TER VERKRIJGING VAN DE GRAAD VAN
DOCTOR IN DE WIS- EN NATUURKUNDE AAN
DE RIJKSUNIVERSITEIT TE UTRECHT, OP GE-
ZAG VAN DEN RECTOR-MAGNIFICUS Dr. H.
BOLKESTEIN, HOOGLEERAAR IN DE FACUL-
TEIT DER LETTEREN EN WIJSBEGEERTE, VOL-
GENS BESLUIT VAN DE SENAAT DER UNIVER-
SITEIT TE VERDEDIGEN TEGEN DE BEDEN-
KINGEN VAN DE FACULTEIT DER WIS- EN
NATUURKUNDE OP MAANDAG 4 FEBR. 1935
DES NAMIDDAGS TE 4 UUR

Gaarne maak ik gebruik van de gelegenheid om enige woorden
van dank te richten tot hen die mijn studiejaren tot een zo aan-
gename tijd gemaakt hebben.

Hooggeleerde Went, niemand heeft een grotere invloed op het
verloop van mijn studie gehad dan gij. Vaak hebt gij mij raad
gegeven en mij geholpen door Uw critiek. Ik zal er steeds dankbaar
voor blijven dat ik onder Uw leiding heb gestudeerd.

Hooggeleerde Koningsberger, Hooggeachte Promotor, al betreur
ik het enerzijds dat het mij niet gegeven was bij Prof. Went te
promoveren, anderzijds beschouw ik het als een groot voorrecht in
de gelegenheid te zijn geweest mijn proefschrift onder Uw leiding
te voltooien. Reeds spoedig na Uw aankomst steldet gij U van
de stand van het werk op de hoogte en sindsdien ondervond ik
voortdurend Uw steun en belangstelling.

Hooggeleerde Jordan, het is voor een botanicus van grote
waarde enigen tijd op Uw Laboratorium als assistent dienst te
doen en in die functie Uw Physiologisch Practicum mee te maken.
Deze tijd werd de voorbereiding tot het bewerken van mijn disser-
tatie.

Hooggeleerde Westerdijk, ieder die Uw colleges en practica volgt
is U dankbaar voor de wijze waarop gij de Phytopathologie tot
een overzichtelijk geheel maakt. Voor mij werd de Baarnse periode
van zeer bijzondere betekenis.

Hooggeleerde Fülle, Zeergeleerde Lanjouw, de excursies welke
gij voor de biologen organiseert behoren tot de grootste aantrek-
kelijkheden van onze studie. Zij zijn in staat zelfs de meest ver-
stokte physioloog enthousiasme voor Uw vak bij te brengen.

Zeergeleerde Du Buy, veel moeite hebt gij U gegeven mij bij
mijn werk behulpzaam te zijn. Ook wist gij mij de ogen te openen
voor tal van algemene vraagstukken.

Zeergeleerde Donk, de weken gedurende welke wij samenwerkten
over de prikkelgeleiding bij Mimosa'pudica behoren tot de aange-
naamste welke ik in Utrecht heb doorgebracht. Het is mij dan
ook een grote vreugde U de resultaten van mijn — met een minder
primitief toestel voortgezette — onderzoek te kunnen toezenden.

Zeergeleerde Pont, Zeergeleerde Bonner, ik dank U voor Uw
grote bereidwilligheid bij de taaicorrectie van dit proefschrift.

Tenslotte ben ik zeer veel dank verschuldigd aan het gehele per-
soneel van Laboratorium en Hortus. In het bijzonder dank ik U,
waarde A. de Bouter, voor de wijze waarop gij de figuren hebt
in orde gebracht.

Of all sensitive plants, Mimosa pudica has always been considered
as the most interesting, owing not only to its complicated and rapid
reaction but also to the fact that the excitation is not confined to
the stimulated part itself but can be conducted through the whole
plant.

The mechanism of movement and excitatory conduction was
beyond the methods of the earlier investigators. What they did
do was to study the means by which the plant could be stimulated,
the contraction of the pulvini serving as an indicator only.

explain the spreading of the excitation. The further experimental
research advanced, however, the less these theories fitted the facts.
In 1900 not one was considered to be really plausible.

Until this time conduction of excitation was thought to be so
rare in plants that no one supposed the sensitive plants to be
equipped with more than one conducting system. In the last decade
it has been shown that there are several modes of conduction, but
the correlation between them remained completely unknown. The
object therefore of the present investigation has been an inquiry
into the relations between the several methods of conduction of
stimulus.

quot;Sir Paul Neile mentioned, that the King had, within
four days past, desired to have a reason assigned, why
the sensitive plants stir and contract themselves upon
being touched; it was resolved that Dr. Wilkins, Dr.
Clarke, Mr. Boyle, Mr. Coelyn and Dr. Goddard, be
curators for examining the fact relating to those plantsquot;.

One of these fellows of the Royal Society, Dr. CLarke, made a
visit to a garden in St. James Park, where such a plant (Mimosa
spec.) was growing under glass. A full account of these investiga-
tions can be found in Micrografia by R. Hooke (1667).
- Clarke (1661) stimulated the plant by various chemical agents
and by cutting the leaf. In the latter case he saw that a green
droplet was exuded, which offered, he thought, an explanation for
the conduction of the stimulus. If a system of vessels throughout
the plant contained the afore-mentioned green fluid, cutting a
vessel should lessen the pressure in all parts, whereupon the
pulvini, if sensitive to a change of pressure, would react. The
transport of a stimulating substance by circulating sap was also
suggested. Both assumptions involved the existence of vessels, which
Clarke could not see, but which, he hoped, would be observed
afterwards. Hooke discovered the vessels in other plants and he
connected this observation with Clarke's theory.

In 1736 du Fay and du Hamel published some observations
on Mimosa. They found that moderate heating, as well as cooling
with ice, stimulated the pinnae. The excitation was conducted
through the pinnae, but did not pass on to other parts of the leaf.
A hundred years later, the stimulating effect of cooling was found
a second time by Fee, and more recently Bose was the third to,
discover it.

Sometime in the i8th century Desfontaines observed the be-
haviour of a plant which he had taken with him in a carriage. At
first the plant reacted, but during the drive it recovered. He pulled
up for a quarter of an hour and then drove on. The plant then
reacted again; so he concluded that the excitability had become
normal again during the rest.

Pfeffer (1873) made some exact investigations of the matter by
stimulating the lower side of the pulvinus mechanically at regular
intervals. If these were less than 2 min. the plant recovered, but
the excitability did not return. According ro Linsbauer (1923)
Pfeffer's statement holds only for certain frequencies of stimulation.

By leading off action currents, Umrath (1927) found that if
the lower side of the pulvinus is stimulated mechanically every
10 sec., some cells react to every other stimulus, but only by giving
an electrical response. He supposed that the refractory period of
the motor cells is lengthened by the frequent stimulation.

Dutrochet (1824) tried to solve the problem of the movement
by dissecting parts of the pulvinus. In this way he found that the
movement is inhibited if the lower side of the pulvinus is lacking.
This was confirmed in 1827 by Burnett and Mayo, who published
Lindsay's experiments, at that time only available as a M.S. (1790)
in the Library of the Royal Society.

By turning a plant upside down before and after reaction, and
measuring each time the divergence of the petiole and the stem,
Brücke (1848) proved conclusively that the fall of the leaf is
effected by the loss of rigidity of a group of cells at the underside
of the pulvinus. The liquid which is thrust out by the cells into the
intercellulair canals expels the air from them and thus brings about
tie change of colour which can be seen in the contracting pulvini,
especially in those of the leaflets (i86j). By exact measurements
Pfeffer (1873) ascertained that the volume of the lower side of
the pulvinus is much reduced after a reaction whereas that of the
upper side remains nearly constant.

