46 THE NATURE OF LIFE 



life. The familiar processes of waste and 

 repair in the body illustrate destructive 

 and constructive metabolism. 



We shall begin with the green plant, 

 on which all other living things depend 

 for food. Here are joined together the 

 simple substances found in air and water 

 to make the materials which every or- 

 ganism needs for its existence. This is 

 the first step in constructive metabolism. 

 Once these materials (such as sugar, 

 starch, fats, and proteins) have been 

 formed by the green plant they can be 

 used by other organisms to build up 

 their own characteristic compounds. 



We may now turn our attention to the 

 activities of the green plant. In the first 

 place it would seem advantageous for 

 it to spread out as much leaf surface 

 as possible, so as to get the maximum 

 amount of air and sunlight. But there 

 is danger that water may evaporate 

 faster from the surface of the leaves than 

 it can be supplied by the root thus caus- 

 ing wilting or even death. We find that 



