is an antagonistic reaction induced by it which opposes the excitatory
contraction. The physiological reaction of a rise of temperature, within
normal range, is expansion and this must oppose the contraction induced
by stimulus. Hence the effect of rise of temperature is complex; it
enhances the excitability which favours contraction, while tending to
oppose this contraction by the induced physiological expansion. As a
result of these opposite reactions there will be a critical temperature,
below which the contractile effect will relatively be greater than
expansion; above the critical point, expansion will be the predominant
effect. The critical temperature will obviously be different in
different organs. The positive curvature may thus be increased by a
slight rise, while it may be neutralised, or even reversed by a greater
rise of temperature.
The induced variation of excitability due to change of temperature is
not the only factor in modifying tropic curvature, for variation of
conductivity also exerts a marked effect.
EFFECT OF TEMPERATURE ON CONDUCTION.
The conducting power of an organ is greatly enhanced with rise of
temperature. Thus in _Mimosa_ the velocity of transmission of excitation
is doubled by a rise of temperature through 9°C. (p. 100). An organ
which is practically non-conducting at a low temperature will become
conducting at a higher temperature.
Thus at a low temperature the organ may be non-conducting, and the
excitatory contraction under unilateral stimulus will remain localised
at the proximal side; this will give rise to a positive curvature. But
under rising temperature, the power of transverse conduction will be
increased and the excitation will be conducted to the distal side. The
result of this will be a neutralisation or reversal into negative
curvature (p. 139). A positive curvature is thus reversed into negative
by change of excitability and conductivity, induced by rise of
temperature; examples of this will be given presently.
PHOTOTROPIC RESPONSE OF TENDRILS.
I shall here adduce considerations which will show that the apparent
anomalies regarding the response of tendrils to light is due to the
variation of transverse conductivity of the organ. In a semi-conducting
tissue, while the excitatory effect of feeble stimulus remains localised
at the proximal side, the effect of stronger stimulus is conducted to
the distal side. This explains the positive phototropic curvature of
tendrils of _Vitis_ and _Ampelopsis_ under feeble light, and its
reversal into negative curvature under intense light.
As the conducting power is increased with rise of temperature it is
evident that at a certain temperature the tropic effect will be exactly
neutralised by transverse conduction. Lowering of temperature, by
reducing the transmission of excitation to the distal side, will restore
the positive curvature. Enhancement of conduction under rise of
temperature will, on the other hand, increase the antagonistic reaction
of the distal side and give rise to a negative curvature.
I shall in verification of the above, describe experiments which I have
carried out on the phototropic response of the tendril of _Passiflora_,
supposed to be insensitive to the action of light.
_Phototropic response of the tendril of_ Passiflora: _Experiment
145._--The tendril was cooled by keeping it for a long time in a cold
chamber, maintained at 15°C. The effect of unilateral light on the
cooled specimen was found to be positive; the tendril was next allowed
to assume the temperature of the room which was 30°C. The response was
now found to have undergone a change into negative. The positive and
negative phototropic curvatures of an identical organ at different
temperatures is seen in the two records given in figure 145.
Neutralisation takes place at an intermediate temperature, and the
organ thus appears insensitive to light.
SEASONAL VARIATION OF PHOTOTROPIC CURVATURE.
[Illustration: FIG. 145.--(_a_) Positive curvature of tendril of
_Passiflora_ at 15°C.; (_b_) negative phototropic curvature at 30°C.]
Reference has been made of the phototropic curvature of _Tropæolum_ and
of Ivy undergoing a change from positive in autumn to negative in
summer. The experiment described above shows that rise of temperature,
by enhancing transverse conductivity, transforms the positive into
negative heliotropic curvature. The reversal of the phototropic
curvature of _Tropæolum_ and Ivy, from positive in autumn to negative in
summer, finds a probable explanation in the higher temperature condition
of the latter season. This inference finds independent support from the
fact previously described (p. 100) that while the velocity of
conduction of excitation in the petiole of _Mimosa_ is as high as 30 mm.
per second in summer, it is reduced to about 4 mm. in late autumn and
early winter.
ANTAGONISTIC EFFECTS OF LIGHT AND OF RISE OF TEMPERATURE.
I have explained the complex effect of rise of temperature on
phototropic curvature. Rise of temperature, within limits, enhances the
excitability, and therefore the positive curvature under light. Its
expansive reaction, on the other hand, opposes the contraction of the
proximal side, which produces the normal positive curvature. Rise of
temperature, as previously stated, introduces another element of
variation by its effect on conductivity. Transverse conduction favoured
by rise of temperature promotes neutralisation and reversal; the
resultant effect will thus be very complicated. I give below account of
an experiment where the induced positive curvature under light underwent
a reversal during rise of temperature.
_Reversal of tropic curvature under rise of temperature: Experiment
146._--The specimen employed for this experiment was a seedling of pea,
enclosed in a glass chamber, the temperature of which could be gradually
raised by means of an electric heater. Provisions were made to maintain
the chamber in a humid condition. The temperature of the chamber was
originally at 29°C., and application of light on one side of the organ
gave rise to positive curvature, followed by complete recovery on the
cessation of light (Fig. 146a). The next experiment was carried out with
the same specimen; while the plant was undergoing increasing positive
curvature under the continued action of light, the temperature of the
chamber was gradually raised from 29° to 33°C. at the point marked with
arrow. It will be seen that the positive curvature became arrested,
neutralised, and finally reversed into negative (Fig. 146b).
[Illustration: FIG. 146.--Effect of rise of temperature on phototropic
curvature. (_a_) normal positive curvature followed by recovery, (_b_)
reversal of positive into negative curvature by rise of temperature at
(H). (Pea seedling).]
_After-effect of rise of temperature: Experiment 147._--The after-effect
of rise of temperature exhibited by this specimen was extremely curious.
The temperature of the chamber was allowed to return to the normal, and
the experiment repeated after an hour; the response was now found to be
negative (Fig. 147a). It appeared probable that the temperature in the
interior of the tissue had not yet returned to the normal, and an
interval of four hours was therefore allowed for the restoration of the
tissue to the normal temperature of the room. The response still
persisted to be negative, as seen in the series of records obtained
under successive stimulations of light of short duration; these negative
responses exhibited recovery on the cessation of light (Fig. 147b).
This reversal of response as an after-effect of rise of temperature was
in this case found to persist for several hours. I experimented with the
same specimen next day when the response was found restored to the
normal positive.
[Illustration: FIG. 147.--After-effect of rise of temperature,
persistent negative curvature: (_a_) response one hour after rise
of temperature; (_b_) series of negative responses after 4 hours
(successive stimuli applied at vertical lines).]
SUMMARY.
Rise of temperature, within limits, enhances the general excitability of
the organ. This has the effect of increasing positive phototropic
curvature. But the physiological expansion induced by rise of
temperature exerts an antagonistic effect.
The transverse conductivity is increased with the rise of temperature;
this favours neutralisation and reversal of phototropic curvature.
Tendrils of _Passiflora_, supposed to be phototropically insensitive,
exhibit positive curvature at low, and negative curvature at a
moderately high temperature.
The change of phototropic curvature exhibited by _Tropæolum majus_ and
Ivy, from positive in autumn to negative in summer, is probably due to
the effect of temperature. Higher temperature with enhanced transverse
conductivity in summer, may thus convert positive into negative
curvature.
The physiological effects of rise of temperature and the stimulus of
light are antagonistic to each other.
Rise of temperature tends to neutralise or reverse the positive
phototropic curvature. The after-effect of temperature is often very
persistent.
XXXVI.--ON PHOTOTROPIC TORSION
_By_
SIR J. C. BOSE,
_Assisted by_
SURENDRA CHANDRA DAS.
In addition to positive or negative curvatures light induces a
responsive torsion. With regard to this Jost says:--
"The mechanics of the torsions have not as yet been fully explained. It
has long been believed that these torsions were occasioned only by the
action of a series of external factors, such as light, gravity, weight
of the organ which individually led to curvatures, but in combination
induced torsions; but later investigations have shown that torsions
might appear when light only was the functional external factor.... If
the torsions cannot generally be regarded as due to the combination of
two curvatures, we are completely in the dark as to the mechanics of
their production."[23]
[23] Jost--_Ibid_--p. 465.
A leaf when struck laterally by light undergoes a twist, so that the
upper surface is placed, more or less, at right angles to the incident
rays; as no explanation was available for this movement, the suggestion
has been made that the particular reaction is for the advantage of the
plant. I shall presently show, that it is possible to reverse this
normal torsion and thus make the upper surface of the leaf move away
from light.
The experiments which I shall presently describe will, it is hoped,
throw light on the obscure phenomenon. I shall be able to show:
(1) that the torsional response is not dependent on the
combination of two curvatures,
(2) that it is also independent of the effect of weight,
(3) that it may be induced not merely by stimulus of light
but by all forms of stimulation,
(4) that the direction of the torsional response depends on
the direction of the incident stimulus and the differential
excitability of the organ, and
(5) that there is a definite law which determines the
torsional movement.
EXPERIMENTAL ARRANGEMENTS.
I shall first describe a typical experiment on the responsive torsion
under the action of light. We have seen that in the pulvinus of
_Mimosa_, light of moderate intensity and of short duration applied on
the upper half induces a slow up-movement, while the stimulus of light
applied below induces a more rapid down-movement. The difference is due
to the fact that the lower half of the pulvinus is relatively the more
excitable. Vertical light thus induces a movement in a vertical plane.
But an interesting variation is induced in the response under the action
of lateral light. A stimulus will be called _lateral_ when it acts on
either the right or left flank of a _dorsiventral_ organ. We shall
presently find that a dorsiventral organ responds to lateral stimulus by
torsion.
The present series of experiments were carried out with the leaf of
_Mimosa_, and in order to eliminate the effect of weight and also for
obtaining record of pure torsion, I employed the following device. The
petiole was enclosed in a hooked support made of thin rod of glass, the
petiole resting on the concavity of the smooth surface. Friction and
the effect of weight is thus practically eliminated; the looped support
prevented up or down movements, and yet allowed perfect freedom for
torsional response. This latter is magnified by a piece of stout
aluminium wire fixed at right angles to the petiole (Fig. 148). The end
of the aluminium wire is attached to the short arm of a recording lever;
there is thus a compound magnification of the torsional movement. The
Oscillating Recorder gave successive dots at intervals which could be
varied from 20 seconds to 2 minutes. Time-relations of the response may
thus be obtained from the dotted record.
