Page:Researches on Irritability of Plants.djvu/145



now describe different experiments carried out for the purpose of observing the effect of varying external conditions—such as the intensity of stimulus, fatigue, and temperature—on the latent period. The mode of procedure adopted was first to take a record giving the latent period under standard conditions, and then to make further records under conditions similar in all respects to the first, except in regard to the one special factor whose influence was to be determined.

As some of the experiments in question necessitated a long period of observation, lasting sometimes over an hour, it became necessary to eliminate one source of possible uncertainty—namely, the effect of electrolytic contact on the pulvinus. It has been shown that there is no variation of excitability induced in the pulvinus where the contact is made with glycerin. It would, however, be preferable to effect direct stimulation without placing either of the electrodes on the pulvinus. In connection with this it was found that if one of the two electrodes were placed on the petiole slightly to the right of the pulvinus, and the other on the stem immediately below it, and a few rapidly alternating shocks passed through, excitation was simultaneous throughout the interposed tract.

This may be demonstrated in a striking manner by