Rotation; I shall now take up the second method, that of Vertical
Rotation, diagrammatic representation of which is given in figure 166V.
The specimen is held vertical and two electrical contacts, A and B, made
with the two lateral sides; it is then rotated round a horizontal axis
perpendicular to the length of the specimen. Rotation may be carried in
a right-handed direction with increasing angle with the vertical. The
point A is thus subjected to enhanced geotropic stimulation and exhibits
increasing electric change of galvanometric negativity; continuous
decrease of angle of inclination to zero by rotation in the reverse
direction causes a disappearance of the induced electric change. The
rotation is next continued in the negative direction by which the point
B is increasingly subjected to geotropic action. B is now found to
exhibit excitatory reaction, the current of response having undergone a
reversal. Rotation to the right and left will be distinguished by plus
and minus signs.
ELECTRIC RESPONSE THROUGH AN ENTIRE CYCLE.
_Experiment 177._--When the specimen is vigorous, characteristic
response with its changing sign may be obtained through an entire cycle
from 0° to +45° to +90°; then back to 45° to 0° to -45° to -90°. With
less vigorous specimens the responses becomes enfeebled under fatigue. I
give below the results of a typical experiment carried out with a
vigorous specimen, the response being distinguished as - when A is
above, and + when A is below, the inversion bringing about a reversal
direction of the responsive current.
+------------------------------------------------------+
|Angle of inclination |+45°|+90°|+45°| 0°|-45°|-90°|
|-------------------------+----+----+----+---+----+----|
|Galvanometer deflection |-19 |-35 |-18 | 0 |+14 | +25|
+------------------------------------------------------+
RELATION BETWEEN ANGLE OF VERTICAL ROTATION AND INTENSITY OF GEOTROPIC
REACTION.
The relation between the angle of inclination and the resulting
geotropic action has already been determined by the Method of Axial
Rotation. The ratio between the geotropic effects at 90° and 45° was
thus found to be 1·49, which is nearly the same as Sin 90°/Sin 45°. I
was next desirous of determining the relative excitations at the two
angles by the Method of Vertical Rotation. It is necessary here to refer
to certain differences of condition in the two methods. In the Axial
Method, the hypothetical statoliths are distributed uniformly through
the length of the cell, and rotation round the long axis causes
displacement of the statoliths, the resulting pressure thus increasing
with the sine of the angle of inclination. But in the case of vertical
rotation through 45° to the right, the statoliths originally at the base
of the cell accumulate to the right hand corner of the cell; a portion
of the basal side of the cell is thus subjected to pressure. When the
angle is increased to 90° the statoliths pass along the whole length
including the basal and apical sides of the cell; but the excitability
of the apical half may prove to be greater than that of the basal half.
Hence excitatory geotropic effect is not likely to vary strictly as in
sine of angle of inclination.
Whatever the reason may be, I find as a result of experiments with 12
different specimens that the mean ratio of the effects at 90° and 45°,
obtained by the Method of Vertical Rotation, is 1·8:1 which is greater
than 1·49:1 obtained by the Method of Axial Rotation, this latter value
being practically the same as Sin 90°/Sin 45°.
SUMMARY.
It is shown that the state of excitation under direct stimulus is
exhibited by an electrical change of galvanometric negativity; the
effect of indirect stimulus induces, on the other hand, an electrical
change of galvanometric positivity. The negative electric change
corresponds to contraction and diminution of turgor; the positive
electric change indicates, on the other hand, an expansion and increase
of turgor.
The electric response to geotropic stimulus is studied by the two
methods of Axial and Vertical Rotation. The upper side of a horizontally
laid shoot is found to undergo an excitatory change of galvanometric
negativity.
In quick reacting organs the latent period of geo-electric response is
about 5 seconds, and the maximum excitation is induced in the course of
2 minutes.
The geo-electric response is due to physiological reaction. The
intensity of response declines with age and is abolished at the death of
the plant.
Under symmetrical conditions, the intensity of geotropic reaction is
found proportional to the sine of the angle of inclination.
Electric investigation shows that the lower half of the pulvinus of
_Mimosa_ is geotropically more excitable than the upper half.
XLI.--THE MECHANICAL AND ELECTRICAL RESPONSE OF ROOT TO VARIOUS STIMULI
_By_
SIR J. C. BOSE.
In the last chapter we studied the electric response of the shoot to the
stimulus of gravity, and found that the excitatory effect of that
stimulus is similar to that of other forms of stimulation. Before taking
up the subject of the geo-electric response of the root to gravitational
stimulus, I shall describe the effects of other forms of stimuli on the
mechanical and electrical response of the root.
In connection with this subject, it should be borne in mind that the
responsive curvature in the root takes place in the sub-apical growing
zone which is separated by a certain distance from the tip. The stimulus
is therefore direct when applied at the responding growing region; it is
indirect when applied at the tip of the root. The intervening distance
between the root-tip and the responsive zone of growth is
semi-conducting or non-conducting.
I shall proceed to give an account of my investigations on the response
of the root to direct and indirect unilateral stimulation. We shall
study:--
(1) The Mechanical response to Direct unilateral stimulus.
(2) The Electrical response to Direct unilateral stimulus.
(3) The Mechanical response to Indirect unilateral stimulus.
(4) The Electrical response to Indirect unilateral stimulus.
MECHANICAL RESPONSE TO DIRECT STIMULUS.
As the geotropic responses of the shoot and the root are opposed to each
other, the object of the investigation is to find out; whether the
response of the root to various stimuli is specifically different from
that of the shoot. We have seen that tissues in general respond to
direct unilateral stimulus by contraction of the proximal and expansion
of the distal side, the tropic curvature being thus _positive_. We shall
now determine whether direct unilateral stimulation of the root induces
a tropic movement which is similar or dissimilar to that exhibited by
the shoot.
_Experiment 178._--In experimenting with roots of various plants I
obtained results which are precisely similar to that of the shoot. The
movement of the root was observed by means of a reading microscope
focussed on the tip of the organ. I employed various forms of stimuli,
mechanical, thermal, and chemical. Unilateral application of these on
one side of the growing region gave rise to a _positive_ tropic
curvature, resulting in a movement towards the stimulus. These
experiments confirm Sachs' observation that unilateral application of
stimulus in the region of growth induces positive curvature of the root.
ELECTRICAL RESPONSE TO DIRECT STIMULATION.
I next undertook an investigation on the electric response of the root
to direct unilateral stimulation.
_Experiment 179._--The terminals of the galvanometer were suitably
connected with the two diametrically opposite points A and B in the
growing region of the root. Stimulus was now applied very near the point
A, the various stimuli employed in different experiments being: (1)
mechanical, (2) chemical, and (3) thermal. In every instance the
excited point A becomes galvanometrically negative. This shows that the
response of the root is in no way different from that of the shoot.
MECHANICAL RESPONSE TO INDIRECT STIMULUS.
Before describing the effect of indirect stimulus on the root, I shall
recapitulate its effects on ordinary tissues. I have shown that the
effect of indirect unilateral stimulus is to induce a movement away from
stimulus. This was shown to be the case with the bud of _Crinum_ (p.
275) and the tendril of _Passiflora_ (p. 291). The mechanical and
electric response to indirect stimulation in the shoot is shown in the
diagrammatic representation (Fig. 170). I shall now proceed to describe
the mechanical response induced by unilateral stimulation of the root
tip. As the responding region of growth is at some distance from the
tip, the stimulation is therefore indirect.
[Illustration: FIG. 170.--Mechanical and electrical response to indirect
stimulation at dotted arrow. In figure to the left, the point A, on the
same side undergoes expansion, with responsive mechanical movement away
from stimulus indicated by continuous arrow. In figure to the right,
indirect stimulus at dotted arrow induces electric response of
galvanometric positivity at A, indicative of increase of turgor and
expansion.]
_Experiment 180._--I employed at first mechanical stimulus of moderate
intensity by rubbing one side of the tip of the root of _Bindweed_; this
induced a movement away from stimulus. Unilateral application of dilute
acid gave rise to a similar response. Thermal stimulus of moderate
intensity also induced responsive movement away from the stimulus (Fig.
171).
Darwin in his _Movements of Plants_ described experiments on the
responsive behaviour of the tip of the radicle. He produced unilateral
stimulation in three different ways, first by attaching minute fragments
of cardboard to one side of the root-tip; this moderate and constant
irritation was found to induce a convexity on the same side of the
growing region, with the resulting negative movement, _i.e._, away from
stimulus. His second method was chemical, one side of the tip being
touched with silver nitrate; the third method of stimulation was a
slanting cut. All these methods induced a movement away from stimulus.
ELECTRICAL RESPONSE TO INDIRECT STIMULATION.
The next investigation was for the determination of the electrical
change induced in the growing region by application of unilateral
stimulus at the root-tip.
[Illustration: FIG. 171.--Diagrammatic representation of mechanical and
electric response of root to indirect stimulus applied at the tip _a_.
Figure to the left shows responsive movement away from stimulus. The
electric response to indirect stimulus is indicated in the figure to the
right; the point on the same side exhibiting galvanometric positivity.
The shaded part indicates the responsive region of growth at some
distance from the tip.]
_Experiment 181._--One of the two electrical connections with the
galvanometer is made at one side of the growing region A, the other
connection being made with the diametrically opposite point B.
Unilateral stimulus was applied at the root tip _a_, of the bean plant
and on the same side as A. I subjected the tip to various modes of
unilateral stimulation. Mechanical stimulation was effected by
emery-paper friction or by pin-prick; chemical stimulation was produced
by application of dilute hydrochloric acid. Thermal stimulation was
caused by the proximity of electrically heated platinum wire. In every
case the response was by _induced galvanometric positivity at A_ (Fig.
171). This electrical variation took place within about ten seconds of
the application of stimulus; the interval would obviously depend on the
length of path to be traversed by the transmitted effect of indirect
stimulation.
The galvanometric positivity at A indicated that there was induced at
that point an increase of turgor and expansion, in consequence of which
the organ would move away from stimulus. Thus both by the mechanical and
electrical methods of investigation we arrive at an identical conclusion
that the effects of unilateral stimulus at the tip of the root gives
rise to a movement, by which the organ is moved away from the source of
stimulus; since tropic movement towards stimulus is termed _positive_,
this opposite response must be regarded as _negative_.
TABLE XXXVI.--EFFECT OF INDIRECT STIMULUS UNILATERALLY APPLIED AT THE
ROOT-TIP.
