178 TEXTBOOK OF BOTANY 



From these figures and the facts in the last few chapters it is evident 

 that 94.63 per cent of the dry matter in the corn plant had its origin in 

 the sugar made from carbon dioxide and water. As the plant stood in the 

 field, only 0.8 of 1 per cent of its weight was due to inorganic salts that 

 had passed into it from the soil, and some of these were present in excess 

 amounts. But one should not infer from this fact that these salts are not 

 of much importance to the plant. It is merely another case in which a 

 relatively small amount of something is essential to the development of 

 the plant. A deficiency is soon reflected in slow growth or even death. 



Such phenomena as the growth of seedlings rooted in pure sand, of a 

 large plant with roots confined in a small pot of soil, or aerial epiphytes 

 and free-floating algae, or, on the contrary, the failure of lawn grass to 

 grow in dense shade even when fertilizers and water are added, may all 

 be adequately explained on the basis of the facts in the last few chapters. 



Summary. In these last four chapters we were concerned primarily 

 with the manufacture and uses of food in plants. The ways in which 

 man and other animals are dependent upon food manufactured in plants 

 were also noted. Both plants and animals are dependent upon food as 

 a source of chemically bound energy which is liberated by respiration, 

 and as a source of building material in assimilation. Owing to the gross 

 similarity of protoplasm in plants and animals their food is essentially 

 the same. It consists of carbohydrates, fats, and proteins, and any of their 

 derivatives that may be used in respiration or assimilation. The range of 

 these substances that can be used by an organism as a source of food 

 depends upon its system of enzymes. Some organisms, for instance, can 

 digest substances that are indigestible to others. 



Assimilation may be considered as the transformation of food into 

 protoplasm, cell walls, and other essential constituents of cells, such as 

 enzymes, chlorophyll, vitamins, and honnones. During this transforma- 

 tion certain inorganic salts are variously combined with the organic com- 

 pounds. Some authors use the term to denote only the transformation 

 of food into protoplasm, thus leaving no general term to designate the 

 transformation of food into other essential parts of the cell. Assimilation 

 should be strictly distinguished from food manufacture and also from 

 digestion. 



The substances in the cell walls of plants are constructed primarily 

 from sugar. An enormous quantity of food is annually converted into 

 cellulose. In the making of protoplasm all types of foods are assimilated, 

 and in addition water and the ions of inorganic salts are incorporated 



