VI.—ON CONDUCTION OF EXCITATION IN PLANTS

By

Sir J. C. Bose.

The plant Mimosa offers the best material for in­ves­ti­ga­tion on conduction of excitation. With regard to this question the prevailing opinion had been that in plants like Mimosa, there is merely a trans­mission of hydro-mechanical disturbance and no trans­mission of true excitation comparable with the animal nerve. I have, however, been able to show that the trans­mission in the plant is not a mechanical phenomenon, but a propagation of excitatory protoplasmic change. This has been proved by the arrest of conduction by the application of various physio­logic­al blocks. Thus local application of increasing cold retards, and finally abolishes the conduct­ing power. The conduct­ing tissue becomes paralysed for a time as an after-effect of application of cold; the lost conduct­ing power may, however, be quickly restored by tetanising electric shocks. The conduct­ing power of an animal nerve is arrested by an electrotonic block, the conductivity being restored on the cessation of the current. I have succeeded in inducing similar electrotonic block of conduction in Mimosa. Conductivity of a selective portion of petiole may also be permanently abolished by local action, of poisonous solution of potassium cyanide.[K]

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.