+--------------------------------------------------------------+
|Effect at the proximal side A in the | Effect at the distal|
| growing region. | side B. |
+--------------------------------------+-----------------------+
|Galvanometric positivity, indicative | Negligible. |
| of increase of turgor and expansion. | |
|--------------------------------------------------------------|
|The corresponding tropic curvature is negative, _i.e._, a |
|movement away from stimulus. |
+--------------------------------------------------------------+
The root-tip when burrowing its way underground comes in contact with
hard substances and moves away from the source of irritation. The
irritability of the root-tip is generally regarded as being specially
evolved for the advantage of the plant. But reference to experiments
that, have been described shows that this reaction is not unique but
exhibited by all plant organs, growing and non-growing. Indirect
stimulus has been shown to give rise, in both shoot and root, to a
_negative_ tropic curvature in contrast to the _positive_ curvature
brought about by direct stimulation; the response of the root is
therefore in no way different from that of vegetable tissues in general.
It will also be seen that an identical stimulus induces two opposite
effects, according as the stimulus is applied at the tip or at the
growing region itself. In the former case, the stimulus is indirect, and
in the latter case it is direct. The results are in strict conformity
with the laws of effects of direct and indirect stimulations that have
been established regarding plant response in general (p. 231).
SUMMARY.
In the root, the responsive region is in the zone of growth. The tip of
the root is separated from the region of response by a semi-conducting
or non-conducting tissue.
Direct unilateral stimulus (applied at the region of growth) induces a
positive curvature by the contraction of the proximal and expansion of
the distal side.
The electrical response to direct unilateral stimulus is galvanometric
negativity of the proximal, and galvanometric positivity of the distal
side.
Indirect unilateral stimulus induces expansion of the proximal side
resulting in negative curvature and movement away from stimulus.
The corresponding electric response induced is galvanometric positivity
of the proximal side.
The responses of the root, to both direct and indirect stimulations, are
precisely similar to those in the shoot. The assumption of specific
irritability of the root as differing from that of the shoot, is without
any justification.
XLII.--GEO-ELECTRIC RESPONSE OF ROOT
_By_
SIR J. C. BOSE,
_Assisted by_
SATYENDRA CHANDRA GUHA.
The effects of various stimuli, direct and indirect, on the response of
the root have been described in the last chapter. These responsive
reactions have been found to be in no way different from those of the
shoot. But the shoot and the root exhibit under the stimulus of gravity,
responsive movements which are diametrically opposite to each other.
These opposite effects of an identical stimulus have been regarded as
due to specific differences of irritability in the two organs, specially
evolved for the advantage of the plant. The root is thus supposed to be
characterised by "positive" and the shoot by "negative" geotropism.
As regards response to other forms of stimuli, the root has been shown
to behave like the shoot. We have now to inquire whether the reaction of
the root to gravitational stimulus is specifically different to that of
the shoot.
The electric method of investigation described in the last chapter,
holds out the possibility of discovering the character of the responsive
reaction induced in the root by its displacement from vertical to
horizontal position; we shall, moreover, be able to make an electrical
exploration of the root-tip and the zone of growth, and thus determine
the qualitative changes of response, induced in two regions of the root
under the action of gravitational stimulus. For the detection of
geotropic action in the shoot, electric contacts were made at two
points diametrically opposite to each other. Displacement of the shoot
from vertical to horizontal position induced excitatory change of
galvanometric negativity at the upper side of the organ, demonstrating
the effect of direct stimulation of that side; this excitatory reaction
of the upper side finds independent mechanical expression in the induced
contraction and concavity of that side of the organ.
I employ a similar electric method for detection of geotropic excitation
of the root, responses to geotropic stimulus being taken at the root-tip
and also at the zone of growth in which geotropic curvature is effected.
I shall now proceed to give a detailed description of the characteristic
electric responses of the tip and of the growing region.
The two diametrically opposite contacts at the tip will be distinguished
as _a_ and _b_, the corresponding points higher up in the growing region
being A and B. When the root is vertical the electric conditions of the
two diametrically opposite points are practically the same. But when the
root is rotated in a vertical plane through +90° a geo-electric response
will be found to take place; the direction of the responsive current
disappears when the root is brought back to the vertical. Rotation
through -90° gives rise again to a responsive current, but its direction
is found reversed.
GEO-ELECTRIC RESPONSE OF THE ROOT-TIP.
_Experiment 182._--I took the root of the bean plant and made two
electric contacts with the diametrically opposite points, _a_ and _b_,
of the root-tip at a distance of about 1·5 mm. from the extreme end.
Owing to the very small size of the tip this is by no means an easy
operation. Two platinum points tipped with kaolin paste are very
carefully adjusted so as to make good electric contacts at the two
opposite sides, without exerting undue pressure. For geotropic
stimulation the root has to be laid horizontal, and as the root of the
bean plant is somewhat long and limp, displacement from the vertical
position is apt to cause a break of the electric contact. This is
avoided by supporting the root from the top and also from the sides; for
the latter purpose, I use paddings of cotton wool.
[Illustration: FIG. 172.--Diagrammatic representation of geo-electric
response of root-tip. The middle figure shows root in vertical position.
Rotation through +90° places _a_ above, which becomes galvanometrically
negative. Rotation through -90°, places _b_ above and makes it
negative.]
After due observance of these precautions the electric response obtained
is found to be very definite; when the root is made horizontal, by
rotation of the root through +90°, the point _a_ is above, and the
responsive current is found to flow from _b_ to _a_, _the upper side of
the tip_ becoming galvanometrically negative; when the root is brought
back to the vertical, the responsive current disappears; rotation
through -90° makes the point _b_ occupy the upper position, and the
responsive current is from _a_ to _b_; the upper side thus exhibits in
every case, an excitatory electric change of galvanometric negativity
(Fig. 172). The root-tip thus exhibits the characteristic response to
direct stimulation. Experiments carried out with 12 different specimens
gave concordant results. The following table gives the absolute values
of electro-motive force induced at the tip under geotropic stimulus.
TABLE XXXVII.--GEO-ELECTRIC RESPONSE OF THE ROOT TIP (_Vicia Faba_).
+--------------------------+
|Specimen.|Induced E. M. F.|
+---------+----------------+
|1 | 0·0005 volt. |
|2 | 0·0011 " |
|3 | 0·0010 " |
|4 | 0·0015 " |
+--------------------------+
ELECTRIC RESPONSE IN THE GROWING REGION.
_Experiment 183._--I next undertook an investigation on the electric
variation induced in the growing region under the stimulus of gravity.
The experimental difficulties are here greatly reduced, since the
available area of contact for galvanometric connection is not so
restricted as in the case of the root-tip. The specimen is securely
mounted so that the root is vertical. It is next rotated in the vertical
plane through +90°, so that the point A in the growing region occupied
the upper position. The electric response in the growing region took
place in a short time and was very distinct. The induced electric change
at A was now galvanometric _positivity_ indicative of increase of
_turgor and expansion_.
The series of experiments were carried out in the following order. The
specimen was first rotated through +90° so that A was above. The
responsive electric variation rendered it galvanometrically positive.
The root was rotated back to neutral position when the current
disappeared. The root was next rotated through -90° and the responsive
current became reversed, the upper B becoming electro-positive (Fig.
173). The alternative rotations through +90° and -90° were carried out
six times in succession with consistent results. The interval allowed
between one stimulation and the next was determined by the period of
complete recovery. Growing fatigue was found to increase this period; at
first it was seven minutes, at the second repetition it was ten minutes,
and at the third time it was prolonged to fifteen minutes.
[Illustration: FIG. 173.--Diagrammatic representation of geo-electric
response of growing region of root. (_a_) Rotation through -90° makes B,
galvanometrically positive. (_b_) Vertical and neutral position. (_c_)
Rotation through +90° places A above and renders it galvanometrically
positive. (_d_) Additive effect on current of response, root-tip a
negative, and growing region A positive.]
I give below the series of electric responses induced by alternate
rotations through +90° and -90°. The upper position was occupied by A in
the odd series, and by B in the even series. In every case the upper
side became galvanometrically positive.
TABLE XXXVIII.--GEO-ELECTRIC RESPONSE OF ROOT IN THE REGION OF GROWTH.
+---------------------------------------------------------------+
| Odd |Galvanometer deflection| Even |Galvanometer deflection|
|series.| A, positive. |series.| B, positive. |
+-------+-----------------------+-------+-----------------------+
| 1 | 20 divisions. | 2 | 18 divisions. |
| 3 | 16 " | 4 | 18 " |
| 5 | 10 " | 6 | 12 " |
+---------------------------------------------------------------+
ADDITIVE ACTION-CURRENT AT THE TIP AND THE GROWING REGION.
It has been shown that under geotropic stimulus the upper side of the
tip, _a_, becomes galvanometrically negative, while the point A, higher
up in the growing region, becomes galvanometrically positive. If now we
make the two galvanometric connections with _a_ and A, the induced
electric difference is increased, and the galvanometric response becomes
enhanced.
_Experiment 184._--The root was at first held vertical, and two electric
contacts made with _a_ and A. In this neutral position there is little
or no current. But as soon as the root was laid horizontal, an
electro-motive response was obtained which showed that _a_ was
galvanometrically negative, and A galvanometrically positive (Fig.
173d). The induced electric response disappeared on restoration of the
root to the vertical position. I give below the results of typical
experiments with a vigorous specimen which gave strong electric
response. It was possible to repeat the geotropic stimulation six times
in succession, the results being perfectly consistent. The responses
taken in succession exhibited slight fatigue, the first deflection being
140 divisions, and the sixth 115 divisions of the galvanometer scale.
TABLE XXXIX.--INDUCED E. M. F. VARIATION BETWEEN THE TIP AND THE GROWING
REGION (_a_ NEGATIVE AND A POSITIVE).
+-------------------------------------------------------+
|Geotropic stimulation. | Resulting electric response.|
+------------------------+------------------------------+
|First stimulation | 140 divisions. |
|Second " | 130 " |
|Third " | 130 " |
|Fourth " | 123 " |
|Fifth " | 127 " |
|Sixth " | 115 " |
+-------------------------------------------------------+
The results of experiments 182 and 183 are summarised as follows:--
(1) the induced galvanometric negativity at root tip indicates
direct stimulation of the tip, and
(2) the induced galvanometric positivity of the growing region
shows that it is the effect of indirect stimulus that reaches it.
