SENSITIVENESS O7' LEAVES. 



419 



1083. In some cases tendrils are not sensitive to contact, but 

 are distinctly apheliotropic, turning- away from the light, and in 

 this wa}' securing for the plant an adequate mechanical support 

 upon some wall or the like. Grape-vines and Virginia creeper 

 furnish good examples of such tendrils. The branches of the 

 tendrils of the grape-vine sometimes clasp around a slender sup- 

 port, somewhat in the same way as an object would be grasped 

 by a thumb and finger. 



The much-branched tendrils of species of Ampelopsis are also 

 apheliotropic ; but when the tips of the branches of the tendrils 

 come in contact with a wall, they become expanded into flat 

 discs which cling to the surface. 



1084. Sensitiveness of petioles. 

 This can be easily examined in 

 the common climbing species of 

 Clematis, in Solanum jasminoides, 

 etc. The leaves circiiiniiutate 

 and, in the case of compound 

 leaves, the separate leaflets also. 

 When young the sides of the 

 petioles are sensitive to touch, 

 bending towards where the pres- 

 sure or compact is. Shortty after 

 clasping the support by means of 

 this bending the petioles increase 

 in thickness, become stronger 

 and tougher than before, and 

 sometimes take on a structure 

 suggestive of a rigid branch. In 

 Gloriosa the sensitiveness is verv 



\f 



marked in the leaf-tips, but only 



on the under surface of the pro- 189 



longed thread-like extremity. 



1085. Sensitiveness of leaf-blades. The fly-trap of Dionsea 

 (considered by some an appendage to the proper leaf-blade) is 

 exquisitely sensitive to anv touch upon the hairs which grow on 

 the faces of the trap. As soon as these are touched the trap 

 instantly closes, and the same effect follows a slight touch on 

 the median line. A cross-section through the loaf shows that 

 the parenchyma is thin- walled. The leaf of the small water- 

 plant Aldrovanda has likewise been shown to be sensitive. 



FIG. 189. Solanum jasminoides. 



