Page:Popular Science Monthly Volume 47.djvu/241

, and motion were each accomplished by separate groups of cells. The receiving, transmitting, and motor zones are not always so distinctly separated, however. In stems, leaves, and other aerial organs the perceptive zone may extend over the greater part of the surface, including the region of curvature. In tendrils the perceptive zone comprises the superficial layers of tissue of the concave side of the organ throughout the region of curvature. It is quite probable in all these instances, however, that the cells receiving the stimulus are not identical with those causing the curvature, or, disregarding the cell, separate masses of protoplasm are differentiated for the performance of each of these functions. A clear conception of this mechanism may be attained by a consideration of the structure and action of a tendril. A tendril is generally of bilateral organization, consisting of a middle layer of pith between two layers of elastic and flexible mechanical tissue (wood); outside the wood, on both the upper and lower sides of the organ, layers of mobile thin-walled parenchyma cells, which in the active condition of the tendril are in a state of high internal tension; covering the parenchyma, a layer of epidermal tissue composed of elastic, thick-walled, but easily extensible cells (see Fig. 5). If a solid body is brought into contact with the concave

 a, epidermis (perceptive zone); b, parenchyma cells which before curvature were similar to f in form; c, e, wood; d, pith; f, parenchyma slightly more elongated than before curvature; g, epidermis of convex side.

side of a tendril, the pressure acts as a stimulus on the ectoplasm—the thin layer of protoplasm lining the walls—of the epidermal cells. The ectoplasm of these cells is connected with that of the thin-walled parenchyma cells at numerous points by means of very fine strands of protoplasm extending through the walls of the intervening cells. An impulse is conducted along the strands to the protoplasm of the thin-walled cells, and their action results in the curvature of the organ. The structure and action of the tendril are generally similar to those of a large number of stems.