Page:Life Movements in Plants.djvu/128

98 Having thus established the physiological character of the transmitted impulse in plants I shall now proceed to give some of the principal results of my earlier and recent investigations on the effects of various agencies on conduction of excitation in plants.

Apart from any question of hydro-mechanical transmission, it is important to distinguish two different modes of transmission of excitation. In a motile tissue contraction of a cell causes a physical deformation and stimulation of the neighbouring cell. Examples of this are furnished by the cardiac muscle of the animal, the pulvinus of Mimosa, and the stamen of Berberis. This mode of propagation may better be described as a convection of excitation.

The conduction of excitation, as in a nerve, is a different process of transmission of protoplasmic change. The conducting tissue in this case does not itself exhibit any visible change of form. In the plant the necessary condition for transmission of excitation to a distance is that the conducting tissue should be possessed of protoplasmic continuity in a greater or less degree. This condition is fulfilled by vascular bundles. There being greater facility of transmission along the bundles than across them, the velocity in the longitudinal direction is very much greater than in the transverse.

For accurate determination of velocity of transmission the testing stimulus should be quantitative and capable of repetition. Abnormal high velocity has been observed in Mimosa by applying crude and drastic methods of stimulation, by a transverse cut or a burn. This is apt to give rise to a very strong hydro-dynamic disturbance, which travelling with great speed, delivers a mechanical blow on the responding pulvinus. Such hydro-dynamic transmission is not the same as physiological conduction.