STEM AND BHANCHEg. 189 



tion. Again, while the pores are usually found 

 on the under surface of the leaf, they are situated 

 on the upper surface of leaves which float on 

 water, like the water-lily and the water-crow- 

 foot ; because in such plants they would be 

 obviously useless for purposes of evaporation on 

 the lower side, which is in contact with the 

 water. Some leaves have the stomata on both 

 sides alike, especially when no one side is much 

 more exposed to sunlight than another. But 

 wherever they are found, they always lie above 

 masses of loose and spongy cell-tissue, in whose 

 meshes and air-spaces evaporation can go on 

 readily. 



On the other hand, as I noted before, leaves 

 which grow in very dry or desert situations 

 require as much as possible to curtail evapo- 

 ration. Such leaves are therefore usually thick 

 and fleshy, and possess a very small allowance 

 of pores. The forms of several leaves, again, 

 are largely dependent upon the necessity for 

 keeping the pores free from wetting, and pro- 

 moting evaporation whenever it is needful for 

 the plant's health and growth ; and this is 

 particularly the case with what are called 

 "rolled leaves," such as one sees in the 

 heaths and the common rock-roses. Many such 

 additional principles have always to be taken 

 into consideration in attempting to account for 

 the various shapes of foliage : indeed, we can 

 only rightly understand the form of any given leaf 

 when we know all about its habits and its native 

 situation. 



The stem, then, besides raising the leaves and 



