7O Elementary Biology. 



to the origin of the cilia. After a variable period of exist- 

 ence in this free-swimming condition the cilia are absorbed, 

 the cell comes to rest, and the reformation of a cellulose coat 

 takes place. 



With regard to the physiology of Protococcus, being a 

 plant possessed of chlorophyll, it decomposes water and 

 carbonic acid in the presence of sunlight, and integrates new 

 protoplasm, setting free oxygen into the air in the process. 

 In this instance, of course, the carbonic acid is obtained 

 from the rain-water. The nitrogenous elements in its com- 



FIG. 17. LIFE-HISTORY OF Protococcus pluvialis. 





a-e, stages in vegetative division \f,g t motile stage. 



position Protococcus obtains from the air or from the sur- 

 rounding objects upon which it may happen to be growing. 

 All the phenomena spoken of as characteristic of plants 

 possessing chlorophyll are exhibited by Protococcus and 

 therefore need not be repeated here. 



When the plant is provided with the necessary substances 

 for its nutrition and life, multiplication takes place. No 

 sexual union or other phenomenon connected with sexual 

 reproduction is, so far as we at present know, manifested by 

 Protococcus. Multiplication is entirely a vegetative process. 

 The protoplasm, when multiplication takes place, becomes 

 divided into two parts, and a cellulose partition is formed 

 between them (fig. 17). Each half may again divide, either 

 simultaneously or successively, and the new cells become 



