﻿70 MacDougal: A Contribution to 



freely curved base sufficient to give an elastic or springing attach- 

 ment of the leaf which is, in consequence, held in its position quite 

 rigidly. 



* 



The portions of the tendril in contact with the support show 

 an increased diameter and a general thickening of the cell walls. 

 The contents of the glucoside cells have nearly disappeared, sug- 

 gesting that this substance functions as reserve material. 



In the outer region of the glucoside cells a ring of libriform 

 tissue is formed which is four or five layers in thickness on the 

 concave side and scarcely half as much on the convex. This 

 ring, as well as the woody, is lignified. The cortex has increased 

 in thickness by simple enlargement of cells. The subepidermal 

 layer has perhaps undergone division forming irregular and much 

 compressed cells. The cells of the epidermal layer of the concave 

 side have undergone enlargement in a radial direction so that this 

 diameter is two or three times as great as the diameter parallel to 

 the surface of the organ. The increase is principally in the form 

 of a greater convexity of the outer wall. The entire concave sur- 

 face of the tendril has epidermal cells forming irregular papillae, 

 consisting of a group of six to fifty cells. Such an arrangement 

 would fasten the organ quite firmly to the support. The epider- 

 mal cells of the convex side have undergone no marked changes 

 except in increase in the axes parallel to the surface and decrease 

 of the convexity of the outer walls. 



General Conclusions. 



Schenk has pointed out that the theory of Darwin, that all 

 tendril-bearing plants were originally twiners, is at least not applic- 

 able to leaf-climbers, and to climbers with tendrils derived from 

 leaves. Leaf-climbers were doubtless derived from species with 

 elongated weak internodes which grow up through the tangles of 

 jungles. The weight of the stem would be supported upon the 

 neighboring plants by means of the leaves and other lateral mem- 

 bers. The movement of the surrounding plants by the wind 

 would tend to produce injuries in the supporting members. The 

 development of the power of response to contact would enable the 