[Illustration: FIG. 148.--Diagrammatic representation for record of
torsional response. H, thin glass hook: A, aluminium wire attached to
petiole for magnification of torsional movement. T, silk thread for
attachment to recording lever.]
With the experimental device just described, we shall be in a position
to study the effect of various stimuli applied at one flank of the
pulvinus--at the junction of the upper and lower halves of the organ.
The observer standing in front of the leaf is supposed to look at the
stem. Torsional response will then appear as a movement either with or
against the hands of the clock. The torsional response, right-handed or
left-handed, will presently be shown to depend on the direction of
incident stimulus. In figure 149, anti-clockwise torsion is recorded as
an up-curve; clockwise rotation is recorded as a down-curve.
ACTION OF STIMULUS OF LIGHT.
_Experiment 148._--The pulvinus of the leaf was stimulated by a
horizontal beam of light thrown in a lateral direction; the areas
contiguous to line of junction of the upper and lower halves of the
anisotropic organ thus underwent differential excitation. When light
struck on the left flank, the responsive torsion was anti-clockwise; the
responsive reaction thus made _the upper and the less excitable half of
the pulvinus face the stimulus_. Figure 149 gives a record of the
torsional movement when light struck the left flank of the organ; on the
cessation of stimulus the response is followed by recovery.
[Illustration: FIG. 149.--Record of torsional response of pulvinus of
_Mimosa pudica_.]
DIRECTIVE ACTION OF STIMULUS.
_Experiment 149._--If now the direction of stimulus be changed so that
light strikes on the right flank instead of the left, the responsive
torsion is found to be reversed, the direction of movement being
clockwise. Here also the responsive movement is such that it is the less
excitable upper half of the organ that is made to face the stimulus. It
will thus be seen that the torsion, anti-clockwise or clockwise, depends
on two factors, namely the direction of stimulus, and the differential
excitability of the organ.
EFFECT OF DIFFERENT MODES OF LATERAL STIMULATION.
I shall now proceed to show that the torsional response is induced not
merely by the action of light, but by all forms of stimulation.
_Effect of chemical stimulation: Experiment 150._--Dilute hydrochloric
acid was at first applied on the left flank of the pulvinus along the
narrow strip of junction of the upper and lower halves. This gave rise
to a responsive torsion against the hands of a clock. Chemical
stimulation of the right flank induced, on the other hand, a torsional
movement with the hands of a clock. Here also the direction of stimulus
is found to determine the direction of responsive torsion.
_Effect of thermal radiation: Experiment 151._--I next employed thermal
radiation as the stimulus; the source of radiation was a length of
electrically heated platinum wire. It is advisable to interpose a narrow
horizontal slit, so as to localise the stimulus at the junction of the
upper and lower halves of the pulvinus. Stimulus applied at the left
flank induced left-handed or anti-clockwise torsion; application at the
right flank gave rise to right-handed torsion.
_Geotropic stimulus._--The stimulus of gravity induces, as I shall show
in a subsequent chapter, a similar responsive torsion, the direction of
which is determined by the direction of the incident stimulus.
EFFECT OF DIFFERENTIAL EXCITABILITY ON THE DIRECTION OF TORSION.
Under normal conditions, the torsional response under light places the
upper surface of the leaf or leaflets at right angles to light. That
this movement is not due to some specific sensibility to light is shown
by the fact that all modes of stimulation, chemical, thermal or
gravitational, induce similar responsive torsion. The torsional response
is, moreover, shown to be determined by the direction of incident
stimulus, and the differential excitability of the organ. This latter
may be reversed by the local application of various depressing agents on
the normally more excitable lower half of the pulvinus. Under this
treatment, the lower half of the pulvinus may be rendered relatively the
less excitable. Lateral application of light now induces a torsional
movement which is the reverse of the normal, so that the upper surface
of the leaf moves away from light. The advantage of the plant cannot,
therefore, be the factor which determines the directive movement; the
teleological argument often advanced is, in any case, no real
explanation of the phenomenon.
In all the instances given above, and under every mode of stimulation,
the responsive movement makes the less excitable half of the pulvinus
face the stimulus. The torsional response is, in reality, the mechanical
result of the differential contraction of a complex organ, which is
fixed at one end and subjected to lateral stimulation. I have been able
to verify this, by the construction of an artificial pulvinus consisting
of a compound strip, the upper half of which is ebonite, and lower half
the more contractile stretched India-rubber; if such a strip be held
securely at one end in a clamp, and if the lateral flank, consisting
half of ebonite and half of India-rubber, be subjected to radiation, and
record taken in the usual manner, it will be found that a torsional
response takes place which is similar to that of the pulvinus of
_Mimosa_. The above experiment was devised to offer an explanation of
the mechanics of the movement. It should, however, be borne in mind in
this connection that the torsional response of pulvinus is brought about
by differential _physiological_ contraction of the organ, the movement
being abolished at death.
From the results given above, we arrive at the following:--
LAWS OF TORSIONAL RESPONSE.
1. AN ANISOTROPIC ORGAN, WHEN LATERALLY EXCITED BY ANY STIMULUS,
UNDERGOES TORSION BY WHICH THE LESS EXCITABLE SIDE IS MADE TO FACE THE
STIMULUS.
2. THE INTENSITY OF TORSIONAL RESPONSE INCREASES WITH THE DIFFERENTIAL
EXCITABILITY; WHEN THE ORIGINAL DIFFERENCE IS REDUCED, OR REVERSED, THE
TORSIONAL RESPONSE UNDERGOES CONCOMITANT DIMINUTION OR REVERSAL.
Having thus established the laws that guide torsional response, I shall
try to explain certain related phenomena which are regarded as highly
obscure. I shall also describe the application of the method of
torsional response in various investigations.
COMPLEX TORSION UNDER LIGHT.
The leaves of the so-called "Compass plants" exhibit very complex
movements, these being modified according to the intensity of incident
light. Thus in compass plants the leaves, under moderate intensity of
light in the morning or in the evening, turn themselves so as to expose
their surfaces to the incident rays. But under intense sun light, the
leaves perform bendings and twistings so that they stand at profile at
midday.
I have not yet been able to secure "Compass plants" at Calcutta. I
shall, however, describe my investigations on the complicated torsional
movements exhibited by certain leaflets by the action of vertical light.
The results obtained from these will show that torsional movements, even
the most complex, are capable of explanation from the general laws that
have been established.
_Torsional movement of leaflet of_ Cassia alata: _Experiment
152._--These leaflets are closed laterally at night but place themselves
in an outspread position at day time. The character of the movement is,
however, modified by the intensity of light. With moderate light in the
morning the leaflets open out laterally. But under more intense light,
the pulvinules of the leaflets exhibit a torsion by which the formerly
infolded surfaces of the leaflets are exposed at right angles to light
from above (Fig. 150). Such complicated movements, in two directions of
space, are also exhibited by other leaflets which are closed at night in
a lateral direction.
[Illustration: FIG. 150.--Leaflets of _Cassia alata_: open in daytime,
and closed in evening.]
For obtaining an explanation of these complex movements under different
intensities of light, we have first to discover the particular
disposition of the two halves of the pulvinule which are unequally
excitable; we have next to explain the responsive movements under the
directive action of moderate and of intense light.
_Determination of differential excitabilities of the organ: Experiment
153._--In the leaflet of _Cassia_ the movement of opening under diffuse
stimulation of light can only be brought about by the contraction of the
outer half, which must therefore be the more excitable. This is
independently demonstrated by the reaction to an electric-shock. On
subjecting the half closed leaflets to diffuse electric stimulation,
they open outwards in a _lateral_ direction. The disposition of the
unequally excitable halves of the pulvinule is thus different from that
of the main pulvinus of _Mimosa_. In the latter, the plane that divides
the two halves is horizontal, the lower half being the more excitable.
Thus in the pulvinule of _Cassia_ the plane that separates the two
unequally excitable halves is vertical, the outer half being the more
excitable than the inner. By inner half is here meant that half which is
inside when the leaflets are closed.
_Effect of strong vertical light: Experiment 154._--When the plant is
placed in a moderately lighted room, the leaflets open out laterally to
the outmost. This is brought about by the contraction of the more
excitable outer half of the organ. If strong light be thrown down from
above, a new movement is superposed, namely, of torsion by which the
leaflets undergo a twist and thus place their inner surface at right
angles to the vertical light. In order to investigate this phenomenon in
greater detail I placed the plant in a well lighted room, the leaflets
being three quarters open under the diffuse light. A very light index
was attached to the leaflet for magnifying the subsequent torsional
movement. A strong beam of parallel light from an arc lamp was thrown
down on the pulvinule from above; this fell at the junction of the more
excitable outer with the less excitable inner half of the organ, the
plane of separation of the two unequally excitable halves being, as
previously explained, vertical. I have shown that under lateral
stimulation, a differentially excitable organ undergoes torsion by which
the less excitable half is made to face the stimulus. Since it is the
inner half of the organ that is the less excitable, the attached leaflet
becomes twisted so as to expose its (former infolded) surface upwards,
at right angles to the incident light.
As a confirmatory test, strong light was made to strike the pulvinule
from _below_ with the result that the leaflets exhibited an opposite
torsion by which their surfaces faced downwards, so as to be at right
angles to light that struck them from below.
Under normal conditions sunlight comes from above; stimulation thus
takes place at the junction of the two differentially excitable halves
of the organ, the plane of separation of which is vertical. The torsion
induced makes the less excitable inner half turn in such a way that the
inner surfaces of the leaflets are placed perpendicular to the incident
light.
ADVANTAGES OF THE METHOD OF TORSIONAL RESPONSE.
The torsional response not only affords a new method of enquiry on the
reaction of various stimuli, but it also possesses certain advantages.
