Certain agents induce excitation in living tissues, the excitatory change being detected by contraction, or by electromotive variation, or by change of electric resistance, and in growing organs by the retardation of the rate of growth. In general, the various stimuli which excite animal tissues also excite vegetable tissues.
It has been shown that every form of stimuli, however diverse, also induces incipient contraction and retardation of the rate of growth. Thus mechanical irritation, such as friction or wound, induces a retardation of growth ([p. 202]); they also induce an excitatory contraction in Mimosa, attended by the fall of the leaf. Different modes of electric stimulation act similarly on both growing and pulvinated organs. The action of light visible and invisible will presently be seen to react on both alike. And in this connection nothing could be more significant than the discriminative manner in which both the pulvinated and the growing organs respond to certain lights and not to others.
In contrast to the contractile effect of stimulus, certain agents induce the antagonistic reaction of expansion. It has been shown that while stimulus induces a retardation, rise of temperature up to an optimum point, induces an acceleration of the rate of growth. I have also referred to the fact that while the autonomous pulsations of Desmodium leaflet exhibit under stimulus a diminution of the extent of the diastolic expansion, warmth on the other hand, induces the opposite effect by diminishing the systolic contraction.
EFFECT OF LIGHT ON PULVINATED ORGANS.
I have referred to the well-known fact that it is the more refrangible portions of the spectrum that are more effective in inducing excitatory reactions and have already given records of the responsive reactions of various lights on growing organs. I shall now give records of the effect of various lights on the pulvinus of Mimosa pudica. The amplitude and time relations of the curves of response will give a more precise idea of the quantitative effects of various lights in inducing excitation.
Fig. 89.—Effect of white light on the pulvinus of Mimosa. Successive dots in this and in the following records are at intervals of 10″. (Magnification 100 times)
Action of white light: Experiment 95.—The source of light was an arc lamp; a pencil of parallel light is made to pass through a trough of alum solution. This process of excluding thermal rays is adopted for the visible rays of the spectrum. Colour filters were also used for obtaining red, yellow and blue lights. The pencil of light is thrown upwards by an inclined mirror on the lower half of the pulvinus. The response is taken by an Oscillating recorder, giving successive dots at intervals of 10 seconds, the magnification employed being 100 times. The pulvinus being subjected to light for 10 seconds gave response by a fall of the leaf (Fig. 89). The response to light is thus found to be essentially similar to that induced by electric stimulus, the only difference being in the relative sluggishness of the reply. Electric shock passes instantaneously through the mass of the pulvinus, stirring up the active tissues to responsive contraction. The latent period is, therefore, as short as 0.1 second and the maximum contraction is effected in about 3 seconds. In the case of the stimulus of light the shock-effect is not so great; excitation, moreover, has to pass slowly from the surface of the pulvinus inwards. Hence the latent period is twelve seconds, and the period of maximum contraction is as long as 90 seconds. As the stimulation is moderate, the recovery is effected in 11 minutes, instead of 16 minutes, which is the usual period for Mimosa to recover from an electric shock. The important conclusion to be derived from this experiment is, that light is a mode of stimulation and that it induces a responsive contraction, similar to that caused by other forms of stimuli. This contractile response under light is exhibited not merely by the motile pulvinus of Mimosa, but by other pulvini as well, such as those of Erythrina indica, and of the ordinary bean plant.
Action of red and yellow lights.—The pulvinus gave little or practically no response to these lights.