CHAPTER SIX 



THE MANUFACTURE OF FOOD 



You will probably remember from your study of physiology 

 that the principal 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 protein which 

 is used by 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 dif- 

 ferences 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, there- 

 fore, 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 

 complex foods from simple substances brings up several questions : 

 What is the method by which plants produce food ? Just what 

 parts of the plants do the work? What constitutes the machin- 

 ery? Out of what materials is the food manufactured? How 

 is the energy suppHed? And what are the conditions under 

 which the process goes on ? 



Photosynthesis. The primary step in the making of food is 

 the building of simple carbohydrates through the process called 

 photosynthesis (Greek: photos, light, and synthesis, putting to- 

 gether). In this process carbon dioxide from the air and water 



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