Life Movements in Plants, Volume II

by Jagadis Chandra Bose

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.