Page:Life Movements in Plants.djvu/144

114 about supra or non-conducting state of the nerve, and this condition was maintained throughout the duration of the current.

I shall next describe a more perfect method for obtaining quantitative results both with plant and animal. In order to demonstrate the universality of the phenomenon, I next used Mimosa pudica instead of Averrhoa, for experiments on plants.

For determination of normal velocity of transmission of excitation and the induced variation of that velocity, I employed the automatic method of recording the velocity of transmission of excitation in Mimosa, where the excitatory fall of the motile leaf gave a signal for the arrival of the excitation initiated at a distant point. In this method the responding leaf is attached to a light lever, the writer being placed at right angles to it. The record is taken on a smoked glass plate, which during its descent makes an instantaneous electric contact, in consequence of which a stimulating shock is applied at a given point of the petiole. A mark in the recording plate indicates the moment of application of stimulus. After a definite interval the excitation is conducted to the responding pulvinus, when the excitatory fall of the leaf pulls the writer suddenly to the left. From the curve traced in this manner the time-interval between the application of stimulus and the initiation of response can be found, and the normal rate of transmission of excitation through a given length of the conducting tissue deduced. The experiment is then repeated with an electric current flowing along the petiole with or against the direction of transmission of excitation. The records thus obtained enable us to determine the influence of the direction of the current on the rate of transmission. I shall presently describe the various difficulties which have to be overcome before the method just indicated can be rendered practical.