Dutrochet also studied the conduction of the stimulus. When
the stem was stimulated with a flame, the excitation passed regions
where a ring of bark had been removed. When a wood cylinder
was removed and the bark left intact as much as possible, the
stimulus did not pass the zone. The conduction thus was confined
to the wood. In 1916 Linsbauer confirmed these results. Dutrochet
thought it improbable that a living tissue effeaed the propagation
of the stimulus since the rapidity seemed independent of the tem-
perature.

I have probably overlooked a number of researches, since in
1839 Meyen mentioned the existence of an extensive literature,
and remarked that a new experiment could hardly be devised. Yet
he discovered an unknown and very fast method of conduction in
the stem, which could be observed when the plant was stimulated
by cutting into the stem just to the wood. The high speed of
conduction reminded him of that of a nervous system, for example
that of the lower animals. Some, who saw him perform his expe-
riment, pitied the plants and asked if it did not hurt them.

The modern investigations cn the conduction of the stimulus
in Mimosa began with Pfeffer (1874). Three different ways of
conduction could be suggested at that time.

First, a stimulating substance might be transported in some way
or other through the plant, but so many adverse arguments could
be raised, that no special experiments were needed to disprove it.
For example by simply bending two leaflets on the basal or apical
end of a pinna, all other leaflets of this pinna can be made to
react in succession, the excitation moving 'basipetally as fast as
acropetally, which, if no wound is made, no substance should do.

Secondly a nervelike system, though not observed anatomically,
might exist. If a nerve is narcotised by ether or chloroform it does
not conduct an excitation. Pfeffer, therefore, narcotised the middle
part of a pinna and stimulated it by cutting a leaflet above the
treated part. The leaflets in the ether did not react, but those below
them did, showing that the conduction was not suppressed.

A third possibility remained, the one which Clarke had already
mentioned and which Hofmeister and Sachs had described very
extensively in their manuals, namely a change of pressure which
could spread along the petiole and stimulate the pulvini.

Pfeffer's theory is different from Clarke's, but the guiding idea
is still the explanation of the appearance of a droplet after cutting
the plant, and much reasoning was required to account for conduc-
tion folloiwing stimulation without wounding, for example elec-
trically (Ritter, 1809 (see Stern, 1924) and Kabsch, 1861).

By microscopic examination Haberlandt (1890) found a net-
work of wide tube-shaped cells in the bark, which he named
quot;Schlauchzellenquot;. They contained a substance which was stained
red by ferric chloride. This was the same reaction which Fee (18 jo)
had found for the much discussed droplet. Thus it was clear that
at least part of the liquid must result from the bark, as already
observed by Meyen (1839). The latter had not attached any im-
portance to the appearance of the droplet, as the plant could still

be stimulated after the removal of the bark. Haberlandt, however,
supposed that Meyen left some of the bark intact.

As the opposing leaflets, which react in pairs, are connected by
the quot;Schlauchzellenquot;, but not by the xylem, Haberlandt concluded
that the excitation was conducted by a change of pressure in the
former turgescent cells and he endeavoured to explain all stimuli
in terms of pressure changes. The stimulating effect of a flame
should thus be due to steamproduction.

Haberlandt's theory has much been critisized, first by Cunning-
ham (1895): H. himself showed that the stimulus could pass a zone
of the petiole which was killed by a jet of steam. Cunningham
pointed out, however, that the quot;Schlauchzellenquot; could not have
remained turgescent after such a treatment.

According to Borzi (1899) quot;Schlauchzellenquot; are lacking in
Neptunia, though a stimulus can be conducted in it.

By mounting a cut plant on a pressure pump and diminishing
or increasing the pressure very suddenly, Mac Dougal (1896) did
not get any reaction. He therefore attacked Haberlandt's as well
as Sachs' theory.

This experiment was repeated by Fitting (1904), who m many
ways attempted to check H.'s idea. A detached leaf recovers if
placed in water. Cutting off a bit of its petiole should not change
the pressure in the vessels nor in the quot;Schlauchzellenquot;, if these are
connected to one another as is supposed by H.'s theory. Never-
theless the leaflets reacted to it.

To ascertain whether the stimulus is conducted by the action
of living cells. Fitting cooled a part of the petiole to 1 C. No
delay in the conduction of the stimulus was found. Thus it was
not effected by living cells. Fitting stimulated the leaf by cutting
the petiole, below the cooled part, and the rapidity of the conduc-
tion was measured by noting the time needed for reaction of the
leaflets.

This is in contradiction with a finding of Bose (1914)- He
stimulated the petiole electrically, and found the reaction of the
main pulvinus to be retarded when the excitation had to pass a
cooled zone, through which it travelled downwards in this experi-
ment. At 2° C. there was no conduction at all.

Fitting supported Pfeffer's view though he was all but satisfied
by it, whereas Bose convinced himself by many experiments that
the stimulus is conducted in a plant in the same way as in a nerve.
As in a nerve tne excitation starts from the cathode and is stopped
by an electrotonic block. Above all it is important that he found

that an action current accompanies the excitation and travels at
the same rate as the latter along rhe petiole as well as along the
stem. Also in other plants, for example ferns, a thermal stimulus
was observed to cause a propagated change of potential.

In 1916 it was observed by Ricca that if the lower part of the
stem is stimulated 'by applying a flame, the stimulus can be con-
ducted to the younger leaves not only through a killed zone, but
also through a short water-filled tube, inserted between the upper
and the lower part of the shoot. This could not be due to a
change of pressure since a small manometer, attached to the tube,
did not show any change. He concluded that a substance must
have passed the tube to stimulate the pulvini above it. This sub-
stance must have been carried along with the transpiration stream.
He made an extract out of slices of the stem, and found that when
the basal end of the petiole of a detached leaf was dipped in this
extract, the leaflets closed after some time. In this way the exis-
tence of such a stimulating substance was proved conclusively.
Ricca used Mimosa Spegazzinii for his investigations.

Fitting (1930) tried to determine the nature of this stimulating
substance by testing various chemicals as to the stimulating effect.
It appears that the extract can be much diluted without losing its
stimu ating effect, but the more diluted it is, the more time it
takes to make the leaflets react.

Since the excitation can also pass downwards, Ricca supposed
that, by the stimulation, some of the substance is set free and is
sucked in through the vessels by the negative pressure in the wood.
He thought it very improbable that yet another method of conduc-
tion could exist, so he endeavoured to prove that Haberlandt's
theory was erroneous. He stimulated the leaflets by dipping them
in water at 70° C., to disprove H.'s supposition that the stimu-
lating effect of heating depends upon the formation of steam. The
bark of the stem was found to be insensitive. He then divided
the basal part of a cut shoot into separate strips of xylem and
phloem. Only when the xylem was stimulated were the leaves
affected. Yet these facts did not prove conclusively that there is
but one way of conduction.

To test Ricca's theory Seidel (1923) made some observations
on the rate of ascent of a Lithium salt as compared with the rate
of conduction of the excitation in a cut shoot. In this way he
found that the transpiration stream is too slow to account for the
conduction. But Snow, in the following year, made nearly the
same experiment with methylene blue, which he proved to ascend

as fast as the excitation. Thus it was indicated that it is possible
that the conduction of the excitation in the stem is effected by a
substance.

Beside the normal conduction Snow found a much faster form,
the quot;high-speed conductionquot;. When he incised a shoot as far as
the cambium, the next leaf fell almost instantaneously, but its
leaflets did not close. This was discussed by Dixon (1924), who
considered it to be a special form of the normal conduction. Be-
cause of the smallness of the wound, too little of the substance
should be sucked up to stimulate the leaflets, and the extraordina-
rily fast transport of the substance should be due to the high nega-
tive pressure in the vessels. Since Ball (1927) has demonstrated the
quot;high-speed conductionquot; in submerged shoots, where a high nega-
tive pressure is not to be expected, Dixon's view is probably not
correct.

