ( x 15. After Sellards.) T. coriacea. ( x 2. After Sellards.)

Whether specifically identical or not, these leaves represent a type distinguished from the other species of the genus bynbsp;the small breadth of the linear-lanceolate or linear-spathulate

lamina, which may be 6�15 cm. in length and 3�12 mm. broad. The lamina is often characterised by transverse foldsnbsp;(fig. 330, C).

The simple fronds included under this specific name are /hharacterised by a strong midrib from which numerous simplenbsp;or forked secondary veins are given off at a right angle ornbsp;slightly inclined. The breadth of the lamina decreasesnbsp;gradually towards the petiole. The Australian species namednbsp;by McCoy Taeniopteris Daintreei, to which Carruthers referrednbsp;the Queensland fossils, has a much narrower and more linearnbsp;form of frond, and for this reason Tenison-Woods instituted anbsp;new specific name. T. Garruthersi represents a form of leaf metnbsp;with in Rhaetic, or possibly Upper Triassic, rocks in S. Africa^nbsp;and Australia. A very similar, perhaps an identical type, wasnbsp;described from Argentina by Geinitz^ as T. mareyiaca: amongnbsp;1 Seward (08) p. 98.nbsp;nbsp;nbsp;nbsp;^ Geinitz (76) PI. ii. figs. 1�3.

many other examples of this form of frond may be mentioned T. immersa^ Nath, from the Rhaetic rocks of Scania and T.nbsp;virgulata from the Rhaetic of Tonkin-.

A comparison of Taeniopteris Carruthersi or various other � species � of Rhaetic fronds with the Jurassic species T. vittatanbsp;illustrates the slight and unimportant differences on whichnbsp;specific separation is based. It is hopeless to attempt to drawnbsp;a satisfactory distinction between the numerous Taeniopterisnbsp;fronds from Upper Triassic and Jurassic rocks.

The simple leaves to which Brongniart applied this name are characteristic of the Inferior Oolite flora of England, andnbsp;examples of the same type are recorded from Jurassic rocks ofnbsp;India, Poland, the Arctic regions, Japan, China, Australia andnbsp;other countriesquot;.

Leaf linear-lanceolate, reaching a length of more than 20 cm. and a breadth of 3 cm. The lamina increases gradually in breadth from thenbsp;base and tapers towards the apex. Numerous secondary veins are givennbsp;off at right angles from a broad midrib : the lateral veins may be simplenbsp;or forked close to their origin, near the margin, or in the intermediatenbsp;portion, of the lamina.

It is exceedingly difficult to use Taeniopteris leaves of this form as evidence in regard to the Jurassic or Rhaetic age ofnbsp;plant-bearing strata. The species T. tenuinervis Brauns, asnbsp;figured by Schenk* from the Rhaetic rocks of Germany andnbsp;Persia, and recorded from several other regions, presents anbsp;close agreement with T. vittata. Oleandridium lentricidi-forme Etheridge � from the Hawkesbury series of Australia isnbsp;another similar leaf The species T. vittata from the Yorkshirenbsp;coast, represented in fig. 332, shows a well-preserved petiolenbsp;with a clean-cut base like that of the petioles of Oleandranbsp;neriiformis and other recent ferns which are detached from thenbsp;rhizome by the action of an absciss-layer.

A broader form of frond with similar venation was described by Lindley and Hutton' as Taeniopteris major. An examinationnbsp;of the type-specimen,from the Inferior Oolite of Yorkshire, nownbsp;in the Manchester Museum, led me to doubt the necessity ofnbsp;specific separation from T. vittata

A smaller frond of the same general type as T. vittata is recorded from Wealden strata of North Germany and Englandnbsp;under the name T. Beyrichii

This generic name was instituted by Stiehler' for impressions of bipinnate sterile fronds, presumably ferns, from Lower Cretaceous rocks near Quedlinburg. The same type ofnbsp;leaf from English Wealden beds had previously been referrednbsp;by Mantell and other authors to Pecopteris, and by Brongniartnbsp;to his genus Lonchopteris^. It is, however, advisable to follownbsp;Nathorst�s example'* and restrict the latter name to Palaeozoicnbsp;species. As already suggested, it would obviate confusion tonbsp;substitute a new generic designation for Lonchopteris in thenbsp;case of Triassic species which are probably members of thenbsp;Osmundaceae. The type-species of Stiehler, Weichselia Ludo-tuicae��, does not differ in any important character fromnbsp;Weichselia Mantelli, the species originally described by Stokesnbsp;and Webb from the Wealden of England as Pecopterisnbsp;reticulata.

1824. Pecopteris reticulata, Stokes and Webb, Trans. Geol. Soc. [2]. Vol. I. p. 423, Pis. XLvi. xLVii.

1828. Lonchopteris Mantelli, Brongniart, Prod. p. 6 ; Hist. veg. foss. p. 369, PI. cxxxi.

1894. Weichselia Mantelli, Seward, Wealden Flora, Vol. i. p. 114. PI. X. fig. 3.

1899. Weichselia reticulata, Fontaine, in Ward, Ann. Eep. U. S. Geol. Surv. p. 651.

^ Seward (00) p. 14. nbsp;nbsp;nbsp;* Schenk (71) PI. xxix.; Seward (94^) A. p. 123.

Stiehler (58) Pis. xii. xiii. nbsp;nbsp;nbsp;^ See p. 576.nbsp;nbsp;nbsp;nbsp;� Nathorst (90).

�� Por figures, see Stiehler loc. cit. and Hosius and Von der March (80) Pis. xLiii. xniv.

8 For synonymy, see Fontaine, in Ward (99) p. 651; Seward (94^) A. p. 114 ; Seward (00) p. 20.

Frond bipinnate, raehis broad ; pinnae very long, of uniform breadth and with prominent axes ; pinnules crowded, entire, with obtuse apex,nbsp;usually oblong but more or less triangular or rounded towards the distal

a, -\

C. nbsp;nbsp;nbsp;Weichselia erratica, Nath. Section of pinna. (After Nathorst.)

ends of the pinnae. The pinnules, which may reach a length of 9 cm., are characterised by a fleshy lamina attached by the whole breadth of thenbsp;base ; the two rows of segments on each secondary rachis are usually

inclined towards one another so that they form with the axis of the pinna a wide-open V instead of lying in one plane (fig. 333, 0). From a mediannbsp;rib are given off numerous anastomosing branches (fig. 333, B).

This characteristic Wealden species is recorded from England, Germany, France, Belgium, Austria, Russia, Bornholm, Northnbsp;America, and Japan. It is by no means certain that Weichselianbsp;Mantelli is a true fern : no satisfactory evidence of fructificationnbsp;has been adduced.

The broad and strong rachis is comparable with that of a Cycadean leaf and the thick lamina suggests a plant ofnbsp;xerophilous habit. I have retained the specific name Mantellinbsp;on the ground of long established usage instead of followingnbsp;Fontaine in his adherence to strict priority.

The name Glossopteris was proposed by Brongniart in 1822^ for an imperfect leaf-impression which he called Filicitesnbsp;{Glossopteris) dubius, but the specimen so named has sincenbsp;been identified as part of a sporophyll of a Lepidostrobus. Thenbsp;author of the genus afterwards published^ a diagnosis, basednbsp;on well-preserved leaves from Permo-Carboniferous rocks innbsp;Australia and India, of the type-species Glossopteris Browniana,nbsp;the Indian examples being distinguished as G. Browniana var.nbsp;indica, while the Australian form was named G. Browniananbsp;var. australasica. Schimper� afterwards raised the Indiannbsp;fossils to specific rank as G. indica though some authors^ havenbsp;continued to consider the two forms as insufficiently distinct tonbsp;be regarded as different species.

Leaves simple, varying considerably in size, shape, and venation characters, but almost without exception characterised by repeatedlynbsp;anastomosing lateral veins. The leaves are of two kinds : (i) foliagenbsp;leaves; apparently always sterile, usually spathiilate, with an obtuse apex,nbsp;a well-marked midrib which may persist to the apex or die out in thenbsp;upper half of the lamina, characterised by its slight prominence andnbsp;comparatively great breadth especially in the basal jjart of the frond. In

most cases the lamina extends as a narrow margin to the leaf-base, but in a few forms there is a short petiole (fig. 334). Though usually spathulate,nbsp;the frond may be linear-lanceolate, or ovate; the apex is sometimes acute.nbsp;Leaves vary in length from 3 to 40 cm. and may in larger forms have anbsp;breadth of 10 cm. Numerous lateral veins curve upwards and outwardsnbsp;to the margin of the lamina or pursue a straight course almost at right-angles to the midrib, (ii) Scale-leaves'^ which differ from the foliage-leavesnbsp;in their much smaller size and in the absence of a midrib; they are deltoid,nbsp;oval or cordate in shape and generally terminate in an acute apex; thenbsp;edge of the lamina may be slightly incurved so that the leaf presents anbsp;convex upper surface supplied with anastomosing veins. Tlie scale-leaves,nbsp;which vary in length from about 1 to 6 cm., probably acted as sporophylls.nbsp;The only evidence as to the nature of the fructification so far obtainednbsp;is represented by empty sporangium-like organs (1-2�1-5 mm. long bynbsp;0-6�0-8 mm. broad) frequently associated with the scale-leaves2.

The leaves, in some cases at least, were borne near together on a cylindrical stem or rhizome which produced branched adventitious roots 2.nbsp;The fossils long known as Vertebraria were recognised by Zeiller'* and bynbsp;Oldham^ as the stems of Glossopteris.

The systematic position of Glossopteris must for the present be left an open question. Though usually sppken of as a fern,nbsp;it is noteworthy that despite the enormous abundance of itsnbsp;foliage leaves in the Permo-Carboniferous strata of India,nbsp;Australia, South Africa, and South America, no single examplenbsp;has been discovered which shows undoubted remains of sorinbsp;or sporangia. Many authors have described fertile leaves ofnbsp;Glossopteris; but it was not until Arber�s discovery of sporangianbsp;in close association with the scale-leaves that any light wasnbsp;thrown on the nature of the reproductive organs.

The probability is that Glossopteris was not a true fern but a member of that large and ever-increasing class, the Pterido-sperms. This opinion is based largely on negative evidence.nbsp;Such sporangia as have been described may have containednbsp;microspores and the plant may have been heterosporous. Thenbsp;occurrence of seeds in association with Glossopteris frondsnbsp;recorded by more than one writer�, though by no meansnbsp;decisive and possibly the result of chance association, is favour-

1 For figures see Zeiller (96) A.; Zeiller (02), (03); Arber (05); Seward (97) A.

able to this view. Dr White�- has suggested that the small leaves described by Zeiller^ as Ottokaria bengalensis from Lowernbsp;Gondwana (Permo-Carboniferous) rocks of India, and similarnbsp;fossils recorded by himself from Brazil as 0. ovalis, may represent � sporangiferous � organs of Glossopteris or Ganganiopteris,nbsp;� both of which are probably pteridospermic.� There is, however, no conclusive evidence in support of this suggestion.

The genus, whatever its position may be, has a special interest for the geologist and for the student of plant distribution; it isnbsp;a characteristic member of a Permo-Carboniferous flora whichnbsp;flourished over an enormous area, including India, South Africa,nbsp;�extending from Cape Colony to Rhodesia and German Eastnbsp;Africa^,�Australia, and South America^ This flora, known asnbsp;the Glossopteris flora, differed considerably in its componentnbsp;genera from that which overspread Europe and North Americanbsp;and some more southern regions in the Upper Carboniferousnbsp;and Permian periods.

The discovery by Amalitzky � of Glossopteris, and other genera characteristic of the Glossopteris flora, in the Upper Permiannbsp;rocks in Vologda (Russia) demonstrates the existence of anbsp;northern outpost of the southern botanical province, andnbsp;Zeiller�s discovery of the genus in the Rhaetic flora of Tonkin�nbsp;shows that Glossopteris persisted beyond the limits of thenbsp;Palaeozoic epoch. Dr David White ^ has recently proposed tonbsp;re-christen the Glossopteris flora the Gangamopteris flora on thenbsp;ground that Gangamopteris is strictly Palaeozoic in its range,nbsp;whereas Glossopteris persisted into the Mesozoic era; this isnbsp;perhaps hardly a sufflcient reason for giving up so wellnbsp;established a title as the Glossopteris flora. A fuller accountnbsp;of this southern flora must be reserved for another volume.

�* Seward (04�) ; Zeiller (97�); Arber (05) p. 17; D. White (07). � Amalitzky (01); Zeiller (98�).

The specific name Browniana is now applied to obtusely pointed leaves which sometimes reach a length of 15 cm., butnbsp;are usually rather shorter. In form and venation they closelynbsp;resemble the leaves of the recent genus Antrophyum andnbsp;species of Acrostichum. The comparatively broad midrib may

be replaced in its proximal portion by several parallel veins; from it are given off numerous lateral veins which form anbsp;reticulum characterised by meshes approximately equal in sizenbsp;and elongated in a direction parallel to the general course ofnbsp;the secondary veins (fig. 334).

The drawings, originall}' published by Zeiller^, reproduced in fig. 335 illustrate the venation and its range of variation ;

the meshes are usually hexagonal and arranged as shown in figs. A and B, but occasionally (fig. 335, C) they follow a morenbsp;steeply inclined course.

Small leaves with a more or less distinct midrib, 2�3 cm. in length, supply transitional stages between foliage- and scale-leaves. In the true scale-leaves spreading and occasionallynbsp;anastomosing veins take the place of the midrib and lateralnbsp;veins of the ordinary frond. McCoy ^ in describing somenbsp;Australian specimens of Glossopteris in IS-II spoke of scale-likenbsp;appendages of the rhizome which he compared with the largenbsp;ramenta of Acrostichum and other ferns. It was, however, Zeiller^

who first recognised the leaf-nature of these scales and adequately described them; additional figures of scale-leaves have been published by Mr Arber� and by myselfh The importance of these small leaves has been considerably increased bynbsp;Mr Arber�s discovery of associated sporangia which, as henbsp;suggests, were probably borne on their lower concave surface.

The sporangia (fig. 336) are compared by Arber with the micro-sporangia of recent Cycads and with the Palaeozoicnbsp;sporangia described by Zeiller as lllsco^^er�^s�a^^u (fig. 256, D);nbsp;the latter are distinguished by the well-defined group of thickernbsp;walled cells representing the annulus of true fern sporangia.nbsp;We know nothing as to the contents of the Glossopteris

sporangia, whether they contained microspores or whether they are the spore-capsules of a homosporous plant.

The rhizome of Glossopteris Browniana has been described in detail by Zeiller, who first demonstrated that the fossilsnbsp;originally assigned by Eoyle^ to the genus Vertebraria representnbsp;the stem of this and, as we now know, of some other species ofnbsp;Glossopteris. Vertebraria occurs in abundance in Permo-Carboniferous strata in association with Glossopteris', the differencesnbsp;between Australian, Indian, and South forms, though expressed

by specific names, are insignificant. The stems are usually preserved in the form of flattened, single or branched, axes sometimes bearing slender branched roots and characterised by one or two,nbsp;or less frequently three, longitudinal grooves or ridges (fig. 337)nbsp;from which lateral grooves or ridges ai'e given off at right angles,nbsp;dividing the surface into more or less rectangular areas 1 cm. ornbsp;more in length. The surface of these areas is often slightlynbsp;convex and in some specimens the outlines of cells may benbsp;detected. Mr Oldham has described some interesting examples

of Yertebraria from India in which the longitudinal and transverse grooves are occupied by a dark brown ferruginous substance or by the carbonised remains of plant-tissues (fig. 338, C, D).nbsp;In transverse section, a Yertebraria cast appears to be dividednbsp;into a number of wedge-shaped segments radiating from anbsp;common centre. Prof Zeiller' has figured specimens of Yerte-braria with portions of Glossopteris fronds still attached.

