FUNCTIONS 229 



process is an advantage or a disadvantage. Here we have 

 the thing which appears more than anything else to have 

 made leaves what they are; they nearly all appear to be 

 designed primarily for this work of food making in the 

 light. In photosynthesis, we may say, we have the leaf's 

 principal excuse for existence. 



Already you have been told nearly as much about pho- 

 tosynthesis as has a proper place in a book like this. The 

 chemistry and the physics of the process are things to be 

 studied elsewhere than in a first brief course in plants. 

 A few new terms at least are needed now, however, and the 

 principal one is the term carbohydrate, the name of that 

 kind of food which photosynthesis produces. 



Carbohydrates form one of the three principal classes of 

 foods and photosynthesis is their manufacture. Photo- 

 synthesis is sometimes called carbohydrate synthesis; it 

 has nothing to do with the manufacture of the other classes 

 of foods. They are made by the protoplasm, and light is 

 not required in the process; they may be made in any 

 living cell. Carbohydrates are, however, the only class of 

 foods which are made entirely out of inorganic materials ; 

 in the manufacture of proteins or fats, carbohydrates are 

 necessary as a basis. 



The two classes of foods besides carbohydrates are 

 the proteins and the fats; we shall consider them when 

 we come to the seeds and the foods stored in them (Chap- 

 ter VIII). In this book the word food is applied only to 

 organic substances, and you can think, of course, of many 

 different kinds of organic substances which we buy in the 

 markets and use for food. Evidently the classification of 

 food into proteins, fats, and carbohydrates is on a different 

 basis from our everyday classification of food into meats, 