A third form of conduction has been discovered by Ball in
submerged shoots. He called it quot;rapid conductionquot;. When such
a shoot was stimulated electrically, a reaction of all main pulvini
could be observed, the excitation travelling at a rate of 2—3 cm.
per sec. in either direction. It could not pass a killed zone, but was
not stopped when a ring of bark was removed. The pith and the
wood, however, had to be intact. The longer the shoot had been
submerged, the faster the rapid conduction travelled. It resembled
the high-speed conduction in the fact that the leaflets never closed,
but the latter affected only the next leaf or at best the next two
leaves, and was never observed except after cutting the shoot. The
rapid conduction, however, followed both upon an electric shock
and burning.

One can easily observe a form of conduction in the pinnae,
which resembles the rapid conduction in the stem. After applying
a stimulus (electric shock or cutting a leaflet), the excitation
travels along the pinna basipetally as fast as it does acropetally.
This is the reason why Pfeffer rejected the idea of conduction by
a stimulating substance, and more recently Snow (19^4 and 1925)
has attacked Ricca's theory on this ground.

Furthermore Snow compared the rate of this conduction with
that of the transpiration stream. When a leaftip was submerged
in a solution of methylene blue and then cut off, the excitation
travelled many times faster than the stain. (This was denied later
by Ricca, 1926). Snow also killed a zone of the petiole by steam
and after that no conduction to the main pulvinus was found.
sno\i- always used cut shoots, while Fitting and Haberlandt per-

formed the same experiment but obtained the opposite results on
intact plants. The difference between the plants of these investi-
gators consists in the fact that the plants of Snow had no negative
pressure in the vessels. From these experiments one can conclude
that under special conditions (a killed zone of the petiole) the
particular form of conduction which is normal in the stem, can
also be observed in the leaf.

In a few cases where the continuous xylem cylinder was broken
but some part of the phloem left intact, rapid conduction of the
excitation through the petiole to the main pulvinus took place.
Herbert (1922) had already observed this, and considered it to be
in support of Haberlandt's theory on the conduction by quot;Schlauch-
zellenquot;. These experiments do not agree at all with Ricca's view
that the excitation can be conducted in the xylem only.

All modern investigators have observed that the rapidity of the
conduction of the excitation in the leaf depends upon the intensity
of stimulation. When a terminal leaflet is cut through, conduction
is much slower than when the end of a pinna is cut. Linsbauer
(1908) made a careful investigation of this matter. He measured
the rapidity of the conduction in the petiole after its stimulation
(i) by touching with a heated platinum wire (rate of conduction
5—9 mm. per sec.), (2) by incision (30 mm. per sec.), (3) 'by cut-
ting it through (130 mm. per sec.). These experiments were con-
tinued bij Umrath (i92ja and b), who found several rates in each
part of the plant and completed the work by measuring the elec-
trical changes accompanying and following the excitation (1928
and 1929). When the petiole was stimulated by the discharge of
a condensîor, the slowest system of conduction reacted and only
a simple action current was found. When it was stimulated in
any other way, however, the result was generally much more com-
plicated.

Bose has shown that, when an electrode is stuck into the petiole
or into the stem of Mimosa, its potential with respect to the earth
is changed when the excitation, being conducted through the plant,
affects the tissue into which the electrode has been stuck. In the
recording of these potential changes, one has a method for tracing
the excitation in the stem and in the petiole. The fact that the
nature of these electric phenomena is unknown should be no reason

not to use them. The cause of the contraction of the cells in the
lower side of the pulvinus is not known either.

The electric response is much more variable than the visible
reaction of the plant, as'was shown by the work of Umrath. Owing
to this fact it is possible to disentangle eventual several ways of
conduction, which all result in the only visible and always identical
reaction namely the movement of the pulvinus. Most of the con-
clusions, which will be derived from the present investigation,
might have been arrived at, even if no potential changes had been
recorded. It would, however, have been a much more difficult task.

Of course the electric phenomena are also interesting on their
own account. Yet it is questionable whether Mimosa is suited to,
the study of action currents in general. Some factors, by which
the potential changes are complicated, will be discussed in chap-
ter III.

The apparatus, which was used to amplify and to record the
potential changes is shown schematically in Fig. A. The electrode
in the earth is connected to the filament of valve I, that on the
petiole, to the grid. Since this is insulated as much as possible, only
a very weak current can flow through the plant. The valve func-
tions best, when the grid is slightly negative to the filament. To
effect this the potential of the filament can be varied by me^.ns of
a potentiometer.

The electrode on the plant was of the silver-silverchloride type.
It could have been stuck in the petiole or in the stem, but I pre-
ferred to make no wound and hung a hook-shaped electrode on
the plant, connection being made by a drop of water. The silver
wire was connected to the grid of valve I by a thin and flexible
copper wire. Thus the petiole was free to quot;move.

When the electrode on the plant becomes o,i V. negative, the
plate current of valve I decreases, the potential difference in the
resistance falls, the grid of valve II becomes more positive and its
plate current increases by ± 3 mAmp. The deflection of the mirror
of the recording galvanometer is registered in the usual way. For
reproduction the photographic records have been copied bij means
of transparent paper. In order to refrain from tedious descriptions
a scheme of each experiment is given next to the record and the
reactions are indicated by uniform symbols. The curves have to
be read from right to left.

The reaction of the ceils under the electrode, which accompanies the con-
duction of excitation, can be observed by the electric response. It will be
shown, however, that not all changes of potential are caused by a reaction
of these cells.

To record the potential changes of two points of the plant at
the same time, two identical instruments were used. Part of the
experiments have been made with an apparatus, slightly different
from that which is represented by Fig. A. The valves I A and I B
were connected to one accumulator, II A and II B likewise, and
only one anode battery was used. The drawback was, that the
potential of the battery lessened as soon as the plate current of one
of the valves II increased, which caused the plate current of the
other valve II to decrease a little. The result was that in the expe-
riments which were made with this apparatus, the rise of one curve
is- accompanied by a slight fall of the other (indicated in the figures
by an asterisk).

The recording galvanometers (Fig. B) were made for the purpose
and have the adventage of being very simple and costing practi-
cally nothing. The zero-point is subject to shifting by magnetic
hysteresis, but there is no objection against using them for qualita-
tive work such as the present. Their period is only about o.oi sec.
This proved to be of great importance.

The action current in the petiole is about o.i Volt. No quite
exact measurements can be made with the apparatus; moreover,
they would be of no importance to the present investigation.

By means of the experiments which will be discussed, it can 'be
shown that there are at least two distinct ways of conduction, both
in the stem and in the leaf. When the plant is stimulated without
wounding, the excitation can only be propagated by one method,
i.e. by the action of living cells. This action is accompanied by a
change 6Î the electric potential at the conducting place. There can
be no objection against calling this change of potential an action
potential. The name action current has also been applied. The
former name is the more correct, but the latter one has always
been used in the literature on this subject.

When a plant is stimulated by wounding, the excitation spreads
in two ways. The first is the action of living cells mentioned above,
the second is due to a stimulating substance, set free at the wound

and transported in the vessels. In these vessels a negative pressure
is commonly found, which accounts for the fact that the substance
can be sucked in quot;basipetally, whereas the transpiration stream
transports it acropetally. It does not merely stimulate the pulvini,
but also those cells w:hich, by their action, can propagate the exci-
tation. When the substance moves on faster than the excitation
can be conducted by the action of the cells, these are stimulated
by the substance, instead of by their neighbours' activity.

