34 FOUNDATIONS OF BIOLOGY 



It is clear that the various forms which follow one another 

 arise by cell division in every case, though this is interrupted 

 once by just the opposite process the complete cytoplasmic 

 and nuclear fusion of two distinct cells to form one cell. 

 This is the process of FERTILIZATION, an expression of a funda- 

 mental phenomenon of protoplasm at the basis of sex and 

 sexual reproduction, which we shall consider at length later. 



Such is the history of Sphaerella. It is apparent that the 

 sequence of diverse forms which arise from one another 

 constitute a life cycle, and although each individual cell in 

 the cycle is a Sphaerella, nevertheless the plant called 

 Sphaerella lacustris comprises all the forms assumed. From 

 one viewpoint we may look upon the cycle as forming an 

 individual of a different or higher order an individual the 

 component cells of which are separate. 



B. METABOLISM IN SPHAERELLA 



We ,now turn our attention from the structure and life 

 history of Sphaerella to the point it was chosen especially to 

 illustrate the metabolism of green plants. It may appear 

 to the reader that a tree or shrub might with more propriety 

 be taken as the example of a typical plant, but, since the 

 fundamental distinction between animals and plants 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. 



Since Sphaerella lives, grows, and multiplies in pools of 

 water exposed to sunlight, it is to this environment that we 

 must look for the materials which it turns into protoplasm, 

 and the energy by which it makes the transformation. And 

 further, since the organism is enclosed in a cell wall, its income 

 and outgo of materials must be in solution in ordei to pass 

 through. 