For instance in studying the response of the leaf of _Mimosa_ under
light, the records were taken of the movement of the leaf in a vertical
plane. But the responsive up-movement, induced by light acting from
above, is opposed by the weight of the leaf. But in the torsional
response, the leaf rests on the hooked glass support and the movement is
thus free from the complicating factor of the weight of the leaf. Again
the pulvinus of _Mimosa_, for example, is sometimes subject to
spontaneous variation of turgor, on account of which it exhibits an
autonomous up or down movement. In the ordinary method of record the
true response to external stimulus may thus be modified by natural
movement of the leaf. But in the torsional method, the autonomous up or
down movement is restrained by the hooked support, and the response to
lateral stimulus is unaffected by the spontaneous movement of the leaf.
The torsional method, moreover, opens out possibilities of inquiry in
new directions, such as the comparison of the excitatory effects of
different stimuli by the Method of Balance, and the determination of the
effective direction of geotropic stimulus.
THE TORSIONAL BALANCE.
A beam of light falling on the left flank of the pulvinus of _Mimosa_
induces a torsion against the hands of the clock. A second beam falling
on the right flank opposes the first movement; the resultant effect is
therefore determined by the effective stimulation of the two flanks. The
pulvinus thus becomes a delicate index by which two stimuli may be
compared with each other. The following experiment is cited as an
example of the application of the method of phototropic balance.
_Experiment 155._--Parallel beam of light from a small arc lamp passing
through blue glass falls on the left flank of the pulvinus; a beam of
blue light also strikes the pulvinus from the right side, and the
intensity of the latter is so adjusted that the resultant torsion is
zero. Blue glass is now removed from the left side, the unobstructed
white light being allowed to fall on the left flank of the pulvinus.
This was found to upset the balance, the resultant torsion being
anti-clockwise. This showed that white light induced greater excitation
than blue light. We next interpose a red glass on the left side, with
the result that the balance is upset in the opposite direction. This is
because the phototropic effect of red light is comparatively feeble.
We may thus compare the tropic effect of one form of stimulus against
a totally different form, phototropic against geotropic action
for example. It is enough here to draw attention to the various
investigations rendered possible by the method of balance. Concrete
examples of some of these will be given in a subsequent chapter.
DETERMINATION OF THE DIRECTION OF STIMULUS.
I have shown that the torsion, clockwise or anti-clockwise, is
determined by the direction of incident stimulus. Hence it would be
possible to determine the direction of incident stimulus from the
observed torsional movement. In the case of light, the direction of
incident stimulus is quite apparent. But it is difficult to determine
the direction of stimulus which is itself invisible. In such cases, the
torsional movement gives us infallible indication of the effective
direction of stimulus. The application of this principle will be found
in a later chapter.
SUMMARY.
Lateral stimulus induces a torsional response in a dorsiventral organ.
This is true of all modes of stimulation.
The responsive torsion is determined by the direction of incident
stimulus, and the differential excitability of two halves of the organ,
the torsion being such that the less excitable half of the organ is made
to face the stimulus.
The twist exhibited by various leaves and leaflets under light finds its
explanation from the demonstrated laws of torsional response.
The direction of incident stimulus may be determined from the responsive
torsion of a dorsiventral organ.
The Method of Torsional Balance enables us to compare the excitatory
efficiencies of two different stimuli which act simultaneously on the
two flanks of the organ.
XXXVII.--RADIO-THERMOTROPISM
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
We have studied the tropic curvature induced by different rays of light.
We saw that while the more refrangible rays of the spectrum were most
effective, the less refrangible rays were ineffective. Below the red,
there are the thermal rays about whose tropic effect very little is
definitely known.
The intricacies of the problem are very great owing to the difficulty of
discriminating the effect of temperature from that of radiation; to this
must be ascribed the contradictory results that have been obtained by
different observers, of which Pfeffer gives the following summary:[24]
[24] Pfeffer--_Ibid_--Vol. III, p. 776.
"In addition to the action of ultra-red rays which are associated with
the visible part of the spectrum, dark heat-rays of still greater wave
length, as well as differences of temperature may produce a thermotropic
curvature in certain cases. Wortmann observed that seedlings of
_Lepidium sativum_ and _Zea Mays_, as well as sporangiphores of
_Phycomyces_ curved towards a hot iron plate emitting dark heat-rays.
Steyer has, however, shown that the sporangiphore of _Phycomyces_ has no
power of thermotropic reaction.... Wortmann observed that the seedling
shoot of _Zea Mays_ was positively, but that of _Lepidium_ negatively,
thermotropic.... Steyer, however, found that both plants were positively
thermotropic. Wortmann has also investigated the radicles of seedlings
by growing them in boxes of saw-dust, one side being kept hot, the other
cold."
It will be noted that in the investigations described above,
thermotropic reaction has been assumed to be the same under variation of
temperature (as in experiments with unequally heated saw-dust), and
under radiation from heated plate of metal. With reference to this Jost
maintains that "so far as we know, thermotropism due to _radiant_ heat
cannot be distinguished from thermotropism due to _conduction_."[25]
[25] Jost--_Ibid_--p. 480.
The effect of temperature, within optimum limits, is a physiological
expansion and enhancement of the rate of growth. The effect of visible
radiation is, on the other hand, a contraction and retardation of
growth. Should radiant heat act like light, the various tropic effects
in the two cases would be similar; the temperature effect would in that
case be opposite to the radiation effect. In order to find whether the
thermal radiation produces tropic curvature similar to that of light, we
have to devise a crucial experiment in which the complicating factor of
rise of temperature on the responding organ is eliminated.
_Experiment 156._--I have described the effect of light applied
unilaterally to the stem of _Mimosa_, at a point diametrically opposite
to the indicating leaf (_Expt._ 104). It was shown that the effect of
indirect stimulus induced at first an erectile movement of the leaf, and
that this was followed by a fall of the leaf on account of transverse
transmission of excitation. In the present experiment I applied thermal
radiation instead of light. The source of radiation was a spiral of
platinum wire heated short of incandescence by means of electric
current. The intensity of incident radiation could thus be maintained
constant, or increased or decreased by approach or recession of the
radiating spiral. The effect of unilateral stimulus of heat-rays was
found exactly similar to that of light; _i.e._, there was at first an
erectile movement due to indirect stimulation, followed by the fall of
the leaf due to transmitted excitation. It will be noticed that under
the particular condition of the experiment, the responding pulvinus was
completely shielded from temperature-variation. The reaction to thermal
radiation is thus similar to that of light.
As regards the effects of rise of temperature and radiation I have shown
that they are antagonistic to each other (pp. 211, 308). Thus in
positive types of thermonastic organs like the flower of _Zephyranthes_,
while rise of temperature induces a movement of opening, radiation
causes the opposite movement of closure. Again, in the negative type
exemplified by _Nymphæa_, rise of temperature induces a movement of
closure; radiation on the other hand, brings about the opposite movement
of opening. The tropic effect of thermal radiation thus takes place in
opposition to that of rise of temperature, and the resultant effect is
therefore liable to undergo some modification, depending on the relative
sensibility of the organ to radiation and to variation of temperature.
The facts that have been given above prove that infra-red radiation is
as effective a mode of stimulation as the more refrangible rays of the
spectrum. Phototropic and radio-thermotropic reactions would therefore
prove to be essentially similar. The following experiments fully confirm
the similarity of the two reactions.
POSITIVE RADIO-THERMOTROPISM.
_Experiment 157._--I shall now describe the normal reaction of a growing
organ to the unilateral stimulus of thermal radiation. Figure 151 gives
a record of response of the stem of _Dregea_ to stimulus of short
duration; the induced curvature is positive or towards the source of
heat. On the cessation of stimulus, there is a recovery which is
practically complete, and which takes place at a slower rate than the
excitatory positive curvature. Repetition of stimulus gives rise to
responses similar to the first. _Successive stimuli of moderate
intensity thus give rise to repeated responses of growth curvature._ An
arbitrary distinction has been made between the responses of pulvinated
and of growing organs. The former is distinguished as a movement of
variation, with its supposed characteristic of repeated response. But
the experiment described shows that this is also met with in the
response by growth curvature. It is only under long continued
stimulation that the curvature is fixed by growth.
[Illustration: FIG. 151.--Positive response to short exposure to
thermal radiation. Successive dots at intervals of 5 seconds.
(_Dregea volubilis._)]
DIA-RADIO-THERMOTROPISM.
The positive curvature is induced by retardation of growth at the
proximal side, and enhancement of growth at the distal side. This
latter effect is, as we have seen, brought about by the effect of
indirect stimulation.
But under long continued action of stimulus, the negative or excitatory
impulse reaches the distal side, inducing diminution of turgor and
retardation of the rate of growth. This leads to neutralisation, the
organ placing itself at right angles to the orienting stimulus.
[Illustration: FIG. 152.--Record of positive, neutral and reversed
negative curvature under continued action of thermal radiation. The
negative response went off the plate. Successive dots at intervals
of 5 seconds. (_Dregea volubilis_).]
_Experiment 158._--This neutralisation is seen in the record given in
figure 152, where under continuous unilateral stimulation, the growing
organ exhibited its maximum positive curvature, after which the movement
became arrested by the arrival of the excitatory impulse at the distal
side, on account of which the first positive curvature became
neutralised. Further continuation of stimulus caused a reversal into
negative in the course of 7 minutes. It will thus be seen that in
inducing phototropic curvature, the heat rays in sunlight play as
important a part as the more refrangible rays of the spectrum.
SUMMARY.
The effects of rise of temperature and of radiation are antagonistic to
each other.
Under unilateral action of thermal radiation a positive curvature is
induced by the retardation of growth at the proximal, and acceleration
of growth at the distal side of the organ.
There is a complete recovery on the cessation of stimulus of moderate
intensity and short duration. Repeated responses may thus be obtained
similar to repeated responses in pulvinated organs. In certain tissues
the power of conduction in a transverse direction is wanting; excitation
remains localised at the proximal side, and the responsive curvature
remains positive.
In other cases, there is a slow conduction of excitation to the distal
side. The result of this under different circumstances is
dia-radio-thermotropic neutralization, or a reversed negative curvature.
In inducing phototropic curvature, the heat rays in sunlight play as
important a part as the more refrangible rays of the spectrum.
