Page:Collected Physical Papers.djvu/406

386 stimulation brings about fatigue, which is removed after a suitable period of rest. Stimulus singly ineffective becomes effective by summation of several. The amplitude of response increases with the intensity of stimulus till a limit is reached. Anæsthetics induce a depression of excitability. A large dose of chloroform and poisonous substances produce a permanent abolition of response with the death of the plant (p. 315). The response is also abolished when the plant is scalded to death (p. 314). The fatal temperature for the plant is about 60° C.

The plant responds to the stimulus of light by contraction on account of which the directly stimulated side of a stem becomes concave seen in the positive heliotropic curvature of stems. The normal electromotive response under light is by an induced change of galvanometric negativity. I succeeded in devising another method by which the excitatory change of the plant-tissue is detected by a diminution of its electrical resistance. The most sensitive device for this purpose is the Quadrant Method which records response to light of so excessively short a duration as that emitted by a single spark (p. 320).

The amplitude of response, by resistivity variation, increases with the intensity light. Very dilute vapour of ether increases the excitability as demonstrated by the enhanced amplitude of response. Strong dose of chloroform, on the other hand, causes depression and death as indicated by gradual diminution and final abolition of response.