The rhizome of Glossopteris, as represented by the Vertehraria casts, is aptly compared by Zeiller*^ with that of the recentnbsp;Polypodiaceous fern Onoclea struthiopteris. Sections of thenbsp;recent stem (fig. 338, E, F) show that the form is irregularlynbsp;stellate owing to the presence of prominent wings whichnbsp;anastomose laterally at intervals as shown by the examinationnbsp;of a series of sections. The leaf-traces are derived from thenbsp;steles of adjacent wings. Fig. 338 (B and A) represents some-1 Zeiller (96) A.nbsp;nbsp;nbsp;nbsp;^ Zeiller (96) A.

what diagrammatically a longitudinal and transverse view of a Vertebraria; the radiating arms represented in the transversenbsp;section (fig. A) are the stem ribs or wings and the segmentsnbsp;between them are intrusions of sedimentary material. Thenbsp;rectangular areas characteristic of the surface of a Vertebrarianbsp;are the intruded segments of rock: these are separated atnbsp;intervals by transverse grooves, which mark the course of

vascular strands given off at each anastomosis of the longitudinal wings to supply the leaves.

Mr Oldham, who discovered the connexion between Glosso-pteris and Vertebraria independently of Dr Zeiller, does not agree with the interpretation of the structural features of thenbsp;rhizome which Zeiller bases on a comparison between Vertebrarianbsp;and Onoclea struthiopteris. Oldham ^ describes Vertebraria as

consisting of a central axis �joined to an outer rind by a series of radial septa,� the spaces between the septa being divided

into chambers by transverse partitions. His view is that the rhizome of Glossopteris was a cylindrical organ and not an

irregularly winged axis like the stem of Onoclea. Zeiller�^ has replied in detail to Oldham�s interpretation and adheres to hisnbsp;original view, that the rhizome consisted of a solid axis withnbsp;radial wings or flanges which at intervals anastomosed transversely in pairs at the nodes. It may, however, be possible thatnbsp;the spaces between the longitudinal and transverse grooves onnbsp;a Vertebraria axis, which have been filled with the surroundingnbsp;rock, were originally occupied in part at least by secondary wood,nbsp;and the transverse strips of carbonaceous material^ lying in thenbsp;grooves may represent medullary-ray tissue and accompanyingnbsp;leaf-traces. The longitudinal striations seen in some specimensnbsp;of Vertebraria on the areas between the grooves may be thenbsp;impressions of woody tissue. It is impossible without the aidnbsp;of more perfectly preserved material to arrive at a satisfactorynbsp;conception of the structural features of a complete Glossopterisnbsp;rhizome.

In the specimen of Glossopteris Browniana shown in fig. 339 several leaves are attached to an axis which showsnbsp;none of the surface-features of Vertebraria. I am indebtednbsp;to the kindness of Dr Mohlengraaff of Delft for the loan of thisnbsp;specimen which was obtained from Permo-Carboniferous rocksnbsp;in the Transvaal. An axis figured by Etheridge� from annbsp;Australian locality bears a tuft of Glossopteris leaves, possiblynbsp;G. Broivniana; in place of the rectangular areas characteristicnbsp;of Vertebraria it shows transversely elongated leaf-scars or, onnbsp;the internal cast, imbricate rod-like projections which Etheridgenbsp;suggests represent vascular bTindles.

It is a question of secondary importance whether or not the fronds which Brongniart spoke of as a variety of Glossopterisnbsp;Browniana should be recognised as specifically distinct. Thenbsp;careful examination by Zeiller of the venation characters has,nbsp;however, afforded justification for separating G. Browniana andnbsp;G. indica. We must admit that the slight and not very constant

differences in the size and form of the meshes produced by the anastomosing of the lateral veins are characters which cannot

B. Glossopteris angustifoUa, Brongniart. (Nat. size.) From Arber, after Feistmantel.

be recognised as having more than a secondary value, though, as a matter of convenience, we employ them as aids to determina-

tion. The arbitrary separation of sterile leaves, which differ by small degrees from one another in form and in the details ofnbsp;venation, by the application of specific names is a thanklessnbsp;task necessitated by custom and convenience; it is, however, idlenbsp;to ignore the artificial basis of such separation. Mr Arber hasnbsp;recently published, in his valuable Glossopteris Flora, annbsp;analytical key which serves to facilitate the description andnbsp;determination of different types of frond

The large leaves of Glossopteris indica, reaching a length in extreme cases of 40 cm. and a breadth of 10 cm., are characterisednbsp;by a rather greater regularity in the arrangement of the

meshes and by the greater parallelism of the upper and lower sides of each mesh (fig. 341) and by less difference in sizenbsp;between the venation meshes than in G. Browniana, the leavesnbsp;of which are usually smaller. The relatively thick epidermisnbsp;consists of rectangular cells with stomata in depressions^.nbsp;The scale-leaves�, rather larger than those of G. Browniana,nbsp;are more or less rhomboidal with rounded angles and reachnbsp;a length of 1'5�6 cm. and a breadth of 1'5�2'5 cm. T henbsp;rhizome is practically identical with that of G. Browniana'^.

This species occuijs in great abundance in the Permo-Carboniferous rocks of India, Australia, and in various parts of

South Africa, and elsewhere. It has been recognised also by Amalitzky^ in Upper Permian beds in Russia and by Zeiller innbsp;the Rhaetic series of Tonkin^.

It is convenient to retain this designation for linear fronds with an acute or obtuse apex and a venation-reticulumnbsp;composed of long and narrow meshes (fig. 340, B). It is by nonbsp;means unlikely, as Arber suggests, that the same plant maynbsp;have produced leaves of the 0. indica type and narrower frondsnbsp;which conform to G. angustifolia. In his description of some

Indian specimens of G. indica, Zeiller draws attention to the variation exhibited in regard to the extent of anastomosingnbsp;between the secondary veins: some examples with very fewnbsp;cross-connexions agree more closely with Taeniopteris thannbsp;with Glossopteris as usually defined�. The venation shown innbsp;fig. 342 illustrates an extreme case of what is almost certainlynbsp;a Glossopteris leaf of the G. angu.stifolia type. This .specimen,nbsp;which was discovered by Mr Leslie in the Permo-Carboniferousnbsp;sandstone of Vereeniging (Transvaal), has been referred to anbsp;variety of Brongniart�s species as G. angustifolia var. taeniopte-roides^ on account of the almost complete absence of any crossconnexions. The reference to Glossopteris, which my friend

Dr Zeiller suggested, is amply justified by the form of the leaf as a whole, by the angle at which the lateral veins leave thenbsp;midrib, a feature in contrast to the wider angle at which thenbsp;lateral veins are usually given off in Taeniopteris (figs. 329, 332),nbsp;and by the similarity to the Indian specimens already mentioned.nbsp;Several authors have described leaves or leaflets under the generic

Fig. 343. Bleclmoxylon talliragarense, Eth.: s, scale-leaves; x, secondary xylem. (After Etheridge. Ax 2; Bx3; C much enlarged.)

name Megalopteris'^ from Carboniferous and Permian rocks which bear a close resemblance to the South African variety,nbsp;but in some cases at least Megalopteris is known to be a pinnatenbsp;and not a simple leaf. The leaf figured by Jack and Etheridgenbsp;as Taeniopteris sp.� from Queensland may also be an example of

1 Dawson (71) A. PI. xvii.; Fontaine and White (80) p. 11; White (95) p. 315; Arber (05^) p. 307, PI. xx.

Glossopteris. Comparison may be made also with the Palaeozoic leaves described in the first instance by Lesquereux and morenbsp;recently by Renault and Zeiller as species of Lesleya'^ (fig. 347).

Under this name Etheridge^ described some specimens from the Permo-Carboniferous Coal-Measures of New Southnbsp;Wales, which he regards as a fern, comparable, in the possessionnbsp;of a cylinder of secondary xylem, with the recent genusnbsp;Botrychium and with Lyginodendron and other members ofnbsp;the Cycadofilices. The slender axis (1�3 mm. in diameter)nbsp;appears to consist of a zone of radially disposed tissuenbsp;(fig. 343, C, x), which is probably of the nature of secondarynbsp;xylem, enclosing a pith and surrounded externally by imperfectlynbsp;preserved remnants of cortex. Unfortunately no anatomicalnbsp;details could be made out, but the general appearance, ifnbsp;not due to inorganic structure, certainly supports Etheridge�snbsp;determination. The stem bore at intervals clusters of linear-lanceolate leaves (reaching 12 mm. in length) in close spiralsnbsp;(fig. 343, A and B); the leaves are characterised by a strongnbsp;midrib and forked secondary veins. Small �pyriform� bodies ofnbsp;the nature of scale-leaves occur in association with the frondsnbsp;(fig. 343, B, s).

In his description of this interesting plant, Etheridge quotes an opinion which I expressed in regard to the comparison of thenbsp;stem with those of Botrychium, Lygmodendron, and othernbsp;genera. No satisfactory evidence has been found as to thenbsp;nature of the fructification. Although the leaves of Blechnoxylonnbsp;are much smaller than those of Glossopteris, I am now disposednbsp;to regard the genus as closely allied or even generically referablenbsp;to Glossopteris. The crowded disposition of the leaves is likenbsp;that in Glossopteris, shown in fig. 339 and in the figures publishednbsp;by Etheridge and by Oldham; the association of scale-leavesnbsp;and foliage-leaves is another feature in common. The absence

' Lesquereux (79) A. PI. xxv.; Renault and Zeiller (88) A. PI. xxiii. See p. 517.

of a reticulum of anastomosing veins can no longer be considered a fatal objection to the suggestion that the Australian typenbsp;may be a species of Glossopteris. If the view that Blechnoxylonnbsp;is not a distinct genus is correct, the occurrence of secondary

xylem is favourable to the opinion already expressed that Glossoptens is more likely to be a Pteridosperm than a true fern.

The data at present available render it advisable to retain Mr Etheridge�s name: the comparison with Glossopteris lacksnbsp;confirmation.

In some Glossopteris leaves the anastomosing secondary veins form a coarser reticulum, as in the example representednbsp;in fig. 344. The name G. retifera was given by FeistmantePnbsp;to Indian fronds of this type; similar forms have been describednbsp;as G. conspicua and G. Tatei. The type illustrated by G. retiferanbsp;is recorded also from Permo-Carboniferous rocks in Zululand^,nbsp;Natal, the Transvaal, Cape Colony, and the Argentine.

In 1847 McCoy� described a leaf-fragment from Permo-Carboniferous rocks in New South Wales as Cyclopteris angustifolia. The type-specimen of this species, which is nownbsp;in the Sedgwick Museum, Cambridge, has been re-describednbsp;by Mr Arberb Subsequently� McCoy instituted the genericnbsp;name Gangamopteris for leaves, like that previously referrednbsp;by him to Cyclopteris, from the Bacchus Marsh Sandstone ofnbsp;New South Wales, but he did not publish a diagnosis of thenbsp;genus until several years later�. FeistmanteP, who hasnbsp;described many species of Gangamopteris from the Lowernbsp;Gondwana strata of India, slightly modified the originalnbsp;diagnosis. The genus is represented by sterile fronds only.nbsp;We know nothing of the stem, and such evidence as is availablenbsp;in regard to the form of the fertile leaves is of a circumstantialnbsp;kind. It is, however, highly probable that Gangamopteris isnbsp;not a true fern but a Pteridosperm.

Leaves simple, sessile, varying in shape; obovate or spathulate, broadly lanceolate or rarely linear; the apex is usually blunt (fig. 345) butnbsp;occasionally gradually tapered. In general appearance a Gangamopterisnbsp;leaf is similar to that of Glossopteris indioa, the chief distinction being the

* McCoy (47). nbsp;nbsp;nbsp;Arber (02^).nbsp;nbsp;nbsp;nbsp;� McCoy (60) p. 107 (footnote).

absence of a midrib. Gangamopteris leaves are on the whole larger than those of Glossopteris; many of them reach a length of 20 cm. and some ofnbsp;the large Indian fronds are nearly 40 cm. long. The venation of Gangamopteris shows a greater uniformity in the size and shape of the meshesnbsp;than that of Glossopteris. The middle of the lamina, especially in thenbsp;lower part, is occupied by a few vertical veins from which branches curvenbsp;upwards and outwards towards the edge of the lamina. The secondarynbsp;veins are connected by frequent anastomoses and agree very closely withnbsp;those of Glossopteris. The lamina becomes narrower towards the base,nbsp;which is either cuneate or in some cases slightly auriculate (fig. 345).

As I have elsewhere pointed out\ the presence or absence of a midrib is not in itself a character of real taxonomicnbsp;importance. In the recent fern Scolopendrium vulgare thenbsp;frond has a prominent midrib, while in S. nigripes there is nonbsp;median rib. Mr Arber has expressed the opinion that �it isnbsp;extremely doubtful whether the genus Gangamopteris shouldnbsp;not be merged in Glossopteris^.� The retention of the two namesnbsp;is, however, convenient, and it would tend to confusion werenbsp;we to carry to its logical conclusion the view that the recognisednbsp;distinction between the two genera may not be a mark ofnbsp;generic difference.

Gangamopteris is confined to Palaeozoic st/ata, a fact which leads White� to speak of the Gangamopteris rather than of thenbsp;Glossopteris Flora. It occurs in South America, South Africa,nbsp;Australia, and India, extending as far north as Kashmir; it hasnbsp;been discovered by Amalitzky in Permian rocks of Russia^.nbsp;The Russian rocks in which Glossopteris and Gangamopterisnbsp;were found are no doubt of Permian age. In Australia, Southnbsp;Africa, Brazil and Argentina, and in the Indian Coal-fields,nbsp;Gangamopteris is a characteristic genus of Lower Gondwananbsp;rocks. These strata are usually spoken of as Permo-Carboniferous in order to avoid the danger of attempting on insufficientnbsp;data a precise correlation with European formations.

Feistmantel speaks of Gangamopteris as most abundant in the Talchir-Karharbari beds, though it is represented also innbsp;the overlying Damuda series. In Australia the genus occurs innbsp;rocks which correspond in position and in their plant fossils

with the Talchir-Karharbari beds of India; similarly, in South Africa and South America the Gangamopteris beds are homo-taxial with those of India and Australia. The leaf describednbsp;by Carruthers' from Brazil as Noeggerathia obovata (the type-specimen is in the British Museum) is no doubt specificallynbsp;identical withOangamopteris cyclopteraides Feist.^ In a papernbsp;by Mr Hayden on Gangamopteris beds in the Vihi Valley,nbsp;Kashmir, evidence is adduced in support of the conclusion thatnbsp;the rocks are � not younger than Upper Carboniferous and maynbsp;belong to the base of that subdivision or even to the Middlenbsp;Carboniferous�.� It would seem that Gangamopteris was anbsp;very widely spread genus during the latter part of the Carboniferous period in the vast Southern Continent to which thenbsp;name Gondwana' Land is often applied, and that it flourished innbsp;the Southern Flora during at least part of the Permian period:nbsp;with other members of the Glossopteris Flora it migrated to thenbsp;North where it has been preserved in Permian rocks of Northernnbsp;Russia. The Glossopteris Flora must have had its birth in thenbsp;Southern hemisphere. The conclusion seems inevitable thatnbsp;the leaves of Glossopteris and Gangamopteris in the shales andnbsp;sandstones of India, South Africa, South America, and Australianbsp;are relics of the vegetation of a continent of which these regionsnbsp;are the disjuncta membra. Darwin wrote to his friend Hookernbsp;in 1881, �I have sometimes speculated whether there did notnbsp;exist somewhere during long ages an extremely isolated continent, perhaps near the South Poleh� It is probable thatnbsp;Gangamopteris is one of the genera which flourished on thisnbsp;continent.

The specimen represented in fig. 345 illustrates the characters of this commonest representative of the genus.

