HOW FOOD IS USED BY PLANTS 329 



food supply from the simple substances carbon dioxide and 

 water and from the more complex compounds, which include 

 nitrogen, phosphorus, and potassium. 



If a plant is not interfered with, the food which it manu- 

 factures is commonly used by it and for its own purposes. 

 For instance, a part of our food is composed of starch from 

 wheat grains ; but if the wheat had not been disturbed, the 

 grains would have fallen to the ground, \vhere, under favor- 

 able conditions, they would have started to grow into new 

 wheat plants. These young plants would have used the food 

 stored up in the grain. By harvesting, milling, baking, and 

 eating the wheat we have appropriated for our own purposes 

 that which otherwise might have fed a young wheat plant. 



There are several purposes for which food is used in the 

 plant. New material is needed for growth, since both the 

 living protoplasm and the cell walls must be built up from 

 food materials. A less obvious use of food is to furnish 

 energy for the life processes of the plant. .V part of the 

 food manufactured by a plant undergoes chemical change 

 by combination with oxygen, which is ordinarily secured 

 through the leaves. This process is called respiration and 

 serves the same purpose in the living plant that respiration 

 does in the living animal. During part of its life the plant 

 stores material in seeds or roots for the nourishment of the 

 next generation of plants. 



331. The kinds of foods. There are live classes of foods: 

 carbohydrates, fats, proteins, mineral salts, and water. The 

 first three are of chief importance. 



The carbohydrates we already know from our study of 

 the manufacture of plant foods in green plants. The process 

 known as photosynthesis is the source of all the carbo- 

 hydrates. The most common carbohydrates are starch and 

 several kinds of sugar. 



The fats, like the carbohydrates, contain carbon, hydrogen, 

 and oxygen, but in differuit proportions. Cottonseed oil and 



