CHAPTER THREE 



THE MANUFACTURE OF FOOD 



You will probably remember from your study of physiology 

 that all the foods used by animals belong to three classes 

 of chemical substances : carbohydrates, fats, and proteins. 

 These same classes of substances constitute the food of plants. 

 A grain of corn contains a supply of starch, oil, and pro- 

 tein for the young plant, and these same foods that are used 

 by animals are accumulated in many plants. The difference 

 in the nutrition of plants and animals Hes, then, not in any 

 differences in the foods used, but in the way their foods are 

 secured. In this chapter the manner in which plants obtain 

 their foods will be discussed. 



Plants the source of all food. Mineral soils and the air 

 do not contain any of the substances that we class as foods. 

 Yet green plants may grow luxuriantly on mineral soils. It 

 follows, therefore, that green plants are able to manufacture 

 their own foods. They can synthesize, or build together, 

 simple substances that they obtain from the soil and air 

 into the complex foods that they require. Animals lack 

 this power. They must have foods that have already been 

 built up, rather than the simple materials of which foods are 

 made. These foods they secure either directly or indirectly 

 from plants. The abihty of plants to manufacture com- 

 plex foods from simple substances brings up several ques- 

 tions : 



What is the method by which plants produce food ? Just 

 what parts of the plants do the work? What constitutes the 

 machinery ? Out of what materials is the food manufactured ? 

 How is the energy supplied ? And what are the conditions 

 under which the process goes on ? 



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