Life Movements in Plants Part 15

_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.

[Ill.u.s.tration: 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 _Pa.s.siflora_, 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 _Tropaeolum 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.

x.x.xVI.--ON PHOTOTROPIC TORSION

_By_

SIR J. C. BOSE,

_a.s.sisted 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 gla.s.s, 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.

[Ill.u.s.tration: FIG. 148.--Diagrammatic representation for record of torsional response. H, thin gla.s.s 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.

[Ill.u.s.tration: 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:--