Unicellular Plants Protophyta. 71 



gradually detached to form new individuals. This process is 

 known as cell-division or fission, and is the simplest known 

 mode of multiplication. These new cells are naturally at 

 first small, and have thinner walls than the older cells ; but 

 increase in age is accompanied by increase in thickness of 

 the cell wall, and increase in size as a whole. 



Protococcus is very widely diffused, because of its power 

 of migration by the possession of a motile stage. Such 

 migrations, of course, take place only in a liquid medium. 

 In addition, however, Protocoaus has the power of retaining 

 its vitality even though dried. The extreme minuteness 

 and lightness of the dried plant manifestly permit of its 

 being carried about by winds, and its area of distribution is 

 thus materially widened. 



There are many different species and varieties of the 

 genus Protococcus, all more or less like each other, and all 

 possessing the general features above enumerated. 



Comparing the life-history of Protococcus with that of 

 Protomyxa, we notice that we have represented the encysted 

 stage in the adult plant, whilst the incipient multicellular 

 condition resulting from rapid division indicates a transi- 

 tion to the Metaphyta, which, as already stated, are merely 

 aggregates of encysted protoplasmic units. Further, the 

 ciliated stage is represented in the motile Protococcus, whilst 

 the amoeboid condition is hinted at in the quiescent naked 

 phase immediately succeeding the withdrawal of the cilia. 

 The plasmodial stage is apparently wanting ; its absence 

 may, perhaps, be explained by the presence of chlorophyll 

 in the protoplasmic unit enabling it to replenish its store of 

 energy directly, without needing to unite with other units, 

 and to undergo molecular rearrangement. 1 



Protococcus is a true plant for three reasons : first, it is 

 invested by a cellulose cr.psule ; secondly, it is possessed of 



1 It is to be noted that in the Myxomycetes, where plasmodium-for- 

 mation is the rule, chlorophyll is wanting. Chlorophyll is also wanting 

 in animals, where the formation of plasmodia is of frequent occurrence. 



