PROTOCOCCUS. 179 



Each single cell is a complete individual, capable of carrying on 

 an independent life. It fairly represents the green plant (such 

 as Pteris) reduced to its lowest terms. (Fig. 92.) 



Like Amoeba and the Infusoria Protococcus, at least in some 

 species, occurs both in a motile or active state in which it moves 

 about, and a quiescent or non-motile state analogous to the en- 

 cysted state of the unicellular, animals. In the latter the motile 

 or active state is the usual or dominant condition and the en- 

 cysted state is rarely assumed. In Protococcus, on the other 

 hand, the motile state is rare, and the ordinary activities of the 

 plant are carried on in the non-motile state. 



Structure. In structure Protococcus is a nearly typical cell 

 (p. 22). It consists essentially of an approximately spherical 

 mass of protoplasm enclosed within a thin woody layer of cellu- 

 lose (cell-wall or cell-membrane), and contains a single nucleus. 

 It also includes one or more chlorophyll-bodies (ckromatophores) 

 (p. 126) by virtue of which it is able to manufacture its own 

 foods, very much after the fashion of the green cells of Pteris. 



In those forms which possess a motile stage the latter con- 

 sists of a spherical, egg-shaped or pear-shaped cell having chro- 

 matophores and a membrane through which two flagella protrude. 

 In the oval forms these are placed near the narrowed end of the 

 cell, and in all cases they are locomotor organs and propel the 

 cell swiftly through the water. (Fig. 92). 



Reproduction. The ordinary method of reproduction in the 

 unicellular plants, as in the unicellular animals, is by cell-division. 

 In Protococcus the sphere becomes divided by a partition into 

 two cells which eventually separate completely one from the 

 other. Very often, however, the separation being incomplete 

 or postponed until after each daughter-cell has in turn become 

 divided, groups or aggregates of cells arise which suggest the 

 first steps in the formation of tissue in the development of higher 

 forms. In the end, however, separation is total and complete, 

 and each cell is therefore not a unit in a body, but is itself a 

 body and an individual (see p. 156). (Fig. 92.) 



The daughter-cells thus produced are the young, or offspring, 

 which have the power to grow and ultimately to divide in their 

 turn. Under favorable circumstances generation may thus fol- 

 low generation in quick succession. Each young cell is actually 