It is clear that the rapidity of the propagation of the action by
a living tissue depends on the condition of the cells. Since it was
found that the rate of the propagation of this action does not
depend on the intensity of the stimulus, one might call it an all-
or-none reaction. On the other hand, when a wound is made, the
stimulating substance will be sucked in the faster the larger the
wound, subject to the negative pressure in the vessels.

Now the phenomenon of conduction in Mimosa pudica becomes
still more complicated by the fact, that even in the most healthy
plant the propagation 'by cell action generally is stopped at several
places. When a wound is made in a leaflet and circumstances are
such that the action is travelling faster than the substance, the
main pulvinus of the stimulated leaf will be affected by the action,
thus reacting before it has been reached by the substance. After
that, however, the action is stopped. In fact I have never observed
it to pass through the main pulvinus into the stem. Hence the other
main pulvini will not be affected, unless the stimulating substance
can get into the stem and stimulate it. The excitation, being once
begun in the stem, may be propagated by the action of cells to
the pulvini. Frequently, however, it is not propagated and they do
not stir before they are stimulated by the substance itself.

It is convenient to study the conduction of the excitation by
the action of cells, in a plant, which is stimulated by applying a
drop of ice water, briefly called quot;icequot; hereafter. It is not thus
wounded and it recovers in a short time. Electric stimulation has
a similar effect, but it causes much more difficulty, especially if
action currents are to be led off.

Action in the Stem. The simplest action currents are found in
the stem. One can stimulate it above or below the electrode. In
both these cases the action currents are identical and due to the
5ame cause, i.e. the reaction of the tissue on which the electrode
is fastened (Fig. i and 2).

In these experiments I endeavoured to let the excitation pass from
the petiole into the stem. The petiole was stimulated with ice, where-
upon the main pulvinus reacted, but no change whatever was found
at the electrode on the stem.

The exact moment of stimulation was not recorded, nor that of
the reaction of the pulvinus. This is indicated in the figures by a
dotted arrow.

Leading off action currents from two electrodes A and B at
the same time is a convenient way of determining the rate of
conduction (Fig. 3).

petiole base
petiole

A see L
)S n

In some experiments the action in the stem was found to be
conducted to the main pulvini. This has also been observed by Ball,
who called it quot;rapid conductionquot;. I tried to let the action be con-
ducted along a decorticated zone, in which I was not successful,
unlike Ball, who found quot;rapid conductionquot; in the stem if only
the pith and the xylem were left intact. Possibly his plants were
in a better condition than mine.

Action in the Petiole. Not only the stem of Mimosa can respond
to a stimulus in this way, but the hypocotyl and the petiole can
do the same. In the latter the action current has a more compli-
cated form. At the moment of the reaction of the main pulvinus,
the curve suddenly falls. When there are two electrodes on the
petiole, both curves naturally fall at the same time (Fig. 4). This

The main pulvinus can be stimulated by touching its lower side.
Then the action of its reacting cells can be propagated to those in
the petiole. In a young leaf this is commonly found. In some expe-
riments, however, the action was confined to the pulvinus (Fig. j).
The curves show the effect of the reaction of the pulvinus on the
potential of the petiole. After that, the lower part of the petiole
was stimulated with ice. The next change of potential was a weak
action current, seemingly a positive variation.

A Positive Action Current? This positive variation was due to.
the reaction of the base of the petiole (not to be confused with the
reaction of the pulvinus). If an electrode had been fastened on this,
part of the petiole, its potential would have dropped with respect
to that of an electrode on the undisturbed part. No electrode was
fastened on the base of the petiole, but the adjoining pulvinus acted
as an elongation of the electrode in the earth, being connected to
it by the stem. Thus, the potential of the earth-electrode being
constant, that of the electrodes on the petiole and on the pinna
must have risen, causing the curves to fall.

Action Currents in General. When a tissue reacts to a stimulus,,
its electric potential with respect to the neighbouring tissue is chan-
ged, and that with respect to the electrode on it is also changed.
The difference between these potential changes causes the action
current. When the middle of a petiole is stimulated, its potential
drops as much with respect to that of its basal, as to that of its
apical part; an electrode on the upper end of the petiole does not
show the intervening disturbance. The fact that the reaction of the
base of the petiole causes the potential of such an electrode to rise
proves that this reacting tissue changes its potential less to that
of the pulvinus than to that of the rest of the petiole. Perhaps
this is correlated with the fact, that when a cell of Nitella is
stimulated and potential changes are led off, these changes depend
on the concentration of KCl in the drop by whidh the electrodes
are connected to the cell. (Harris and Osterhout, 1929).

In the experiment of Fig. 5 one of the electrodes was fastened
on a pinna and the other on the upper part of the petiole. The basal
end of the petiole was stimulated with ice. The passing of the
action at electrode B was accompanied by a normal action current
(curve B). This case it is absolutely certain that the action did not
pass into the pinna. The action was occasionally found to pass into
the secondary pulvini. Passage of the action into the pinnae is
only passable in a young leaf and is very rare. Curve A shows an.

action current even in this case, however. This must be due to the
reaction~df the upper end of the petiole. It seems that this changes
its potential with respect to the rest of the petiole more than to
the secondary pulvinus. In some other experiments this action cur-
rent of curve A was much weaker.

In the experiment of Fig. 6, in contrast to that of Fig. j, the
action of the pulvinus was propagated to the petiole. The action
current of its base is seen to start about one second after the reac-
tion of the pulvinus.

The experiment of Fig. 7 is much like that of Fig. 5. In the
former the petiole was stimulated close to its upper end. The action
travelled downwards to electrode B (normal action current in
curve B), and upwards to the upper end of the petiole (small action
current in curve A). Long before these action currents have finished,
both curves show a rapid fall, due to the action of the base of
the petiole.

In the experiment of Fig. 8. approximately the same may be
seen. In this case the pulvinus was not stimulated beforehand, so
its reaction, indicated by the very sudden fall of the curves, fol-
lows immediately upon the beginning of the action of the base of
the petiole.

A Monophasic Action Current in the Petiole. Since apparently
the action current in the petiole is more or less diphasic if led off
in the way I have described, one may inquire whether it is not
possible to get a really monophasic response. This was found indeed
to be practicable by letting the action be stopped by a cooled zone.
Fig. 9 represents such an experiment. First the basal part of the

petiole was stimulated (action current in curve B) and the pulvinus
reacted (which is shown by the curves though the petiole was pre-
vented from falling). The action does not pass on to electrode A,
as it cannot pass through a zone which is at a temperature of
below ± 10° C. or above ± 50° C. Thereafter it was stimulated
near the secondary pulvini, and the excitation travelled down to
electrode A. It appears that the potential of the reacting cells
between the unreactive cooled portion and the reactive non-treated
portion, drops as much with respect to the former (cooled cells)
as to the latter (untreated cells). Therefore a monophasic action
current was found in this experiment.

The Action of the Base of the Stem. Generally the action in
the stem is stopped by some unknown cause before it reaches the
earth, but sometimes a diphasic action current may be obtained
from it. Fig. lo shows such a diphasic action current in curve B,

whereas in curve A only the response of the base of the stem is
seen. (It is quite possible that it is the response of the base of the
hypocotyl; the exact site of this potential change was not found).
The same is shown in Fig. ii. In this experiment, although the
stem was stimulated only lo mm. below the electrode, the action
was not conducted up to electrode B but only downwards.

