[Chap. XII THE FOOD OF PLANTS 107 



considered. When a seed or a tuber is placed in a moist chamber and a 

 young plant grows from it, what substances in the seed or tuber dis- 

 appear as the cells of the new plant are made? They are exactly the 

 same kinds of carbohydrates, fats, proteins, and derived compounds that 

 the animal uses as food. Parts of the tuber, especially the cell walls, 

 remain but may be digested and used as food by certain bacteria and 

 fungi. From such facts as these, together with other facts that will be 

 discussed in later chapters, botanists regard the food of green plants to 

 be identical with that of animals. 



The process by which the green plant obtains food, however, is unique. 

 The food is not obtained from the soil, from the air, from water, or 

 from other organisms. It is made within the plant from simple inorganic 

 compounds obtained from the soil, air, and water. In the next few chap- 

 ters we shall consider material and energy transformations involved in 

 the food-making processes of green plants, the influence of the environ- 

 ment upon these processes, the consequent behavior of the plant in cer- 

 tain environments, and the vital importance of these transformations to 

 the whole biological world and to industry. 



Summary. In the foregoing discussion we have considered two very 

 different concepts of the food of living organisms, with special reference 

 to green plants. The older view proposed by Aristotle is based upon the 

 assumption that food is something that enters the organism from with- 

 out. A more recent concept of food is based upon a knowledge of the 

 physiology of organisms, that is, the ways in which substances are used 

 within the organism regardless of whether they are obtained from the 

 outer world or are made within the body. It limits food to such sub- 

 stances as carbohydrates, fats, proteins, and their derivatives which mav 

 be used in cell construction and as a source of energy within the 

 organism. 



Such a concept of food emphasizes the fundamental similarity in the 

 nutritive processes of the protoplasm in all plants and animals. The older 

 concept denies this fundamental similarity in organisms, and excludes 

 all the modern concepts of the food of green plants. 