XXXVIII.--RESPONSE OF PLANTS TO WIRELESS STIMULATION
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
A growing plant bends towards light, and this is true not only of the
main stem but also of its branches and attached leaves and leaflets.
Light affects growth, the effect being modified by the intensity of
radiation. Strong stimulus of light causes a diminution of the rate of
growth, but very feeble stimulus induces an acceleration. The tropic
effect is very strong in the ultra-violet region of the spectrum with
its extremely short wave length, but the effect declines practically to
zero as we move towards the less refrangible rays--the yellow and the
red with their comparatively long wave length. As we proceed beyond the
infra-red region, we come across the vast range of electric radiation,
the wave lengths of which vary from 0·6 cm., the shortest wave I have
been able to produce, to others which may be miles in length. There thus
arises the very interesting question, whether plants perceive and
respond to the long ether waves including those employed in signalling
through space.
At first sight this would appear to be very unlikely; for the most
effective rays are in the ultra-violet region with wave length as short
as 20 × 10^{-6} cm.; but with electric waves used in wireless signalling
we have to deal with waves 50 million times as long. The perceptive
power of our retina is confined within the very narrow range of a
single octave, the wave lengths of which lie between 70 × 10^{-6} cm.
and 35 × 10^{-6} cm. It is difficult to imagine that plants could
perceive radiations so widely separated from each other as the visible
light and the invisible electric radiation.
But the subject assumes a different aspect, when we take into
consideration the _total_ effect of radiation on the plant. Light
induces two different effects which may broadly be distinguished as
external and internal. The former gives rise to movement; the latter
finds no outward manifestation, but consists of an 'up' or assimilatory
chemical change, with concomitant increase of potential energy. Of the
two reactions then, one is dynamic attended by dissimilatory 'down'
change; the other is potential, associated with the opposite 'up'
change. In reality the two effects take place simultaneously; but one of
these becomes predominant under definite conditions.
The modifying condition is the _quality_ of light; with reference to
this I quote the following from Pfeffer: "So far as is at present known,
the action of different rays of the spectrum gives similar curves in
regard to heliotropic and phototactic movements, to protoplasmic
streaming and movements of the chloroplastids as well as the photonastic
movements produced by growth or by changes of turgor. On the other hand,
it is the less refrangible rays which are most active in
photo-synthesis."[26] The dynamic and potential manifestations are thus
seen to be complementary to each other, the rays which induce
photo-synthesis being relatively ineffective for tropic reaction and
_vice versâ_.
[26] Pfeffer--Vol. II, p. 104.
Returning to the action of electric waves, since they exert no
photo-synthetic action they might conceivably induce the complementary
tropic effect. These considerations led me to the investigation of the
subject fourteen years ago, and my results showed that very short
electric waves induce a retardation of rate of growth; they also produce
responsive movements of the leaf of _Mimosa_, when the plant was in a
highly sensitive condition.[27] The energy of the short electric waves
is very feeble, and undergoes great diminution at a distance; hence the
necessity of employment of a specimen of plant in a highly sensitive
condition.
[27] "Plant Response"--p. 618 (1905).
I resumed my investigations on the subject at the beginning of this
year. I wished to find out whether plants in general perceived and
responded to the long ether waves which reached it from a distance. The
perception of the wireless stimulation was to be tested not merely by
the responsive movement of sensitive plants, but by diverse modes of
response given by all kinds of plants.
Stimulus induces, as we have seen, three different types of response in
plants. It causes excitation in sensitive plants like _Mimosa_, in
consequence of which the leaf undergoes a fall; this is the mechanical
response to stimulus. Stimulus also induces electric response in plants,
both sensitive and ordinary, the excited tissue undergoing an electric
change of galvanometric negativity. Finally, the effect of stimulus on
growing plants is a variation in the rate of growth, an acceleration
under feeble, and a retardation under strong stimulus. I undertook to
investigate the effect of electric waves on plants by the methods of
mechanical and of electrical responses, and also by that of induced
variation of growth.
[Illustration: FIG. 153.--Diagrammatic representation of method employed
for obtaining response to wireless stimulation. Transmitting apparatus
seen to the right. Receiving aerial connected to upper part of plant,
the lower part of the plant or the flower-pot being connected with the
earth.]
THE WIRELESS SYSTEM.
For sending wireless signals, I had to improvise the following
arrangement, more powerful means not being available. The secondary
terminals of a moderate sized Ruhmkorff's coil were connected with two
cylinders of brass, each 20 cm. in length; the sparking took place
between two small spheres of steel attached to the cylinders. One of the
two cylinders was earthed, and the other connected with the aerial 10
meters in height. The receiving aerial was also 10 meters in height and
its lower terminal led to the laboratory, and connected by means of a
thin wire to the experimental plant growing in a pot; this latter was
put in electric connection with the earth (Fig. 153). The distance
between the transmitting and receiving aerial was about 200 meters, the
maximum length permitted by the grounds of the Institute.
MECHANICAL RESPONSE OF _Mimosa_.
_Experiment 159._--One of the leaves of _Mimosa_ was connected with the
aerial by means of a thin tinsel of loose wire, which did not interfere
with the free movement of the leaf. This latter was attached to the
recording lever. Wireless signals induced a responsive fall of the leaf
(Fig. 154) which was gradual as under action of light, and not so abrupt
as under a mechanical blow.
[Illustration: FIG. 154.--Mechanical response of leaf of _Mimosa_ to
electric wave.]
[Illustration: FIG. 155.--Electric response of _Mimosa pudica_ to
wireless stimulation.]
THE ELECTRIC RESPONSE.
_Experiment 160._--The leaf of _Mimosa_ was in this experiment held
securely, and two electrical connections made, one with the less
excitable upper and the other with the more excitable lower half of the
pulvinus. The incident ether-wave induced an electric response in the
pulvinus, the more excitable lower half exhibiting galvanometric
negativity. On the cessation of stimulus there was a recovery (Fig.
155).
It is not at all necessary to employ the sensitive _Mimosa_ for
exhibition of electric response; for this is universally exhibited by
all plants. The only condition for electric response is that the points
of electric contacts should be made with two unequally excitable areas
in the plant. This may be secured by artificial means as by causing
'injury' to one point of contact.[28] It is however much better to take
advantage of the natural difference of excitability of two different
areas in the organ as in the pulvinus of _Mimosa_. This difference of
excitability is also found between the inner and outer sides of a hollow
tubular organ as in the peduncles of various lilies. I was thus able to
secure specimens which were far more sensitive to the action of electric
waves than the pulvinus of _Mimosa_.
[28] "Comparative Electro-Physiology"--p. 149.
EFFECT OF WIRELESS STIMULATION ON GROWTH.
There now remains the very interesting question as to whether the effect
of long ether waves induce any variation of growth. The results given
below show that growing plants not only perceive but respond to the
stimulus of electric waves. The effects to be presently described are
exhibited by all plants.
I shall, however, content myself in describing a typical experiment
carried with the seedling of wheat. The specimen was mounted on the
Balanced Crescograph, and the growth exactly balanced. This gives a
horizontal record; an acceleration of growth above the normal is, in
the following records, represented by a down curve, and a retardation
by an up-curve.
_Effect of feeble stimulus: Experiment 161._--I first studied the effect
of feeble stimulus. This was secured by decreasing the energy of sparks
of the radiator. The response was an acceleration of rate of growth as
seen in figure 156a. The analogy of this with the accelerating effect of
sub-minimal intensity of light (p. 224) is very remarkable.
[Illustration: FIG. 156.--Record of responses to electric wave by the
Balanced Crescograph (_a_) response to feeble stimulus by acceleration
of growth, (_b_) response to strong stimulus by retardation, (_c_)
responses to medium stimulation--retardation followed by recovery.
Down-curve represents acceleration, and up-curve retardation of growth:
(Seedling of wheat.)]
_Effect of strong stimulus: Experiment 162._--The maximum energy
radiated by my transmitter, as stated before, was only moderate. In
spite of this its effect on plants was exhibited in a very striking
manner. The balance was immediately upset, indicating a retardation of
the rate of growth. The latent period, _i.e._, the interval between the
incident wave and the response, was only a few seconds (Fig. 156b). The
record given in the figure was obtained with the moderate magnification
of 2,000 times only. But with my Magnetic Crescograph, the magnification
can easily be raised ten million times; and the response of plant to the
space signalling can be exalted in the same proportion.
Under an intensity of stimulus slightly above the sub-minimal, the
responses exhibit retardation of growth followed by quick recovery, as
seen in the series of records given in Fig. 156c.
A remarkable peculiarity in the response was noticed during the course
of the experiments. Strong stimulation by ether waves gives rise, as we
have seen, to a very marked retardation of the rate of growth. Repeated
stimulation induces fatigue, and temporary insensitiveness of the organ.
Under moderate fatigue the effect is a prolongation of the latent
period. Thus in a particular experiment the plant failed to give any
response to a short signal. But after an interval of five minutes a
marked response occurred to the wireless stimulus that had been received
previously. The plant had perceived the stimulus but on account of
fatigue the latent period was prolonged, from the normal 5 seconds to as
many minutes.
SUMMARY.
Plants not only perceive, but also respond to long ether waves employed
in signalling through space.
Mechanical response to wireless stimulation is exhibited by the leaf of
_Mimosa pudica_.
All plants give electric response to the stimulus of long ether waves.
Growing plants exhibit response to electric waves by modification of
rate of growth. Feeble stimulus induces an acceleration, while strong
stimulus causes a retardation of the rate of growth.
The perceptive range of the plant is far greater than ours; it not only
perceives but responds to the different rays of the vast ethereal
spectrum.
XXXIX.--GEOTROPISM
_By_
SIR J. C. BOSE.
No phenomenon of tropic movement appears so inexplicable as that of
geotropism. There are two diametrically opposite effects induced by the
same stimulus of gravity, in the root a movement downwards, and in the
shoot a movement upwards. The seeming impossibility of explaining
effects so divergent by the fundamental reaction of stimulus, has led to
the assumption that the irritability of stem and root are of opposite
character. I shall, however, be able to show that this assumption is
unnecessary.