A Petiole Acting as an Electrode on the Stem. In the experiment
of Fig. 12 the stem was stimulated and the action current was led
off in the ordinary way (curve B). The other electrode was fastened
on a petiole, Which acted as an elongation of it. A weak action
current appears in curve A at the moment that the action passes
the pulvinus without stimulating it. At first one wonders why this
action current is so weak, but it should be remembered that it only
expresses the difference between two potential changes. On the
one hand we have the change in pvotential of the reacting tissue

to that ot the still undisturbed part of the stem and on the other
hand the change to that of the pulvinus.

Refractory Period. After a reaction, no conduction is possible
for some minutes. To conduct the action from the stem to t'he
pulvinus and from rhe pulvinus to the petiole a still longer rest is
needed. This does not mean that the single cells cannot react during
this period, but only that they cannot propagate the excitation
to one another, either by the weakness of their reaction or by a
decreased excitability. In this connection the work of Umrath
(1927) must be mentioned. He found that when a pulvinus is sti-
mulated mechanically every 10 sec., it reacts visibly to the first
stimulus. After that the reactivity does not return, even for some
time after the leaf has resumed its normal position. Yet every
20 sec. an action current appeared at an electrode, which had been
stuck in the lower side of the pulvinus. Now it is quite possible
that the motor tissue cannot be stimulated by slight mechanical
stmiulation, but only by the action of those cells which give action
currents every 20 sec. The action, however, will not be propagated
unless they are in full quot;tonusquot;, which can hardly be expected if
thegt; are reacting every 20 sec. (s. page 53).

Theories on Conduction by Action. We do not know anything
about the way in which the action is propagated. Ball supposes
that a little of the stimulating substance, which Ricca showed the
cells of Mimosa to contain, is ejected by the reacting cells and
causes their neighbours to react in their turn and so on. He made
no special experiments to investigate this problem.

According to Bose, however, the conduction by living cells is
identical with that in the nerve. One of Bose's arguments in favour
of his theory is the fact that the rate of conduction depends greatly
upon the temperature (Qio = ± I can confirm this statement.
Though conduction in a nerve is very much faster and though it
is accomplished in a single cell, whereas it is almost certain that
in a plant many cells have to cooperate, it is a remarkable fact
that the conducting cells of a plant react by an action current and
are at the same time sensitive to electric stimulation.

Electrical Stimulation. To stimulate a nerve one may apply a
constant voltage during a comparatively long time, for example
I sec. The weakest, still stimulating voltage is called the rheobase.
Stimulating the nerve by twice the rheobase, one finds that to excite
it, the potential (twice the rheobase) must be applied during a cer-
tain minimum time, the so-called chronaxy, (which is not 0,5 sec.
but many times less). The excitability of a nerve is now usually

expressed by its clironaxy, since it has been found that this is
much more typical than its rheo'base because of the dependence of
the latter on such casual factors as the resistance of the circuit.
Moreover the chronaxy was found to be related to the rate of
conduction.

In view of these facts I made a few measurements of the chro-
naxy of the petiole of Mimosa. It was found to be approximately
0,05 sec., which is about 100 times as great as that of some nerves.
As this investigation is barely started, one should not attach much
value to it. Probably the chronaxy is not related in the same man-
ner to the rate of conduction in a tissue, consisting of many cells,
as it is related to that in a single axon of a nerve.

Umrath (1925 c) made a great many experiments concerning
the chronaxy of all parts of Mimosa. That of the petiole was found
by him to be 0,2—0,4 sec. He compared these results and those
of his investigations on nerves and muscles with a theory on con-
duction of excitation. Part of his work on this subject can be found
in Planta ; (1928).

Since Hill and Osterhout (1930) have shown that one . part
of a cell of Nitella can be stimulated by the action current of the
other part in case an intervening zone is killed, and even that the
action can pass on in this manner from cell to cell, valuable sup-
porting evidence has been advanced in favour of Bose's theory.

When a wound is made, by burning or cutting a leaflet, not
only an action current can be derived from an electrode on the
petiole, but in addition a very irregular variation of potential,
which may last several minutes, and which cannot be observed after
mechanical stimulation. To distinguish this potential change from
the action current it will be referred to as quot;the variationquot;. It will
be demonstrated in this chapter, that the variation is due to the
effect of Ricca's stimulating substance.

quot;The Variationquot;. Figs. 13 and 14 represent experiments in
which a leaf was stimulated by cutting a leaflet. The action cur-
rents s'how that the action was conducted downwards to the pul-
vinus resulting in the fall of the leaf. Before the action itself was
finished, the variation started. To disentangle the two kinds of

indica tes
/the closure of
the basal leaflets
of the pmnae when
the excitaliorr tra-
vel I es acropetally

potential changes the petiole should be stimulated beforehand with
ice. (Fig. 15). Then the two components of the curves are shown
apart and one may notice that in this experiment the variation
travels much slower than the action current, or strictly speaking,
that the factor which causes the variation, is transported at a
slower rate than the action of the cells. The variation is not delayed
by a preliminary stimulation of the petiole with ice. In the expe-
riment of Fig. 15 as well as in that of Fig. 14 it reaches the elec-
trode B about 32 sec. after the cutting of the leaflet. About 7 sec.
later some more leaves fall and at the same time the apparent
positive action current of the base of the stem (see page 68) causes
a break in the curves. Still later the leaflets of the other leaves close
up, and thus all parts of the plant react to the stimulus. In many
cases the secondary pulvini do not react at all.

In these experiments the electrodes were fastened on the petiole
of the stimulated leaf and so the excitation passed them in a
basipetal direction. In the experiment of Fig. 16 another leaf was
stimulated and the excitation passed the electrodes in an acropetal
direction. The first potential change to be observed in the curves
is the action current of the base of the stem. 10 sec. later the action
travels up through the petiole, affecting the electrodes, and is
followed by the variation. Fig. 17 represents a similar experiment,
the difference being that the petiole was stimulated with ice before-
hand, as in the experiment of Fig. 15.

The Variation is Conducted through a Killed Zone. When part
of a petiole has been killed by heat (Fig. 18) or when it is cooled
down to approximately 5° C. (Fig. 19), the variation, as contrasted
with the action, can still be transmitted. One may conclude that
it is due to the effect of a substance which is transported through
this part of the petiole. In these experiments the leaf was wounded
by burning instead of by cutting.

Ricca's Stimulating Substance. This substance is identical with
that which was extracted from the plant by Ricca (1916), as is
s'hown in the next experiment (Fig. 20).

A well-rested detached leaf was placed with the basal end of
the petiole in the extract. Both the action current and the variation
were led off by the electrode on the petiole and by that on the
pinna.

Since the action current always precedes the variation, it is
necessary to suppose that those cells, which propagate the excita-
tion by their action, are stimulated by the substance. It happens
occasionally that they do not oropagate it in the normal way. In

Fig. 21 for example, the cells under electrode A only reacted after
the substance had reached them. It looks as if a zone between
the electrodes had been killed, but 30 min. afterwards an excellent
conduction was found (Fig. 22).

In the above mentioned experiments the excitation was conduc-
ted from the leaflets of one leaf to those of another. It is clear from
the facts which have been recorded in the preceding chapter, that
this was not effected by the action of cells. Yet in all the petioles
and in the stem the action was found to be propagated as is shown
by the action currents. It was conducted downwards through the
pinna but stopped at its basal end, the petiole not reacting with
potential changes till the substance reached its distal end. The
action was then conducted through the petiole and affected the
main pulvinus. The other main pulvini of the shoot only reacted
after the substance had been transported to them (in some cases
they were stimulated by the action of the cells in the stem; see
3age 64). Thereupon the action travelled up through the petioles,
3ut did not in general pass out to the pinnae (although this was
observed occasionally in a young leaf). Thus the closing leaflets
indicate the arrival of the substance in the pinnae.

