Page:Life Movements in Plants.djvu/230

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XV.—EFFECT OF MECHANICAL STIMULUS ON GROWTH


By


Sir J. C. Bose.


Amongst the various stimuli which induce excitation in Mimosa may be mentioned the irritation caused by rough contact, by prick, or wound. Friction causes moderate stimulation, from which the excitated pulvinus recovers within a short time. But a prick or a cut induces a far more intense and persistent excitation; the recovery becomes protracted, and the wounded pulvinus remains contracted for a long period.

I shall now describe the effect of mechanical irritation on growth. For moderate stimulus, I employ rough contact or friction; more intense stimulation is caused by a prick or a cut.

EFFECT OF MECHANICAL IRRITATION.

Experiment 76.—In this experiment, I took a peduncle of Zephyranthes, which had a normal rate of growth of 0.18 μ per second. I then caused mechanical irritation by rubbing the surface with a piece of card-board. The mechanical stimulation was found to have caused a retardation of growth, the depressed rate being 0.11 μ per second, or three-fifths the normal rate. As this particular mode of stimulation was very moderate, the normal of rate growth was found to be restored after a short period of rest. After 15

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