From these facts it will be seen that the tip perceives the stimulus and
thus undergoes excitation, and that owing to the intervening tissue
being a semi-conductor of excitation, it is the positive impulse that
reaches the growing region and induces there an expansion and a convex
curvature.
GEO-PERCEPTION AT THE ROOT TIP.
The results given above fully confirm Charles Darwin's discovery that it
is the root tip that perceives the stimulus of gravity[37]; he found
that removal of the tip abolished the geotropic response of the root.
Objection has been raised about the shock-effect of operation itself
being the cause of abolition of response. But subsequent observations
have shown that Darwin's conclusions are in the main correct.
[37] "This view has been the subject of a considerable amount
of controversy. Wiesner denies the localisation of geotropic
sensitiveness. Czapek, on the other hand, supports Darwin's
theory. Recently Picard has attacked the problem in a new way
(and) concludes that not only the root tip but also the
entire growing zone is capable of perceiving gravitational
stimuli.... As both Picard's experimental method and his
interpretation are open to criticism, the author has repeated
his experiments with a more satisfactory apparatus. He finds
that in _Vicia Faba_, _Phaseolus multeflorus_ and _Lupinus
albus_, both apex and growing zone are geotropically
sensitive, the former being by far the more sensitive of the
two, and the curvature of the growing zone being without a
doubt largely induced by secondary stimuli transmitted from
the apical region. Charles Darwin's views were therefore in
the main correct."--Haberlandt--_Ibid_, p. 748.
The experiments which I have described on the geo-electric response of
the root tip and of the growing region offer convincing proof of the
perception of the stimulus at the tip, and the transmission of the
effect of indirect stimulus to the growing region. These experiments
exhibit in an identical _uninjured_ organ: the excitatory reaction at
the upper side of the tip, the cessation of excitation, and the
excitation of the opposite side of the tip, following the rotation of
the organ through +90°, 0° and -90°. The effect at the growing zone is
precisely the opposite to that at the tip, _i.e._, an expansive
reaction which results from the effect of indirect stimulus, in contrast
to the contractile reaction due to direct stimulation.
We may now proceed a step further and try to obtain some idea of the
difference in the mechanics of geotropic stimulation of the shoot and of
the root, to account for the different responses in the two organs. The
reason of this difference lies in the fact that in the shoot the
perceptive and responding region is one and the same; every cut-piece of
stem exhibits the characteristic geotropic curvature. In the root the
case is different; for the removal of the sensitive root-tip reduces or
abolishes the geotropic action; the region of maximum geotropic
perception is thus separated from that of response. It must be borne in
mind _that this holds good only in the case of gravitational stimulus_,
for the decapitated root still continues to respond to other forms of
stimulation such as chemical or photic.
The cause of this difference in the reactions to geotropic and other
stimuli lies in the fact that in the latter case, energy is supplied
from outside. But in geotropism the force of gravity is by itself
inoperative; it is only through the weight of the cell contents that the
stimulus becomes effective. Want of recognition of this fundamental
difference has led many observers in their far-fetched and sweeping
attempt, to establish an identity of reaction of the root to geotropic
and photic stimulations, in spite of facts which plainly contradict it.
Thus the root moves away from the incident vertical line of gravity; but
under light, the root very often moves towards the stimulus. The
negative phototropic response of the root of _Sinapis_ is an exceptional
phenomenon for which full explanation has been given in page 376.
We shall next consider whether the particular distribution of the
falling starch-grains (which offers a rational explanation of geotropic
stimulation) in the shoot and in the root, is capable of furnishing an
explanation of the different geotropic responses in the two organs. In
this connection, the results of investigation of Haberlandt and Nemec
are highly suggestive. Haberlandt finds statoliths present in the
responding region of the stem; the geotropic stimulation of the stem is
therefore direct. Nemec's investigation on the distribution of
statoliths in the root show, on the other hand, that it is the central
portion of the root cap that contains the falling starch grains, and
this would account for the indirect geotropic stimulation of the root.
The theory of statoliths is, however, not essential for the explanation
of the opposite geotropic effects in the shoot and in the root. The
observed fact, that the perceptive region in the root is separated from
the responding region, is sufficient to explain the difference of
geotropic action in the two organs. Through whatever means the stimulus
of gravity may act, it is inevitable, from the fact that the stimulation
of the shoot is direct and of the root indirect, that an identical
stimulus should in two cases induce responsive reactions of opposite
signs.
It will thus be seen that the postulation of two different
irritabilities in the shoot and in the root is wholly unnecessary and
unwarranted by facts. For the irritability of the root has been shown to
be in no way different from that of other organs; an uniformity is thus
found to exist in the reaction of all vegetable tissues.
SUMMARY.
On subjection of the tip of the root to the stimulus of gravity, the
upper side exhibits excitatory reaction of galvanometric negativity.
This shows that the root-tip undergoes direct stimulation.
The electric response in the growing region above the stimulated point
of the root-tip is positive, indicative of increase of turgor and
expansion. This is due to the effect of indirect stimulus.
The stimulus of gravity is perceived at the root-tip; it is the effect
of indirect stimulus that is transmitted to the responding region of
growth.
In contrast with the above is the fact that the growing region of the
shoot is both sensitive and responsive to geotropic stimulus.
As the effects of direct and indirect stimulation on growth are
antithetic, the responses of shoot and root to the direct and indirect
stimulus must be of opposite signs.
There is no necessity for postulating two different irritabilities for
the shoot and the root, since tissues in general exhibit positive or
negative curvatures according as the stimulus is direct or indirect.
XLIII.--LOCALISATION OF GEO-PERCEPTIVE LAYER BY MEANS OF THE ELECTRIC
PROBE
_By_
SIR J. C. BOSE,
_Assisted by_
SATYENDRA CHANDRA GUHA.
The obscurities which surround the phenomenon of geotropism arise: (1)
from the invisibility of the stimulating agent, (2) from want of
definite knowledge as to whether the fundamental reaction is contractile
or expansive, and (3) from the peculiar characteristic that the stimulus
is only effective when the _external_ force of gravity reacts
_internally_ through the mass of contents of the sensitive cells.
The experiments that have been detailed in the foregoing chapters will
have removed most of the difficulties. But beyond these is the question
of that power possessed by plants of _perceiving_ geotropic stimulus by
means of certain localised sense organs, which send out impulses in
response to which neighbouring cells carry out the movement of
orientation in a definite direction. Are the sensitive cells diffusely
distributed in the organ or do they form a definite layer? Could we by
the well established method of physiological response localise the
sensitive cells in the interior of the organ? As the internal cells are
not accessible, the problem would appear to be beyond the reach of
experimental investigation.
It is true that post-mortem examination of sectioned tissues under the
microscope enables us to form a probable hypothesis as regards the
contents of certain cells causing geotropic irritation; we have thus the
very illuminating theory of statoliths propounded by Noll, Haberlandt
and Nemec. But for the clear understanding of the _physiological
reaction_ which induces the orientating movement, it is necessary to get
hold, as it were, of a single or a group of sensory cells _in situ_ and
in a condition of fullest vital activity; to detect and follow by some
subtle means the change induced in the perceptive organ and the
irradiation of excitation to neighbouring cells, through the entire
cycles of reaction, from the onset of geotropic stimulus to its
cessation.
The idea of obtaining access to the unknown geo-perceptive cell in the
interior of the organ for carrying out various physiological tests would
appear to be very extravagant; yet I could not altogether give up the
thought that the obscure problem of geotropic action might be attacked
with some chance of success, by means of an electric probe which would
explore the excitatory electric distribution in the interior of the
organ. But the experimental difficulties which stood in the way were so
great that for a long time I gave up any serious attempt to pursue the
subject. And it is only when the present volume is going through the
press that the very first experiments undertaken proved so highly
successful that I am able to give a short account of the more important
results, which cast a flood of light on the obscurities of geotropic
phenomena. The new method has opened out, moreover, a very extensive
range of investigation on the activities of cells in the interior of an
organ, and enabled me to localise the conducting 'nerve' which transmits
excitation in plants. These and other results will be given in the next
volume.
[Illustration: FIG. 174.--Diagrammatic representation of the
geo-perceptive layer in unexcited vertical, and in excited horizontal
position. (See text.).]
METHOD OF EXPLORATION BY THE ELECTRIC PROBE.
The principle of the new method will be better understood if I first
explained the steps of reasoning by which I was led to discover it. The
experiments described in Chapter XL showed that the upper surface of a
horizontally laid shoot exhibits sign of excitation by induced
galvanometric negativity; that this was due to the stimulus of gravity
was made clear by restoration of the plant-organ to the vertical
position, when all signs of electric excitation disappeared. Now the
skin of the organ on which the electrode was applied could not be the
perceptive organ, for the removal of the epidermis did not abolish the
geotropic action; the perceptive layer must therefore lie somewhere in
the interior. As every side of a radial organ undergoes geotropic
excitation, the geo-perceptive cells must therefore be disposed in a
cylindrical layer, at some unknown depth from the surface. In a
longitudinal section of the shoot, they would appear as two straight
lines G and G´ (Fig. 174). In a vertical position the geo-perceptive
layer will remain quiescent but rotation through +90° would initiate the
excitatory reaction. Let us first centre our attention to the
geo-perceptive layer G, which occupies the upper position. This
sensitive layer perceives the stimulus and is therefore the focus of
irritation; the state of excitation is, as we have seen, detected by
induced galvanometric negativity, and the electric change would be most
intense at the perceptive layer itself. As the power of transverse
conduction is feeble, the excitation of the perceptive layer will
irradiate into the neighbouring cells in radial directions with
intensity diminishing with distance. Hence the intensity of responsive
electric change will decline in both directions outwards and inwards.
The distribution of the excitatory change, initiated at the perceptive
layer and irradiated in radial directions is represented by the depth of
shading, the darkest shadow being on the perceptive layer. Had
excitation been attended with change of light into shade, we would have
witnessed the spectacle of a deep shadow (vanishing towards the edges)
spreading over the different layers of cells during displacement of the
organ from vertical to horizontal; the shadow would have disappeared on
the restoration of the organ to the vertical position.
Different shades of excitation in different layers is, however, capable
of discrimination by means of an insulated electric probe, which is
gradually pushed into the organ from outside. It will at first encounter
increasing excitatory change during its approach to the perceptive layer
where the irritation will be at its maximum. The indicating galvanometer
in connection with the probe will thus indicate increasing galvanometric
negativity, which will reach a maximum value at the moment of contact of
the probe with the perceptive layer.