The difficulty of relating geotropic curvature to a definite reaction to
stimulus is accentuated by the fact that the direction of the incident
stimulus, and the side which responds effectively to it are not clearly
understood; nor is it known, whether the reaction to this stimulus is a
contraction, or its very opposite, an expansion.
Taking the simple case of a horizontally laid shoot, the geotropic
up-curvature is evidently due to differential effect of the stimulus on
upper and lower sides of the organ. The up-curvature may be explained by
one or the other of two suppositions: (1) that the stimulus of gravity
induces contraction of the upper side; or (2) that the fundamental
reaction is not a contraction but an expansion and this of the lower
side. The second of these two assumptions has found a more general
acceptance.
Tropic curvatures in general are brought about by the differential
effect of stimulus on two sides of the organ. Thus light falling on one
side of a shoot induces local contraction, the rays being cut off from
acting on the further side by the opacity of the intervening tissue. But
there is no opaque screen to cut off the vertical lines of gravity,[29]
which enter the upper side of a horizontally laid shoot and leave it by
the lower side. Though lines of force of gravity are transmitted without
hindrance, yet a differential action is found to take place, for the
upper side, where the lines of force enter, becomes concave, while the
lower side where they emerge becomes convex. Why should there be this
difference?
[29] I shall in what follows take the _direction_ of vertical lines
of gravity as that of movement of falling bodies, from above
towards the centre of the earth.
For the removal of various obscurities connected with geotropism it is
therefore necessary to elucidate the following:
1. The sign of excitation is, as we found, a contraction and concomitant
galvanometric negativity. Does gravitational stimulus, like stimulus in
general, induce this excitatory reaction?
2. What is the effective direction of geotropic stimulus? In the case of
light, we are able to trace the rays of light which is incident on the
proximal side and measure the angle of inclination. In the case of
gravity, the invisible lines of force enter by one side of the organ and
leave by the other side. Assuming that the direction of stimulus is
coincident with the vertical lines of gravity, is it the upper or the
lower side of the organ that undergoes effective stimulation?
3. What is the law relating to the 'directive angle' and the resulting
geotropic curvature? By the directive angle (sometimes referred to as
the angle of inclination) is meant, as previously explained, the angle
which direction of stimulus makes with the responding surface.
4. We have finally to investigate, whether the assumption of opposite
irritabilities of the root and the shoot is at all justifiable. If not,
we have to find the true explanation of the opposite curvatures
exhibited by the two types of organs.
Of these the first three are inter-related. They will, however, be
investigated separately; and each by more than one method of inquiry.
The results will be found to be in complete harmony with each other.
I propose in this and in the following chapters to carry out the
investigations sketched above, employing two independent methods of
enquiry, namely, of mechanical and of electrical response. I shall first
describe the automatic method I have been able to devise, for an
accurate and magnified record of geotropic movement and its time
relations.
THE GEOTROPIC RECORDER.
The recorder shown in figure 157 is very convenient for study of
geotropic movement. The apparatus is four-sided and it is thus possible
to obtain four simultaneous records with different specimens under
identical conditions. The recording levers are free from contact with
the recording surface. By an appropriate clock-work mechanism, the
levers are pressed for a fraction of a second against the recording
surfaces. The successive dots in the record may, according to different
requirements, be at intervals varying from 5 to 20 seconds. The records
therefore not only give the characteristic curves of geotropic movements
of different plants, but also their time durations. For high
magnification, I employ an Oscillating Recorder, the short arm of the
lever being 2·5 mm., and the long arm 250 mm., the magnification being a
hundredfold; half that magnification is, however, sufficient for general
purposes.
[Illustration: FIG. 157.--The Quadruplex Geotropic Recorder.]
DETERMINATION OF THE CHARACTER OF GEOTROPIC REACTION.
The observed geotropic concavity of the upper side of a horizontally
laid shoot may be due to excitatory _contraction_ of that side, or it
may result from passive yielding to the active responsive _expansion_ of
the lower side. The crucial test of excitatory reaction under geotropic
stimulus is furnished by investigations on geo-electric response. When a
shoot is displaced from vertical to horizontal position, _the upper side
of the organ is found to undergo an excitatory electric change of
galvanometric negativity_ indicative of diminution of turgor and
_contraction_. The electric change induced on the lower side is one of
galvanometric positivity, which indicates an increase of turgor and
expansion. The tropic effect of geotropic stimulus is thus similar to
that of any other mode of stimulation, _i.e._, a contraction of the
upper (which in the present case is the proximal) and expansion of the
lower or the distal side. The vertical lines of gravity impinge on the
upper side of the organ which undergoes effective stimulation.
In order to show that the concavity of the upper side is not due to the
passive yielding to the expansion of the lower half, I restrained the
organ from any movement. I have explained that excitatory electric
response is manifested even in the absence of mechanical expression of
excitation; and under geotropic stimulus, the securely held shoot gave
the response of galvanometric negativity of the upper side. Hence the
fundamental reaction under geotropic stimulus is excitatory
_contraction_ as under other modes of stimulation.
Finally, I employed the additional test of induced paralysis by
application of intense cold. Excitatory physiological reaction is, as we
know, abolished temporarily by the action of excessive cold.
_Experiment 163._--I obtained records of mechanical response to
determine the side which undergoes excitation under geotropic stimulus,
the method of discrimination being local paralysis induced by cold. I
took the flower-scapes of _Amarayllis_ and of _Uriclis_, and holding
them vertical applied fragments of ice on one of the two sides. I then
laid the scape horizontal, first with cooled side below, the record
showed that this did not affect the geotropic movement. But on cooling
the upper side, the geotropic movement became arrested, and it was not
till the plant had assumed the temperature of the surroundings that the
geotropic movement became renewed. Figure 158 shows the effect of
alternate application of cold, on the upper and lower sides of the
organ.[30] In the shoot, therefore, it is the upper side of the organ
that becomes effectively stimulated. Before proceeding further I shall
make brief reference to the highly suggestive statolithic theory of
gravi-perception.
[30] "Plant Response"--p. 505.
[Illustration: FIG. 158.--Effect of alternate application of cold on the
upper and lower sides of the organ. Application of cold on upper side
(down-pointing arrow) induces arrest of geotropic movement. Application
below (up-pointing arrow) causes no arrest.]
THEORY OF STATOLITHS.
With regard to the perception of geotropic stimulus there can be no
doubt that this must be due to the effect of weight of cell contents,
whether of the sap itself, or of the heavy particles contained in the
cells, exerting pressure on the sensitive plasma. The theory of
statoliths advocated by Noll, Haberlandt and Nemec (in spite of certain
difficulties which further work may remove) is the only rational
explanation hitherto offered for gravi-perception. The sensitive plasma
is the ectoplasm of the entire cell, and statoliths are relatively heavy
bodies, such as crystals and starch grains. Haberlandt has found
statoliths in the apo-geotropic organs like stems.[31] When the cell is
laid horizontal, it is the lower tangential wall which has to support
the greater weight, and thus undergo excitation. In the case of
multicellular plants laid horizontally, the excitation on the upper side
is, as we have seen, the more effective than on the lower side. This
inequality, it has been suggested, is probably due to this difference
that the statoliths on the upper side press on the inner tangential
walls of the cells while those on the lower side rest on the outer
tangential walls.
[31] Haberlandt--"Physiological Plant Anatomy"--p. 603.
When the organ is held erect, the action of statoliths would be
symmetrical on the two sides. But when it is laid horizontal a complete
rearrangement of the statoliths will take place, and the differential
effects on the upper and lower sides will thus induce geotropic
reaction. This _period of migration_ must necessarily be very short; but
the reaction time, or the latent period, is found to be of considerable
duration. "Even in rapidly reacting organs there is always an interval
of about one to one and a half hours, before the horizontally placed
organ shows a noticeable curvature, and this latent period may in other
cases be extended to several hours."[32] This great difference between
the _period of migration_ and the _latent period_ offers a serious
difficulty in the acceptance of the theory of statoliths. But it may be
urged that the latent period has hitherto been obtained by relatively
crude methods, and I therefore undertook a fresh determination of its
value by a sensitive and accurate means of record.
[32] Jost--_Ibid_, p. 437.
DETERMINATION OF THE LATENT PERIOD.
As regards the interpretation of the record of geotropic movement, it
should be borne in mind that after the perception of stimulus a certain
time must elapse before the induced growth-variation will result in
curvature. There is again another factor which causes delay in the
exhibition of true geotropic movement; for the up-movement of stems, in
response to the stimulus of gravity, has to overcome the opposite down
movement, caused by weight, before it becomes at all perceptible. On
account of the bending due to weight there is a greater tension on the
upper side, which as we have seen (p. 193), enhances the rate of growth,
and thus tends to make that side convex. The exhibition of geotropic
response by induced contraction of the excited upper side thus becomes
greatly delayed. In these circumstances I tried to discover specimens in
which the geotropic action would be quick, and in which the retarding
effect of weight could be considerably reduced.
_Geotropic response of flower stalk of Tuberose: Experiment 164._--For
this I took a short length of flower stalk of tuberose in a state of
active growth; the flower head itself was cut off in order to remove
unnecessary weight. After a suitable period of rest for recovery from
the shock of operation, the specimen was placed in a horizontal
position, and its record taken. The successive dots in the curve are at
intervals of 20 seconds, and the geotropic up-movement is seen to be
initiated (Fig. 159) after the tenth dot, the latent period being thus 3
minutes and 20 seconds, the greater part of which was spent in
overcoming the down-movement caused by the weight of the organ.
[Illustration: FIG. 159.--Geotropic response of flower stalk of tube
rose: preliminary down-movement is due to weight.]
[Illustration: FIG. 160.--Geotropic response of petiole of _Tropæolum_:
latent period shorter than 20 seconds.]
_Geotropic response of petiole of_ Tropæolum: _Experiment 165._--I
expected to obtain still shorter latent period by choosing thinner
specimens with less weight. I therefore took a cut specimen of the
petiole of _Tropæolum_, and held it at one end. The lamina was also cut
off in order to reduce the considerable leverage exerted by it. The
response did not now exhibit any preliminary down-movement, and the
geotropic up-movement was commenced within a few seconds after placing
the petiole in a horizontal position (Fig. 160). The successive dots in
the record are at intervals of 20 seconds and the second dot already
exhibited an up-movement; the latent period is therefore shorter than 20
seconds. It will thus be seen that the latent period in this case is of
the same order as the hypothetical period of migration of the
statoliths.
