Very naturally the first studies made in this subject attempted to discover an arrangement in plants comparable to a simplified neuro-muscular system of the animal. Expectations of this character were of course bound to meet with signal disappointment; a fact that should have been apparent if the history and widely different purpose of the animal and vegetal organism had been taken into consideration. Parallelisms between the reactions of plants and animals even to the same class of stimulus are to be accepted with great caution. Thus it has recently become apparent that the heliotropism of animals as investigated by Loeb is widely different from the heliotropism, or phototropism, of plants both as to the features of light acting as stimuli in the separate cases, and the general nature of the consequent reactions.
Recent papers by Nemec on the transmission of impulses in plants, and the discussions of geotropism and the organs of equilibrium of plants by Noll, Czapek and Haberlandt have awakened much interest in the mechanism of irrito-motility of plants.
Not fully appreciating the significance of the diffused and generalized forms of perception organs, much effort has been directed toward fixing on specialized protoplastic tracts, with functions analogous to nerves. The quest has not yet met with decided success in any single instance. We have, however, arrived at the general conclusion that the ectoplasmic layers of the protoplasts of peripheral cells function as sensory organs, and that impulses are transmitted between the motor and sensory zones by these layers and their interprotoplastic threads. As to the nature of the impulse, one can only hazard a meaningless guess that it may consist in a chain of chemical, catalytic or osmotic disturbances.
Two noteworthy attempts have been made to ascribe the function of transmission of impulses to specially differentiated structures. The first was by Haberlandt who dealt with the transmission of impulses in
Mimosa, the common 'sensitive plant' of the tropics, cultivated in conservatories. An impulse set up at the tip of a pinnule of one of these plants is conducted through petioles, stems and branches to a distance of a meter at a rate varying from 6 to 31 mm. per second. A study of the structure of the plant reveals the presence of a connected series of long tube-like cells in the fibro-vascular bundles, usually turgid, and containing relatively small protoplasts. It is argued that impulses take the form of hydrostatic disturbances communicated through the system