This type agrees closely with G. cyclopteroides in size and in the form of the leaf, hut it is distinguished by the flatternbsp;form of the arch formed by the lateral veins, by their greaternbsp;inclination to the margin of the lamina, and by the morenbsp;acutely pointed apex of the lamina. This species, though notnbsp;very sharply distinguished from G. cyclopteroides, is importantnbsp;as coming from beds which have been assigned on other thannbsp;palaeobotanical evidence to an Upper or possibly a Middlenbsp;Carboniferous horizon h

We have no definite information in regard to the nature of the reproductive organs of Gangamopteris, but such evidence asnbsp;there is supports the view expressed by Dr White ^ and sharednbsp;by some other authors that Gangamopteris and Glossopterisnbsp;should be assigned to the Pteridosperms. Despite the abundance of Gangamopteris leaves, no fertile specimen has beennbsp;discovered. This negative evidence may prove to be as correctnbsp;as that which led Stur� to exclude Neuropteris, Alethopteris andnbsp;Odontopteris from the ferns. The only evidence of a positivenbsp;kind is that furnished by Dr David White in his recent Reportnbsp;on the Palaeozoic Flora of South Brazil. This author describesnbsp;some small Aphlebia-like leaves under two new generic namesnbsp;Arhe7'ia* and Derhyella^. The differences between the two setsnbsp;of specimens, so far as can be determined from the reproductions of imperfect impressions, are slight, and it is by no meansnbsp;clear that a distinction of generic rank exists. These scale-leaves are on the average about 2 cm. in length; the lamina isnbsp;oval or rounded and has more or less prominent lobes. Innbsp;Derhyella there are indications of anastomosing veins. Thenbsp;specimens referred to Arberia minasica are, as White pointsnbsp;out, very similar to the fossil described by Feistmantel fromnbsp;Lower Gondwana rocks of India as probably a portion of annbsp;inflorescence of Noeggerathiopsis^. Feistmantel�s specimen is

^ White (08) p. 537, PI. viii. figs. 8-10. nbsp;nbsp;nbsp;� Ihid, p. 543, PI. ix. figs. 1-3.

represented in fig. 346: the curled lobes may have originally borne seeds. In the Brazilian examples the abruptly truncatednbsp;lobes �bear evidence of separation from reproductive bodies.�nbsp;An important point is the association of these scale-leaves withnbsp;Gangamopteris fronds and with gymnospermous seeds of thenbsp;Samaropsis type. On the leaves assigned to Derbyella auritanbsp;circular depressions occur at the base of the lobes which arenbsp;described as probably due to sporangia.

Dr White�s discovery gives us increased confidence in expressing the view that Gangamopteris bore its reproductivenbsp;organs on specialised leaves very different from the sterilenbsp;fronds; it also strengthens the suspicion that the genus is anbsp;member of the class of seed-bearing fern-like plants.

Pig. 346. Arberia sp. { = Noegyerathiopsis of Feistmantel). (Nat. size. After Peistmantel.)

This generic designation was instituted by Lesquereux^ for simple oval-linear leaves from the Coal-Measures of Pennsylvania. The leaves so named are probably genericallynbsp;identical with the specimen doubtfully assigned by Brongniart^nbsp;to the Coal-Measures, and made by him the type of the genusnbsp;Cannophyllites on the ground of a resemblance to the leaves ofnbsp;the recent flowering plant Ganna. Fig. 347 illustrates thenbsp;form of a Lesleya leaf from the Coal-basin of Gard, named by

Grand�Eury L. simplicinervis^, a type in which the veins are frequently unbranched and not repeatedly forked as in mostnbsp;examples of the genus (fig. 329, C). The features of the genusnbsp;are, the oval-linear or lanceolate shape of the presumably simplenbsp;frond, its entire or, in one species at least {L. Delafondi, Zeill.),nbsp;finely dentate margin, the stout rachis giving off at a verynbsp;acute angle numerous dichotomously branched secondary veins.

In L. Delafondi (fig. 329, C), described by Zeiller^ from the Lower Permian of Autun, the frond may reach a length ofnbsp;more than 20 cm. and a breadth of 8 cm. Similar species arenbsp;represented by L. ensis^ from the coal-field of Commentry,nbsp;and L. g^'andis^ from Upper Carboniferous rocks of Northnbsp;America. The genus is characteristic of Upper Carboniferousnbsp;and Lower Permian strata: the form of the leaf and the direction

' Grand�Eury (90) A. PI. viii. fig. 5. nbsp;nbsp;nbsp;^ Zeiller (90) p. 166, PI. xiii. fig. 2,

� Renault and Zeiller (88) A. PI. xxiii. fig. 6. nbsp;nbsp;nbsp;* Lesquereux, loc. cit.

of the secondary veins suggest comparison with Glossopteris, but in Lesleya there are no cross-connexions between the veins.nbsp;Nothing is known as to the fructification, a fact which naturallynbsp;evokes the opinion that the genus is a Pteridosperm^ and not anbsp;true fern. Some years before the discovery of Pteridosperms,nbsp;Grand�Eury^ suggested that Lesleya might be a Gymnosperm ;nbsp;his opinion being based on the woody nature of the rachis andnbsp;on the simple venation of Lesleya simplicinervis.

In their monograph of fossil plants from the Bunter Series of the Vosges, Schimper and Mougeot� described some pinnatenbsp;leaves of ferns as species of the genus Neuropteris. In 1869nbsp;Schimper^ placed these in a new sub-genus Neuropteridium, innbsp;order to draw attention to the fact that their fronds appearnbsp;to be simply pinnate and not bipinnate or tripinnate as innbsp;Neuropteris. The type-species of Neuropteridium is N. grandi-folia Sch. and Moug. from the Bunter Sandstones of the Vosges.nbsp;The genus includes Triassic European specigs and the widelynbsp;distributed Permo-Carboniferous species from Brazil� originallynbsp;described by Carruthers as Odontopteris Plantiana. It isnbsp;probable that some Carboniferous plants, particularly speciesnbsp;from the lower members of the formation, referred to the genusnbsp;Cardiopteris, are not generically distinct from the Indian andnbsp;southern hemisphere type Neuropteridium validum (= Odontopteris Plantiana).

Fronds pinnate, linear ; a broad rachis bears pinnules which may be either semicircular or broadly linear with an entire or lobed margin. Thenbsp;longer pinnules may exceed 6 cm. in length. The pinnules agree withnbsp;those of Neuropteris in being attached by the median portion of the laminanbsp;and not by the whole base, which is more or less auriculate. In somenbsp;cases the repeatedly forked veins diverge from the centre of the pinnulenbsp;base ; in others there is a midrib which persists for a short distance only,nbsp;and in some species the more persistent median vein gives the segments anbsp;closer resemblance to those of Neuropteris. Fructification unknown, withnbsp;the exception of obscure indications of sporangia (?) on the fertile leaves ofnbsp;a Triassic species.

The specimen represented in fig. 348 illustrates the main features of Neuropteridium validum. This species is referred tonbsp;by Dr White^ as iVquot;. Plantiarmm on the ground of priority, andnbsp;with a view to perpetuate the name of the English engineernbsp;Nathaniel Plant who discovered the species in a Brazilian Coalfield in the province of Rio Grande do Sul. Feistmantel�s specificnbsp;nanie is however retained as being much better known. Annbsp;examination of Mr Plant�s specimen in the British Museumnbsp;led me� to speak of the Brazilian species as identical withnbsp;N. validum described by Feistmantel from Lower Gondwananbsp;rocks of India. Zeiller* had previously drawn attention to thenbsp;resemblance between the two sets of specimens. The frond ofnbsp;N. validum may exceed 50 cm. in length. The lower pinnulesnbsp;may be entire and semicircular in form while the upper andnbsp;larger segments, which may reach a length of 5 or 6 cm., arenbsp;characterised by broad lobes (fig. 348).

This type is represented in the flora of the Talchir-Karharbari series (Lower Gondwana) of India�, in Permo-Carboniferous rocks of Brazil and Argentine�, and in the sandstones of Vereeniging on the borders of the Transvaalnbsp;and Cape Colony. It is a characteristic member of the Glossop-teris Flora and occurs in association with Glossopteris andnbsp;Gangamopteris.

This species has been figured by Schimper and Mougeot�^ from the Bunter of the Vosges and more fully described by

2 White (08) p. 483. s geward (03) p. 83. � Feistmantel (79).nbsp;nbsp;nbsp;nbsp;.nbsp;nbsp;nbsp;nbsp;� Kurtz (94).

Blanckenhorn^ from the Bunter beds of Commern. The pinnate leaves reach a length of 65 cm.; the lower semicircular pinnulesnbsp;pass gradually into broadly linear segments characterised by annbsp;auriculate base and a Neuropteris type of venation (fig. 354,nbsp;D', E). In the example reproduced in fig. 349 from one of

Blanckenhorn�s figures, the fronds are attached to a short and thick rhizome bearing roots and portions of old petioles.

An example of another Triassic species is afforded by Neuro-pteridiuni grandifolium Schimp, and Moug., which agrees very closely with N. validum in the size and shape of the pinnules.

The occurrence in Lower Mesozoic European rocks of fronds hardly distinguishable from the older southern species may benbsp;regarded as favourable to the view already expressed, that somenbsp;at least of the Permo-Carboniferous plants migrated north ofnbsp;the Equator. The resemblance between the Vosges Triassicnbsp;species of Schizoneura^ and the examples of this genus recordednbsp;from the Lower Gondwana rocks of India affords additionalnbsp;evidence of a northern migration.

Our knowledge of the reproductive organs of Neuropteri-dium is practically nil. There is no doubt that Zeiller^ and Blanckenhorn� are correct in regarding the Bunter frondsnbsp;assigned by Schimper and Mougeot to the genus Crematopterisnbsp;as the fertile leaves of Neuropteridium intermedium or somenbsp;other species from the same horizon. These fronds bear crowdednbsp;pinnules similar to those of Neuropteridium intermedium,nbsp;N. Voltzii^, and other species, exhibiting on the exposed surfacenbsp;numerous carbonaceous spots which may be the remains ofnbsp;sporangia.

Schimper� applied this generic name to Lower Carboniferous fronds of a simple-pinnate habit which had previously beennbsp;described as species of Gyclopteris. Cardiopteris frondosa maynbsp;serve as a typical example. This species, originally described bynbsp;Goeppert as Gyclopteris frondosa (fig. 350), is recorded fromnbsp;Lower Carboniferous rocks in the Vosges district� in Silesia,nbsp;Moravia��, and Thuringia�. The pinnules, which are attachednbsp;in opposite pairs to a broad rachis, vary in length from 2 tonbsp;10 cm. and have a breadth of 2 to 8 cm.; in manner of attachmentnbsp;and venation they agree with those of Neuropteridium validum.nbsp;The venation is very clearly shown in a drawing of some largenbsp;pinnules figured by Stur�.

1 Vol. I. p. 292. nbsp;nbsp;nbsp;2 Zeiller (00^).nbsp;nbsp;nbsp;nbsp;� Blanckenhorn (85) p. 129, PI. xxi.

* Blanckenhorn loc. cit. The specimens figured by this author are in the Strassburg Museum, as are also some of those figured by Schimper and Mougeot.

^ Ibid. nbsp;nbsp;nbsp;* Britsch, K. (97).nbsp;nbsp;nbsp;nbsp;Stur (75) A. PI. xiv. fig. 1.

is shown in fig. 350 on a slightly reduced scale, was originally figured by Schimper from an unusually good example in thenbsp;Strassburg Museum. Schimper�s drawing hardly does justicenbsp;to the original specimen.

A frond bearing rather narrower pinnules, alternately placed on the rachis, which Fritsch has described as Gardiopterisnbsp;Hochstetterii var. franconica from the Culm of Thuringia, bearsnbsp;a close resemblance to Neuropteridium validum but differs innbsp;the entire margin of the pinnules. An Upper Carboniferous

species from Russia described by Grigoriew^ as Neuropteris, cf. cordata var. densineiira, represents another form of similar habit.

Schuster^ has recently proposed a new generic name Ulvopteris for a fragment of a pinna from the Coal-Measuresnbsp;of Dudweiler in Germany bearing large pinnules, which henbsp;compares with those of Gardiopteris and species of Rhacopteris.nbsp;The specimen appears to be indistinguishable from some of

those already referred to as conforming to Neuropteridium, and it is difficult to recognise any reason for the creation of a newnbsp;generic name.

We cannot hope to arrive at any satisfactory decision in regard to the precise affinity between Neuropteridium validumnbsp;and species referred to Gardiopteris and other genera so long asnbsp;portions of sterile fronds are the only tests at our disposal. Itnbsp;is difficult to determine whether a specimen consisting of annbsp;axis bearing pinnules represents a large pinna of a bipinnatenbsp;frond or if it is a complete pinnate leaf. There is, however,nbsp;no adequate reason for supposing that the presumablynbsp;pinnate fronds from the Gondwana Land rocks are genericallynbsp;distinct from the Lower Carboniferous European speciesnbsp;Gardiopteris frondosa. Granting the probability that bothnbsp;genera are Pteridosperms and closely allied to one another, thenbsp;two generic names may be retained on the ground of long usagenbsp;and in default of satisfactory evidence confirmatory of genericnbsp;identity. Gardiopteris would thus stand for a type of frondnbsp;characteristic of the Lower Carboniferous strata of Europe,nbsp;while Neuropteridium is retained for the Southern speciesnbsp;iV. validum, and for others from the TriSs of the Vosges.

This name was proposed by PresP for large leaf-like impressions having a pinnate or pinnatifid form and characterisednbsp;by a confused irregular type of venation, or by a fine superficialnbsp;striation or wrinkling which simulates veins. Gutbier hadnbsp;previouslj^ described similar fossils as Fucoides, and othernbsp;authors have described Aphlebiae as species of Rhacophyllum,nbsp;Schizopteris, and other general The term Aphlebia is retained,nbsp;not as denoting a distinct genus but (i) as a descriptive namenbsp;for detached leafy structures similar to those figured by Presl,nbsp;which are now recognised as laminar appendages of the petiolesnbsp;of ferns or fern-like fronds, and (ii) as an epithet for highlynbsp;modified pinnules which frequently occur at the base of the

primary pinnae of Pecopteroid and Sphenopteroid fronds (e.g. Dactylotheca plumosa, fig. 293)^.

Modified pinnules, similar in their reduced and deeply dissected lamina to those represented in fig. 293, are frequentlynbsp;found at the base of the primary pinnae of Palaeozoic species ofnbsp;Sphenopteris and other genera of Pteridosperms or ferns,includingnbsp;members of the Coenopterideae. Potoni�^ gives a list of variousnbsp;types of Aphlebiae in his paper on these organs. A strikingnbsp;case has recently been described by Zeiller in a French Uppernbsp;Carboniferous species, Sphenopteris MathetP. It would seemnbsp;that the larger examples of Aphlebiae are more frequentlynbsp;associated with the compound leaves of Pteridosperms thannbsp;with those of Ferns*.

As examples of the larger types of Aphlebiae reference may be made to Aphlebia crispa (Gutb.)�, which reaches a length ofnbsp;nearly 60 cm. and has the form of a more or less triangularnbsp;pinnate leaf divided into decurrent deeply lobed segments, tonbsp;a similar species represented by A. Germari {= Schizopterisnbsp;lactuca Germ.)� which simulates the leaves of endive {Gichoriumnbsp;endivia Li), and to some large forms figured by Grand�Eury�� asnbsp;species of Schizopteris.

Aphlebiae such as that figured by Kidston� as Rhacophyllum crispum, with narrow ultimate segments, might easily benbsp;mistaken for the impressions of an alga.

The term Aphlebia may be applied also to the Cyclopteroid pinnules on the petioles of some species of Neuropteris, Odonto-pteris and Archaeopteris. Goebel� has referred to the applicationnbsp;by Potonie and other authors of the term Aphlebioid to thenbsp;pinnules which serve as bud-protecting organs in recent frondsnbsp;of Gleichenia (fig. 226, p. 290); he expresses the opinion that itnbsp;is superfluous and misleading to make use of a special designation for structures which are undoubtedly modified pinnules.nbsp;In the case of fossils it is, however, convenient to employnbsp;the term Aphlebia as a descriptive name for modified pinnules

1 Page 406. nbsp;nbsp;nbsp;� Potoni� (03) p. 162.nbsp;nbsp;nbsp;nbsp;3 Zeiller (06) Pis. vr. vii.

* Arber (06). nbsp;nbsp;nbsp;^ Renault and Zeiller (88) A.; Zeiller (88) A. PI. li.

