IRRITABILITY AND MOVEMENT IN PLANTS. 231 



pulse, 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 con- 

 sideration 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 tis- 

 sue (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 inter- 

 nal 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 



Fig. 5. Diagram of the Longitudinal Section of a Tendril in the Eegion of Curva- 

 ture, a, epidermis (perceptive zone) ; b, parenchyma cells which before curvature were 

 similar to f in form ; c, e, wood ; d, pith ; /, 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. 