It will be understood that the surface electric reaction under geotropic
stimulus, which we hitherto obtained, would be relatively feeble
compared to the response obtained with direct contact with the maximally
excited perceptive layer. When the probe passes beyond the perceptive
layer the electric indication of excitation will undergo decline and
final abolition. The characteristic effects described above are to be
found only under the action of gravitational stimulus; they will be
absent when the organ is held in a vertical position and thus freed from
geotropic excitation.
I have hitherto spoken of the excitatory effect of the upper layer;
there must be some physiological reaction on the lower perceptive layer,
though of a different character, represented diagrammatically by
vertical shading. Had the physiological reaction on the lower side of a
radial organ been the same as on the upper, geotropic curvature would
have been an impossibility, for similar reactions on opposite sides
would, by their antagonistic effects, have neutralised each other.
After this preliminary explanation, I shall give a detailed account of
the experiments and results. It is to be borne in mind that the
investigation I am going to describe presupposes no hypothesis of
geotropic action. I start with the observed fact that an organ under the
stimulus of gravity, exhibits responsive movement. I ascertain the
nature of the underlying reaction by electric tests; I have, in my
previous works, fully demonstrated that the excitatory contractile
reaction is detected by electro-motive change of galvanometric
negativity, and the opposite expansive reaction by a change of
galvanometric positivity. With the electric probe I ascertain whether
geotropic irritation is diffuse, or whether it is localised at any
particular depth of the organ. I map out the contour lines of
physiological reaction with its heights and depths of excitation.
I shall now proceed to describe the results of electric exploration into
the interior of the organ. The trouble I foresaw, related to the
irritation caused by the passage of the probe, and the after-effect of
wound on variation of excitability.
THE ELECTRIC PROBE.
[Illustration: FIG. 175.--The Electric Probe. Figure to the left
represents one electric contact made with sepal of _Nymphæa_, and the
other, with the flower-stalk by means of the probe; the included
galvanometer is represented by a circle. Figure to the right an enlarged
view of the probe.]
The wound-irritation is, however, reduced to a minimum by making the
probe exceedingly thin. A fine platinum wire 0·06 mm. in diameter passes
through a glass tubing drawn out into a fine capillary, and fused round
one end of the platinum wire which protrudes very slightly beyond the
point of fusion; the exploring electrode is thus insulated except at the
protruded sharp point of the platinum wire. The length of the capillary
is about 6 mm., just long enough to pass the experimental plant-organ
transversely from one end to the other; the average diameter of the
capillary is about 0·15 mm. The other end of the platinum wire comes out
of the side of the tubing and is led to one terminal of the
galvanometer, the other being connected with an indifferent point in the
organ. The probe can be gradually pushed into the plant-organ by
rotation of a screw head, one complete rotation causing a forward
movement through 0·2 mm. (Fig. 175).
_Wound-reaction._--I have shown that a prick acts as a mechanical
stimulus, and in normal excitable tissues induces an excitatory change
of galvanometric negativity. This wound-reaction increases with the
extent of the wound, and the suddenness with which it is inflicted. On
account of the fineness of the probe, it insinuates itself into the
tissue rather than make any marked rupture; the probe again is
introduced very gradually; with these precautions the wound-reaction is
found to be greatly reduced. The immediate effect of the prick is a
negative deflection of the galvanometer, which declines and attains a
steady value in the course of about 5 minutes.
_Effect of wound on excitability._--I have shewn (p. 81) that severe
wound caused by transverse section induced a temporary abolition of
irritability in _Mimosa_, but that the normal excitability was restored
in the course of an hour. A prick from a thick pin was shown to depress
temporarily the rate of growth, the normal rate being restored after an
interval of 15 minutes (p. 202). In the case of geo-electric
excitability, the depressing effect of the passage of the probe, I find,
to disappear in the course of about 10 minutes.
For a choice of experimental material we have to find specimens which
are not merely geotropically sensitive, but also exhibit large electric
response under stimulus. In both these respects the shoot of
_Bryophyllum_ and the flower stalk of _Nymphæa_ give good results.
ELECTRIC EXPLORATION FOR GEO-PERCEPTIVE LAYER BY MEANS OF THE PROBE.
_Experiment 185._--I shall now proceed to give a detailed account of the
experiments. The first specimen employed was the shoot of
_Bryophyllum_, one contact being made with the side of the stem, and the
other with an indifferent point on the leaf which was always held
vertical. In a particular experiment, the probe was introduced into the
stem through 0·4 mm. and a feeble galvanometric negativity was induced
as the wound-effect. After an interval of 5 minutes, this attained a
steady value of -15 divisions. On the rotation of stem through +90°, the
point A was above and a very much larger deflection of -82 divisions was
obtained, being the result of summation of wound and geo-electric
effects. On restoration of the plant to vertical position the
geo-electric reaction disappeared, leaving the persistent wound reaction
of -15 divisions unchanged. The true geo-electric reaction at a point
0·4 mm. inside the stem was thus -67 divisions which is the difference
between -82 and -15 divisions. I obtained in this manner the excitatory
reactions at different layers of the organ. The following table gives
true values of geo-electric reaction at different layers of the stem as
the probe entered it by steps of 0·4 mm.
TABLE XL.--SHOWING THE GEO-ELECTRIC REACTION AT DIFFERENT DEPTHS OF
THE ORGAN (_Bryophyllum_).
+-------------------------------------------+
|Position of the| Geo-electric excitation |
|probe. |(galvanometric negativity).|
+---------------+---------------------------+
| Surface | 5 divisions. |
| 0·4 mm. | -20 " |
| 0·8 " | -24 " |
| 1·2 " | -22 " |
| 1·6 " | -18 " |
| 2·0 " | -14 " |
| 2·4 " | -10 " |
| 2·8 " | -5 " |
| 3·2 " | 0 " |
+-------------------------------------------+
The results given above, typical of many others, show that there is a
definite layer in the tissue which undergoes maximum excitation under
the stimulus of gravity, and that this excitation irradiates with
diminishing intensity in radial directions inwards and outwards.
_The geo-perceptive layer may thus be experimentally localised by
measuring the depth of intrusion of the probe for maximum deflection of
galvanometric negativity._
_Localisation of geo-perceptive layer in_ Nymphæa: _Experiment 186._--I
employed the same method for the determination of the perceptive layer
of a different organ namely, that of the flower stalk of _Nymphæa_. The
electric reaction in _Nymphæa_, even under the prevailing unfavourable
condition of the season, was moderately strong, being about three times
greater than in _Bryophyllum_. A dozen observations made with different
specimens gave very consistent results of which the following may be
taken as typical. The probe was in this case, as in the last, moved by
steps of 0·4 mm. at a time. Other examples will be given later where
readings were taken for successive steps of 0·2 mm.
TABLE XLI.--SHOWING THE DISTRIBUTION OF INDUCED GEO-ELECTRIC EXCITATION
IN DIFFERENT LAYERS (_Nymphæa_).
+--------------------------------------------+
|Position of probe.|Galvanometric deflection.|
+------------------+-------------------------+
| Surface | 0 divisions. |
| 0·4 mm. | -16 " |
| 0·8 " | -42 " |
| 1·2 " | -20 " |
| 1·6 " | -10 " |
| 2·0 " | -2 " |
| 2·4 " | 0 " |
+--------------------------------------------+
It will be seen that as in _Bryophyllum_, so in _Nymphæa_, the
geo-electric excitation increased at first with increasing depth of the
tissue till at a depth of 0·8 mm. of the particular specimen the induced
excitation attained a maximum value. The excitatory effect then
declines till it vanished at a depth of 2·4 mm.
The depth of layer at which maximum excitation takes place varies to
some extent, according to the thickness of the shoot. Thus while in a
thin specimen of _Bryophyllum_ 3·6 mm. in diameter the geo-perceptive
layer was found at a depth of 0·6 mm., it occurred at the greater depth
of 0·8 mm. in a thicker specimen, 5 mm. in diameter. In _Nymphæa_ also
the perceptive layer was found at a depth of 0·8 mm. in a thin and at a
depth of 1·4 mm. in a thick specimen.
Having thus succeeded in localising the geo-perceptive layer by
experimental means, it was now possible to examine the anatomical
characteristics of the layer by examining it under the microscope. I
also wished to find out from microscopic examination, the cause of
certain differences noticed in the determinations of the perceptive
layer in _Bryophyllum_ and in _Nymphæa_. In the former the probe always
encountered the maximally excited geo-perceptive layer from whichever
point of the surface it entered the organ; this indicated that the
sensitive layer in _Bryophyllum_ was continuous round the axis. In
_Nymphæa_, however, the probe occasionally missed the sensitive layer;
but a new point of entry led to successful localisation of the
perceptive layer; this was probably due to the particular layer not
being continuous but interrupted by certain gaps.
MICROSCOPIC EXAMINATION OF THE MAXIMALLY EXCITED LAYER.
The specimens were taken out after the electric test, and the transverse
sections made at the radial line of the passage of the probe. Thus in a
particular experiment with _Bryophyllum_ the point of maximum geotropic
excitation was found to be at a distance of 0·8 mm. from the surface. By
means of the micrometer slide in the stage and the micrometer eye-piece,
the internal layer 0·8 mm. from the surface was examined; the particular
sensitive layer S was recognised as the _continuous_ 'starch sheath' or
endodermis containing unusually large sized starch grains (Fig. 176).
These often occurred in loosely cohering groups of 8 to 10 particles,
and their appearance is very different from the small sized irregularly
distributed grains in other cells.
Examination of the microscopic section of the flower stalk of _Nymphæa_
showed that the 'starch sheath' was not continuous but occurred in
crescents above the vascular bundles which are separated from each
other. The occasional failure of electric detection of the perceptive
layer is thus due to the probe missing one of the crescents, which with
intervening gaps, are arranged in a circle.
[Illustration: FIG. 176.--Transverse section showing continuous
geo-perceptive layer S; enlarged view S' of cell of endodermis
containing group of large starch grains. (_Bryophyllum_).]
I give below a number of experimental determinations of the
geo-perceptive layer in different specimens together with the
micrometric measurement of the distance of the 'starch sheath' from the
surface, the transverse section being made at the place where the probe
entered the shoot. Eight different determinations are given, three for
_Bryophyllum_ and five for _Nymphæa_.
TABLE XLII.--SHOWING THE POSITION OF THE GEO-PERCEPTIVE LAYER AND
OF 'STARCH SHEATH' IN DIFFERENT SPECIMENS.
