Page:Researches on Irritability of Plants.djvu/158

 For successive determinations, with the same specimen, of the periods required for the transmission of excitation through a given length of conducting-tissue, did not differ from each other by so much as one-twentieth of a second and were often actually identical.

For the purpose of these experiments I used by preference the petiole of Mimosa, for the reason that in this the conducting-strands situated in the fibro-vascular bundle would be more continuous and evenly distributed than in a branching specimen. In order to determine the velocity of transmission, the stimulus of induction-shock is applied to the petiole at a distance d from the responding pulvinus. Let us suppose t to be the true time taken by the excitation to reach the pulvinus; the initiation of the responsive movement will however be further delayed by the latent period of the pulvinus L. The total time-interval T observed to elapse between the application of stimulus and the initiation of response will therefore be the true time t plus the latent period L. To obtain the true time we have to subtract the latent period L from the observed interval T, thus. The velocity of transmission is then found by dividing the distance by the true time. The necessary data are therefore the distance between the stimulated point and the pulvinus, the time-interval between the application of stimulus and the initiation of response, and the latent period of the individual pulvinus.

In making these determinations the apparatus employed is the same as that for the determination of the latent period. As in these experiments we have to measure time which may be several seconds in duration, the recording-plate is made to travel at the relatively slow rate of 2 cm. each second or thereabouts. The vibrating recorder must be selected according to the degree of accuracy that