68 



General Botany 



Fig. 38. Guinea grass, a plant grown in the tropics for fodder. Vertical leaves expose a 

 large surface to the sunlight in spite of the crowding. 



occur on both surfaces, while in most horizontal leaves the 

 stomata are confined to the lower surface (page 27). Vertical 

 leaves are hkely to be of the same color on both surfaces, while 

 horizontal leaves are generally of a darker green on the upper 

 surface. 



The difference in the color of the two sides of a horizontal leaf 

 is due in part to the presence of a larger amount of chlorophyll 

 in the compact palisade layers of the mesophyll than in the loose 

 spongy layer beneath. In vertical leaves the similarity of struc- 

 ture in the mesophyll on each side, and the fact that both surfaces 

 of the leaf are equally illuminated, account for the sameness of 

 color of the two surfaces. The color of leaves is sometimes modi- 

 fied by the presence of hairs, wax, or drops of resin. 



Motile leaves. The leaves of which we have been speaking 

 have their positions rather definitely fixed when they reach ma- 

 turity. There is another class of leaves, however, in which the 