+-----------------------------------------------------------+
| Specimen. | Distance of | Distance of the |
| | geo-perceptive layer | starch sheath |
| | from surface. | from surface. |
| | (Method of | (Microscopic |
| | electric probe.) | measurement.) |
+---------------+----------------------+--------------------+
|_Bryophyllum_: | | |
| | (1) 0·6 mm. | 0·6 mm. |
| | (2) 0·8 " | 0·8 " |
| | (3) 0·8 " | 0·8 " |
| _Nymphæa_: | | |
| | (1) 0·6 " | 0·6 " |
| | (2) 0·8 " | 0·8 " |
| | (3) 0·8 " | 0·8 " |
| | (4) 1·0 " | 1·0 " |
| | (5) 1·4 " | 1·4 " |
+-----------------------------------------------------------+
Thus in all specimens examined, the experimentally determined
geo-perceptive layer coincided with the 'starch sheath.' The theory of
statoliths thus obtains strong support from an independent line of
experimental investigation. The statolithic theory has been adversely
criticised because in simpler organs the geotropic action takes place
in the absence of statoliths. There is no doubt that the weight of
the cell contents may in certain cases be effective in geotropic
stimulation; it may nevertheless be true that "at a higher level of
adaptation, the geotropically sensitive members of the plant-body are
furnished with special geotropic sense-organs--a striking instance of
anatomico-physiological division of labour."[38]
[38] Haberlandt--_Ibid_, p. 597.
In the instances of _Bryophyllum_ and _Nymphæa_ given above, the
geo-perceptive layer localised by means of the electric probe is
definitely found to be the endodermis containing large sized starch
grains.
INFLUENCE OF SEASON ON GEO-ELECTRIC RESPONSE.
I shall now describe certain modifications in response, which result
from the change of season and also from condition of high temperature.
Physiological reactions, generally speaking, are much affected by
different seasons; thus the seedlings of _Scirpus Kysoor_ exhibit a very
rapid rate of growth of 3 mm. per hour in August, but a month later the
growth-rate declines to only 1 mm. per hour. I find similar depression
of growth with the advance of season in seedlings of _Zea Mays_, where a
very rapid fall in growth takes place in the course of a fortnight. The
intensity of geotropic responses, both mechanical and electrical, of
_Tropæolum_ declines rapidly in the course of a month from February to
March (p. 454). The flowers of _Nymphæa_ began to appear by the end of
June when the flower stalks exhibited strong geo-electric response. But
later in the season, by July and the beginning of August, the response
underwent continuous decline, and by the end of August the response was
nearly abolished.
Much time had to be spent in perfecting the apparatus, and it was not
till the beginning of August that the investigations could be properly
started; the responsive indications were, however, marked and definite,
though relatively feeble compared to those obtained at the beginning of
the season. The decline of the geo-electric response was to a certain
extent also due to the prevailing high temperature.
_Effect of high temperature._--I shall in the next chapter describe
experiments which show that geotropic response is diminished under rise
of temperature. The specimens employed for localisation of
geo-perceptive layer exhibited, as stated before, a decline of
geo-electric response with the advance of the season. This may partly
be due to unfavourable season, and partly to high temperature. In the
middle of the season the responses were extremely feeble on warm days,
but on cool mornings they became suddenly enhanced, to decline once more
by the middle of the day. I could sometimes succeed in enhancing the
sensitiveness by placing the specimen in a cold chamber. It thus
appeared that certain internal change unfavourable for geo-perception
takes place at high temperatures, and that the sensitive condition could
sometimes be restored by artificial cooling. But later in the season,
the internal change, whatever it may be, had proceeded too far, and
artificial cooling did not restore the sensitiveness of the specimen.
What are the physico-chemical concomitants which distinguish insensitive
specimens, in which the electric indications had declined almost to the
vanishing point?
TEST OF INSENSITIVE SPECIMENS.
I shall now describe the various physico-chemical concomitants which
accompany the condition of relative insensibility. I have found three
different tests: the electric, the geotropic, and the microscopic, by
which the sensitive could be distinguished from the insensitive
condition. The following tests were made on insensitive specimens.
_Electric test: Experiment 187._--By the end of August the geo-electric
indications given by the probe had, as stated before, almost
disappeared. The tonic condition of the specimen, _below par_, was
independently revealed by the response to prick of the probe: this, in
vigorous specimens, is by an electric response of galvanometric
negativity. But the response to prick in sub-tonic specimens is very
different. I find that when the physiological condition of the tissue
falls _below par_, the sign of response undergoes a reversal into one of
_galvanometric positivity_. The same reversal under condition of
sub-tonicity was also shown to take place in growth, where under the
stimulus of light a positive acceleration took place, instead of normal
retardation of growth (p. 221). In the present investigation, the
insensitive specimens were found to give abnormal positive electric
response to the stimulus of prick made by the probe. The prick-effect in
fact often gave me previous indication as to the suitability of the
particular specimen for exhibition of geo-electric response.
_Test of geotropic reaction: Experiment 188._--I took four different
specimens of _Bryophyllum_ and _Nymphæa_, and held them horizontal.
These plant organs had, earlier in the season, exhibited very strong
geotropic effect, the shoot curving up through 90° in the course of ten
hours or less. But these specimens obtained later in the season
exhibited very feeble curvature, which hardly amounted to 10 degrees,
even after prolonged exposure to geotropic action for 24 hours.
_Test of microscopic examination._--I next made sections of
_Bryophyllum_ and _Nymphæa_ and on examining them under the microscope
discovered certain striking changes. A fortnight ago the group of large
starch grains stained with iodine were the most striking feature of the
starch sheath. But now these starch grains could not be found in any of
the numerous specimens examined. The presence of the starch grains thus
appears to be associated with the sensitiveness of the perceptive layer.
REACTION AT LOWER SIDE OF THE ORGAN.
There remains now the important question of the physiological change
induced on the lower side of the horizontally laid shoot. The
physiological reaction of two sides of the organ must be different,
since the upper side exhibits contraction and the lower side expansion.
It may be urged that the effect of one of the two sides might result
from the passive yielding to the definite reaction induced on the
opposite side. Investigation by the electric method enables us, however,
to discriminate the two reactions from each other, since the electric
response characteristic of the induced physiological change takes place
in the organ, even under condition of restraint by which movement is
prevented. We shall therefore investigate the geo-electrical reaction on
the lower side of the securely held organ, and find out whether the
induced electric change undergoes any variation in different layers from
below upwards. There are two different ways in which the electric
explorations of the lower side of the organ may be carried out. In the
first method, the probe is introduced from below, and successive
readings for geo-electric response taken as the probe enters the organ
by successive steps. It is understood that the true geotropic effect is
found from difference of galvanometer readings in vertical and
horizontal positions. In the second method, the probe is introduced from
above, and successive readings for the response taken for different
positions of the probe as it enters the organ from the upper side and
comes out ultimately at the lower side. This I shall call the METHOD OF
TRANSVERSE PERFORATION. The intrusion of the probe on the upper side
gives, as we have seen, increasing negative deflection of the
galvanometer which reaches a maximum at the perceptive layer. Passage of
the probe to still greater depths give deflections which decline to
zero. But when the probe comes within the influence of the perceptive
layer of the under side, the electric indication, as we shall presently
find, undergoes a reversal.
ELECTRIC EXPLORATION OF THE LOWER SIDE OF THE ORGAN.
I shall first describe the results obtained from the first method, the
probe entering the organ from the lower side.
_Experiment 189._--The investigation was carried out with the stem of
_Bryophyllum_, and the flower stalk of _Nymphæa_. The probe was made to
enter the organ through 0·4 mm. and the geo-electric effect found, on
rotation of the flower stalk of _Nymphæa_ from the vertical to the
horizontal, was a deflection of +6 divisions of the galvanometer. _The
change induced at the lower side by geotropic stimulus is thus
galvanometric positivity, indicative of enhancement of turgor, and, of
expansion._ Intrusion of the probe through 0·6 mm. gave rise to an
increased positive geo-electric response. That the sign of electric
response depended on the relation of the side of the organ to the
vertical lines of gravity was demonstrated by alternate rotation of the
plant through +90° and -90°, the probe remaining at a definite position.
Rotation through +90° brought A above, and rotation through -90° brought
A below. When the probe was in the _up_ position the geo-electric
response was negative, but when rotation through -90° brought it
_below_, the response became positive. Thus with an identical contact in
the plant, the electric response underwent reversal from negative to
positive. This will be understood from the following table.
+--------------------------------------------------+
|Position of the | Galvanometer | Galvanometer |
| probe inside | deflection: | deflection: |
| the organ. | A _above_. | A _below_. |
+----------------+----------------+----------------+
| 0·4 mm. | -8 divisions. | +5 divisions. |
| 0·6 mm. | -16 " | +10 " |
+--------------------------------------------------+
It will thus be seen that physiological change induced at any point is
modified by its relation to vertical lines of gravity. When the point is
above, the induced change is _negative_, when below, the induced change
is _positive_.
I shall next describe the variation of effect at different layers of the
under side of the organ.
_Experiment 190._--A complete set of readings of the geo-electric
reaction at different layers of the organ was taken, as the probe
entered the lower side by successive steps of 0·2 mm. The following
table gives the results obtained with a specimen of _Nymphæa_.
TABLE XLIII.--ELECTRIC EXPLORATION OF DIFFERENT LAYERS ON THE LOWER SIDE
OF THE ORGAN (_Nymphæa_).
+-----------------------------+
|Position of the|Galvanometer |
| probe. | deflection. |
+---------------+-------------+
| Surface | 2 divisions.|
| 0·2 mm. | 4 " |
| 0·4 " | 8 " |
| 0·6 " |16 " |
| 0·8 " |20 " |
| 1·0 mm. |32 " |
| 1·2 |16 " |
| 1·4 " |12 " |
| 1·6 " | 4 " |
| 1·8 " | 0 " |
+-----------------------------+
It is thus seen that just as in the upper so also in the lower side, the
electric variation undergoes at first an increase which attains a
maximum; beyond this point the electric change undergoes a rapid
decline. The induced electric change on the upper and lower sides are,
however, different, galvanometric _negativity_ in one case and
_positivity_ in the other.
The maximum galvanometric _negativity_ of the upper side was found to
occur at the geo-perceptive layer. We may next inquire about the
anatomical characteristic of the layer in the lower side of the organ
which exhibits the maximum galvanometric _positivity_. Microscopic
section of the specimen employed in the above experiment showed the
particular layer to be the starch crescent which lies above the vascular
bundle. Thus the same geotropic layer which when placed above shows the
maximum galvanometric negativity, exhibits maximum positivity when
placed below.
