Page:Life Movements in Plants.djvu/274

244 ofDesodium pulsation, and in the acceleration of rate of growt n growing organs (p. 237).

Certain agnts induce xcitation in living tissues, the exciatory change being let eted by contraction, or by olectromotive variation, or by change of electric resistance, an in growing or^ans by the rtardation of tho rate of growth. In general, the various stimuli which xcite animal tssus also xcite vgetablo tissues It has bn shown that erery form uf slimuli, hmwerer diverse, also indncrs incipient rntuetion und rrturdation Thus mechanieal irritation, such or woun, induces a retardation of growth they also induce an excitatory contraction in of the rle of yroth frictio (p. A2); Mimosa, attended by the fall of the laf of oleetrie stimulation Dillernt modes bothgrowing act similarly on an pulvinated organs. The action of light visible and in- visile will presently be seen to react on both aliko in this connoetion nothing could bo more significaut than the discrimninative manner And in which both the pulvinate an the growing organs respond to certain lights anl not to thers In contrast to the contractilo etfeet of stimnulus, certain agents luce tho antagonistic reaction of hasbn shown that whilo stimulus induces a retardation, rise of temperature up to acceleration of the rate of growth. I have also referred to th fact that while the autonomous pulsations of Dsmudiun lat exibit under «timulus a diminution of the extent of the diastolic expansion, warmth on the other hand, in- ducs th opposite elfect by liminishin; the systolic con- expasion It an optimu point, induces an traction

I have referred to the well-known fact that it is the more refrangible portions of the spectrum that are more ef- fectivo in inducing excitatory reactions and have already given