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 
