CHAPTER XXX 

 FOOD. THE NUTRITIVE CYCLE 



You have just noted that photosynthesis is the manufac- 

 ture of only one kind of food carbohydrate food. Evi- 

 dently, it is important for us to find out what other kinds 

 of food there are. We have been studying inorganic phe- 

 nomena of nature, especially with reference to their rela- 

 tions to life. Now if we are to see how this inorganic 

 realm of nature is related to life in the most important 

 way of all, we must surely study food. For it is by means 

 of food that the actual stuff of life (protoplasm} and the 

 structures that it inhabits (bodies] are built up out of the 

 inorganic materials of earth. 



What Food Is. So let us first ask ourselves, what is 

 food? It is a popular rather than a scientific term, and 

 does not have a very precise meaning. Ordinarily it 

 refers to anything which, when taken into the body, con- 

 tributes to the growth, energy, or repair of the body. 

 So we may take this as our "working definition" of food. 

 But in applying this definition, some questions arise. Is 

 water a food? We shall have to admit that it contributes 

 to the growth of the body. Is air a food ? We shall have 

 to admit that it is essential to the release of energy in the 

 body. And yet air and water are quite different from the 

 things we ordinarily think of as food such things as 

 bread, meat, and potatoes. 



232 