METHOD OF TRANSVERSE PERFORATION.
_Experiment 191._--I next carried out a complete exploration of the
interior of the organ along the diameter. The probe started from the
upper surface, and came out at the lower by successive steps of 0·2 mm.,
the corresponding geo-electric effects being observed at each step. It
has to be borne in mind that the successive readings were obtained by
rotation from vertical to +90° (A above); the rotation was never carried
out in the negative direction through -90°. But the probe entering from
above passed the central axis, and entered a region where the
galvanometric indication was transformed from negative to positive. The
following table gives the results obtained with the flower stalk of
_Nymphæa_.
TABLE XLIV.--SHOWING THE INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE
FLOWER STALK OF _Nymphæa_ (diameter = 6·8 mm.)
+---------------------------------+
| Position of | Galvanometer |
| probe. | deflection. |
+--------------+------------------+
| Surface ... | - 10 divisions. |
| 0·2 mm. ... | - 26 " |
| 0·4 " ... | - 40 " |
| 0·6 " ... | - 50 " |
| 0·8 " ... | - 62 " |
| 1·0 " ... | - 72 " |
| 1·2 " ... | - 88 " |
| 1·4 " ... | -108 " |
| 1·6 " ... | - 72 " |
| 1·8 " ... | - 44 " |
| 2·0 " ... | - 30 " |
| 2·2 " ... | - 18 " |
| 2·4 " ... | - 10 " |
| 2·6 " ... | - 5 " |
| 2·8 " ... | - 2 " |
| 3·0 " ... | 0 " |
| 3·2 " ... | 0 " |
| 3·4 " ... | 0 " |
| 3·6 mm. ... | 0 divisions. |
| 3·8 " ... | 0 " |
| 4·0 " ... | 0 " |
| 4·2 " ... | + 2 " |
| 4·4 " ... | + 4 " |
| 4·6 " ... | + 5 " |
| 4·8 " ... | + 11 " |
| 5·0 " ... | + 22 " |
| 5·2 " ... | + 38 " |
| 5·4 " ... | + 46 " |
| 5·6 " ... | + 39 " |
| 5·8 " ... | + 32 " |
| 6·0 " ... | + 24 " |
| 6·2 " ... | + 18 " |
| 6·4 " ... | + 12 " |
| 6·6 " ... | + 6 " |
| 6·8 " ... | + 3 " |
+---------------------------------+
[Illustration: FIG. 177.--Curve of geo-electric excitation in different
layers of _Nymphæa_. Ordinate represents geo-electric excitation;
abscissa, distance from upper surface of flower stalk. The diagrammatic
section underneath shows the position of geo-perceptive layer
(starch-sheath) corresponding to maximum induced galvanometric
negativity and positivity on the two sides.]
[Illustration: FIG. 178.--The curve of geo-electric excitation in
different layers of _Bryophyllum_.]
A curve constructed from the data given above is seen in figure 177. The
diameter of the flower stalk was 6·8 mm. The negative geo-electric
reaction is seen to undergo an increase till it attains a climax at the
depth of 1·4 mm. It then undergoes a continuous diminution till it
becomes zero at the depth of 3 mm.; this neutral zone extends through 1
mm. When the probe enters a depth of 4·2 mm. measured from the upper
side, it enters a region affected by the perceptive layer situated on
the under side, the opposite physiological reaction being indicated by
induced electric change of galvanometric positivity. This positivity
reaches a climax at a depth of 5·4 mm. measured from the upper side, and
1·4 mm. when measured from the lower side. The points of maximum
positivity and negativity are situated symmetrically on the opposite
sides of the organ. The electric variation of maximum positivity on the
lower side is comparatively feeble, less than half the corresponding
maximum negativity on the upper side. Microscopic section showed that
the geo-perceptive layers were the same as the starch-crescents.
_Experiment 192._--I carried out similar experiments with the shoot of
_Bryophyllum_. The results are given in Table XLV; the curve of the
electric distribution along the diameter is seen in figure 178. The
characteristics of this curve are the same as that of _Nymphæa_. The
maximum galvanometric negativity occurred at the depth of 0·6 mm., and
of positivity at a corresponding point on the opposite side.
TABLE XLV.--SHOWING INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE STEM
OF _Bryophyllum_ (diameter = 3·6 mm.).
+--------------------------------+
| Position of | Galvanometric |
| probe. | deflection |
+---------------+----------------+
|Surface | 0 divisions.|
|0·2 mm. | -24 " |
|0·4 " | -45 " |
|0·6 " | -63 " |
|0·8 " | -21 " |
|1·0 " | - 9 " |
|1·2 " | - 6 " |
|1·4 " | - 3 " |
|1·6 " | 0 " |
|1·8 " | 0 " |
|2·0 " | 0 divisions.|
|2·2 " | 0 " |
|2·4 " | + 3 " |
|2·6 " | + 4 " |
|2·8 " | + 9 " |
|3·0 " | +36 " |
|3·2 " | +21 " |
|3·4 " | + 9 " |
|3·6 " | 0 " |
+--------------------------------+
Microscopic examination showed that the electric maxima in _Bryophyllum_
coincided with the diametrically opposite points in the continuous
endodermic ring. In _Bryophyllum_ as in _Nymphæa_, the excitatory
galvanometric negativity of the upper geo-perceptive layer is greater
than the induced positivity of the lower layer in the ratio of about
2:1. But in a depressed condition of the tissue, the excitatory reaction
is the first to disappear and the positive reaction persists, though
with diminished intensity.
The geo-electric distribution in vigorous specimens seems to indicate
that under the stimulus of gravity a marked excitatory reaction
(contraction) takes place in the layer of cells contiguous to the upper
geo-perceptive layer, and, a less marked positive reaction (expansion)
occurs in layers contiguous to the lower perceptive layer.
It is remarkable that physiological reaction of opposite kinds should
occur on the upper and lower sides of an organ under the identical
stimulus of gravity. The difference of reaction may conceivably be
connected with the fact that the vertical lines of gravity enter by the
upper, and leave by the lower side of the organ. The statolithic
particles rest on the inner tangential walls of the perceptive cells of
the upper layer, and on the outer tangential walls of the lower layer.
Similar difference of physiological reactions of a polar character are
also known in responses of plants under the action of an identical
electric current; here with different ionic distributions, contraction
takes place at the kathode, and expansion at the anode.
The geo-electric reactions that have been described were obtained under
unfavourable conditions of climate and of temperature. But under better
conditions the reaction becomes very greatly enhanced, as would appear
from the following account of results which I obtained on two separate
occasions in the beginning of August. The season had not become quite as
unfavourable as towards the end of the month, but the prevailing sultry
weather had caused great depression of the geo-electric excitability.
On the first occasion referred to, thunderstorm had broken out at night,
and it was refreshingly cool in the morning. It was with the utmost
surprise that I noted the astonishing violence of the geo-electric
response which the plants gave that morning; the maximum response
hitherto obtained was about 100 divisions of the galvanometer scale; but
on the present occasion the displacement of the plant, from vertical to
horizontal position, induced responsive deflection so great that the
galvanometer spot of light flew off the scale of 3,000 divisions. I was
at first incredulous of the results and wasted the valuable occasion in
trying to discover some hidden source of error. Subsequent tests showed
that my misgivings were groundless, and that the extraordinary large
deflection was really due to geo-electric reaction. On the second
favourable occasion, which lasted for three hours (during the cool hours
of the morning), I was able to secure a number of important
observations. Thus displacement of the flower stalk of _Nymphæa_ through
+90° was immediately followed by geo-electric response, the deflection
being about 3,000 divisions of the scale. The latent period hardly
exceeded a second; the return of the plant to the vertical position was
quickly followed by electric recovery which was complete. The above
results were obtained with the same specimen time after time without a
single failure. The successive responses showed no sign of fatigue.
Another remarkable effect was noticed during gradual increase of the
angle of inclination. Nothing happened till a critical angle was
reached, which was roughly estimated to be about 33°; when this critical
angle was exceeded by a single degree, there was a sudden precipitation
of geo-electric response. The experiments were repeated time after time
with the identical result. It appeared as if some frictional resistance
obstructed the displacement of the geotropic particles accumulated at
the basal end of the cell, and it was not till the organ had been
tilted beyond 33° that this resistance to sliding was overcome.
SUMMARY.
The electric distribution induced in an organ under the stimulus of
gravity may be mapped out by means of an exploring Electric Probe.
The induced galvanometric negativity of the upper side of an organ
(indicative of excitation) undergoes variation in different layers of
the organ. The excitatory reaction attains a maximum value at a definite
layer, beyond which there is a decline.
The geo-perceptive layer is experimentally localised by measuring the
depth of intrusion of the probe for maximum deflection of galvanometric
negativity.
The geo-perceptive layer thus determined is found to be the starch
sheath which contains a number of large-sized starch grains.
The power of geo-perception undergoes seasonal variation. It is also
lowered by high temperature.
The geo-electric response undergoes decline with growing sub-tonicity of
the specimen; such specimens exhibit abnormal positive electric response
under the stimulus of prick and feeble curvature under geotropic
stimulus. The large-sized starch-grains, normally observed in the
endodermis, are found to disappear in specimens which have become
geo-electrically insensitive.
The electric response of the lower side of the organ to gravitational
stimulus is of opposite sign to that of the upper side. The electric
distribution on the lower side exhibits variations in different layers,
the maximum positivity occurring at the perceptive layer. In vigorous
specimens the excitatory negative electric change on the upper side is
greater than the positive electric change on the lower side. Depressed
condition of the tissue is attended by a relatively greater decline of
the negative in comparison with the positive.
The induced electric variation on the upper and on the lower side
indicates that the layers of tissue contiguous to the upper perceptive
layer undergoes contraction, while those contiguous to the lower
perceptive layer undergoes expansion.
XLIV.--ON GEOTROPIC TORSION
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
I have explained that in a dorsiventral organ, lateral application of
various stimuli induces a responsive torsion by which the less excitable
side is made to face the stimulus (p. 403). I shall in this chapter show
that the effect of stimulus of gravity is in every respect similar to
other forms of stimulation.
[Illustration: FIG. 179.--Diagram of arrangement for torsional response
under geotropic stimulus. The less excitable upper half of pulvinus is,
in the above figure, to the left and the torsional response is
clockwise.]
