Page:Life Movements in Plants.djvu/230



Amongst tho various stimuli which induce excitation in .llimusa may be mentioned the irritation caused by rough contact, by prick, or wound. Friction causes mode 'atc stimu- lation, from which the oxcitated pulvinus recovers within a short limo. But. a prick or a cut induces a far more in- it‘IlHt‘ and persistent excitation; the recovery becomes pro- t'acicd, and the wounded pulvinus remains Contracted for a long period.

I shall now describe the eti'ect of mechanical irritation on growth. For moderate stimulus, I employ rough con- tact. or friction; more intense stimulation is caused by a prick or a cut.

[drpminmzl 76.—~In this experiment, I took a peduncle ul' Zuphgﬂtnlhm, which had a normal rate of growth of 0'18 it 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 ,1. per second, or three-tit'ths 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