UTILIZATION OF FOOD IN THE PLANT 251 



a very large part of the human race is dependent upon it 

 for their principal food. Rye flour is used for bread in some 

 European countries. Beans and peas contain starch and more 

 protein than almost any other common vegetable food. They 

 are therefore particularly valuable as a substitute for meat in 

 hot countries or in places where meat is scarce. 



Corn and oats are used for human food, but not to so great 

 an extent as some of the other grains. They are usually fed 

 to animals which may then be used as food. The lean meat 

 of animals is particularly rich in proteins, as the fatty parts 

 are in fats, but flesh is poor in carbohydrates. 



The enormous importance of the industries which are pro- 

 ducing these supplies these great quantities of stored plant 

 food may be understood by reference to the table of the 

 quantities and values of agricultural products in the United 

 States, given in section 248. 



273. Use of food by plants. If food which is stored is not 

 used by men or removed in some other way, the plant will 

 finally use it in its processes of growth. It was said above 

 that beans contain much protein material, and this is in an 

 insoluble form. If a bean is planted and starts to grow, the 

 protein grains in the bean will be digested by the aid of an 

 enzyme and dissolved, after which they may be carried to the 

 growing parts, where the protein material will form additional 

 living protein protoplasm. 



This is a wonderful process, for the digested and dissolved 

 proteins are dead materials, but the protoplasm which is formed 

 from them is a living substance. Therefore a dead material has 

 become a living one. Proteins are the only materials in which 

 life may exist, and the only known way by which non-living 

 protein may become living material is for it to unite with some 

 already living substance and thus become a part of it. This 

 process whereby non-living protein material unites with the 

 living protoplasm and thus becomes living material is called 

 assimilation a process the exact nature of which is not known. 



