42 



Science of Plant Life 



Fig. 29. Guinea grass, a plant grown in the tropics for fodder. Note the vertical 

 leaves and the large amount of leaf surface exposed by the plant to the light. 



in this position by their sheathing bases rather than because 

 of a response to light. These plants usually occur in dense 

 growths, and the vertical position of the leaves permits the 

 light to penetrate to their bases. This has the advantage of 

 allowing photosynthesis to go on throughout the entire 

 length of the leaves. 



Differences in vertical and horizontal leaves. Vertical 

 leaves differ from horizontal leaves in several particulars : 



In vertical leaves the mesophyll may be composed of 

 spongy tissue, or it may be composed entirely of palisade 

 cells. More rarely there are palisade layers on both sides, 

 with a spongy layer between. In contrast, a horizontal leaf 

 usually has a palisade layer beneath the upper epidermis, and 

 the lower portion of the mesophyll is composed of loosely 

 arranged cells. In vertical leaves stomata usually occur on 