I may state here that I have been successful in devising an electric
method for the determination of the latent period, in which the
disturbing effect of the weight of the organ is completely eliminated.
Applying this perfect method, I found that the latent period was in some
cases as short as a second. The experiment will be found fully described
in a later chapter.
THE COMPLETE GEOTROPIC CURVE.
The characteristics of the geotropic curve are similar to those of other
tropic curves. That is to say the susceptibility for excitation is at
first feeble; it then increases at a rapid rate; in the third stage the
rate becomes uniform; and finally the curvature attains a maximum value
and the organ attains a state of geotropic equilibrium (cf. page 353).
The period of completion of the curve varies in different specimens from
a few to many hours.
_Experiment 166._--The following record was obtained with a bud of
_Crinum_, the successive dots being at intervals of 10 minutes. After
overcoming the effect of weight (which took an hour), the curve rose at
first slowly, then rapidly. The period of uniformity of movement is seen
to be attained after three hours and continued for nearly 90 minutes.
The final equilibrium was reached after a period of 8 hours (Fig. 161).
[Illustration: FIG. 161.--The Complete Geotropic curve (_Crinum_).]
For studying the effect of an external agent on geotropic action, the
period of uniform movement is the most suitable. Acceleration of the
normal rate (with enhanced steepness of curve) indicates that the
external agent acts with geotropism in a concordant manner; depression
of the rate with resulting flattening of the curve shows, on the other
hand, the antagonistic effect of the outside agent.
DETERMINATION OF EFFECTIVE DIRECTION OF STIMULUS.
The experiments which have been described show that it is the upper side
(on which the vertical lines of gravity impinge) that undergoes
excitation. The vertical lines of gravity must therefore be the
direction of incident stimulus. This conclusion is supported by results
of three independent lines of inquiry: (1) the algebraical summation of
effect with that of a different stimulus whose direction is known, (2)
the relation between the directive angle and geotropic reaction, and (3)
the torsional response under geotropic stimulus.
EFFECT OF ALGEBRAICAL SUMMATION.
_Experiment 167._--A flower bud of _Crinum_ is laid horizontally, and
record taken of its geotropic movement. On application of light on the
upper side at L, the responsive movement is enhanced, proving that
gravity and light are inducing similar effects. On the cessation of
light, the original rate of geotropic movement is restored (Fig. 163).
Application of light of increasing intensity from below induces, on the
other hand, a diminution, neutralisation, or reversal of geotropic
movement.
Light acting vertically from above induces a concavity of the excited
upper side in consequence of which the organ moves, as it were, to meet
the stimulus. The geotropic response is precisely similar. In figure 162
the arrow represents the direction of stimulus which may be rays of
light or vertical lines of gravity.
[Illustration: FIG. 162.--Stimulus of light or gravity, represented by
arrow, induces up curvature as seen in dotted figure.]
[Illustration: FIG. 163.--The effect of super-imposition of photic
stimulus. The first, third, and fifth parts of the curve, give normal
record under geotropic stimulus. Rate of up-movement enhanced under
light L.]
ANALOGY BETWEEN THE EFFECTS OF STIMULUS OF LIGHT AND OF GRAVITY.
In geotropic curvature we may for all practical purposes regard the
direction of stimulus as coinciding with the vertical lines of gravity.
The analogy between the effects of light and of gravity is very
close[33]; in both the induced curvature is such that the organ moves so
as to meet the stimulus. This will be made still more evident in the
investigations on torsional geotropic response described in a subsequent
chapter. The tropic curve under geotropic stimulus is similar to that
under photic stimulus. The tropic reaction, both under the stimulus of
light and of gravity, increases similarly with the 'directive' angle.
These real analogies are unfortunately obscured by the use of arbitrary
terminology used in description of the geotropic curvature of the shoot.
In figure 163 records are given of the effects of vertical light and of
vertical stimulus of gravity, on the responses of the horizontally laid
bud of _Crinum_. In both, the upper side undergoes contraction and the
movement of response carries the organ upwards so as to place it
parallel to the incident stimulus. Though the reactions are similar in
the two cases, yet the effect of light is termed _positive_
phototropism, that of gravity _negative_ geotropism. I would draw the
attention of plant-physiologists to the anomalous character of the
existing nomenclature. Geotropism of the shoot should, for reasons given
above, be termed _positive_ instead of _negative_, and it is unfortunate
that long usage has given currency to terms which are misleading, and
which certainly has the effect of obscuring analogous phenomena. Until
the existing terminology is revised, it would perhaps be advisable to
distinguish the geotropism of the shoot as _Zenithotropism_ and of the
root as _Nadirotropism_.
[33] Exception to this will be found in page 336, where explanation
is offered for the difference.
RELATION BETWEEN THE DIRECTIVE ANGLE AND GEOTROPIC REACTION.
When the main axis of the shoot is held vertical, the angle made by the
surface of the organ with lines of force of gravity is zero, and there
is no geotropic effect. The geotropic reaction increases with the
directive angle; theoretically the geotropic effect should vary as the
sine of the angle. I shall in the next chapter describe the very
accurate electrical method, which I have been able to devise for
determination of relative intensities of geotropic action at various
angles. Under perfect conditions of symmetry, the intensity of effect is
found to vary as the sine of the directive angle. This quantitative
relation fully demonstrates that geotropic stimulus acts in a definite
direction which coincides with the vertical lines of gravity.
The conditions of perfect symmetry for study of geotropic action at
various angles will be fully described in the next chapter. In the
ordinary method of experimentation with mechanical response the organ is
rotated in a vertical plane. The geotropic movement is found increased
as the directive angle is increased from zero to 90°.
DIFFERENTIAL GEOTROPIC EXCITABILITY.
It has been shown that geotropic stimulus acts more effectively on the
upper side of the organ. The intensity of geotropic reaction is,
moreover, modified by the excitability of the responding tissue. It is
easy to demonstrate this by application of depressing agents on the more
effective side of the organ. The rate of geotropic up-movement will be
found reduced, or even abolished by the local application of cold,
anæsthetics like chloroform, and of poisonous potassium cyanide
solution.
The different sides of a dorsiventral organ are unequally excitable to
different forms of stimuli. I have already shown (p. 85) that the lower
side of the pulvinus of _Mimosa_, is about 80 times more excitable to
electric stimulus than the upper side. Since the effect of geotropic
stimulus is similar to that of other forms of stimuli, the lower side of
the pulvinus should prove to be geotropically more excitable than the
upper side. This I have been able to demonstrate by different methods of
investigation which will be described in the following chapters.
Under ordinary circumstances, the upper half of the pulvinus is, on
account of its favourable position, more effectively stimulated by
geotropic stimulus; in consequence of this the leaf assume a more or
less horizontal position of "dia-geotropic" equilibrium. But when the
plant is inverted the more excitable lower half of the organ now
occupies the favourable position for geotropic excitation. The leaf now
erects itself till it becomes almost parallel to the stem. The response
of the same pulvinus which was formerly "dia-geotropic" now becomes
"negatively geotropic"; but an identical organ cannot be supposed to
possess two different specific sensibilities. The normal horizontal
position assumed by the leaf is, therefore, due to differential
geotropic excitabilities of the two sides of a dorsiventral organ.
I have explained (p. 401) that when the pulvinus of _Mimosa_ is
subjected to lateral stimulation of any kind, it undergoes a torsion, in
virtue of which the less excitable half of the organ is made to face the
stimulus. Experiments will be described in a subsequent chapter which
show that geotropic stimulus also induces similar torsional response.
The results obtained from this method of enquiry give independent proof:
(1) that the lower half of the pulvinus is geotropically the more
excitable, and (2) that the direction of incident geotropic stimulus is
the vertical line of gravity which impinges on the upper surface of the
organ.
SUMMARY.
The stimulus of gravity is shown to induce an excitatory reaction which
is similar to that induced by other forms of stimulation. The direct
effect of geotropic stimulus is an incipient contraction and retardation
of rate of growth.
The upper side of a horizontally laid shoot is more effectively
stimulated than the lower side, the excited upper side becoming concave.
Electrical investigation also shows that it is the upper side that
undergoes direct stimulation.
Tropic reactions are said to be positive, when the directly stimulated
side undergoes contraction with the result that the organ moves to meet
the stimulus. According to this test, the geotropic response of the stem
is _positive_.
The geotropic response is delayed by the bending down of the
horizontally laid shoot. Reduction of weight is found to shorten the
latent period; in the case of the petiole of _Tropæolum_ this is shorter
than 20 seconds. The latent period of geotropic response is found to be
of the same order as the "migration period" of the hypothetical
statoliths.
The complete geotropic curve shows characteristics which are similar to
tropic curves in general.
In a dorsiventral organ the geotropic excitabilities of the upper and
lower sides are different. In the pulvinus of _Mimosa_ the geotropic
excitability of the lower half is greater than that of the upper half.
The differential excitabilities of a dorsiventral organ modifies its
position of geotropic equilibrium.
XL.--GEO-ELECTRIC RESPONSE OF SHOOT
_By_
SIR J. C. BOSE,
_Assisted by_
SATYENDRA CHANDRA GUHA, M.Sc.
The experiments that have been described in the preceding chapter show
that the upper side of a horizontally laid shoot undergoes excitatory
contraction, in consequence of which the organ bends upwards. The
fundamental geotropic reaction is, therefore, not expansion, but
contraction which results from all modes of stimulation.
In confirmation of the above, I wished to discover and employ new means
of detecting excitatory reaction under geotropic stimulus. In regard to
this, I would refer to the fact which I have fully established that the
state of excitation can be detected by the induced electromotive change
of galvanometric negativity. This electrical indication of excitation
may be observed even in plants physically restrained from exhibiting
response by mechanical movement.[34]
[34] "Comparative Electro-Physiology," p. 20.
