METABOLISM OF ORGANISMS 31 



secreted by the protoplasm and is composed of cellulose: a 

 carbohydrate especially characteristic of plants. It is evident that 

 the organism is a single cell. (Fig. 12.) 



Since Protococcus is a single cell, we find reproduction pre- 

 sented in its simplest terms: under favorable conditions the cell 

 divides and the two resultant cells sooner or later separate. Some- 

 times, however, several divisions occur before the cells are sep- 

 arated and thus there is formed, as it were, a temporary multi- 



Fig. 12. — Protococcus vulgaris, a unicellular green plant. Separate indi- 

 vidual and temporary cell groups formed by cell division. 



cellular body, but one without any physiological division of labor 

 between the cells because all are independent in their vital ac- 

 tivities. Moreover, this tendency to form temporary cell groups 

 suggests the type of step that was taken when the multicellular 

 body was established during plant evolution. 



We may now turn our attention to the point Protococcus was 

 chosen especially to illustrate — the characteristic life processes 

 of green plants. At first glance it may appear that a multicellular 

 plant, such as a tree or shrub, would afford a more suitable ex- 

 ample, but since the fundamental distinction between plants and 

 animals is chiefly a question of metabolism, there are advantages 

 in studying it in a single cell, where one's attention is not dis- 

 tracted by root, stem, and leaf. 



2. Food Making 



Since Protococcus lives, grows, and multiplies in moisture ex- 

 posed to sunlight, it is to this environment that we must look for 

 the materials with which it constructs protoplasm, and the energy 

 which it employs in the process. And, furthermore, since the organ- 

 ism is enclosed within a cell wall, its income and outgo must be ma- 

 terials in solution in order to pass through to the living protoplasm. 



In short, Protococcus takes materials from its surroundings in 

 the form of simple compounds, as carbon dioxide, water, and 

 mineral salts, which are relatively stable and therefore practically 



