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. [Illustration: 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