The Conduction from Petiole to Pinnae Vice Versa. Most of
the experiments on which this representation of the conduction
in Mimosa is based, have still to be discussed. Some of the other
experiments have only partly been explained. In those, represented
by Figs. 16, 17, 21 and 22 the leaflets only closed after the varia-

tion appeared at the distal electrode, the interval between the
moment of stimulation and the reaction of the leaflets being
wholly independent of the action current.

In Fig. 23 the transition from pinna to petiole is illustrated. One
electrode was fastened on a pinna 2 mm. from its base and the
other on the upper end of the petiole. The leaflets on the tip of
the pinna were cut, which caused the leaflets to close in pairs. The
excitation then travelled downwards to A and the action current
was found to coincide with the closure of the adjoining leaflets.
In' this experiment it took a very long time before the variation
appeared at A. This experiment shows that the action does not
appear in the petiole until after the variation had passed A. If we
assume that the variation is due to the stimulating substance, it
follows that this substance carries the excitation through those parts
of the plant, throug;h which the action is not conducted.

The same phenomenon was found at the transition from the
main pulvinus to the stem (Fig. 24). The action current of the
lower side of the pulvinus, into vhich a needle had been stuck,
shows a graph, different from those belonging to the action currents
led off from the petiole and from the stem. This is not due to the
fact that the leaf was stimulated by cutting a leaflet as the same
graph would have been produced, if the pulvinus had been stimu-
lated mechanically or with ice. TTie excitation did not appear in
the stem until the variation had reached the pulvinus.

Is the Conduction Delayed in a Cooled Zone? After the fore-
going it may be understood why Bose and Fitting in their experi-
mental results and deductions disagreed as to the rate of conduc-
tion in a cooled zone. Bose stimulated the distal end of the petiole
electrically and noted the time which elapsed till the fall of the
leaf. As no wound was made, the excitation was conducted by the
action of cells only, and was delayed, and might even have been
stopped, by a cooled zone. Fitting, however, wounded a petiole
at its basal end and noted the closure of the leaflets. Now it has
been shown that these cannot react unless the stimulating substance
has reached the pinnae. It is clear that the transport of the sub-
stance by the transpiration stream does not depend upon the tem-

perature of the petiole. Hence Fitting did not find any corre-
lation of the temperature and the rate of conduction. He concluded
that the excitation is not conducted by the action of living cells.

Instead of cutting a petiole as did Fitting, I cut the leaflets of
one leaf and noted the closure of those of another. In some experi-
ments a zone of the petiole, either of the cut or of the uncut leaf,
was cooled. In neither case was the reaction found to be delayed.
The conduction of action was beyond doubt suppressed in the coo-
led zone, but in the experiments in which no zone was cooled, it
stopped at the distal end of the petiole, so that the conduction of
action was of no importance to the result of the experiment. The
transport of the substance takes place independently of the cooling
of any particular part of the petiole.

The Variation Marks the Presence of the Stimulating Substance.
It has been mentioned repeatedly that the variation is an indication
of the presence of the stimulating substance and in fact another
explanation of the above mentioned results can scarcely be sug-
gested. Nevertheless it is hard to visualize the substance being

sucked in through the vessels as fast as the variation was found
to travel downwards. Experiments were next made to ascertain
whether the two are really correlated.

Acropetal Conduction. Ricca and Snow have observed that in
the stem the rate of excitatory conduction depends upon the rate
of transpiration. They found that a stain, when sucked in by a
cut shoot, is transported at the same rate as that with which the
excitation is conducted when a stimulating extract is sucked in.

When the 'basal end of a detached leaf is dipped in the extract,
an action current is aroused in the petiole, but it does not pass
out to the pinnae and the leaflets. These react after the variation
has reached the pinnae. According to our supposition the substance
s-hould also have reached them at that time. To determine if this
were the case I added a concentrated solution of methylene blue
to the extract. The moment the leaflets started closing, the petiole
was cut through at its upper end and in 8 out of 16 cases the stain
was found at the cut. As it was found in only a few vessels it may
readily have been overlooked in the other 8 cases.

In another similar experiment 4 leaves were kept in damp air
and another 4 in air of 50% humidity. The leaflets of the former
closed 50, 35, 75 and 85 sec. after application of the extract, res-
pectively. Those of the latter after 10, 20, 8 and 13 sec. In this
experiment the conduction of the excitation evidently depended
on the rate of transpiration.

To ascertain whether the rate of movement of the variation
also depends on the rate of transpiration, the pinnae of a leaf of
an intact plant were enclosed in a glass case to check the transpi-
ration as much as possible (Fig. 25). After about half an hour
another leaf of the plant was stimulated by burning with a flame.
The case then was removed and after 35 min. the experiment was
repeated (Fig. 26). In the former case the variation travelled much
slower than in the latter and the water in the vessels can hardly
have done otherwise. The stimulus in the experiment of Fig. 25
was not weaker, but rather stronger than in that of Fig. 26. In
identical experiments the difference in the rate of conduction was
often less. The difficulty is that one never has control over the
concentration of the stimulating substance in the vessels. Fitting
has shown that the leaflets of a detached leaf, the petiole of which
is dipped into an extract, close sooner the more concentrated the
extract. The rate of the transpiration stream is thus not the only
factor to be taken into account. It is probable that by burning
a leaf more of (he stimulating substance enters the vessels and is

transported to the stem, than by cutting it. A more concentrated
extract will be sucked up by the other leaves. It can be thus under-
stood that the interval between the reaction of the main pulvini
and the closure of their leaflets may depend upon the way in
which the plant is stimulated.

Basipetal Conduction. The part of Ricca's theory which has
been most generally attacked is the assumption that the substance
is sucked in from the wound through the pinna and the petiole.
This could be caused only by a negative pressure in the vessels.
This negative pressure can be lessened by cutting the shoot and
placing it in a dish with water and by checking the transpiration.

A leaf was stimulated by burning (Fig. 27). After this the stem
was cut below the main pulvinus and the leaf was placed in damp
air. Some hours later it was stimulated a second time (Fig. 28).
This time the variation was not found at the electrode. Yet the
substance must have reached the top of the petiole as the conduc-
tion by action was observed at the electrode. It is likely that this

is due to the expansion of the sap upon burning the leaflets. The
result of this experiment confirms Snow's supposition that lack of
negative pressure accounted for his failure to get any basipetai
conduction through a killed zone of the petiole. Snc«- experimented
on cut shoots (1924).

Ricca found that a leaf can be stimulated by dipping it into
water at 70° C. The excitation is then conducted as if the leaf
had been stimulated by burning. I heated a zone of the petiole
to 88° C. and found that the substance, which is set free at that
place, was carried down to the stem. (Fig. 29). To be certain that
heating causes the sap to move downwards, the following experi-
ment was made (Fig. 30). Part of the petiole was cooled down while
the temperature of a point just below this zone was measured by
means of a thermo-needle. The needle was also cooled, of course,
though less. Then the pinnae were dipped into water of about
85° C. whereupon the temperature of the needle fell nearly 1° C.
This can only be due to a basipetai movement of the sap, probably
effected partly by the expansion of heated water and partly by
the negative pressure.

The negative pressure can be observed by cutting off the top of
the shoot and replacing it by a water-filled capillary connected

with a piecequot; of bicycle-valve tubing. In most cases the water was
sucked in.

It is tjuite possible that the roots of potted plants are m a worse
condition than those of plants, rooted in the ground. I do not
know whether the pressure in the vessels of the latter is ordinarily
positive or negative.

Positive pressure inside a potted plant can be obtained by heating
its roots to about 40° C., while the air is damp. When a positive
pressure has been established in this way, it often can be detected
by the abnormal appearance of the younger leaves, the pinnae being
curved and even twisted, the leaflets bending their tips downwards.
This remarkable appearance of the plant may be due to the change
of pressure from negative to positive as a result of which different
torsioneffects develop.