3 Renault and Zeiller (88) A. PL xxiv. nbsp;nbsp;nbsp;^ Grand�Eury (90) A. PI. xix.

scale-leaves of considerable size (fig. 351) which occur in the leaf-axils or as ochrea-like stipules on the fronds of Gunneranbsp;(a tropical and subtropical Dicotyledonous genus) bear a verynbsp;close resemblance to some Palaeozoic Aphlebiae, e.g. Aphlebianbsp;crispa (Gutb.). The recent and fossil scale-leaves may benbsp;regarded as similar in function as in form; moreover the delicatenbsp;coiled fronds of Palaeozoic Pteridosperms or ferns, like those ofnbsp;some recent flowering plants, may have been kept moist by anbsp;secretion of mucilage. The pinnatifid stipules of Marattianbsp;fraxinea (fig. 241, B, p. 317) resemble certain fossil Aphlebiae,nbsp;and the wrinkled surface of the recent stipules presents annbsp;appearance similar to that which in some fossil forms has beennbsp;erroneously described as veining. It is not improbable thatnbsp;mantle-leaves of such recent ferns as Polypodium quercifoliumnbsp;(fig. 234, M, p. 303) are comparable with some fossil Aphlebiaenbsp;which may have served as humus-collectors for Palaeozoicnbsp;epiphytes.

The filiform appendages on the petioles of the recent fern Hemitelia capensis (fig. 235, p. 304) have often been comparednbsp;with the aphlebioid leaflets of fossil fronds.

Potoni� who has discussed the nature of Aphlebiae regards them as vestiges of a once continuous lamina, which formed anbsp;winged border to the branched axes of more primitive forms ofnbsp;fronds. It is possible that the pinnules between the pinnae onnbsp;the rachis of Archaeopteris and the Cyclopteroid leaflets ofnbsp;Neuropteris and Odontopteris may have the morphologicalnbsp;significance attributed to them by Potoni�. In some casesnbsp;it is probable that the Aphlebiae, whether vestiges or not,nbsp;served the purpose of protecting either the whole frond ornbsp;individual pinnae. Aphlebiae, though especially characteristicnbsp;of Palaeozoic leaves, are occasionally met with in the form ofnbsp;modified pinnules at the base of the primary pinnae onnbsp;Mesozoic ferns, e.g. in Goniopteris hymenophylloides^.

In some fern fronds the lowest pinnule of each pinna differs in shape or size from the normal ultimate segments,nbsp;but it would be almost affectation to extend the use of thenbsp;term Aphlebia to such pinnules. The Jurassic speciesnbsp;^ Seward (00) PI. xxi. fig. 1.

Cladophlebis lobifolia (Phill.) is a case in point In this fern, which some authors speak of, without sufficient reason,nbsp;as Dicksonia lobifolia^, the lowest pinnule is large and differentnbsp;in shape from the others.

Fig. 352. A. Sphenopteris obtusiloba. Pinnule. (Enlarged. After Zeiller.) B, C. S. obtusiloba. nat. size. After Zeiller.)

E. nbsp;nbsp;nbsp;Sphenopteris furcata { = Diplotme7na furcatum). (Slightly enlarged. After Zeiller.)

Sphenopteris is one of the many generic names which we owe to Brongniart�. It is the generic designation used for anbsp;great number of Palaeozoic and later fronds, most of which arenbsp;those of true ferns while some Palaeozoic species are undoubtednbsp;Pteridosperms. The genus, which is purely provisional, includes

members of widely different families possessing pinnules of the same general type, such as is represented in some recent speciesnbsp;of Davallia, Asplenium, and other ferns.

The fronds of Sphenopteris may be bipinnate, tripinnate, or quadri-pinnate; the rachis may be dichotomously branched or the branching may be of the pinnate type characteristic of most recent ferns. The pinnulesnbsp;are small; they vary considerably in shape even in a single frond, but thenbsp;chief characteristics are: the lobed lamina, contracted and often wedge-shaped at the base (fig. 352), the dichotomously branched veins radiatingnbsp;from the base or given off from a median rib at an acute angle. Thenbsp;lamina may be divided into a few bluntly rounded lobes (fig. 352, C) ornbsp;deeply dissected into linear or cuneate segments (fig. 352, A, B, E).

Examples of Sphenopteroid leaves have already been described under the genera Goniopteris, Onychiopsis, Ruffordia,nbsp;etc. Among the numerous examples of Sphenopteris speciesnbsp;from the Carboniferous rocks mention may be made of Sphenopteris obtusiloha Brogn.^(fig. 352, A�C), which occurs in thenbsp;Middle and Lower Coal-Measures of Britain 2. This type isnbsp;characterised by the almost orbicular, oval or triangular pinnulesnbsp;which may reach a length of 15 mm.; they are occasionallynbsp;entire, but more usually divided into 3 to 5 rounded lobes. Thenbsp;forked veins radiate from the base of the pinnule. The rachisnbsp;may be dichotomously branched. Fructification unknown.

The species S.furcata Brongn.�, characteristic of the Middle and Lower Coal-Measures of Britain (fig. 352, E), is referrednbsp;to under Stur�s genus Diplotmema* in which it is includednbsp;by some authors solely because of the dichotomous habit ofnbsp;branching of the pinnae.

The pinna represented in fig. 353 illustrates a similar type of pinnule. This species, which is very common in the Calciferousnbsp;Sandstone of Scotland, was described by Bindley and Huttonnbsp;as Sphenopteris affinis^.

The fronds of Sphenopteris affinis were' discovered by Mr Peach� in a fertile condition, but he regarded the reproductivenbsp;organs as those of a plant parasitic on the Sphenopteris fronds.

�* Zeiller (88) A. p. 147, Pis. iv. v.; Kidston (86) p. 80. nbsp;nbsp;nbsp;�* See p. 535.

Kidston^ substituted Stur�s genus Calymmatotheca for Spheno-pteris on the ground that the sporangia figured by Peach under the name Staphylopteris Peachii hear a close resemblance

to the organs which Stur described as valves of an indusium in his species Calymmatotheca Stangeri^. An examination of Stur�s

specimens by Miss Benson^ and by Prof. Oliver and Dr Scott has confirmed Star�s interpretation of the appendages at thenbsp;tips of the fertile pinnae as valves of an indusial or cupularnbsp;structure. The superficially similar bodies on the fertile pinnaenbsp;of (S', affinis are however true sporangia, and cannot legitimatelynbsp;be-included in the genus Calymmatotheca as described by Stur.nbsp;For this reason Miss Benson institutes a new genus Telangium,nbsp;the type-species of which, T. Scotti from the Lower Coal-Measures of Lancashire, is based on petrified material. Thenbsp;Scotch species Sphenopteris affinis (= Calymmatotheca affiinis ofnbsp;Kidston) is also transferred to Telangium', the sporangia arenbsp;considered by Miss Benson to be microsporangia. This with othernbsp;species is no doubt correctly included in the Pteridosperms.nbsp;A complete frond of Sphenopteris aiffinis, showing a regularnbsp;dichotomy of the main axes, is represented by an admirablenbsp;drawing in Hugh Miller�s Testimony of the Rocks^.

Some of the Palaeozoic species of Sphenopteris probably represent the fronds of true ferns, but others are known to havenbsp;been borne by Pteridosperms. (S. Hoeninghausi (fig. 290, C,nbsp;p. 399) is the foliage of Lyginodendron, and Scott� speaks ofnbsp;three species, S. dissecta, S. elegans, and S. Linkii as the leavesnbsp;of Heterangium. Grand�Eury^ has recorded the occurrence innbsp;French Coal-Measures of seeds in association with othernbsp;Sphenopteroid fronds.

The discovery of sporangia on the fronds of several Palaeozoic species of Sphenopteris and Pecopteris has led to the institution of new generic names, which indicate an advance innbsp;knowledge beyond the stage implied by the use of thosenbsp;provisional designations based solely on the form and venationnbsp;of the pinnules. Other names have been created by authors innbsp;place of Sphenopteris and Pecopteris on the ground that anbsp;striking feature in the mode of branching of fronds is sufficientlynbsp;important to justify generic recognition even in the absence

of fertile specimens. As examples of designations based primarily on the branch-system of compound leaves, the genera Mariopteris, Diplotmema, and Palmatopteris may be brieflynbsp;considered (fig. 354 A�C). Dr Kidston^ is of opinion that thenbsp;creation of new genera for purely vegetative characters of frondsnbsp;is of no real advantage, and he prefers to retain the oldernbsp;provisional names for species known only in the sterile condition.nbsp;On the other hand, if we are sufficiently familiar with specimensnbsp;large enough to enable us to recognise a well-defined morphological character, it may serve a useful purpose to employ anbsp;generic designation for features which may have a phylogeneticnbsp;value. A comparative examination of Palaeozoic, Mesozoic, andnbsp;recent compound fronds, including both Pteridosperms and truenbsp;ferns, brings to light certain distinguishing features characteristic of the older types which, as Potoni� maintains^, point tonbsp;the derivation of the pinnate habit from a primitive dichotomous system of branching. For a more complete discussion ofnbsp;this question reference should be made to Potoni�s suggestivenbsp;papers. Among recent ferns Matonia and Bipteris, twonbsp;survivals from the past, afford instances of fronds with anbsp;branching system of the dichotomous type.

Similarly, in Gleichenia, Lygodium, and more rarely in species of Polypodiaceae (e.g. Davallia aculeata, fig. 232)nbsp;dichotomy is a striking feature of the fronds. In the greatnbsp;majority of recent ferns the fronds have assumed a pinnatenbsp;habit. Among Palaeozoic fern-like fronds dichotomous branching of the main rachis and of the pinnae is much more common.nbsp;Potoni� draws attention to several other features whichnbsp;distinguish Palaeozoic fronds from the majority of laternbsp;species: the frequent occurrence of pinnules borne directly onnbsp;the main rachis (fig. 354, D), and of modified pinnules ornbsp;Aphlebiae on the rachis and petiole, are characters to which henbsp;attributes an evolutionary significance. The main point is thatnbsp;a comparative examination of leaf-form affords evidence innbsp;favour of the view that the modern type of frond, with itsnbsp;naked rachis bearing two rows of pinnae, has been derived fromnbsp;a less specialised type in which the distinction between thenbsp;^ Kidston (Ol^) p. 191.nbsp;nbsp;nbsp;nbsp;^ Potoni� (95); (99),

parts of the leaf is much less evident. The primitive leaf was probably a dichotomously branched axis provided with anbsp;continuous lamina which eventually became broken up intonbsp;separate lobes or pinnules.

As the dichotomy of the frond became less regular, a pinnate habit was acquired, as is clearly seen in many Palaeozoic typesnbsp;which constitute connecting links between forked and pinnatenbsp;fronds (fig. 354, D). The Aphlebiae may be remnants of thenbsp;once-continuous lamina on the petiole, and the normal pinnulesnbsp;borne on the rachis may be regarded as the attributes of frondsnbsp;in which the division of physiological labour had not reachednbsp;the stage which characterises the leaves of recent ferns.

This name, which is due to Zeillerh is applied by him to Palaeozoic fronds characterised by a double bifurcation of thenbsp;rachis of the primary pinnae. Mariopteris muricata (= Peco-pteris muricata Schloth.) may be taken as the type of the genus.nbsp;This species is common in the Lower and Middle Coal-Measuresnbsp;of Britain and rare in the Upper Coal-Measures^. It isnbsp;described by Kidston� as one of the most polymorphic andnbsp;widely distributed Coal-Measure species. The pinnules asnbsp;seen in fig. 364, B, are of the Sphenopteroid type. No fertilenbsp;specimens are known, but it is significant that Grand�Eury^nbsp;has recorded the association of Mariopteris muricata and seeds.

The main rachis gives off alternate naked branches, each of which bifurcates at its apex into two short naked axes, and thesenbsp;are again forked, the ultimate branche.s having the form ofnbsp;bipinnate pinnae provided with large Sphenopteroid pinnulesnbsp;(fig. 354, B). Zeiller includes in Mariopteris some speciesnbsp;which Stur�* referred to his genus Diplotmema. Possibly somenbsp;of the Palaeozoic fronds with a zigzag rachis may have beennbsp;climbers like Lygodium.

Diplotmema.

This generic name is employed by Zeiller� and other authors in a more restricted sense than that in which it was originally

Z). Zeilleri. Pinnule. ( x 3. After Zeiller.) Neuropteris macrophylla. (British Museumnbsp;N. macrophylla. Pinnule. (Slightly enlarged.

N. Scheuchzeri. (Slightly reduced. After Kidston.) AUoiopteris Essinghii. (Enlarged. After Potoni�.)

used by Stur. The Upper Carboniferous species Sphenopteris furcata Brongn. (fig. 352, E) may serve as the type. This speciesnbsp;occurs in the Middle and Lower Coal-Measures of Britain^.

The main rachis gives off branches as in Mariopteris, but in Diplotmema each naked lateral branch is forked at its apexnbsp;into two opposite pinnae bearing deeply dissected Sphenopteroidnbsp;pinnules. Zeiller^ and Stur have recorded fertile specimensnbsp;of Diplotmema, but in no' case have actual sporangia been

Eig. 355. A. Cephalotheca mirabilis, Nath. Fertile pinnae. (Partially restored. After Nathorst.)

discovered. In the species Diplotmema Zeilleri Stur (fig. 354, C, C') two Aphlebiae occur at the base of each secondary axis�.nbsp;It has been pointed out by Potoni� that in Diplotmema furcatumnbsp;the equal dichotomy of the lateral branches is not characteristic

of the frond as a whole. In the case of branches higher on the rachis the dichotomy becomes unequal and the forked axis isnbsp;gradually replaced by a simple pinna (fig. 354, A). For this typenbsp;of frond, Potoni� proposed the generic name Palmatopteris innbsp;place of Diplotmema, which he discards. The long comparatively slender rachis of P. furcata suggests comparison withnbsp;the liane species of Lygodium'^.

Cephalotheca.

This genus was proposed by Nathorst^ for some peculiar bipinnate fertile fronds from the Upper Devonian rocks ofnbsp;Bear Island. The pinnae bear slender forked ultimate segments represented by a few detached fragments (fig. 355, B),nbsp;associated with the rachises. The fertile pinnae are given offnbsp;in opposite pairs from the main axis over which they arenbsp;concrescent (fig. 355, A). A mop-like cluster of sporangia isnbsp;borne on the lower surface and close to the base of a fertilenbsp;pinna: the exannulate sporangia are compared with those ofnbsp;Scolecopteris. Nathorst compares Cephalotheca with a Belgiannbsp;species of Upper Devonian age described by Cr�pin� asnbsp;Rhacophyton condrusorum and by Gilkinet^ as Sphenopterisnbsp;condrusorum. A similar fossil is also described by Baily � asnbsp;Filicites lineatus from the Kitorkan Grits of Ireland.

The position of Cephalotheca cannot be definitely determined from the available data, but it is more probable that it wasnbsp;a seed-bearing Pteridosperm and not a true fern. Zeiller�nbsp;has recently expressed the same opinion.

Thinnfeldia.

The genus Thinnfeldia, founded by Ettingshausen in 1852 on some Hungarian Liassic specimens, though frequently included in the Filicales, cannot be said to occupy that positionnbsp;by virtue of any well-authenticated filicinean features. It is by

no means improbable that many of the species referred to this genus are closely allied to Palaeozoic Pteridosperms.nbsp;Thimifeldia may be briefly defined as follows :

Fronds simple and pinnatifid, pinnate or bipinnate: rachis broad and occasionally dichotomously branched. Pinnules often fleshy or coriaceous ;nbsp;broadly linear, entire or lobed, provided with a midrib from which simplenbsp;or forked secondary veins are given off at an aciite angle : or the laminaenbsp;may be short and broad without a midrib and traversed by severalnbsp;slightly divergent and forked veins.