The direction of force of gravity is fixed, and we have to arrange
matters in such a way that the geotropic stimulus should act on the
dorsiventral organ in a lateral direction. In the following experiments
the pulvinus of _Mimosa_ is taken as the typical dorsiventral organ.
For lateral stimulation, the plant is placed on its side, so that the
vertical lines of gravity impinge on one of the two flanks of the organ.
In regard to this, I shall distinguish two different positions, _a_ and
_b_. In the _a-position_, the apex of the stem and the upper half of the
pulvinus are to the left of the observer, and in _b-position_, the apex
of the stem and the less excitable upper half of the pulvinus are to the
right. The arrangement for obtaining record of the torsional response
under _a-position_ is shown in figure 179.
_Torsional response in a- and b-positions: Experiment 193._--When the
leaf is in _a-position_, the geotropic torsion is found to be with the
movement of the hands of a clock. In the _b-position_, on the other
hand, the torsion is against the hands of a clock. In both these cases
the _geotropic torsion makes the less excitable upper half of the
pulvinus face the vertical lines of gravity_. The incident stimulus is
vertical, and it is the upper flank, consisting of the upper and lower
halves of the pulvinus (on which the vertical lines of gravity impinge)
that undergoes effective stimulation.
_Algebraical summation of geotropic and phototropic effects: Experiment
194._--We are, however, able to adduce further tests in confirmation of
the above. If the direction of the incident geotropic stimulus is
vertical, and should it act more effectively on the upper flank, it
follows that stimulus of light acting from above would enhance the
previous torsional response due to geotropism. In the above case, the
lines of gravity and the rays of light coincide. The effect of rays of
light acting from below should, on the other hand, oppose the geotropic
torsion. The additive effect of stimulus of light and gravity is seen
illustrated in figure 180. The first part of the curve is the record of
pure geotropic torsional movement. Light from above is applied at L;
the rate of movement is seen to become greatly enhanced. Light is next
cut off, and the enhanced rate induced by it is also found to disappear,
the response-curve being now due solely to geotropic action. The effect
of geotropism in opposition to phototropism will be found in the
following experiments, where the opposing action of light of different
intensities is seen to give rise to a partial, to an exact, or to an
over-balance.
[Illustration: FIG. 180.--Additive effect of stimulus of gravity G, and
of light L. Application of light at--L increases torsional response.
Removal of light restores original geotropic torsion.]
[Illustration: FIG. 181.--Algebraical summation of geotropic and
phototropic actions. Light applied below at--L, opposes geotropic
action. Cessation of light restores geotropic torsion. Cessation of
light is indicated by L within a circle.]
BALANCE OF GEOTROPIC BY PHOTOTROPIC ACTION.
_Photo-geotropic balance: Experiment 195._--I shall here describe in
detail the procedure for obtaining an exact balance. A parallel beam of
light from a small arc lamp is reflected by means of an inclined mirror,
so as to act on the pulvinus below. An iris diaphragm regulates the
intensity of incident light. The first part of the curve is the record
of geotropic torsional movement. Light of a given intensity was applied
below at a point marked -L (Fig. 181); this is seen to produce an
over-balance, the phototropic effect being slightly in excess. The
intensity of incident light was continuously diminished by regulation of
the diaphragm till an exact balance was obtained as seen in the
horizontal part of the record. It is with great surprise that one comes
to realise the fact that the effect of one form of stimulus can be so
exactly balanced by that of another, so entirely different, and that the
stimulus of gravity could be measured, as it were, in candle powers of
light! After securing the balance, light was cut off, and the geotropic
torsion became renewed on the cessation of the counteracting phototropic
action.
[Illustration: FIG. 182.--Application of white light at--L in opposition
causes reversal of torsion. Red light R, is ineffective, and geotropic
torsion is restored. Reapplication of white light causes once more the
reversal of torsion.]
_Comparative balancing effects of white and red lights: Experiment
196._--White light was at first applied at -L in opposition to geotropic
movement. The intensity of light was stronger than what was necessary
for exact balance, and its effect was at first to retard and then
reverse the torsional response due to geotropism. When thus
overbalanced, red glass was interposed on the path of light at R. As the
phototropic effect of this light is feeble or absent, the geotropic
torsion became predominant as seen in the subsequent up-curve. The red
glass was next removed substituting white light at -L to act once more
in opposition; the result is seen in the final over-balance, and
reversal of torsion (Fig. 182).
[Illustration: FIG. 183.--Effect of coal gas on photo geotropic balance.
Geotropic torsion, G, is exactly balanced by opposing action of light
-L. Application of coal gas at C, at first caused enhancement of
phototropic action with resulting reversal. Prolonged application
induced depression of phototropic reaction, geotropic action thus
becoming predominant.]
_Effect of coal gas on the balance: Experiment 197._--The method of
balance described above opens out new possibilities in regard to
investigations on the relative modifications of geotropic and
phototropic excitabilities by a given external change. Traces of coal
gas are known to enhance the phototropic excitability of an organ while
continued absence of oxygen is found to depress it. The experiment I am
going to describe shows: (1) the enhancement of phototropic excitability
on the introduction of coal gas, and (2) the depressing effect of excess
of coal gas and of the absence of oxygen. After obtaining the normal
curve of geotropic torsion, light was applied below at -L, and exact
balance was obtained in the course of two minutes as seen in the top of
the curve becoming horizontal. Coal gas was now introduced in the
plant-chamber at C. This induced an enhancement of phototropic effect
with resulting over-balance seen in the reversal of torsion. This
enhancement persisted for more than three minutes. By this time the
plant-chamber was completely filled with coal gas, and the resulting
depression of phototropic action is seen in the second upset of the
balance, this time in favour of geotropic torsion (Fig. 183). It would
seem that the cells which respond to light are situated nearer the
surface of the organ than those which react to geotropic stimulus. Hence
an agent which acts on the organ from outside, induces phototropic
change earlier than variation in geotropism.
SUMMARY.
Under lateral action of geotropic stimulus, a dorsiventral organ
undergoes torsional response by which the less excitable half of the
organ is made to face the stimulus.
The direction of incident geotropic stimulus is the same as the
direction of vertical lines of gravity. Under geotropic stimulus it is
the upper side of the organ that undergoes effective stimulation.
The effects of gravity and of light become algebraically summated under
their simultaneous action. Light may be made to act in opposition to the
stimulus of gravity. By suitable adjustment of the intensity of light,
the two torsions become exactly balanced.
This state of balance is upset by any slight variation in one of the
opposing stimuli.
The relative modification of geotropic and phototropic excitabilities by
an external agent, is determined by the resulting upset of the
photo-geotropic balance.
XLV.--ON THERMO-GEOTROPISM
_By_
SIR J. C. BOSE.
I shall in this chapter investigate the effect of variation of
temperature on geotropic response. We have to bear in mind in this
connection, that for the exhibition of geotropic curvature two
conditions are necessary: (1) the presence of a perceptive organ to
undergo excitation under the stimulus of gravity, and (2) the motility
of the organ. A motile organ, including both the pulvinated and growing,
will exhibit no geotropic effect on account of the depression of the
power of perception through seasonal or other changes, or in the entire
absence of the perceptive organ. The organ may, on the other hand,
possess the geo-perceptive apparatus, but no visible movement can take
place in the absence of motility of the tissue.
As regards the modifying influence of temperature on geotropic
curvature, the effect will depend on two factors:
(1) the influence of variation of temperature on
geo-perception by the sensitive layer, and
(2) the modifying effect of temperature variation
on the motile reaction.
I have in Chapter XLIII adduced facts which appear to show that the
power of geo-perception declines at high temperatures. As regards motile
reaction, we have seen that in _Mimosa_ it increases from a minimum to
an optimum temperature beyond which there is a depression (p. 55). As
the optimum temperature for geo-perception is not necessarily the same
as that for responsive curvature, the result is likely to be very
complex.
The case becomes simpler after the attainment of maximum curvature.
Enhanced temperature has a tendency to diminish the tropic curvature, as
we found in the arrest and reversal of phototropic curvature under the
application of warmth (p. 393); it appears as if rise of temperature
induced a relatively greater expansion of the contracted side of the
organ.
I shall now describe the effect of rising temperature on geotropic
curvature in general, including torsion. A horizontally laid shoot
curves upwards under geotropic action; a dorsiventral organ, owing to
the differential excitabilities of its upper and lower sides, places
itself in the so-called dia-geotropic position. A dorsiventral organ,
moreover, exhibits a torsional movement under lateral stimulus of
gravity.
In the geotropic movements we are able, as stated before, to distinguish
three different phases (cf. Fig. 161). In the first, the movement
initiated undergoes an increase; in the second, the rate of movement
becomes more or less uniform; and in the last phase, a balance takes
place between the tropic reaction, and the increasing resistance of the
curved or twisted organ to further distortion.
The question now arises whether this position of geotropic equilibrium
is permanent, or whether it undergoes modification in a definite way by
variation of temperature. I shall proceed to show that the position of
equilibrium undergoes a change in one direction by a rise, and in the
opposite direction by a fall of temperature. I shall use the term
_thermo-geotropism_ as a convenient phrase to indicate the effect of
temperature in modification of geotropic curvature and torsion.
I shall first deal with the effect of variation of temperature on
geotropic torsion. Under the continued action of stimulus of gravity the
torsion increases till it reaches a limit; for the twisted organ
resists further distortion and a balance is struck when the twisting and
untwisting forces are equal and opposite. In this state of equilibrium
the effect of an external agent, say of variation of temperature, will
bring about an upset of the balance. The torsion will be increased if
the external agent induces an enhancement of geotropic action; it will,
on the other hand, be decreased when it induces a diminished reaction.
[Illustration: FIG. 184.--Magnet M causes deflection of the needle _n
s_, suspended by a thin wire. Increase of magnetisation of M increases
deflection, while decrease of magnetisation diminishes the deflection.]
A physical analogy will make this point clear; imagine a small magnetic
needle suspended by a thin wire; the earth's directive force is supposed
to be annulled by the well known device of a compensating magnet. A
second and larger magnet M is now placed at right angles to the
suspended needle; N will repel _n_ and attract _s_, and a deflection
will be produced, the deflecting force of the magnet M being balanced by
the force of torsion of suspending wire (Fig. 184).