When such a plant is stimulated by wounding, the excitation is
hardly ever conducted to the other leaves, and at the electrodes

on the petiole of the stimulated leaf the variation is found to be
lacking or to be smaller and to travel at a lesser rate. In some
cases no variation is found at all (Fig. 31).

The experiment was repeated when the temperature of the roots
had fallen to 22° C. (Fig. 32).

The most convincing evidence for Ricca's assumption, that the
stimulating substance is sucked in from the wound, was obtained
by the following experiment (Fig. 33). A cut shoot was mounted
on a water-filled tube connected to a water-suctionpump. When
the outlet of the pump was closed and the water turned on, the
sap in the vessels was put under a pressure of 2 atmospheres. When
one of the leaves was now stimulated with a flame, the burned
leaflets only reacted. In this experiment the action did not travel
through the entire pinna When afterwards the outlet of the
pump was opened, the pressure became negative, and shortly there-
after the remaining open leaflets of the stimulated pinna closed
in pairs. Thereupon the petiole was reached by the substance and

From other observations it seems possible to ascribe this to the age of
the leaf.

the action was aroused in it. It was- conducted past the electrodes
and affected the main pulvinus. The variation followed the action
current.

As has been shown in all experiments the appearance of the
variation depends upon the transpiration stream. It is certainly
not due to a slow form of conduction by action of liVing cells as
has been suggested by Umrath (1928).

The electrical variation must, however, be due to some change
in the cells under the electrode. These cells may be identical to
those which conduct the action. It is remarkable that, though the
electric phenomena enable us to study the conduction by action of
living cells, the anatomical path of this conduction is still unknown.
Bose supposes the phloem to be the path of conduction (1925).
I have made no investigations into this matter.

The way in which the potential is changed in a part of'the
petiole, while it is 'being cooled, is shown in Fig. 34. The first top
of the curve is probably identical with the action current. The
second part is reminiscent of the variation.

In a few similar experiments a part of the petiole was cooled
gradually in the course of some minutes. The potentia! of the elec-
trode in the cooled part remained constant during the cooling until

the treatment itself caused stimulation. It then dhanged as in the
experiment of Fig. 34.

It is well known that when a cell is cooled suddenly, the proto-
plasmic movement is stopped. The more gradually the cooling is
performed, the greater the change of temperature has to be to
effect this. This phenomenon may be correlated with t^he stimu-
lating effect of cooling, as is supposed by Umrath (1934).

The action current is due to a temporary alteration of the
electric properties of the cell. It is quite possible that a low tempe-
rature as well as the presence of the stimulating substance causes
the prolongation of this alteration i.e. the variation. Not all cells
will be affected by the stimulating substance and they will recover
the sooner the less of it has got into the sap of the vessels. This
may cause the irregular appearance of the variation, although it
is wholly an unproved supposition.

In some experiments a leaf was stimulated by cutting a pinna.
Occasionally the main pulvinus was observed to react almost at
once, i.e. after i—3 sec. The fall of the leaf preceded the closure
of the basal leaflets of the cut pinna. Neither an action current
nor a variation was found to accompany the conduction (Fig. 35).
They follow it and travel through the petiole in their usual way.

It has been mentioned above that the action of a pulvinus, which
has been stimulated mechanically, can pass out to the petiole.
Sometimes the same can be observed in these experiments. The
pulvinus is then stimulated by some unknown change brought about
by cutting a pinna, and the action, being thus aroused in it, is
conducted acropetally. The basal electrode therefore is the first
to react, whereas the variation moves basipetally (Fig. 36). Umrath
(1928) was deceived by this phenomenon. Since he led off from
only one electrode on the petiole, he did not observe that the action
is conducted up instead of downwards. He concluded from the
appearance of the action current, that this rather fast method of
conduction is effected by the action of some special cells, which
can be stimulated in no other way than by cutting a pinna or a
petiole.

As this method of conduction was most frequently observed quot;in
plants with little or no negative pressure in the vessels, it seems

probable that the rapidly moving excitation is not due to the mo-
vement of a substance.

Conduction by the action of living cells is not probable either,
as the excitation can pass through a cooled part of the petiole.

through which the action is not conducted (Fig. 37). The action
in the petiole, as started at the pulvinus, is limited to the part
below the cooled zone (action current at B). At last the substance
reaches the distal end of the petiole and an action current appears
at A.

We have then to return to the old view that a stimulus can be
conducted by a change of pressure either in the xylem or in the
phloem. Of these two possibilities the former is the less probable.
In the first place the (negative) pressure was very low in some
of these experiments. In the second place the fast conduction was
never observed in a leaf when part of the petiole had been killed
on the previous day, even if the leaflets were expanded and looked
fresh. In the third place neither Mac Dougal, Fitting nor I suc-
ceeded in stimulating a plant By changing the pressure in the ves-
sels of the stem.

In these experiments the pressure in the phloem was always equal
to that in the xylem as both were cut through, whereas in intact
plants they are widely different. The failure of these efforts is
therefore no argument against Haberlandt's assumption that the
stimulus can be conducted through the quot;Schlauchzellenquot;.

The quot;high-speed conductionquot; in the stem (Snow, 1924) has much
in common with the fast conduction in the leaf which was dis-
cussed above. Only by cutting the plant can both be found.
Although the application of a flame is generally a more efficient
stimulus, these two hig'h-speed conductions are not obtained by
burning the leaf or the stem.

It has been mentioned in the first chapter that until recently
nearly all investigators have supposed that conduction of excitation
such as can be observed in sensitive plants, does not exist in other
plants. The latter indeed had, according to scientific opinion of
earlier times, no use for such a conducting system. In itself, this is
not remarkable, since in the latter the action can be detected by
electric changes only. Nevertheless Fee (1858), in a treatise on
the movements of leaves, suggested that some plants might be
sensitive to a stimulus without being able to show it: quot;Aussi pen-
sions nous qu'il existe des plantes, à tissues aussi excitables que
ceux de la Sensitive, qui cependant ne peuvent se mouvoir faute

d'organes appropriés aux mouvement. Ce n'est pas assez que d'avoir
la faculté, il faut encore avoir l'instrumentquot;

Bose has led off action currents from plants, which do not react
in any visible way to the stimuli to whidh he subjected them.

Umrath did the same (1929), but some of these potential changes
differ greatly from the action currents w'hich I have found in
Mimosa.

As 1 had observed that wounding Mimosa causes a substance
to be transported in the vessels, which by its stimulating effect,
conducts the excitation without the assistance of living cells, I
thought it desirable to ascertain whether the potential changes in
other plants are real action currents, due to conduction by action
of cells.

The stem of Vitis discolor can be stimulated with ice. The
action is propagated at a rate of 9 mm. per sec., up as well as
downwards (Fig. 38). It was never observed to pass out to another
internode. When the same internode was stimulated another time,
after i,j min., the action travelled more slowly. When the interval
between two stimuli was only 20 sec., the action was not conducted
at all.

Also in the tendril of Vitis gongylodes an action current was
found when it was stimulated with ice. When the tendril was cut,
the potential change was much like the variation, which in Mimosa
is due to the stimulating substance (Fig. 39).

The question whether the electric phenomena in Vitis may be
identified with those in Mimosa remains to be investigated.

1) And he added: „Ainsi le phoque (seal), aussi intelligent peut-être que ie
chien, ne peut, faute de pieds, s'éloigner du bord de la mer, n'accomplissant
que des actes peu nombreux, purement instinctifsquot;.