The genus is chiefly characteristic of Upper Triassic, Rhaetic, ami Jurassic floras, though it was in all probability representednbsp;in Permian floras. Several species, many of which are valueless, are recorded also from Cretaceous and Tertiary formations.nbsp;Search should be made for fertile specimens or for evidencenbsp;as to the association of seeds with Thinnfeldia fronds.

Some Permian fossils from Kansas which Sellards^ has made the type of a new genus, Glenopteris, appear to benbsp;indistinguishable generically from leaves of Lower Mesozoicnbsp;age universally recognised as typical examples of Thinnfeldia.

Thinnfeldia odontopteraides (Morris)^. Figs. 356�358.

This is a very variable species as regards the shape and size of the ultimate segments and their venation. It is a type ofnbsp;extended geographical range characteristic of Rhaetic or Uppernbsp;Triassic rocks in Australia, South Africa, India, South America,nbsp;and various European localities.

Frond bipinnate ; the broad rachis may be dichotomously branched. Pinnules with a thick lamina which may be almost semi-circular in form,nbsp;deltoid, broadly oval or broadly linear, and often confluent at the base.nbsp;Short and broad pinnv�es occur on some fronds directly attached to thenbsp;main rachis between the piimae. The longer and narrower pinnulesnbsp;(fig. 356, C), resembling those of the Palaeozoic genus Alethopteris, have anbsp;well-defined midrib, while the smaller segments are characterised bynbsp;several slightly divergent veins which spring directly from the rachisnbsp;(fig. 356, A). Epidermal cells polygonal or, above the veins, rectangularnbsp;in shape ; stomata, which are slightly sunk, occur on both the upper andnbsp;lower epidermis. Fertile specimens unknown.

The portion of a lohed pinnule shown in fig. 356, B, illustrates a form of segment intermediate between the linearnbsp;type with a midrib and a row of shorter pinnules withoutnbsp;a median vein. Fig. 356, D, represents another instance ofnbsp;variation in the arrangement of the veins in segments ofnbsp;different sizes. Various specific and generic names have beennbsp;assigned to Thinnfeldia fronds of Rhaetic age on the ground ofnbsp;the occurrence of pinnules longer and narrower than those

usually associated with T. odontopteroides] but in view of the range of variation met with in a single leaf it is advisable tonbsp;extend rather than to restrict the boundary of what we arenbsp;pleased to regard as a specific type.

The name Thinnfeldia lancifolia has been applied by Morris to fossils from Australia which may be identified withnbsp;T. odontopteroides, and the same designation is employed by

Szajnocha and by Solms-Laubach ^ for Rhaetic specimens from South America. Similar fronds are described by Geinitz� asnbsp;Thinnfeldia tenuinervis from Argentine Rhaetic strata.nbsp;0dontopte7'is macrophylla Curran, T. falcata Ten.-Woods, Glei-chenia lineata Ten.-Woods, and Cardiopteris Zuberi Szaj.nbsp;afford other examples of what are probably closely allied forms�.

Some exceptionally large examples of T. odontopteroides are figured by FeistmanteP from the Hawkesbury series of Newnbsp;South Wales in which the bipinnate frond has a breadth of

^ Geinitz (76) PI. i. � Seward (08) p. 95. nbsp;nbsp;nbsp;^ Feistmantel (90) A. PI. xxiv.

25�30 cm. A specimen from the Molteno beds of South Africa, probably of Rhaetic age, represented in fig. 357, illustrates anbsp;smaller leaf with pinnules of the linear type, some of whichnbsp;are partially divided into shorter pinnules with forked veins.nbsp;The example represented in fig. 358, from Cyphergat (S. Africa),nbsp;shows two equal branches of a rachis with small contiguousnbsp;segments.

Some specimens figured by Zeiller' from the Rhaetic strata of Tonkin as Pecopteris {Bernouillia ?) sp. may be portions ofnbsp;Thinnfeldia fronds, and the large leaves which he refers tonbsp;Ctenopteris Sarreni differ but slightly from the Australiannbsp;specimens described by Feistmantel as T. odontopteroides.

Under this name Ettingshausen described the type-specimen of the genus from Lower Lias strata at Steierdorf in Hungary.nbsp;He assigned the plant to the Coniferae on the ground of anbsp;resemblance of the pinnules to the phylloclades of Phyllocladus.nbsp;Thinnfeldia rhomboidalis bears a close resemblance to T. odon-topteroides, but the pinnules are usually longer and narrower,nbsp;as shown in the English specimen from the Lower Lias of

Thinnfeldia I'homboidalis, Ettings. Slightly reduced. From an English Liassic specimen in the British Museum. [M.S.]

Dorsetshire represented in fig. 359. The darker margin of the pinnules shown in fig. 360, C, gives the impression of a revolutenbsp;lamina, but a microscopical examination points to a thickernbsp;cuticle at the edge of the segments.

The species is recorded from Jurassic rocks of France, Germany, Italy, India, Australia, and elsewhere h

Jurassic, Cretaceous and some Tertiary species referred to Thinnfeldia, but many of these are probably not genericallynbsp;identical with T. odontopteroides or T. rhomhoidalis. Mr Berry �nbsp;in a paper on The American species referred to Thinnfeldianbsp;concludes that the genus is �a rather indefinite one...andnbsp;badly in need of revision.� He regards the Middle and Uppernbsp;Cretaceous American species as Conifers related to Phyllocladusnbsp;and probably forming a link between the Podocarpeae andnbsp;Taxeae: for these forms he proposes the generic name Proto-phyllocladus. The opinion has been expressed elsewhere^ thatnbsp;this � problematicaP � genus rests on an unsatisfactory basis;nbsp;the available data do not justify the use of a name whichnbsp;implies the existence in North American Cretaceous floras ofnbsp;a type related to the New Zealand and Tasmanian Conifernbsp;Phyllocladus. We are not in a position to assign a singlenbsp;species of Thinnfeldia to the Filicales or the Gymnosperms.

A leaflet from Jurassic rocks of Poland figured by Eaci-borski^ shows what this author regards as the impression of a circular sorus: no sporangia have been found. A specimennbsp;in the British Museum�, which is said to oome from Rhaeticnbsp;beds in Queensland, shows a row of contiguous polygonalnbsp;prominences on each side of the midrib which reseznble thenbsp;sori of a fern; but until sporangia are discovered we cannotnbsp;determine the precise nature of this apparently fertile frond.

A species described by Fontaine� from the Potomac beds (Wealden-Jurassic) of North America as Thinnfeldia. variabilisnbsp;affords a good example of a plant which cannot be identifiednbsp;with any degree of confidence either as a fern or a seed-bearingnbsp;type. Mr Berry draws attention to the former application ofnbsp;this name by Velenovsky to a distinct Lower Cretaceousnbsp;Bohemian species and proposes for Fontaine�s plant the namenbsp;T. Fontainei; he maintains that no one has doubted the fern-nature of the Potomac plant. T. variabilis may indeed be anbsp;fern, but the evidence is not such as to preclude legitimatenbsp;doubts as to the correctness of this suggestion. Solms-Laubach�,

^ Berry (03). nbsp;nbsp;nbsp;� Seward (04) p. 31.nbsp;nbsp;nbsp;nbsp;� Hollick and Jeffrey (09) p. 24.

in referring to Schenk�s view that Thinnfeldia and its allies may represent a group intermediate between Ferns and Gymno-sperms, admits that it is a possible supposition; he is, however,nbsp;inclined to consider Lomatopteris and Gycadopteris, � generanbsp;especially comparable with Thinnfeldia,� as more probablynbsp;ferns.

At this point we may conveniently consider a series of genera which occupy an equally uncertain position and bearnbsp;a very close resemblance to Thinnfeldia.

The generic name Lomatopteris was proposed by Schimper^ for some bipinnate fronds originally described by Kurr^ fromnbsp;Jurassic rocks of W�rtemberg as Odontopteris (?) jurensisnbsp;(fig. 360, A). I have elsewhere expressed the opinion� thatnbsp;this German species may be identical with Thinnfeldia rhom-hoidalis Ett. Kurr�s type-specimen, a portion of which isnbsp;reproduced in fig. 360, A, consists of a frond or large pinna

characterised by a prominent and broad rachis giving off alternate linear pinnae bearing bluntly rounded, contiguousnbsp;and basally concrescent pinnules having a thick or revolutenbsp;border and a central rib. The lateral veins are visible in thenbsp;ultimate segments of Kurr�s fossil. Saporta^ has describednbsp;several species, which he refers to Schimpers genus, fromnbsp;French Jurassic strata; it is, however, difficult to recognisenbsp;some of the examples represented in his illustrations as specifically distinct forms. This author notices the resemblance ofnbsp;Loniatopteris to Thinnfeldia, not only in habit but in thenbsp;structure of the epidermal cells^. In Loniatopteris and innbsp;Thinnfeldia the cells have straight and not sinuous wallsnbsp;and the slightly sunken stomata are surrounded by a ringnbsp;of epidermal cells. Salfeld� has recently described portionsnbsp;of fronds from Jurassic rocks of South-West Germany, whichnbsp;he identifies as Lomatopteris jurensis. He disagrees with mynbsp;view that Lomatopteris does not differ sufficiently from Thinnfeldia to be accorded generic autonomy, chiefly on the groundnbsp;that the folded-over edge of the pinnules is a distinguishingnbsp;feature of Loniatopteris. There is, however, no difference, innbsp;appearance at least, between the leaflets of some species ofnbsp;Thinnfeldia, amp;.g. T. rhomboidalis trom Liassic rocks of England hnbsp;and those referred to Lomatopteris. In a later paper, Salfeld'nbsp;describes some Portlandian fragments from North Germany asnbsp;Lomatopteris Schimperi, identifying them with a Wealden fossilnbsp;of somewhat doubtful affinity, which Schenk� makes the typenbsp;of his species. The Portlandian specimens are described asnbsp;tripinnate, with thick decurrent obtusely terminated pinnulesnbsp;with a revolute edge. The general form of the frond is verynbsp;similar to that of L. jurensis. Salfeld publishes a photographnbsp;of a large specimen which he describes as fertile and a drawingnbsp;of a piece of a pinna: the latter is reproduced in fig. 360, B.nbsp;He speaks of sori occurring in two rows, probably attached tonbsp;lateral veins, in the groove between the midrib and the revolutenbsp;edge of the lamina. The sporangia are described as �nicht

1 Saporta (73) A. nbsp;nbsp;nbsp;^ Sohenk (67) A.nbsp;nbsp;nbsp;nbsp;3 galfeld (07) p. 192.

naher bekannt^� An examination of the figures reveals nothing as to the nature of the � sori.� The specimens arenbsp;considered by Salfeld to afford decisive evidence against thenbsp;view that Lomatopteris and Thinnfeldia are generically identical.nbsp;Nothing has so far been published which constitutes a validnbsp;argument in favour of retaining Schimpers generic name.

Cycadopteris.

Zigno^ founded the genus Cycadopteris on Italian Jurassic impressions regarded by Schimper as indistinguishable fromnbsp;Lomatopteris. As Solins-Laubach� points out, the supposednbsp;sori of Cycadopteris described by Zigno are not convincing.nbsp;There appear to be no satisfactory reasons for separatingnbsp;Cycadopteris from Lomatopteris, nor do the fronds describednbsp;under these names exhibit any important differences fromnbsp;Thinnfeldia.

Ptilozamites.

Nathorst^ founded this genus on a remarkable series of specimens from the Rhaetic Coal-beds of Scania and assignednbsp;it to the Cycadophyta. The species Ptilozamites Heeri may benbsp;taken as a representative type. The leaves are linear andnbsp;.simply pinnate. In the example shown on a much reducednbsp;scale in fig. 361 the frond is 53 cm. long and 2T cm. broad.nbsp;The upper edge of each pinnule is straight or slightly concave;nbsp;the lower edge is rounded; the veins are slightly divergentnbsp;and dichotomously branched (fig. 356, E, p. 539). In some ofnbsp;Nathorst�s specimens the broad rachis is forked as in manynbsp;Thinnfeldias.

As a comparison of fig. 356, A and E, shows, the pinnules of some specimens of Thinnfeldia odontopteroides are identicalnbsp;with those of Ptilozamites. In the latter genus the rachis isnbsp;either unbranched or occasionally forked, while in Thinnfeldianbsp;the branching may be of the dichotomous or pinnate type. Innbsp;Ptilozamites the segments appear to be always without a mid-

name Ptilozamites should perhaps be retained for such long and narrow fronds as that shown in fig. 361: no speciesnbsp;included in Thinnfeldia is known in which the rachis reachednbsp;so great a length without branching. The habit of Ptilozamites Heeri predisposes one in favour of Nathorst�s opinionnbsp;that the fronds are Cycadean: we have no information in regardnbsp;to the nature of the reproductive organs.

This name was instituted by Saporta�, at Brongniart�s suggestion, for Liassic species characterised by pinnules likenbsp;those of Thinnfeldia, but distinguished by the bipinnate habitnbsp;of the frond. Saporta compares the genus with the Palaeozoicnbsp;leaves known as Odontopteris, and with Italian Jurassic plantsnbsp;referred by Zigno to his genus Dichopteris.

The name Ctenozamites is applied by Nathorst^ to the type of frond which Saporta, Zeiller, and other authors refer tonbsp;Ctenopteris. Nathorst instituted Ctenozamites for fossils agreeing in the form and venation of the pinnules with his genusnbsp;Ptilizamites but differing in being bipinnate and not pinnate.

Fronds of Ctenopteris are characteristic of the Jurassic and Khaetic series; they are known only in the sterile condition.nbsp;As Zeiller� says, Ctenopteris may be a member of the Cycado-filices, an extinct group founded on Palaeozoic plants combining Cycadean and Filicinean characters, and some of whichnbsp;are now known to be Pteridosperms. It is probable that thenbsp;genus is not a true fern: it is more likely to be a membernbsp;of the Cycadophyta or of some generalised extinct group.

1 Saporta (73) A. p. 352. nbsp;nbsp;nbsp;^ Nathorst (78) p. 122.nbsp;nbsp;nbsp;nbsp;^ Zeiller (03) p. 52.

Frond bipinnate, broad rachis giving off branches at an acute angle; pinnules broadly linear, slightly falcate, with several slightly divergentnbsp;forked veins.

A frond very similar to the Lower Lias specimen from Dorsetshire represented in fig. 362 was described by Leckenbynbsp;as Ctenis Leckenbyi (Bean MS.) from the Inferior Oolite ofnbsp;Yorkshire�. Leckenby recognised the possibility of a Cycadean

affinity, but regarded the bipinnate habit as an objection. The branched fronds of the Australian Cycad Bowenia supply annbsp;answer to this objection. Several good examples of Ctenopterisnbsp;cycadea are figured by Schenk ^ from Rhaetic rocks of Persia.nbsp;Zeiller�s Tonkin Rhaetic species, C. Sarrani^, affords a strikingnbsp;illustration of the difficulty of drawing a clear line of separationnbsp;between Ctenopteris and some species of Thinnfeldia.

Gtenopteris is in all probability very closely related to Thinnfeldia and Ptilozamites.

This genus was proposed by Zigno^ for some large specimens from the Jurassic plant-beds of Northern Italy.

The bipinnate leaves are characterised by the great breadth of the rachis which is dichotomously branched in the distal region (fig. 36.3); thenbsp;linear pinnae reach a considerable length. Pinnules relatively small,nbsp;oblong and slightly contracted at the base; the decurrent and confluentnbsp;lamina forms a narrow wing to the main axis. Veins slightly divergentnbsp;and forked, as in Ptilozamites.

A specimen of this species in the Padua Museum has a total length of 83 cm. It has been elsewhere suggested^ thatnbsp;a fragment figured by Zigno as a fertile example of this type isnbsp;probably part of a frond of the Osmundaceous fern Todites.nbsp;Since this opinion was expressed I have had an opportunitynbsp;of examining the actual specimen at Padua; the circularnbsp;patches described by Zigno as sori appear to be irregularities innbsp;the matrix and not an original feature.

