410 



VEGETABLE PHYSIOLOGY 



from the wound and a new root-tip was developed upon each. 

 As soon as the new tip was formed, the rootlets showed a 

 power of reacting to the stimulus of gravitation, and the 

 curvature resulted in the usual place. If we turn to the 

 reaction of the leaf of Dioncea to contact, we find that the 

 whole leaf may be somewhat roughly handled without 

 closing, so long as no contact is made with the hairs, three 

 in number (fig. 160), which arise upon a particular portion 



FIG. 160. LEAF OF Dioncea musclpula. 



1, open ; 2, closed : a lateral view, b, surface view; 3, one of the sensitive 

 spines ( X 50) ; 4, glands on the surface of the leaf ( X 100). 



of the blade. So soon, however, as one of these is touched, 

 the leaf closes. 



In many leaves the cells of the upper epidermis are convex 

 on their upper surfaces, and a ray of light passing through 

 them is brought to a focus somewhere in the palisade par- 

 enchyma, with such distinctness that it is possible to use 

 a piece of detached epidermis in the fashion of a number 

 of lenses placed side by side. Haberlandt has attributed 

 a perceptive function to these cells, which he calls ocelli. 



