274 LEAVES 



sugar furnishes either all or an important part of the constituents 

 of which all other plant substances are composed. Even if light 

 is essential to the formation of proteins, which are formed so 

 abundantly in leaves and carried away like the photosynthetic 

 sugar to other parts of the plant for use, sugar is an important 

 constituent of proteins and is therefore a basal substance in their 

 formation. Since animals obtain their food either directly or 

 indirectly from plants, it is evident that the photosynthetic sugar 

 is also the basal food for animals. 



The various metabolic changes in the plant have to do with 

 providing living protoplasm, a frame work, reserve foods, secre- 

 tions, and energy. 



Protoplasm. It is in connection with protoplasm, the living 

 substance of both plants and animals, that the metabolic processes 

 occur. Protoplasm not only transforms substances enclosed 

 within it but by means of enzymes which it secretes it is also able 

 to act on substances with which it is not in contact. Within the 

 protoplasm sugar is synthesized by the chloroplasts, and starch, 

 proteins, fats, and many other plant products are constructed. 

 At the same time substances are being constructed in the proto- 

 plasm, substances are also being decomposed, so that within the 

 protoplasm substances resulting from both constructive and 

 destructive processes are always present. So it is not at all 

 strange that many kinds of substances are present in the proto- 

 plasm. Some, like starch and some proteins, are insoluble, while 

 many, like the sugars and acids, are dissolved in the nuclear and 

 cell sap. That protoplasm can transform substances with which 

 it is not in contact is well illustrated in seeds, where enzymes 

 secreted by the embryo diffuse out to the endosperm and trans- 

 form it into soluble forms of food. 



One of the important constructive processes of protoplasm is the 

 formation of protoplasm. As the plant grows and more cells are 

 formed, more of the elements of chromatin, nucleoli, cytoplasm, 

 and all other protoplasmic structures must be formed. 



As to the chemical composition of protoplasm in its living 

 state, we have no definite knowledge. Chemical analyses of 

 dead protoplasm show that a large number of substances are 

 present, of which proteins are the largest and most essential part. 

 The different kinds of proteins vary in composition, but all are 

 composed chiefly of carbon, hydrogen, oxygen, and nitrogen. In 



