Page:Life Movements in Plants.djvu/274

of Desmodium pulsation, and in the acceleration of rate of growth in growing organs (p. 237).

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.

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