CHAPTER XXI 

 FURTHER DISCUSSION OF DEPENDENT PLANTS 



341. Nutrition, the leading problem. "Getting a living is 

 the first business of life, and food is the basis of a living ; for 

 the body derives both its substance and its energy from its 

 food " (Needham). This statement is equally true of plants 

 and of animals, and the greatest problem of living things is 

 how they are to obtain an adequate supply of proper food. 

 Much of what has already been said about plants has to do in 

 one way or another with their nutrition. A good deal has also 

 been said about the dependence of plants upon one another 

 and upon animals, both in relation to food and to reproduc- 

 tion. The discussion of the bacteria and other fungi (Chap- 

 ters XI, XIV, and XV), though it deals with structures and 

 reproduction, constantly presents the relationships of these 

 organisms to their food material. There are, however, some 

 further aspects of interdependence which we shall consider. 



342. Kinds of dependence for nutrition. Plants such as forest 

 trees, ordinary grasses, cereals, and many others familiar in the 

 farm and garden, which possess chlorophyll, can manufacture 

 their own food (Chapter II). Since these green plants are 

 dependent for materials from which to make food rather than 

 for organized food, it is common to consider them as inde- 

 pendents. This does not mean that they can live independent 

 of anything outside themselves, but that if carbon dioxide, 

 water, certain mineral salts containing nitrogen, potassium, 

 etc., proper light and temperature, are available to them, they 

 can use these things in constructing nourishing foods. These 

 primary foods which are. made by green plants are directly or 

 indirectly the basis of the foods of all plants which are not 

 green, and of all the food of animals, which in this wide sense 



371 