Brongniart� instituted the genus Pachypteris for some imperfectly preserved English Jurassic fossils from Whitby,nbsp;which he described as P. lanceolata. Specimens have sincenbsp;been described �* from the Inferior Oolite rocks of the Yorkshirenbsp;coast. Brongniart described the pinnules as being withoutnbsp;veins or as possessing only a midrib. It is almost certain thatnbsp;the apparent absence of veins in most specimens� is due to thenbsp;fleshy nature of the segments and that the species P. lanceolatanbsp;should be transferred to Dichopteris.

Krasser� has described a species from Cretaceous rocks of the island of Lesina, off the Dalmatian coast, as Pachypteris dal-

matica which is very similar in habit to the English specimens and to Zigno�s Dichopteris visianica. One of Krasser�s specimensnbsp;is practically identical with Dichopteris lanceolata (Brongn.),

while in others the small pinnules are replaced in some of the pinnae by a continuous lamina with a few distal serrations.nbsp;The latter form a link between the Dichopteris and Thinnfeldianbsp;type of segment. Krasser gives a full r�sum� of opinionsnbsp;expressed by other authors in regard to the position of Pachy-pteris (= Dichopteris) and decides in favour of a Cycadeannbsp;alliance.

A French Jurassic plant which Saporta' made the type of a new genus Scleropteris, and described as S. Pomelii, appearsnbsp;to be indistinguishable from Dichopteris.

Dichopteris, though conveniently retained as a distinct genus, agrees so closely, in the broad and forked rachis and innbsp;the fleshy pinnules, with Thinnfeldia that it would seem reasonable to regard the two genera as members of the same group.

Several authors have drawn attention to the striking resemblance in form and venation between the fronds of thenbsp;Palaeozoic genus Odontopteris and those of Ctenopteris andnbsp;Thinnfeldia. In Odontopteris, as in Neiir-opteris, another Palaeozoic genus, the rachis occasionally bifurcates as in Thinnfeldianbsp;and Dichopteris, and the ultimate segments of some species ofnbsp;Odontopteris (fig. 366, A) are practically identical with those ofnbsp;Thinnfeldia and Ptilozamites.

Odontopteris is probably a Pteridosperm. There is no adequate reason for supposing that this group of plants whichnbsp;played a prominent part in the Permo-Carboniferous floras wasnbsp;no longer in existence during the Mesozoic era.

Brongniart^ instituted the genus Odontopteris for compound fronds from the Coal-Measures characterised by pinnules attached by the whole breadth of the base and traversed bynbsp;numerous forked veins. Odontopteris is very rare in Britishnbsp;Carboniferous rocks and �appears to be restricted to the Middlenbsp;and Upper Coal-Measures�.�

Fronds large, bipiiinate or tripinnate, the main raohis, which may be dichotomously branched, bears long linear pinnae with broadly linear ornbsp;deltoid pinnules, acute or blunt, attached by the whole of the base ; thenbsp;lower margin of the lamina, which is usually entire and rarely lobed (e.g.

Odontopteris osmundaeformis) \ is often decurrent on the axis of the pinna. The basal pinnule of each pinna is frequently attached by a contractednbsp;base, and the lamina may differ in form from that of the normal segments.

Pinnules often occur on the main rachis, and in some species the petiole bears modified pinnules which are larger than the ultimate segments ofnbsp;the pinnae and in some cases Cyclopteroid in shape. The pinnules arenbsp;traversed by numerous dichotomously branched veins; if a midrib isnbsp;present it dies out in the basal part of the lamina. In some speciesnbsp;(genus Mixoneura) pinnules of the Neuropteroid type, characterised by anbsp;well-defined midrib, occur in association with typical Odontopteroidnbsp;pinnules on the same pinna.

The species represented in fig. 364, C, D, from the Middle Coal-Measures of Barnsley, Yorkshire, illustrates the form andnbsp;venation of the Odontopteris type of pinnule. Another species,nbsp;0. Reichiana Gutb.h is also recorded by Kidston from the Lowernbsp;Coal-Measures of Lancashire. Some unusually good specimensnbsp;of the type-species of the genus Odontopteris minor, Brongn.,nbsp;have been figured by Zeiller^ from the Coal-Measures of Blanzynbsp;(fig. 365) which show the dichotomy of the main axis and thenbsp;occurrence of Aphlebiae on the petiole. The late Dr Weiss�nbsp;divided Odontopteris into two sections, Xenopteris and Mixoneura, the pinnules of the former having the form shown in.nbsp;fig. 364, D; while in species of the latter sub-genus some ofnbsp;the pinnules are identical in form and venation with those ofnbsp;Neuropteris except that they are attached by the whole breadthnbsp;of the base. Zeiller^ employs Mixoneura as a generic designation. In an American species 0. Wortheni Lesq.� the pinnulesnbsp;bear numerous hairs like those on some species of Neuropterisnbsp;(fig. 373, p. 570). The large size of the fronds of Odontopteris suggested to Weiss� that they were borne on the stems of tree-ferns,nbsp;but Grand�Eury�s� examination of specimens in the Coal-bedsnbsp;of central France led him to picture the plant as bearing a tuftnbsp;of leaves on a short subterranean stem. Renault and Zeiller�,nbsp;on the other hand, obtained evidence in the Commentry Coalfield of fronds borne on elongated stems which grew on thenbsp;ground and were supported by stronger plants. Stur� was thenbsp;first to suggest that Odontopteris should be excluded from the

� Zeiller (06) Pis. xix.-xxii.; (OO^) p. 100, fig. 73. nbsp;nbsp;nbsp;� Weiss, C. B. (70).

8 Renault and Zeiller (88) A. p. 219. nbsp;nbsp;nbsp;*nbsp;nbsp;nbsp;nbsp;Stur (84).

ferns. Grand�Eury�s* supposed fertile pinnules of Odontopteris do not afford any satisfactory evidence of the sporangial naturenbsp;of the small swellings which he figures at the ends of the veins.nbsp;This author pointed out several years ago that the petioles ofnbsp;some species of Odontopteris possess the anatomical features ofnbsp;Myeloxylon, a type of leaf-stalk which is now known to belongnbsp;to Pteridosperms. In a recent paper Grand�Eury^ records thenbsp;association of Odontopteris fronds with small seeds (Odontoptero-carpus), a discovery which leaves little or no doubt as to thenbsp;Pteridospermic nature of the genus. The fronds of Odontopterisnbsp;are very similar in habit to those of Neuropteris, anothernbsp;Pteridospermic genus.

The similarity between some Odontopteris and Thinnfeldia leaves, to which attention has already been called, is well illustrated by 0. genuina Grand�Eury^, a pinnule of which is represented in fig. 366, A. Odontopteris is a fairly widespread genusnbsp;in Upper Carboniferous and Lower Permian rocks, and is recordednbsp;also from Triassic strata: it is represented in the Coal-fields ofnbsp;North America and in several parts of Europe^.

In some fronds included in Odontopteris the pinnae are characterised by a broad irregularly lobed lamina which alsonbsp;forms a winged border to the rachis. Examples of this form arenbsp;afforded by Odontopteris Browni Sew.� from the Burghersdorpnbsp;Series (Triassic ?) of Cape Colony, and 0. Fischeri described bynbsp;Brongniart� from the Permian of Russia. The Russian speciesnbsp;would perhaps be more appropriately placed in the genusnbsp;Gallipteris, as Weiss�� suggests; the absence of venation innbsp;0. Browni renders generic identification unsatisfactory.

^ Weiss, C. E. (69); Goeppert (64) A.; Potoni� (93) A, (04); Lesquereux (80) A., p. 124; White (99) p. 125.

Brongniart^ instituted this genus for certain species of supposed ferns previously referred to the genera Pecopteris,nbsp;Alethopteris, and Neuropteris. Callipteris is a characteristicnbsp;Permian plant which is almost certainly a Pteridosperm.

those of Neuropteris. The drawings of fertile segments published by Weiss^ afford no indication of reproductive organs. Potoni�^ figures some pinnules of Gallipteris conferta in whichnbsp;the thick lamina is covered with sinuous grooves probablynbsp;made by some insect larvae; as he suggests, similar markingsnbsp;may have been mistaken for the remains of sori. The occurrence of Gallipteris fronds recorded by Weber and SterzeFnbsp;in association with Medullosa stems in the Lower Permiannbsp;of Saxony is in accordance with Grand�Eury�s conclusion.

Fronds reaching 1 metre in length, bipinnate or tripinnate, main rachis frequently exhibiting a combination of dichotomous and pinnatenbsp;branching. Pinnae linear, usually crowded, decurrent on the rachis ; thenbsp;pinnules on the lower side of the pinnae are continued on to the rachis.nbsp;Pinnules of the Pecopteroid type, entire or slightly lobed, or of thenbsp;Sphenopteroid type and more or less deeply dissected (fig. 360 (1, D), thenbsp;lamina of adjacent pinnules concrescent ; on the lower pinnae thenbsp;lamina may be continuous as in an Alethopteris pinnule. A midribnbsp;may extend almost to the bluntly rounded apex of the ultimate segments,nbsp;giving off oblique, simple, or forked veins, the lowest of which arise directlynbsp;from the rachis ; in the Sphenopteroid forms the lateral veins are givennbsp;off at a more acute angle.

A striking feature of the genus is the occurrence of pinnules on the main rachis, as in Odontopteris. Zeiller has wiselynbsp;extended the application of Gallipteris to fronds possessing thisnbsp;character irrespective of the entire or lobed form of thenbsp;ultimate segments. He found among the numerous examplesnbsp;of the genus obtained from Autun'* and Lodeve^ transitionalnbsp;fo-rms connecting such species as C. conferta (fig. 367) andnbsp;G. Pellati Zeill. (fig. 366, C) in which the Pecopteroid pinnulesnbsp;are slightly lobed, with G. lyratifolia (Goepp.) (fig. 366, D),nbsp;G. flabellifera'^ (Weiss), and G. Bergeroni Zeill. characterised bynbsp;deeply lobed Sphenopteroid segments.

' Weiss (69) Pis. vi. vii. nbsp;nbsp;nbsp;^ Potoni� (93) A. PI. i. figs. 1, 2.

2 Weber and Sterzel (96) p. 99. nbsp;nbsp;nbsp;^ Zeiller (90) p. 84.nbsp;nbsp;nbsp;nbsp;* Zeiller (98*).

This polymorphic species (fig. 367) is one of the most characteristic Permian plants. The oval-linear pinnules, attachednbsp;hy the whole base, occur on both pinnae and rachis; thisnbsp;feature, the thick texture of the lamina, and the linear, obliquelynbsp;set, pinnae render the fronds easily recognisable. The frondsnbsp;bore seeds.

In a recent account of some Permian plants from Germany, Schuster^ refers a portion of a frond to CaUipteris confertanbsp;(Sternberg) var. polymorpha Sterzel, which is characterised bynbsp;unusually large and polymorphic pinnules. In size and shapenbsp;the pinnules recall those of Neuropteridium validum Feist.

Callipteridium.

The name Callipteridium, created by Weiss^as a sub-genus of Odontopteris, is applied by Zeiller and other authors to anbsp;few Upper Carboniferous and Permian species characterised bynbsp;the occurrence of simply pinnate pinnae on the main rachisnbsp;between the bipinnate primary pinnae. Single pinnules arenbsp;borne directly on the rachis of the primary pinnae between thenbsp;pinnate branches. The form and venation of a typical pinnulenbsp;are shown in fig. 366, B. Callipteridium pteridium, originallynbsp;recorded by Schlotheim as Filicites pteridiusquot;^, has been fullynbsp;described by Renault and Zeiller from unusually large specimensnbsp;found in the Commentry Coal-field�. This species illustratesnbsp;the peculiar morphological features of the genus. The mainnbsp;rachis of the tripinnate fronds, several metres long, shows anbsp;combination of dichotomous and pinnate branching; from thenbsp;zigzag and forked axis are given off bipinnate pinnae and,nbsp;between these, shorter pinnate branches. The pinnules closelynbsp;resemble those of Callipteris conferta but reach a greater length;nbsp;the pinnules borne on the rachises of the lateral branches differnbsp;from the others in their broader base and more triangular lamina.

No fertile specimens have been found. It is probable that Callipteridium was not a true fern, and that White is correctnbsp;in including it among the Pteridosperms.

Archaeopteris.

In 1852 Forbes� published a brief description of some supposed fern fronds, found by the Geological Surveyors ofnbsp;Ireland in Upper Devonian rocks of Kilkenny, under the namenbsp;Cyclopteris hihernica. The Irish specimens were more fullynbsp;described by Baily� in 1858. Fronds of the same type werenbsp;referred by other authors to Cyclopteris, Adiantites or Noeg-gerathia, until Schimper^ proposed the generic name Palaeo-pteris on the ground that the fronds described by Forbes and

5 Forbes (53) p. 43. nbsp;nbsp;nbsp;� Baily (59) p. 75.nbsp;nbsp;nbsp;nbsp;^ Schimper (69) A. p. 473.

Baily are distinguished by the nature of their fertile pinnae from the sterile leaves included in Brongniart�s provisionalnbsp;genus Cycloptei�is. The earlier use of Palaeopteris by Geinitz

for an entirely different plant led Dawson* to institute the genus Archaeopiens. The genus Archaeopteris may be definednbsp;as follows:

Fronds bipinnate, reaching a considerable length (90 cm.); the stout rachis bears long linear pinnae; sterile pinnules obovate or cuneate withnbsp;an entire, lobed, fimbriate, or laciniate lamina traversed by divergentnbsp;diohotomously branched veins. The fertile pinnae usually occur on thenbsp;lower part of the rachis; pinnules with a much reduced lamina bearnbsp;numerous fusiform or oval exannulate sporangia (fig. 369, A, E, H), sessilenbsp;or shortly stalked, singly, or in groups of two or three. The base of thenbsp;petiole is characterised by a pair of partially adnate stipules (fig. 369, C, D),nbsp;and single pinnules or scales occur in some species on the rachis betweennbsp;the pinnae and on the petiole.

The specimen from Kilkenny represented in fig. 368 has a length of over 80 cm. The upper pinnae bear numerousnbsp;imbricate obovate pinnules (fig. 369, A, B) with an entire ornbsp;very slightly fimbriate margin, while on the shorter lowernbsp;pinnae the ultimate segments are reduced to a slender axisnbsp;bearing numerous fusiform sporangia, 2�3 mm. in length.nbsp;Kidston^ has pointed out that sporangia occasionally occurnbsp;on the edge of ordinary pinnules, and he first recognised thenbsp;stipular nature of the scale-like appendages which Baily noticednbsp;on the swollen petiole base (5 cm. broad) of the Irish speciesnbsp;(fig. 369, C). Eestorations of Archaeopteris hiherrdca have beennbsp;figured by Baily� and by Carruthers^, but the description ofnbsp;the fertile pinnae by the latter author requires modification innbsp;the light of Kidston�s description of the Dublin specimens.

Archaeopteris is recorded from Upper Devonian rocks of the South of Ireland, Belgium, Germany, Southern Russia, Bearnbsp;Island, and Ellesmere Land in the Arctic regions, Canada,nbsp;Pennsylvania, and elsewhere. Many of the specimens describednbsp;under different names bear a close resemblance, which in somenbsp;cases probably amounts to specific identity, to A. hibernica.