A.nbsp;When Mimosa pudica is stimulated without being wounded,
for example by applying a drop of water at less than io° C., the
excitation is conducted by the action of living cells and accom-
panied by potential changes, referred to as action currents.

The action of the cells can be propagated through the stem, the
petioles, the pinnae and the pulvini. In most plants it is stopped
at several places, for example at the transition from pinna to secon-
dary pulvinus.

The rate at which it is conducted depends upon the temperature.
It does not pass through a killed part of the petiole nor through
a zone which is cooled to approximately 5° C.

The conduction by action of cells naturally depends upon the
condition of these cells. The action is best conducted in damp air
in a young shoot.

The conduction by living cells may be identified with Ball's
quot;rapid conductionquot; in the stem and with Umrath's quot;slow conduc-
tionquot; in the leaf. Most of Bose's work is related to it. He observed
the action current and found most of the above mentioned proper-
ties of this kind of conduction.

B.nbsp;When a plant is stimulated 'by wounding, for example by
burning a leaflet, the excitation can be conducted by the action
of cells as well as by the transport of a stimulating substance,
which Ricca has demonstrated to be set free at the wound.

The cells which propagate the excitation by their action, are
stimulated by this substance.

By means of the substance the excitation can be conducted
through a killed zone and through such parts of the plant, as do
not propagate it by the action of cells.

The presence of the substance is indicated by a change of poten-
tial, referred to as quot;the variationquot;.

The substance is sucked in from the wound by the negative
pressure in the vessels, and is transported by the transpiration
stream. By changing the pressure from negative to positive it can
be prevented to 'be sucked in.

C.nbsp;When a plant is stimulated by cutting a pinna, the exci-
tation can be conducted by the action of cells, by the transport
of the stimulating substance, and also by a third, very fast mecha-
nism of conduction by which only the main pulvinus is affected.
It is not accompanied by potential changes. It was observed in
young leaves, especially in damp air.

It passes a cooled zone of the petiole but was never found to
be conducted through a killed part.

It may be comparable to Snow's quot;high-speed conductionquot; in
the stem, and to Umrath's quot;fast conductionquot; in the leaf.

The probability of a relation between Haberlandt's quot;Schlauch-
zellenquot; and this fast conduction has been discussed.

No mechanism of conduction should be referred to as quot;normal
conductionquot;.

The author wishes to take this opportunity to thank Prof. F. A.
F. C. Went, at whose suggestion this work was commenced, and
Prof. V. J. Koningsberger, the present Director of the Institute,
for their valuable aid and criticism.

6.nbsp;Brücke, E., Ueber die Bewegungen der Mimosa pudica. Archiv f.
Anatomie und Physiologie, 1848, p. 434.

Leaves of Mimosa pudica. Quart. J. Sc. Lit. Art. New Series,
III, p. 76. (1827).

9.nbsp;Can dol e, A. P. de, Physiologie végétale, p. 866. (1832).
id. Clarke, see Hooke.

in the Motor Organs of Leaves. Ann. Roy. Bot. Garden, Calcutta.
6, part I. (1895).nbsp;^ , „

16.nbsp;Fée, A. L. A., Mimosa pudica. Mémoire physiologique et organographique

sur la Sensitive et les plantes dites sommeillantes. Mém. Soc.
Mus. hist. nat. Strasbourg. 4, p. 69. (1850).

sur Porliera hygrometrica. Bull. Soc. Bot. France, j, p. 4JI. (1858).
Fitting, H., Weitere Untersuchungen zur Physiologie der Ranken.
Cap. 5. Jahrb. wiss. Bot. 39, p. 424. (1904).

Jahrb. wiss. Bot. 72, p. 700. (1930).
Haberlandt, G., Das Reizleitende Gewebesystem der Sinnpflanze.
Leipzig. (1890).

Harris, E. S. and W. J. V. Osterhout, The Death Wave in
Nitella. IL Applications of Unlike Solutions. J. Gen. Physiol. 12,
P- 35 5- (19^9)-

Variations. J. Gen. Physiol. 13, p. $47. {1930).
Hofmeister, W., Die Lehre von der Pflanzenzelle. p. 315, Leipzig

Kabsch, W., Anatomische und physiologische Untersuchungen über
einige Bewegungserscheinungen im Pflanzenreiche. Bot. Zt. 19,
p. 362. (1861).

Linsbauer, K., Ueber Reizleitungsgeschwindigkeit und Latenzzeit
bei Mimosa pudica. Wiesner Festschrift, p. 396. (1908).

D. Bot. Ges. 32, p. 609. (1914).
---Ueber die Interferenz der Stossreizen und über Ermüdungser-
scheinungen an Blattgelenken von Mimosa pudica. Jahrb. wiss.
Bot. 62, p. 283. (1923).
Mac Dougal, D. T., The Mechanism of Movement and Transmission
of Impulses in Mimosa and other „Sensitivequot; Plants: A Review
with some Additional Experiments. Bot. Gaz. 22; p. 293, (1896).
Meyen, F. J. F., Pflanzenphysologie. p. ji6 seq. (1839).
Pfeffer, W., Physiologische Untersuchungen: Mimosa pudica. Leipzig

wiss. Bot. 9, p. 308. (1874).
R i c c a, U., Solution d'un problème de physiologic: la propagation
de stimulus dans la Sensitive. Arch. Ital. d. Biol. 65. (1916).

z. allg. Bot. 2, p. 557. (19^3):
Snow, R., Conduction of Excitation in Stem and Leaf of Mimosa
pudica. Proc. Roy. Soc. B, 96, p. 349. (1924).

Stern, K., Elektrophysiologie der Pflanzen. Berlin. (1924).
Umrath, K., Ueber die Erregungsleitung im Blatte von Mimosa
pudica. Sitzber. Wien. Akad. Math. Nat. Kl. I, 134, p. 29. (i92ja).

19-
20.

22.

25-
26.

28.
•29-

30.

31-

32-

33-

34-

35-
,6.

37-
38.

39-

40.

41.

42-

Dassen, M., Verhandeling ter beantwoording van de vrage: Wat weet
men met zekerheid van de bewegingen die men aan de bladen van vele
planten waarneemt? Nat. Verhand, d. Holl. Maatschappij d. Weten-
schappen Haarlem. Oct. 22. (1835).

Goebel, K., Die Entfaltungsbewegungen der Pflanzen und deren teleologischen
Deutung. Cap. 9. Die Sensitiven. Jena. (1920).

De theoretische voorstelling van de plagiotropie, zoals die door
Metzner werd gegeven, houdt geen rekening met het feit dat de
geonastie (Rawitscher) onder invloed van de lengtekracht langzaam
toe- of afneemt, zodat ook de stand, waarin geotropie en geonastie
in evenwicht zijn, met de tijd verandert.nbsp;(

De theorie van Selig Hecht over de correlatie van de lichtsterkte
en de gezichtsscherpte bij insecten wordt door het experiment niet
afdoende bewezen.

Ten onrechte trekt Winterstein uit zijn proeven de concliisie
dat de cH van het bloed en niet de CO2 spanning de ademhaling
reguleert.

Het voorkomen van Nothomonokotylen onder de Polycarpicae
wijst niet op een verwantschap van de Monokotylen met deze groep.

De geographische verspreiding van Mercurialis perennis wordt
niet in de eerste plaats door het gehalte van de grond aan vrije kalk
bepaald.

Er is geen grond om met Hasselbaum aan te nemen dat de
Mycorrhiza van Empetrum voor de stikstofvoeding van deze
plant van betekenis is.

Het is door de proeven van Menon niet bewezen dat de op appel
en de op aardappel parasiterende schimmels éénzelfde protopecti-
nase afscheiden; zijn proeven bewijzen echter wel dat de specialisa-
tie op andere gronden moet berusten.

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