TEXTBOOK OF BOTANY 



about fifteen times the weight of all the fish hatched in the 

 same river in the course of a year. The plankton includes 

 various one-celled algae more or less like Protococcus, as 



well as many species 

 that form small colo- 

 nies. Algse as large 

 as Spirogyra are not 

 ordinarily thought of 

 as belonging to the 

 plankton ; the use of 

 the term is commonly 

 limited to organisms 

 that are microscopic 

 or but Httle larger. 

 The plankton algae, 

 building up their 

 bodies as we have 

 seen that Spirogyra 

 does out of the simple 

 substances that are 

 dissolved in the river, 

 lake, or sea water, 

 themselves serve as 

 food for the many 

 kinds of minute ani- 

 mals in the plankton. 

 Both the plankton 

 algae and the plank- 

 ton animals in turn are used as food by the larger water 

 animals, especially the fish, and the latter in their turn are 

 an important item of human food. We are ourselves con- 

 cerned, therefore, with anything that affects the number and 

 the growth of the plankton organisms. 



FIG. 21. A red alga (Dasya elegans). 