ELECTRIC RESPONSE TO STIMULUS.
Before giving account of the results of investigations on the detection
of geotropic excitation by means of electric response, I shall describe
a few typical experiments which will fully explain the method of the
electrical investigation, and show the correspondence of mechanical and
electric responses. I have explained how tropic curvatures are brought
about by the joint effects, of contraction of the directly excited
proximal side A, and the expansion of the distal side B. In the diagram
of mechanical response to stimulus (Fig. 164a) the excitatory
contraction is indicated by - sign, and the expansion, by + sign. The
resulting movement is, therefore, towards the stimulus as shown by the
curved arrow.
I shall now describe the corresponding electric effects in response to
unilateral stimulus. We have to determine the induced electrical
variation at the proximal side A, and at the distal side B.
[Illustration: Fig. 164.--Diagrammatic representation of the mechanical
and electrical response to direct unilateral stimulation indicated by
arrow:--
(_a_) Positive mechanical response (curved arrow) due to
contraction of directly stimulated A, and expansion of
indirectly stimulated B.
(_b_) Electric response of induced galvanometric negativity
of A under direct stimulation.
(_c_) Electric response of induced galvanometric positivity
at the distal point B.
(_d_) Additive effects of direct and indirect stimulations;
galvanometric negativity of the directly stimulated proximal
A, and galvanometric positivity of the indirectly stimulated
distal point B.]
_Electric response to direct stimulation: Experiment 168._--For the
determination of electric response at the directly excited proximal side
A, we take a shoot with a lateral leaf. The point A, which is to undergo
stimulation, is connected with one terminal of the galvanometer, the
other terminal being led to an indifferent or neutral point N on the
leaf. Application of any form stimulus at A, gives rise to an electric
current which flows through the galvanometer from the neutral to the
excited point A (Fig. 164b). _The directly stimulated point A thus
becomes galvanometrically negative._ The "action" current lasts during
the application of stimulus and disappears on its cessation.
_Electric response to indirect stimulation: Experiment 169._--We have
also seen that application of stimulus at A causes indirect stimulation
of the distal point B resulting in an increase of turgor and expansion.
The corresponding electric change of the indirectly stimulated point B
is found in the responsive current, which flows now through the
galvanometer from the indirectly stimulated B to the neutral point N
(Fig. 164c). _The indirectly stimulated point thus becomes
galvanometrically positive._
Having thus obtained the separate effects at A and B, we next modify the
experiment for obtaining the joint effects. For this purpose the neutral
point N is discarded and A and B connected directly with the indicating
galvanometer. On stimulation of A that point becomes negative and B
positive, and the current of response flows through the galvanometer
from B to A. The deflection is increased by the joint electrical
reactions at A and B (Fig. 164d).
The results may thus be summarised:--
TABLE XXXIII.--ELECTRIC RESPONSE TO DIRECT UNILATERAL STIMULUS.
+---------------------------------------------------------+
| Electrical change at the | Electrical change at the |
| proximal side A. | distal side B. |
+----------------------------+----------------------------+
| Galvanometric negativity | Galvanometric positivity |
| indicative of contraction | indicative of expansion |
| and diminution of turgor. | and increase of turgor. |
|---------------------------------------------------------+
| The corresponding tropic curvature is positive movement |
| towards stimulus. |
+---------------------------------------------------------+
Galvanometric negativity is thus seen to indicate the effect of direct
stimulus, and galvanometric positivity that of indirect stimulus. We
thus see the possibility of electric detection of the effects of
geotropic stimulation. This method would, moreover, enable us to
discriminate the side of the organ which undergoes greater excitation.
EXPERIMENTAL ARRANGEMENTS FOR OBTAINING GEO-ELECTRIC RESPONSE.
Returning to the investigation on electric response to geotropic
stimulus, the specimen of plant is at first held erect; two electrodes
connected with a sensitive galvanometer are applied, one to an
indifferent point, and the other to one side of the shoot. The
sensitiveness of the galvanometer was such that a current of one
millionth of an ampere produced a deflection of the reflected spot of
light through 1,000 divisions of the scale. An action current is
produced on displacement of the plant from vertical to horizontal
position.
_Non-polarisable electrodes._--The electrical connections with the plant
are usually made by means of non-polarisable electrodes (amalgamated
zinc rod in zinc-sulphate solution and kaolin paste with normal saline).
I at first used this method and obtained all the results which will be
presently described. But the employment of the usual non-polarisable
electrodes with liquid electrolyte is, for our present purpose,
extremely inconvenient in practice; for the plant-holder with the
electrodes has to be rotated from vertical to horizontal through 90°.
The reliability of the non-polarisable electrode, moreover, is not above
criticism. The zinc-sulphate solution percolates through the kaolin
paste and ultimately comes in contact with the plant, and seriously
affects its excitability. The name non-polarisable electrode is in
reality a misnomër; for the action current (whose polarising effect
is to be guarded against) is excessively feeble, being of the order of a
millionth of an ampere or even less; the counter polarisation induced by
such a feeble current is practically negligible.
The idea that non-polarisable electrodes are meant to get rid of
polarisation is not thus justified by the facts of the case. The real
reason for its use is very different; the electrical connections with
the plant has to be made ultimately by means of two metal contacts. If
we take two pieces of metal even from the same sheet, and put them in
connection with the plant, a voltaic couple is produced owing to slight
physical differences between the two electrodes. Amalgamation of the two
zinc rods with mercury reduces the electric difference but cannot
altogether eliminate it.
I have been able to wipe off the difference of potential between two
pieces of the same metal, say of platinum, and by immersing them in
dilute salt solution from a voltaic couple. The circuit is kept complete
for 24 hours, and the potential of the two electrodes by this process is
nearly equalised. A perfect equality is secured by repeated warming and
cooling of the solution and by sending through the circuit, alternating
current which is gradually reduced to zero. I have by this means been
able to obtain two electrodes which are iso-electric. The specially
prepared electrodes (made of gold or platinum wire) are put in
connection with the plant through kaolin paste moistened with normal
saline solution. Care should be taken to use opaque cover over the
plant-holder, so as to guard against any possible photo-electric action;
moistened blotting paper maintains the closed chamber in a uniform humid
condition.
The direct method of contact described above is extremely convenient in
practice; the resistance of contact is considerably reduced, and there
is no possibility of its variation during the necessary process of
rotation of the plant for subjecting it to geotropic action.
[Illustration: FIG. 165.--Diagrammatic representation of geo-electric
response. The middle figure represents vertical position. In figure to
the right rotation through +90° has placed A above with induced electric
change of galvanometric negativity of A. In the figure to the left,
rotation is through -90° A being below; the electric response is by
induced galvanometric positivity of A. For simplification of diagram,
vertical position of sepal is not always shown in the figure.]
GEO-ELECTRIC RESPONSE OF THE UPPER AND LOWER SIDES OF THE ORGAN.
We have next to discover the electric change induced by geotropic
stimulus on the upper and lower sides of the organ. For this purpose it
is necessary to find a neutral point which is not affected by the
inclination of the organ from vertical to horizontal position. For the
present experiment, I employed the flower of the water lily _Nymphæa_,
the peduncle of which is sensitive to geotropic action. One electrical
contact is made with a sepal, which is always kept vertical; the other
electric contact is made at the point A, on one side of the flower stalk
(Fig. 165). On making connections with a sensitive galvanometer a very
feeble current was found, which was due to slight physiological
difference between the neutral point, N, and A. This natural current
may be allowed to remain, the action current due to geotropism being
_superposed on it_; or the natural current may be neutralised by means
of a potentiometer and the reflected spot of light brought to zero of
the scale.
_Induced electric variation on upper side of the organ: Experiment
170._--While the sepal is held vertical, the stalk is displaced through
+90° so that the point A is above. Geotropic stimulation is at once
followed by a responsive current which flows through the galvanometer
from N to A, the upper side of the organ thus exhibiting excitatory
reaction of galvanometric negativity (Right-hand figure of 166). When
the stalk is brought back to vertical position geotropic stimulation
disappears, and with it the responsive current.
_Electric response of the lower side: Experiment 171._--The stalk is now
displaced through -90°; the point A, which under rotation through +90°
pointed upwards, is now made to point downwards. The direction of the
current of response is now found to have undergone a reversal; it now
flows from A on the lower side to the neutral point N; thus under
geotropic action _the lower side of the organ exhibits galvanometric
positivity_ indicative of increase of turgor and expansion (Left-hand
figure 166).[35]
[35] For detailed account cf. Chapter XLIII.
Having thus found that the upper side of the organ under geotropic
stimulus becomes galvanometrically negative, and the lower side,
galvanometrically positive, we make electric connections with two
diametrically opposite points of the shoot A and B, and subject the
organ to alternate rotation through +90° and -90°. The electro-motive
changes induced at the two sides now became algebraically summated. I
employ two methods for geotropic stimulation: that (1) of Axial
Rotation, and (2) of Vertical Rotation.
[Illustration: FIG. 166.--Diagrammatic representation of the Method of
Axial Rotation H, and of Vertical rotation V (see text).]
METHOD OF AXIAL ROTATION.
In the method of Axial Rotation, the organ is held with its long axis
horizontal (Fig. 166 H). We have seen that the geotropic action
increases with the angle which the responding surface of the organ makes
with the vertical lines of gravity. When the organ is held with its
length horizontal, the angle made by its two sides, A and B, with the
vertical is zero and there is thus no geotropic effect. There is,
moreover, no differential effect, since the two sides are symmetrically
placed as regards the vertical lines of force. The plant is next rotated
round its long axis, the angle of rotation being indicated in the
circular scale. When the rotation is through +90°, A is above and B
below; this induces a differential geotropic effect, the upper side
exhibiting excitatory electric change of galvanometric negativity.
_Experiment 172._--I shall, as a typical example, give a detailed
account of experiments with the petiole of _Tropæolum_ which was found
so highly excitable to geotropic stimulus (p. 434). The specimen was
held horizontal with two symmetrical contacts at the two sides, the
electrodes being connected in the usual manner with the indicating
galvanometer. When the plant is rotated through +90° there is
an immediate current of response, the upper side becoming
_galvanometrically negative_. This excitatory reaction on the upper side
finds, as we have seen, mechanical expression by contraction and
concavity, with positive or up-curvature.
