PLANTS. 55 



cavity by their roots extending in search of water. 

 They not infrequently grow downward forty or 

 fifty feet after penetrating the walls of a well. 



LEAVES STRUCTURE AND FUNCTION. 



If it can be said that one part of a plant is more 

 useful than another, then the most important part 

 is the leaves. They grow upward and outward 

 into the air and light. They spread their flat 

 bodies into numberless shapes, and the most beau- 

 tiful forms. The leaves of most plants are very 

 thin, compared with their other dimensions. It is 

 necessary that they should be, since their function 

 is to gather air and light, and to evaporate water. 

 During normal growth, there is a great deal of 

 water evaporated. Since the amount depends 

 upon the quantity in the soil which can be absorbed 

 by the roots and sent up to the leaves, plants in 

 very dry regions have very thick, fleshy leaves, 

 which expose less surface compared with the bulk 

 of the plant. Such, for instance, are the cacti, 

 whose native home is the dry, arid region of west 

 central United States. 



Some forms of the cactus plant have no true 

 'leaves at all. The large, green, fleshy stalk per- 

 forms the functions of both leaves and stem, and 

 acts also as a water reservoir from one rainy season 

 to another. If these plants had thin and highly 

 divided leaves, so much surface would be exposed 

 that evaporation would carry away the water too 



