7'HE MOVEMENTS OF PLANTS. 179 



come in contact with a solid body, it will quickly become 

 concave on the side touched, and thus will wrap about the 

 object, if it be of suitable size. This curvature is due first to 

 the shortening of the cells upon the concave side and later to 

 unequal growth on the convex and concave sides. Finally 

 this effect extends to all parts of the tendril, which begins to 

 curve. As both ends are fast, it is a mechanical necessity 

 that the curves become spiral coils, both right- and left- 

 handed, accompanied by a twisting of the tendril on its axis 

 (fig. 69). After the coils are formed the tissues of the 

 tendril become thick-walled and rigid, so that the plant is 

 attached to the support by a spiral spring. 



Other tendrils do not nutate, but are negatively helio- 

 tropic, and by contact their tips are stimulated to develop 

 disks which apply themselves closely to the support and send 

 into its irregularities short outgrowths from the surface cells. 

 Such plants are adapted to support themselves by walls, tree- 

 trunks, etc. The Japanese ivy and one form of the Virginia 

 creeper are notable examples. 



The coiling of the leaf-stalks is not unlike the first curva- 

 tures described for tendrils (fig. 100). 



EXERCISE XLIII. 



To show effect of contact as a stimulus to tendrils. 



Stroke with a pencil the concave side of the tip of a tendril of passion 

 vine, squash, wild cucumber, or balsam-apple, on a. warm day or in a 

 hothouse, and observe curvature which follows in a few minutes. 



252. (B) Movements of turgor. — The movements already 

 described are confined to members which are growing, either 

 throughout, or in some part. As turgor cau affect only tissues 

 whose cell-walls are elastic (If 156), the movements pro- 

 duced directly by variation in turgor can occur only in such 

 mature members as are provided with special motor organs. 

 In almost all cases these are leaves. Stimuli which regulate 