[Illustration: FIG. 167.--Diagrammatic representation of the
geo-electric response of the shoot (see text).]
The differential stimulation of A and B disappears on rotation of the
axis back to zero position, and the induced electro-motive response also
disappears at the same time. If now the axis be rotated through -90°, A
will become the lower, and B the upper and the excited side. The
electro-motive change is now found to have undergone a reversal, B
becoming galvanometrically negative. This induced electro-motive
variation under geotropic stimulus is of considerable intensity often
exceeding 15 millivolts. The characteristic electric change is shown
diagrammatically in figure 167 in which the middle figure shows the
symmetrical or zero position. On rotation through +90° (figure to the
right) A occupies the upper and B the lower position. A is seen to
exhibit induced change of galvanometric negativity. Rotation through
-90° reverses the current of response, as B now occupies the upper and A
the lower position.
CHARACTERISTICS OF GEO-ELECTRIC RESPONSE.
There are certain phenomena connected with the electric response under
geotropic stimulus which appear to be highly significant. According to
statolithic theory
"Geotropic response begins as soon as an organ is deflected from its
stable position, so that a few starch-grains press upon the ectoplasts
occupying the walls which are underneath in the new position; an actual
rearrangement of the starch-grains is therefore not an essential
condition of stimulation. As a matter of fact, the starch-grains do very
soon migrate on to the physically lower walls, when a positively or
negatively geotropic organ is placed horizontally, with the result that
the intensity of stimulation gradually increases attaining its maximum
value when all the falling starch-grains have moved on to the lower
region of the ectoplast. The time required for the complete
rearrangement of the statoliths may be termed the period of migration;
its average length varies from five to twenty minutes in different
organs."[36]
[36] Haberlandt--_Ibid_--p. 598.
Stimulation, according to the statolithic theory, is induced by the
displacement of the particles. The diameter of the geotropically
sensitive cells is considerably less than 0·1 mm.; and the stimulus will
be perceived after the very short interval taken by the statoliths to
fall through a space shorter than 0·1 mm. This may be somewhat delayed
by the viscous nature of the plasma, but in any case the period for
perceptible displacement of the statoliths should be very short, about a
second or so, and the latent period of perception of stimulus should be
of this order.
The mechanical indication of response to stimulus is delayed by a period
which is somewhat indefinite; for the initiation of responsive growth
variation will necessarily lag behind the perception of stimulus.
[Illustration: FIG. 168.--Geo-electric response of the petiole of
_Tropæolum_.]
_Experiment 173._--The mechanical response with its drawbacks is thus
incapable of giving an accurate value of the latent period. The
electrical method of investigation labours under no such disadvantage,
since the excitation is here detected even in the absence of movement.
The perception of stimulus will thus be followed by response without
undue delay. I shall in this connection give a record of electric
response of the quickly reacting petiole of _Tropæolum_, when the angle
of inclination is increased from zero to 90°. The responsive movement of
the galvanometer spot of light was initiated in less than 5 seconds and
the maximum deflection was reached in the course of 90 seconds. The
angle was next reduced to zero, and the deflection practically
disappeared in the further course of a minute and a half (Fig. 168).
There was a small "excitation remainder". But with vigorous specimens
the recovery is complete.
[Illustration: FIG. 169.--Geo-electric response of the scape of
_Uriclis_.]
The latent period of quickly reacting petiole of _Tropæolum_ is thus
about 5 seconds, a value which is more consonant with the idea of
particles inducing excitation by their fall through an exceedingly short
distance. In very sluggish organs latent period may be as long as a
minute (Fig. 169), which is considerably shorter than an hour, the
generally accepted value. Further even in the electric response, the
latent period will be delayed beyond the period of perception. For this
perception takes place in some unknown sensitive layer in the interior
of the tissue, while electric contact is made with the epidermis
outside. It is obvious that certain time must elapse before the
excitation, initiated at the sensitive layer, should reach the
epidermis. Under ideal conditions of experiment which will be described
in a subsequent chapter, the latent period for geotropic excitation, I
find, to be sometimes as short as a second.
PHYSIOLOGICAL CHARACTER OF GEO-ELECTRIC RESPONSE.
The intensity of the electro-motive variation is found to depend on the
physiological vigour of the specimen. The _Tropæolum_ plant, used for
most of the above experiments, are at the best condition of growth in
Calcutta in February; after this the plants begin to decline in March
and die off by the end of April.
_Experiment 174._--In February the intensity of electric response was
nearly double of that in March; it was only in March that I made
quantitative determination of the induced electro-motive force between
the upper and lower contacts on rotation of the specimen from zero to
90°. The E. M. F. was determined by the potentiometer method. I give
below the following typical values obtained with two different
specimens:--
Specimen Induced E. M. F.
(1) 12 millivolts.
(2) 15 "
In the most favourable season the induced electro-motive force is likely
to exceed the above value very considerably.
_Effect of Age._--While a young petiole gave the above value, an old
specimen from the same plant exhibited no response. The plants were in a
dying condition in April and all indications of electrical reaction were
found abolished. The physiological character of the response was also
demonstrated by first obtaining the normal electric response in a
vigorous specimen; after death, by immersion in boiling water, the
plant gave no electric response to geotropic stimulus.
EFFECT OF DIFFERENTIAL EXCITABILITY OF THE ORGAN.
I have hitherto described the geo-electric effect of radial and
isotropic organs. The induced E. M. F. at 90° was found practically the
same whether A was above and B below, and _vice versâ_. In the
mechanical response of the pulvinus of _Mimosa_, the geotropic
excitability was, however, found to be greater in the lower half than in
the upper (p. 440). I wished to investigate the question of differential
geotropic excitability anew, by means of electric response.
_Experiment 175._--Electric connections with the galvanometer were made
with the upper and lower halves of the pulvinus, the organ being placed
in the vertical or neutral position. The angle of inclination was then
increased to 90° in the positive and negative directions alternately.
TABLE XXXIV.--DIFFERENCE OF GEO-ELECTRIC RESPONSE OF UPPER AND LOWER
HALVES OF THE PULVINUS OF _Mimosa_.
+---------------------------------------------------------+
| Specimen. | Position of particular | Induced E. M. F. |
| | half of pulvinus. | |
+-----------+--------------------------+------------------+
| (1) | { Upper half above | 23 millivolts. |
| | { Lower half above | 30 " |
| | | |
| (2) | { Upper half above | 16 " |
| | { Lower half above | 29 " |
+---------------------------------------------------------+
In the former case the upper half of the pulvinus occupied the
up-position; in the second case the up-position was occupied by the
lower half of the pulvinus. In both cases strong electric responses were
obtained, the upper point of contact being always galvanometrically
negative. There was, however, a difference between the two responses,
the excitatory electro-motive variation was invariably greater when the
lower half of the organ occupied the favourable up-position. This will
be seen from the results of two typical experiments in table given
above.
The electrical mode of investigation thus leads to confirm the result
obtained with mechanical method that the lower half of the pulvinus of
_Mimosa_ is geotropically more excitable than the upper half.
RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.
In the Method of Axial Rotation, the condition of the experiment is
ideally perfect; in the neutral position the sides A and B are both
parallel to the vertical lines of gravity, and are little affected by
geotropic reaction. As the specimen is rotated on its long axis the
vertical component of the force of gravity increases with the angle of
inclination. The hypothetical statolithic particles will become
displaced all along the cell, and the vertical pressure exerted by them
will also increase with the angle.
The geo-electric response will then afford us a measure of the intensity
of excitation induced at various angles of inclination. The mechanical
response on account of its inherent defects does not afford us the true
relation between the angle of inclination and intensity of geotropic
reaction. But the electric method of inquiry is free from the defects of
the mechanical method.
_Experiment 176._--The specimen was rotated so that the angle of
rotation was 45°, and the maximum electric response observed. The angle
was next increased to 90° and the reading for the enhanced response
taken. The ratio of the geo-electric response at 90° and 45°, thus
affords us a measure of the effective stimulations at the two angles.
I give below a table which gives results obtained with 24 different
specimens.
TABLE XXXV.--RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.
+-------------------------------------------------------------+
|No. of specimen.| Galvanometric deflection. |Ratio b/a.|
| +---------------------------------+ |
| | (_a_) at 45° | (_b_) at 90° | |
+----------------+---------------+-----------------+----------+
| 1 | 70 divisions | 110 divisions | 1·5 |
| 2 | 30 " | 45 " | 1·5 |
| 3 | 90 " | 126 " | 1·4 |
| 4 | 70 " | 100 " | 1·4 |
| 5 | 21 " | 33 " | 1·6 |
| 6 | 30 " | 50 " | 1·6 |
| 7 | 12 " | 20 " | 1·6 |
| 8 | 14 " | 20 " | 1·4 |
| 9 | 10 " | 16 " | 1·6 |
| 10 | 45 " | 75 " | 1·5 |
| 11 | 25 " | 40 " | 1·6 |
| 12 | 14 " | 20 " | 1·4 |
| 13 | 13 " | 20 " | 1·5 |
| 14 | 30 " | 50 " | 1·5 |
| 15 | 38 " | 54 " | 1·4 |
| 16 | 50 " | 75 " | 1·5 |
| 17 | 55 " | 90 " | 1·5 |
| 18 | 13 " | 20 " | 1·5 |
| 19 | 17 " | 25 " | 1·4 |
| 20 | 80 " | 130 " | 1·5 |
| 21 | 15 " | 22 " | 1·4 |
| 22 | 45 " | 75 " | 1·5 |
| 23 |135 " | 220 " | 1·6 |
| 24 | 55 " | 93 " | 1·5 |
+-------------------------------------------------------------+
| Mean ratio = 1·49 |
+-------------------------------------------------------------+
The mean ratio 1·49 may thus be regarded as the relative
geotropic effects at 90° and 45°; this is practically the
same as Sin 90°/Sin 45° = 1·4. Hence we arrive at the
following law:
_The intensity on geotropic action varies as the sine of the
directive angle._
METHOD OF VERTICAL ROTATION.