Physiological and anatomical investigations on Mimosa pudica

Abstract

The plant is a multicellular organism, and hence necessity arises for inter­communication and inter-action between more or less distant organs. I have shown elsewhere (1) that this is accomplished in the plant, as in the animal, in two different ways, by translocation of matter and by transmission of motion. The first is secured by the slow movement of fluids carrying chemical substances in solution, such as occurs in the circulation of sap. The second is brought about by the rapid propagation of protoplasmic excitation, such as the nervous impulse in the animal. The evolution of a nervous system for the transmission of excitation in the living organism is a matter of much theoretical importance. It is therefore of interest to investigate the trans­mission of excitation in the undifferentiated protoplasm of the plant. In the course of my investigations on nervous reactions in plants during the last twenty years (2, 3, 4, 5) many important facts have been discovered which throw light on the more complex phenomena in the animal. A suitable subject for investigation is Mimosa pudica, in which the pulvinus acts as a motile effector, the excitation being conducted along definite channels. It is unfortunate that in most investigations on the nature of the transmission of excitation in plants, hydro-mechanical disturbance was involved by the crude and drastic methods of stimulation employed, which led to erroneous conclusions. For the determination of the true character of the transmission, it is essential that the stimulus should be such as to produce no mechanical disturbance. The important characteristics of the impulse, moreover, can only be determined by accurate measurement of its normal velocity and the changes induced in it under physiological variations. It is therefore necessary to obtain, by means of an automatic recorder, reliable measurements of the period of transmission in the conducting tissue, as well as the latent period of the effector. These requirements are fulfilled by my Resonant Recorder (2), which enables time-intervals as short as 0·005 sec. to be determined with very great precision (3).