The state of equilibrium will however be disturbed by variation of the
magnetic force of M. It is known that a rise of temperature diminishes
magnetisation while lowering of temperature increases it. Hence the
deflecting force of the magnet will be diminished under rise of
temperature with concomitant diminution of deflection of the needle and
the torsion of the wire. Fall of temperature, on the other hand, will
cause an increase of deflection and of torsion. The physical
illustration given above will help us to understand how the
physiological effect of variation of temperature may bring about changes
in geotropic curvature and torsion.
TROPIC EQUILIBRIUM UNDER VARYING INTENSITIES OF STIMULUS.
The following experiment will show that the position of tropic
equilibrium is not fixed but subject to variation under changes of
effective stimulation.
[Illustration: FIG. 185.--Effect of variation of intensity of light on
phototropic equilibrium. Increase of intensity of light from L to L'
produces an increased positive curvature and a new state of balance.
Diminished intensity of light _l_ brings about a new balance at a lower
level. The cessation of light (_l_ within a circle) restores the normal
position of the organ.]
_Experiment 198._--I have explained how a maximum tropic curvature is
induced under continued action of light. Employing the pulvinus of
_Erythrina indica_ I applied light on the upper half of the pulvinus:
(1) of medium intensity L, (2) of strong intensity L', and (3) of feeble
intensity _l_. The source of light was an arc lamp; the intensity of
light was varied by means of a focussing lens, which gave a parallel, a
convergent or a divergent beam, with corresponding increase or
diminution of intensity of light. Light was in each case continued till
equilibrium was reached. Inspection of figure 185 shows that the
position of equilibrium depends on the intensity of stimulation; the
balance is 'raised' under increased and 'lowered' under decreased
intensity.
In the case of geotropism the stimulus is constant, but its tropic
effect, we shall presently see, undergoes variation with changing
temperature.
EFFECT OF VARIATION OF TEMPERATURE ON GEOTROPIC TORSION.
_Modification of geotropic torsion: Experiment 199._--The _Mimosa_ plant
was placed on its side, so that the pulvinus was subjected to lateral
geotropic action. In response to this it underwent torsion, the upper
half of the pulvinus tending to place itself so as to face the vertical
lines of gravity. This torsional response was recorded as an
up-movement; on the attainment of equilibrium the record became
horizontal. The plant was now subjected to a cyclic variation of
temperature, and the resulting variation of torsion recorded at the same
time. The temperature of the plant chamber was gradually raised from the
normal 30° to 34° C. and then allowed to return to the normal; finally
the temperature was lowered to 26°C. Rise of temperature was effected by
means of an electrical heater placed inside the chamber with a vessel of
water placed above it. Care has to be taken that the rise of temperature
is gradual, since a sudden variation often acts as a stimulus. The water
in the vessel not only keeps the chamber in a humid condition but also
prevents sudden fluctuation of temperature. After the temperature had
been raised to 34°C., the heating current was stopped and the door of
the plant chamber gradually opened, so as to allow the temperature to be
restored to the normal. Cooled air was next introduced into the chamber
till the temperature fell to 26°C. Figure 186 exhibits clearly the
effect of variation of temperature on geotropic torsion. The maximum
torsion had been attained at 30°C. and the first part of the record is
therefore horizontal. Warmth was applied at H, and after a latent period
of ten minutes, the geotropic torsion underwent a continuous diminution
till a new state of equilibrium was reached at 34°C. This took place
shortly after the stoppage of the heating current at (H). On return to
normal temperature the torsional balance was restored to its original
position of equilibrium. Application of cold at C, is seen to bring
about a new state of balance with an increase of geotropic torsion.
[Illustration: FIG. 186.--Effect of variation of temperature on
geotropic torsion. Application of warmth at H diminishes the geotropic
torsion; return to normal temperature (H) restores the original torsion;
cooling at C, increases the geotropic torsion.]
The position of geotropic equilibrium is thus seen to be modified by
variation of temperature, the tropic effect being diminished with the
rise, and enhanced with the fall of temperature.
It may be thought that the phenomenon just described may not be
different from ordinary thermonasty, exhibited by the perianth leaves of
_Crocus_ and _Tulip_ in which a rise of temperature induces a movement
of unfolding, and a fall of temperature brings about the opposite
movement of closure. In these cases the movement is determined solely
by the natural anisotropy of the organ, and not by the paratonic action
of a directive external force. Thus the inner side of the perianth
leaves undergoes an expansion with rise of temperature attended by the
opening of the flower; this movement of opening does not undergo any
change on holding the flower in an inverted position.
But the torsional movement of the leaf of _Mimosa_, and the induced
variation of torsion under change of temperature are not solely
determined by the natural anisotropy of the organ; it is, on the
contrary, regulated by the directive action of the stimulus of gravity.
The pulvinus in normal position does not exhibit any geotropic torsion
and in the absence of an antecedent torsion change of temperature cannot
induce any variation in it. It is only after the pulvinus had become
torsioned under the lateral action of geotropic stimulus that a
responsive variation is induced in it by the action of changing
temperature.
The change in torsion is, moreover, determined in reference to the
paratonic action of incident geotropic stimulus. This will be clearly
understood from the tabular statement given below.
TABLE XLVI.--SHOWING THE EFFECT OF RISE OF TEMPERATURE ON GEOTROPIC
TORSION.
+-------------------------------------------------------------------+
|Position of the organ.| Geotropic effect. | Effect of rise of |
| | | temperature. |
+----------------------+---------------------+----------------------+
|Right flank above: |Right-handed torsion.|Left-handed torsional |
| (_a_) position. | | movement (untwist). |
|Left flank above: |Left-handed torsion. |Right-handed torsional|
| (_b_) position. | | movement (untwist). |
+-------------------------------------------------------------------+
By right flank in the above table is meant the side of the pulvinus to
the right of the observer facing the leaf of the plant held in the
normal position. When the plant is laid on its left side in the
_a_-position, the right flank will be above and the responsive torsion
under geotropic stimulus becomes right handed or with the hands of a
clock (Cf. Fig. 179). When the plant is laid on its right side, the left
flank will be above and the geotropic torsion becomes left handed or
against the hands of the clock.
It will be seen from the above that in whatever way the experimental
condition may be varied, the movement in response to variation of
temperature is determined in relation to the antecedent geotropic
torsion. The geotropic effect whether left-handed or right-handed
torsion is always diminished by the rise of temperature, and enhanced by
the fall of temperature.
VARIATION OF APO-GEOTROPIC CURVATURE UNDER THERMAL CHANGE.
I shall now proceed to show that variation of temperature not merely
induces variation of geotropic torsion but also of geotropic curvature.
I shall first demonstrate the effect of thermal change on geotropic
curvature of the shoot, and then demonstrate its effect on dia-geotropic
curvature of leaves.
_Experiment 200._--A specimen of _Tropæolum majus_ grown in a small
flower pot, is laid on its side. Under geotropic action the shoot
becomes curved, the upper side becoming concave and the lower side
convex. The end of the stem is attached to the recording apparatus; when
the plant is subjected to a rise of temperature, the movement induced
shows that the geotropic effect has undergone a diminution, the
curvature exhibiting a flattening; lowering of temperature, on the other
hand, increases the geotropic curvature. Other instances of this will be
found in a subsequent chapter. The diurnal movement of the 'Praying
Palm' is a striking example of the effect of variation of temperature in
modification of geotropic curvature (p. 30). Rise of temperature is
thus shown to diminish geotropic torsion of dorsiventral organs, and the
apo-geotropic curvature of radial organs. We have next to study the
effect of temperature variation on the dia-geotropic equilibrium of
leaves.
EFFECT OF VARIATION OF TEMPERATURE ON DIA-GEOTROPIC EQUILIBRIUM.
In the normal position of the plant, the leaf of _Mimosa_ assumes, under
geotropic action, an equilibrium position which is approximately
horizontal. I shall proceed to show that this position of equilibrium
also undergoes appropriate variation under changing temperature, the
leaf undergoing a fall during rise, and an erection during fall of
temperature.
I stated that the torsional response is one of the means of recording
geotropic effect and its variations. In the ordinary position of the
plant, the geotropic variation will be indicated by the responsive up or
down movement of the leaf in a vertical plane. Taking the leaf of
_Mimosa_, we have thus the means of studying the effect of variation of
temperature by two independent means of inquiry, namely, by record of
ordinary responsive movement in a vertical plane, and also by record of
torsional response. The variation of temperature which induces these
movements may be simultaneously recorded by means of a differential
metallic thermometer. The Multiplex Recorder employed for this research
consists of three recording levers. A photographic reproduction of the
apparatus will be found in a subsequent chapter (see Fig. 190). The
first lever is attached to the leaf of _Mimosa_ placed in the normal
position; the second lever records the torsional response of _Mimosa_
leaf, the plant being placed on its side; the third lever attached to
the differential metallic thermometer gives a continuous record of
variation of temperature.
[Illustration: FIG. 187.--Simultaneous record (_a_) of variation of
temperature, (_b_) of up or down movement of leaf of _Mimosa_, and (_c_)
of variation of torsion. Rise of temperature is attended by fall of leaf
and diminution of torsion, fall of temperature inducing the opposite
effect.]
_Effect of variation of temperature: Experiment 201._--Special
arrangement was made for gradual variation of temperature in the plant
chamber. Two rectangular metallic vessels each 50 × 30 × 6 cm. were
placed on opposite sides of the plant chamber, and warm water was made
to circulate through them; this device ensured a steady rise of
temperature. The flow of warm water was then stopped and the plant
chamber was allowed to cool down; the fall of temperature was at first
moderately rapid, but later on the rate of cooling became extremely
slow; on account of this the temperature of the plant chamber, towards
the end of the experiment remained higher than the normal temperature
outside. The rate of rise and fall of temperature during the entire
course is illustrated in the thermo-graphic (_a_) tracing (Fig. 187);
the record (_b_) exhibits the movement of the leaf in a vertical plane,
rise of temperature being attended by a diminution of geotropic
curvature resulting in the fall of the leaf, the fall of temperature
inducing the opposite effect. In record (_c_) is seen the responsive
variation of geotropic torsion, rise of temperature inducing a
diminution and fall of temperature causing an enhancement of torsion.
The results obtained by diverse methods thus prove that the geotropic
effect is diminished under rise, and increased under fall of
temperature.
SUMMARY.
The position of equilibrium under geotropic action is not fixed but
undergoes change with variation of temperature.
The geotropic curvature and torsion are increased by lowering of
temperature, and decreased by rise of temperature. This is equally true
of apo-geotropic and dia-geotropic curvatures.