A. Jacksoni originally described by Dawson' and more recently by Smith and White^ from Devonian rocks of Maine, thenbsp;Canadian type A. gaspiensis Daws., and some species figurednbsp;by Lesquereux� from Pennsylvania, are examples of formsnbsp;which present a striking similarity in habit to the Irish species.nbsp;The Belgian Devonian fossils named by Cr�pin^ Palaeopterisnbsp;hibernica var. minor are regarded by him as probably identicalnbsp;with Goeppert�s species Cyclopteris Roemeriana from the neighbourhood of Aachen. Heer recorded Archaeopteris Roemeriananbsp;from Upper Devonian beds in Bear Island, and Nathorst�, whonbsp;has published a more complete account of the Arctic forms,nbsp;draws attention to the resemblance of some of them tonbsp;A. hibernica. A species described by Schmalhausen� fromnbsp;the Upper Devonian of Southern Russia as A. archetypusnbsp;(fig. 369, D) appears to differ from A. hibernica in the slightlynbsp;less reduced lamina of the fertile segments. This species hasnbsp;been more adequately illustrated by NathorsU from materialnbsp;collected in Ellesmere Land: he is unable to confirm Schmal-hausen�s statement that the pinnae are spirally disposed.

The species A. fimbriata (fig. 369, G) described by Nathorst from Bear Island is characterised by the more deeply dissectednbsp;lamina of the sterile pinnules. In A. fissilis Schmal. fromnbsp;Russia and Ellesmere Land the lamina (fig. 369, E, F) is cutnbsp;up into filiform segments: a fertile pinnule of this speciesnbsp;is represented in fig. 369, E.

Some sterile impressions figured by Krasser� from Palaeozoic strata (Lower Carboniferous or Upper Devonian ?) in thenbsp;province of Nanshan in China as Noeggerathia acuminijissanbsp;are considered by Zeiller� to be portions of an Archaeopterisnbsp;or Rhacopteris frond. The resemblance to the former genusnbsp;is however by no means close enough to warrant a referencenbsp;to Archaeopteris. The sterile specimens described by Stur'quot;nbsp;from the Culm of Altendorf as species of Archaeopteris are

� Nathorst (02). nbsp;nbsp;nbsp;� Schmalhausen (94).nbsp;nbsp;nbsp;nbsp;7 Nathorst (04).

probably not generically identical with the Irish and Arctic species. The dichotomous branching of the rachis in A. Tscher-maki and A. Dawsoni is a feature unknown in Archaeopteris.

Fio. 369. A. Archaeopteris hibernica. Fertile pinna. Dublin Geological Survey Museum. (Reduced. After Kidston.)

C. nbsp;nbsp;nbsp;A. hibernica. Base of petiole. (Dublin Museum. After Kidston.}

D. nbsp;nbsp;nbsp;A. archetypus. Base of petiole: Ellesmere Land. (Afternbsp;Nathorst. f nat. size.)

E. nbsp;nbsp;nbsp;A. fissilis. Sporangia. (Slightly enlarged. After Schmal-hausen.)

F. nbsp;nbsp;nbsp;A. fissilis. Sterile pinnule. Ellesmere Land. (Slightly enlarged. After Nathorst.)

H. nbsp;nbsp;nbsp;Archaeopteris sp. Ellesmere Land. (After Nathorst. f nat.nbsp;size.)

In the absence of fertile pinnae the separation of Archaeopteris from Rhacopteris is by no means easy.

Archaeopteris was regarded by Carruthers as a fem closely allied to recent species of Hymenophyllaceae, but this conclusion was based upon an inteipretation of the fertile segmentsnbsp;which Kidston^ has shown to be incorrect. The latter authornbsp;regarded the presence of stipules and the structure of thenbsp;exannulate sporangia as evidence of a Marattiaceous alliance.nbsp;In a later reference to Archaeopteris, Kidston expresses thenbsp;opinion that the genus is not a true fern but a member of thenbsp;Cycadofilices or Pteridosperms, a view shared by Grand�Eury=nbsp;and doubtless by many other palaeobotanists. The sporangianbsp;of Archaeopteris appear to be of the same type as those ofnbsp;Dactylotheca (fig. 290, E, p. 399). Schmalhausen gave expressionnbsp;to his disagreement with Nathorst and other authors whonbsp;referred Archaeopteris to the Marattiaceae by proposing thenbsp;distinctive group-name Archaeopterideae.

There can be little doubt that the reproductive organs of Archaeopteris so far discovered are microsporangia, and that thenbsp;plant bore seeds. The sporangia are larger than those of anynbsp;known fern and, as Kidston points out, they are similar to thosenbsp;of Crossotheca which he has shown to be microsporangia of thenbsp;Pteridosperm Lyginodendron. The presence of stipules innbsp;Archaeopteris hibernica, A. fimhriata, A. archetypus (fig. 369,nbsp;D) and probably throughout the genus does not materiallynbsp;affect the question of taxonomic position. Stipules are anbsp;characteristic feature of Marattiaceae and, in a reduced form,nbsp;of Osmundaceae, but similar appendages are borne at the basenbsp;of the petiole of the Cycad Ceratozamia. The occurrence ofnbsp;Aphlebiae on the rachis of Archaeopteris is a feature shared bynbsp;the fronds of Neuropteris and other Pteridosperms.

The fronds for which Brongniart� created this genus, though suspected by Stur in 1883 as wrongly classed amongnbsp;the ferns, have only recently been shown to be the leaves ofnbsp;Pteridosperms. As yet only one case is recorded in which

Neuropteris pinnae occur in organic connexion with seeds but it is almost certain that the genus as a whole must be

placed in this generalised group. Renault^ pointed out that the petioles of Neuropteris fronds from Autun had the anatomicalnbsp;features of Myeloxyloii (petiole of Medullosa). Since Kidston�snbsp;important discovery of seed-bearing pinnae of iY. heterophylla,nbsp;Grand�Eury^ has recorded the association of Neuropteris frondsnbsp;with seeds in French Coal-fields. By some of the older authorsnbsp;Neuropteris was compared with Osinunda because of a similaritynbsp;in venation. In the frequent dichotomy of the frond andnbsp;in the occurrence of pinnules on the rachis, Neuropteris closelynbsp;resembles Odontopteris^: there can be little doubt as to thenbsp;close relationship of the Pteridosperms possessing these twonbsp;types of foliage. Neuropteris may be defined as follows:

Fronds reaching a considerable size, probably in some cases a length of 10metres^; bi- or tri-pinnate; the rachis may be diohotomously branchednbsp;(figs. 354, D; 370) ; both rachis and petiole bear single pinnules, those onnbsp;the latter frequently differ from the normal leaflets in their larger Cyolo-pteroid laminae (fig. 370). Pinnules entire, rarely slightly lobed, broadlynbsp;linear, attached by a small portion of the base, which is usually morenbsp;or loss cordate. In A. Orangeri Brongn. the pinnules are attached bynbsp;a short pedicel�. The midrib always dies out before reaching the blunt ornbsp;pointed apex of the lamina and gives off at an acute angle numerousnbsp;secondary veins characterised by their arched course and repeated forking.

Potoni� describes the secondary veins of the pinnules of Neuropteris pseudogigantea^ as occasionally anastomosing, anbsp;feature which may be regarded as a step towards the reticulatenbsp;venation of the closely allied genus Linopteris.

Renault^ described some petrified pinnules of Neuropteris in which the mesophyll shows a differentiation into uppernbsp;palisade tissue and lacunar tissue below; the lower epidermis isnbsp;infolded at intervals where grooves (probably stomatal) occurnbsp;like those on the leaves of an Oleander {Nerium oleander).

The rachises of Neuropteris fronds are described by Grand�-Eury under the generic name Aidacopteris^.

' Renault (76). nbsp;nbsp;nbsp;Grand�Eury (08).nbsp;nbsp;nbsp;nbsp;3 White (99) p. 128.

� Potoni� (99) p. 113. nbsp;nbsp;nbsp;^ Renault (82) A. Vol. in. ; Zeiller (90) p. 139.

This species is characteristic of the Lower Coal-Measures of Britain; it occurs also in the Middle Coal-Measures and isnbsp;a common type in Upper Carboniferous rocks in various partsnbsp;of the world. The fronds are large and tripinnate, the rachis

Fig. 371. Neuropteris heterophylla. From a specimen in the Manchester Museum. J iiat. size. M.S.

is often dichotornously branched and Cyclopteroid pinnules may occur on the petiole. The pinnules, 5�20 mm. in lengthnbsp;and 3�8 mm. broad, have a rounded apex (fig. 354, E, p. 535).nbsp;As shown in fig. 371 which represents a primary pinna, the

1 Brongniart (22) A. PI. ii. fig. 6. For synonymy, see Kidston (03) p. 773; Zeiller (88) A. p. 261.

small pinnules on the lower branches are gradually replaced in the upper portion of the specimen by falcate segments.

The rachis of the large fronds of this species illustrates the dichotomous habit of many Neuropteris fronds, also the occurrence on the petiole of large Cyclopteroid pinnules (cf fig. 370).nbsp;The small piece of a pinna reproduced in fig. 372 shows thenbsp;slender attachment of the segments, the blunt apex, and the

Neuropteroid venation. Single pinnules of this species may be distinguished from those of N. Scheuchzeri by the blunter apex,nbsp;the absence of the pair of small Cyclopteroid pinnules on thenbsp;same branch and by the absence of hairs. N. macrophylla isnbsp;characteristic of the Upper Coal-Measures of Britain.

Fragments of this well-known Coal-Measure species were figured by Scheuchzer in his Herbarium Dihivianum^ as Lithos-munda minor, and by Lhywd (Luidius^) as Phyllites mineralis asnbsp;early as 1760. Neuropter�is Scheuchzeri, so named by Hoffmannnbsp;in 1826, is a type which many authors have described undernbsp;different names. Lesquereux� figured it as H. hirsuta from thenbsp;Coal-fields of Pennsylvania, and under the same name it isnbsp;recorded by Fontaine and White�� from Permian rocks ofnbsp;Virginia. The oval patches on the surface of a pinnule described by these authors as sori are certainly not of that nature.nbsp;The same species is described by Bunbury� from Nova Scotia

as N. cor data Brongn. var. angustifolia. For a full synonymy of the species reference should be made to lists published bynbsp;Kidston�, White��, and Zeiller�.

The large tripinnate fronds are characterised by the long linear- or oval-lanceolate pinnules (fig. 373)� with a pointednbsp;apex and numerous bristle-like hairs on the lamina; two muchnbsp;smaller Cyclopteroid segments occur at the base of the pinnaenbsp;which are terminated by the linear leaflets (fig. 354, F, p. 535).

Neuropteris Scheuchzeri is characteristic of the Upper and Middle Coal-Measures of Britain and is recorded from several

� Lesquereux (79) A. PI. viii. nbsp;nbsp;nbsp;�* Fontaine and White (80) p. 47.

�� White (99) p. 132. nbsp;nbsp;nbsp;* Zeiller (88) A. p. 251.nbsp;nbsp;nbsp;nbsp;� See Vol. i. p. 45.

localities in North America and the Continent. Zalessky' has recently recorded the species from the Coal-Measures of Donetz.nbsp;The frequent occurrence of detached pinnules points to anbsp;caducous habit. Even single leaflets can, however, he identifiednbsp;by their large size, the pointed apex, and hairy lamina. Thenbsp;hairs are preserved as fine oblique lines simulating veins;nbsp;they were so described by Roemer^ who took them for crossconnexions between the secondary veins and referred the pinnules to Gutbier�s genus Dictyopteris.

Another example of Neuropteris with hairy pinnules is described from the Commentry Coal-field by Renault andnbsp;Zeiller as N. horrida^. The oval-linear, bluntly rounded,nbsp;pinnules are characterised by a median band of hairs on eachnbsp;surface and a narrower strip at the edge of the lamina.

This generic name was created by Brongniart in 1828^ for specimens which he believed to be complete single leaves ofnbsp;orbicular or reniform shape similar to those of Trichomanesnbsp;reniforme. The lamina is traversed by numerous dichotomouslynbsp;branched veins which spread from the centre of the base.

It was suspected by Bindley and Hutton � that certain Cyclopteris leaves belonged to the frond of a species ofnbsp;Neuropteris, and some years later Lesquereux� concluded thatnbsp;Brongniart�s genus was founded on orbicular leaflets of Neuropteris. In 1869 RoehH figured a specimen of Neuropterisnbsp;bearing Cyclopteroid pinnules on its rachis. It is now universally admitted that Cyclopteris is not a distinct genus and thatnbsp;the specimens so named were borne as modified pinnules on thenbsp;main rachis of Neuropteris and Odontopteris. It is, however,nbsp;convenient to retain the name for detached leaflets whichnbsp;cannot be referred to the fronds on which they were borne. Anbsp;specimen found by Mr Hemingway in the Upper Coal-Measures

of Yorkshire and described in 1888^ affords a striking example of the large size attained by what was probably a frond ofnbsp;Neuropteris. The piece of main rachis reached a length ofnbsp;over 120 cm. and bore five pairs of Cyclopteris pinnules, somenbsp;of which were 7 cm. long and 5 cm. broad. The completenbsp;frond must have reached a length of at least 4 metres. Fig. 370nbsp;shows some typical Cyclopteroid leaflets on the petiole of anbsp;Neuropteris frond.

The Upper Palaeozoic fronds included in this genus are more familiar as species of Dictyopteris. Potoni�^ has, however,nbsp;pointed out that the creation of this name by Lamouroux innbsp;1809 for a genus of Brown Algae which is still retained, makesnbsp;it advisable to fall back upon the designation Linopteris.nbsp;Uutbier � proposed the genus Dictyopteris in 1835: Linopterisnbsp;was first used by PresU in 1838. The fronds so named arenbsp;identical with species of Neuropteris except in the anastomosisnbsp;of the secondary veins; Linopteris bears to Neuropteris thenbsp;same relation as Lonchopteris bears to Alethopteris. As innbsp;Neuropteris, Cyclopteroid pinnules occur on the petioles of Linopteris, but the veins form a fine reticulum. Grand�Eury� recordsnbsp;the association of Linopteris Brongniarti with seeds belongingnbsp;to the genus Hexagon ocarpon, a fact which points to thenbsp;Pteridosperm nature of the foliage.

Some fertile pinnules of Linopteris Schutzei (Roemer) are described by Zeiller� from Autun as bearing on the undernbsp;surface of the lamina two rows of long and pointed sporangia,nbsp;probably united in groups. The presumption is that these arenbsp;microsporangia.

Fig. 374 is a reproduction of a careful drawing, originally published by Zeiller', of a pinnule of the type-specimen ofnbsp;Gutbier�s species Linopteris neuropteroides. This species differsnbsp;from Linopteris obliqua, instituted by Bunbury� for specinrens

obtained by Lyell ^ from the Coal-Measures of Nova Scotia, in the smaller size of the meshes. Linopteris obliqua occurs in thenbsp;Upper and Middle Coal-Measures of Britain; it is recorded bynbsp;Zeiller from Asia Minor, by Lesquereux^ from Pennsylvania,nbsp;and by other authors from several European localities. 'Thenbsp;pinnules frequently occur detached from the frond and likenbsp;those of some species of Neuropteris were caducous. Linopterisnbsp;is rare in British strata.

Alethopteris.

The name Alethopteris, instituted by Sternberg^, is applied to compound fronds often reaching a considerable size, exhibitingnbsp;the following features:

The linear pinnules are attached by the whole breadth of the base, with the lower edge of the lamina decurrent and usually continuous withnbsp;that of the next pinnule (figs. 290, A, p. 399; 375). The ultimate segmentsnbsp;are entire, with an acute or rounded apex and often characterised by a fairlynbsp;thick lamina convex on the upper surface. From a prominent midrib,nbsp;continued to the apex of the pinnule, numerous simple and forked secondarynbsp;veins are given off at a wide angle, the decurrent portion of the laminanbsp;being supplied by veins direct from the axis of the pinna. In the uppernbsp;part of a frond or primary pinna the pinnules may be replaced by anbsp;continuous, lobed, or entire simple lamina. The main rachis occasionallynbsp;exhibits dichotomous branching, but the fronds are for the most partnbsp;constructed on the pinnate plan. Single Cyclopteroid pinnules'* occur onnbsp;the petiole of some species of the genus.

