[Chap. XVI SYNTHESIS OF FATS AND PROTEINS 147 



teins and their derivatives appear to be the primary chemical bases of 

 different protoplasms, different hereditary miits, and different species 

 of plants and animals. The specificity of organic synthesis in plants and 

 animals and its constancy in repetition, cell after cell and generation 

 after generation, are among the most important facts of nature. Without 

 this specificity and constancy, organisms as we know them would not 

 exist. 



What Becomes of the Food Made by Plants? 



The answer to this question is the central theme of the next two 

 chapters. It may be briefly summarized here, however, as an introduc- 

 tion to what is to follow. 



Some of the food is oxidized within the cells of the plant. The amount 

 of food that is consumed in plants by oxidation differs with the kind of 

 plant and also with the environment of the plant. Under excellent grow- 

 ing conditions about one-fourth of the food made by a corn plant is 

 oxidized in the plant during its lifetime. In a single growing season about 

 one-third of the food made by a young apple tree is oxidized within the 

 tree. However, plants may be placed in environments in which they 

 oxidize food faster than they make it. 



Some of the food is converted into substances of which the cells of 

 the plant are composed. Here again, the amount so used varies with the 

 kind of plant and with the environment. In the com plant referred to 

 above, about one-half of the food is used in this manner. 



The food that is made by the plant and not used by it in either of these 

 two ways of course accumulates within the plant. It is frequently re- 

 ferred to as "storage food." The amount of this accumulated food may 

 be great or small, depending upon the kind of plant and its environment. 

 It is the source of the food of most animals and numerous non-green 

 plants. Many of the non-green plants and some of the simpler animals, 

 because of their unique enzvmes, can also digest and use the cell walls 

 as food. 



The relative proportion of the kinds of food that accumulate in a 



plant is conditioned by the hereditary constitution of the plant. In Table 



3 are shown the amounts of glucose, sucrose, starch, protein, and fat in 



the leaves of several well-known plants. The analyses were made after a 



period of rapid photosynthesis in detached leaves. Accumulated foods 



are expressed in percentages of total dry weight of the leaves.^ 



^ Data from B. N. Singh, K. N. Lai, and K. Prasad, Proc. Indian Acad. ScL, VIII B, 

 301, 1938. 



