Leaves in Relation to Light 43 



both surfaces, while in most horizontal leaves the stomata 

 are confined to the lower surface (page 18). Vertical leaves 



FiG. 30. Various positions taken by leaflets of lima bean : A , position in intense 

 light ; B, position in diffuse light ; C, position in darkness. 



are likely 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 to the presence of a larger amount of chlorophyll 

 in the compact palisade layers of the mesophyll than in the 

 loose spongy layers beneath. In vertical leaves, the similarity 

 of structure 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. 



Motile leaves. The leaves of which we have been speaking 

 have their positions rather definitely fixed when they reach 

 maturity. There is another class of leaves, however, in which 

 the positions of the blades are not fixed but are changed ac- 

 cording to the intensity and direction of light. A familiar 

 example is the roadside sweet clover. At night the three 