In certain species of Alethopteris the pinnules appear to have been deciduous as in DidymocUaena among recent fernshnbsp;A piece of cuticle from the upper surface of a pinnule ofnbsp;Alethopteris Orandini (Brongn.) figured by Zeiller^ shows verynbsp;deafly the polygonal form and straight walls of most of thenbsp;epidermal cells, those above the veins being almost rectangular.nbsp;The position of the sunken stomata is revealed by small circularnbsp;spaces surrounded by a circle of cells.

The absence of fertile specimens of this common genus of Upper Carboniferous plants led Stur� to exclude it from thenbsp;ferns. Although no seeds have so far been found in organicnbsp;connexion with an Alethopteris frond, it is certain that somenbsp;species, probably all, represent the foliage of Pteridosperms.nbsp;Renault was the first to describe petrified specimens of Alethopteris fronds exhibiting the anatomical structure of Myeloxylonnbsp;(leaf-axis of Aledullosa). The calcareous nodules from Englishnbsp;Coal-seams contain numerous fragments of the Myeloxylonnbsp;type of rachis bearing Alethopteroid pinnules.

The constant association of the fronds of Alethopteris lonchitica and Trigonocarpon seeds noticed by Mr Hemingwaynbsp;in the Coal-Measures of Yorkshire led him to regard the speciesnbsp;as seed-bearing; it has since been recognised as the foliage ofnbsp;the Pteridosperm Medtdlosa anglica^.

Grand�Eury'^ has recorded the association in French Coalfields of species of Alethopteris with Trigonocarpon and Pachy-testa seeds.

This species, described by Schlotheim in 1820 as Filicites lonchiticus and previously figured by Scheuchzer'^, is abundantnbsp;in the Middle and Lower Coal-Measures of Britain�. It isnbsp;characterised by large tripinnate fronds, probably quadripinnatenbsp;in the lower part, bearing primary pinnae of a more or less

triangular form divided into pinnate branches replaced in the apical region by linear segments. The pinnules, 8�30 mm. longnbsp;and 3�5 broad, are linear- or oval-lanceolate with an obtusenbsp;apex; the upper margin of the lamina is slightly contracted atnbsp;the base, while the lower edge is decurrent.

This species, figured by Parkinson in 1811, closely resembles A. lonchitica, but is distinguished by the more crowded and relatively longer pinnules which are joined to one anothernbsp;by a narrow connecting lamina (Fig. 375). The secondarynbsp;veins in A. Serlii are rather finer and more numerous. Grand�-Eury- records the association of the seed Pachytesta with frondsnbsp;of this species in the Coal-Measures of St Etienne.

^ For synonymy, see Kidston (94) p. 596; (03) p. 806; White (99) p. 117. 2 Grand�Eury (04).

A. Serin is very abundant in the Upper Coal-Measures bub rare in the Middle Coal-Measures of Britain^.

Lonchopteris.

This name was proposed by Brongniart^ for sterile fronds from Upper Carboniferous rocks which are practically identicalnbsp;with species of Alethopteris, but differ in the reticulate venationnbsp;of the pinnules. It has been pointed out in a previous chapter*nbsp;that Lonchopteris is usually used for Palaeozoic species, thenbsp;Wealden leaves, which were placed in this genus by Brongniart,nbsp;being transferred to Weichselia.

There can be little doubt as to the close relationship of Lonchopteris with Alethopteris: both may be referred to thenbsp;Pteridosperms. Lonchopteris rugosa Brongn.quot;* (fig. 290, B, p. 399)nbsp;and L. Bricei Brongn., both British species, are fairly commonnbsp;in Upper Carboniferous strata. In L. rugosa, a Middle Coal-Measures species, the anastomosing secondary veins form polygonal meshes (fig. 290, B, p. 399) smaller than those of L. Bricei.

Pecopteris.

Reference has already been made to this genus in the chapter on Marratiales, so far as regards certain species ofnbsp;fertile fronds the sporangia of which res�mble those of recentnbsp;Marattiaceae. It is, however, by no means safe to assume thatnbsp;such Pecopteris fronds were borne on stems having the anatomical characters of ferns. The sporangia in some at least of thenbsp;species may have contained microspores. In one Upper Carboniferous species usually referred to Pecopteris, P. Pluckeneti,nbsp;Schlot., Grand�Eury� has recorded the occurrence of seeds onnbsp;the pinnules of the ordinary fronds. This species will benbsp;referred to in Volume iii. The substitution of such genericnbsp;names as Ptychocarpus, Asterotheca, Hawlea, Dactylothecanbsp;and others for the purely provisional designation Pecopterisnbsp;indicates a step towards a conclusion as to natural affinity.nbsp;The probability is that Pecopteris, as applied to Palaeozoic

3 Page 494. nbsp;nbsp;nbsp;-gt; Kidston (94) p. 596.nbsp;nbsp;nbsp;nbsp;Grand�Eury (05).

species, in many cases stands for the compound fronds of true ferns, but the possibility of the inclusion of those ofnbsp;Pteridosperms in the same category is by no means excluded.nbsp;The designation Pecopteris may conveniently be retained fornbsp;sterile bipinnate, tripinnate, or quadripinnate fronds bearingnbsp;pinnules having the following characteristics:

Lamina short, attached to the rachis by the whole of the base and at a �wide angle, w'ith the edges parallel or slightly converging towards thenbsp;usually blunt apex; adjacent pinnules may be continuous basally by anbsp;narrow lamina. A well-marked midrib extends to the apex and gives offnbsp;simple or forked lateral veins almost at right angles (fig. 352, D, p. 529).

Hydathodes like those on the leaflets of Polypodium vulgare, and other recent ferns * are occasionally seen at the ends of the lateral veins ofnbsp;Pecopteris pinnules.

In addition to the examples of Palaeozoic fronds with the Pecopteris form of pinnule referred to in chapter xxii., thenbsp;species Pecopteris arborescens naay be briefly described.

The species named by Schlotheim Filicites arborescens in 1804 is characteristic of the Upper Coal-Measures and isnbsp;recorded also from Permian strata*.

P'ronds large; the rachis, which may reach a breadth of 3 cm.^, gives off long ovoid-lanceolate pinnae in two alternate rows (fig. 376); pinnulesnbsp;small, 1-5�4 mm. long and 1�2 mm. broad, contiguous, with roundednbsp;apex, attached approximately at right angles ; the upper surface of thenbsp;lamina is slightly convex and may be hairy�. The fertile pinnules,nbsp;identical in shape with the sterile, bear groups of ovoid examiulatenbsp;sporangia (synangia). The midrib extends to the apex of the pinnule andnbsp;gives off simple veins at a wide angle (fig. 352, D).

Our knowledge of the reproductive organs is very meagre. Grand�Eury described the synangia as consisting of 3�5nbsp;sporangia borne on a central receptacle; sporangia have beennbsp;described also by Stur�, Renault and Zeiller^ and Potoni�*,

1 Potoni� (922). (93) p_ 54^ nbsp;nbsp;nbsp;2 For synonymy, see Kidston (88) p. 366.

but no fertile British specimens are recorded. Stur places this species in the genus Scolecopteris, and Potoni� regards thenbsp;sporangia found by him on Permian fronds, which may benbsp;identical with Pecopteris arborescens, as conforming to those ofnbsp;the Asterotheca type. It is impossible to decide on the evidence available whether this species is a Pteridosperm or anbsp;fern, but there is a natural inclination in doubtful cases to givenbsp;preference to the first of these two choices.

The numerous fronds from Carboniferous and Permian rocks described as species of Pecopteris exhibit a considerable rangenbsp;of variation in the form of the pinnules. In many species thenbsp;pinnules are of the type represented in fig. 352, D; in othersnbsp;the lamina of the ultimate segments is slightly contracted atnbsp;the base and the secondary veins are given off at a more acute

angle, as in Pecopteris polymorpha, Brongn.^ In Pecopteris unita, Brongn., already described as Ptychocarpus unitP, thenbsp;pinnules are joined together except in the apical region. Somenbsp;fronds included in Pecopteris possess pinnules in which Peco-pteroid and Sphenopteroid features are combined; P. Sterzeli,nbsp;Zeill.� and P. Pluckeneti, Schlot. are examples of fronds innbsp;which the pinnules are lobed as in Sphenopteris, but the basenbsp;of the lamina is only slightly contracted and the venation is notnbsp;that of typical Sphenopteris species.

The species to which Potonie has applied the generic name Alloiopteris* also illustrates the impossibility of drawing anbsp;sharp line between Pecopteris and Sphenopteris. The frondsnbsp;already described in chapter xxv. under the designationnbsp;Corynepteris bear pinnules with a contracted base; in somenbsp;species the lamina is lobed, but in others (fig. 354, G) it isnbsp;entire with a midrib nearer one edge than the other. Thenbsp;species which Potoni� assigns to Alloiopteris, like many othernbsp;Sphenopteroid and Pecopteroid fronds, are characterised bynbsp;the occurrence of an abnormal pinnule (aphlebia) at the basenbsp;of each pinna (fig. 354, G, p. 535). Young fronds of Pecopterisnbsp;are occasionally met with showing very clearly the circinatenbsp;vernation of the pinnae as in the leaves of Cycas and Angiopterisnbsp;represented in fig. 220, p. 283. The genus Spiropteris was creatednbsp;by Schirnper� for coiled unexpanded fronds of fossil ferns ; it isnbsp;however superfluous to apply a distinctive term to specimensnbsp;of this kind.

The designation Pecopteris is employed chiefly for leaves of Palaeozoic age which are unknown in the fertile state, or donbsp;not afford sufficient evidence as to the nature of the sporangianbsp;to justify the substitution of a special generic name. Manynbsp;Mesozoic species have also been referred to Pecopteris, but mostnbsp;of these are more appropriately included in Brongniart�s laternbsp;genus Cladophlebis. The pinnules of Cladophlebis, as Brongniartnbsp;pointed out, are intermediate between Pecopteris and Neuro-pteris; they are usually attached by the whole breadth of

the base, as in Pecopteris, but the more acute origin, more arched form, and more frequent dichotomy of the lateral veinsnbsp;are features shared by Neuropteris. As a rule, Mesozoic sterilenbsp;fronds with straight or folded, entire or dentate pinnules arenbsp;of the Cladophlebis type: this genus is especially characteristicnbsp;of Rhaetic and Jurassic floras. Examples of Cladophlebisnbsp;pinnules are shown in figs. 256, 257 (pp. 340, 342). It is tonbsp;be regretted that authors do not make more use of the genericnbsp;name Cladophlebis in describing sterile fronds, instead ofnbsp;following the misleading and unscientific practice of employingnbsp;such genera as Pteris, Asplenites, and others on wholly insufficient grounds.

LIST OF WOEKS EEFERRED TO IN THE TEXT.

Amalitzky, W. (01) Sur la d�couverte, dans les dep�ts permiens sup�rieurs du Nord de la Russie, d'une Acre GHossopterieime amp;c.nbsp;Com.'pt. Rend. vol. cxxxii. p. 591.

Arber, E. A. N. (02) On the distribution of the Glossopteris Flora. Geol. Mag. vol. ix. p. 346.

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Zenker, J. C. (37) Scolecopteris elegans, Zenk. ein neues fossile.s Farrngewachs mit Fruotifloationen. Linnaea, vol. xi. p. 509.

INDEX

The Index includes the names of Authors and plants mentioned in this volume. No references are, however, given to the following Authors, whosenbsp;names occur too frequently to render special reference of use to the reader:nbsp;A. Brongniart, E. Kidston, A. G. Nathorst, H.Potoni�, B. Eenault,D. H. Scott,nbsp;H. Graf zu Solms-Laubaoh, D. Stur, W. C. Williamson, E. Zeiller.

INDEX
S. stipata 367	T. gigantea 489
Spieker, T. 69	T. immersa 492
Spiropteris 579	T. jejunata 485, 488
Spirorbis 102-104	T. Jourdyi 489
Sprengel, A. 412	T. lata 489
Stangeria paradoxa 307	T. Lescuriana 487
Staphylopteris Peachii 531	T. major 494
Stauropteris 433, 434, 465-469	T. marantacea 407, 408
S. burntislandica 468, 469	T. mareyiaca 491
S. oldhamia 444, 450, 465-468	T. missourieiisis 485
Stejf'ensia silesiaca 404	T. multinervis 486-488
Steinhauer, H. 126-128, 228, 229	T. Newberriana 488
Stenzel, C. G. 375, 417, 418, 435,	T. spatulata 489, 490
450-453	T. superba 489
Sternberg, C. von 105, 110, 124, 126,	T. tenuinervis 489, 492
198, 413, 573	T. virgnlata 492
Sterzel, J. T. 366, 402, 412, 413	T. vittata 485, 489, 492-494
Stiehler, A. W. 494	Tafialla graveolens 40, 75
Stigniaria 66, 141, 153, 158, 226-247,	Tansley, A. G. 16, 280, 310, -
256, 261, 265	Telangium 532
S. andbathra 231	T. Scotti 532
S. ficoides 158, 159, 174, 226-232,	Teratophyllum aculeatum 301
236-239, 246, 247, 256, 261	Thamnocladm 27
S. ficoides minuta 255	Thammpteris 326,329-331,33'
S. fiexuosa 239	T. Schlechtendalii 329, 330,
S. inaeqtmlis 174	Thaumatopteris 385
S. radiculosa 157-160	T. Brauniana 385
S. rimosa 226	T. Miiensteri 386
S. stellata 247	T. Schenki 385
Stigmariopsis 205, 208, 233-239	Theobroma 209
S. anglica 235	Thinnfeldia 537-552, 556
Stokes and Webb, 494	T. falcata 540
Slopes, Marie C. 436	T. Fontainei 543
Strasburger, E. 398	T. lancifolia 539
Stroviatoptens 291	T. odontopteroides 538, 541-i
Stur, D. See note, p. 609	T. rhomhoidalis 542-545
Sturiella 324	T. tenuinervis 540
Sub-Sigillariae 203	T. variabilis 482, 543
Sudworth, G. B. 134	Thoday, D. 6
Sykes, M. Gladys 23, 47	Thomas, A. P. W. 32, 13, 1
Syringodendron 198, 204, 205, 221,	Thomas, Ethel N. 239
226, 233, 238	Thompson, D�Arcy W. 131,
S. esnostense 204	Thyrsopteris 295, 296, 369
S. striatum 198	T. elegans 289, 294, 308, 36i
Szajnooha, L. 540	T. elongata 378 T. Murrayana 367
Taeniopteris 485-494, 508, 509	T. rarinervis 369
T. Beyrichii 494	T. schistorum 366
T. Garnoti 485, 488, 490	Tmesipteris 4, 12-25
T. Carrutliersi 491	T. tannensis 17
T. coriacea 488, 490	Todea 267, 337, 341, 468
T. Daintreei 490, 491	T. australis 346

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ERRATA IN VOL. I

CHAPTER XIP.

phyllum.

BT

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13

16

CHAPTER XIII.

H,

xni]

20

chyma succeeded externally by an ill-defined phloem (fig. 119, A, p) surround the xylem and a fairly distinct endodermis (fig. 119,nbsp;A and B, e) encloses the whole. To Mr Boodle^ is due the

[.

-px

px

23

the �sporophylls� of JVnesipteris are branches and not eaves,

25

CHAPTEE XIV.

A. HOMO SPORE AE.

B. HETEROSPOREAE.

.a

quot;'� nbsp;nbsp;nbsp;ta a. o�M,. H.,w� �4 Bo,.�nbsp;nbsp;nbsp;nbsp;M.B.,

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tropical

40

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41

than occurs in the majority of vascular plants in which stem and root have more pronounced structural peculiarities. Anbsp;pericycle, 2�6 cells in breadth, encloses the xylem and phloem

ws.

bands and this is succeeded by an endodermis, 2�3 cells broad, With vaguely defined limits. In L. clavatum, as in L. alpinum,nbsp;another British species, the broad cortex is differentiated into

xiv]

45

and the line of dehiscence is determined in some species at least by the occurrence of smaller cells in the wall. In

Si

transverse sections of cones in which the sporangia are strongly saddle-shaped, the sporophylls may appear to bear two sporangia.