NUTRITION OF PliOTOCOCCUS. 181 



one half of the parent cell and contains a moiety of whatever 

 that contained. Here, therefore, as in Amaha^ the problems 

 of heredity, uncomplicated by the occurrence of sex, are reduced 

 to their lowest terms. 



In some kinds of Protococciis the quiescent cells, under 

 special circumstances, which are not well understood, t^ive rise 

 to the motile forms {zoospores) referred to above. Cilia, or 

 rather flagella, are formed, and the protoplasmic mass with its 

 included chromatophores swims actively about in the water. 

 After a time these motile cells may come to rest, lose their fla- 

 gella and divide into two or more daughter-cells, each of which 

 in its turn may become a motile cell and repeat the process, or, 

 under other conditions, develop into the ordinary quiescent cell. 



In some species of Protococciis in which there is a motile 

 stage another form of reproduction, a kind of rudimentary 

 gamogenesis, has been observed. In this process two of the 

 motile cells (gametes) meet, fuse (conjugation)^ lose their flagella, 

 become encysted (see p. 161), and ultimately give rise to the 

 ordinary cells of Protococciis^ both non-motile and motile. 

 This process, however, has not yet been observed in the species 

 under consideration. 



Physiology. Our actual knowledge of the physiology of 

 Protococciis is very small. But the study of comparative plant 

 physiology gives every reason to believe that the essential phys- 

 iological operations of this" simple plant are fundamentally of 

 the same character as in the higher green plants, such as Pteris. 



Nutrition. The income of Protococciis^ when growing in 

 its natural habitat on tree-branches, moist bricks, and the like, 

 is difficult to determine. But as it is able to live also in ordi- 

 nary rain-water, we are able to set down its probable income 

 under those conditions with some deo^ree of accuraev. There 

 is do doubt that it absorbs water and carl)on dioxide by dif- 

 fusion through the cellulose wall, and that these substances 

 are used in the manufacture of starch, which, if stored up, 

 makes its appearance in the form of small graTiulos within the 

 chromatophores. This process takes place only in the light and 

 through the agency of the chlorophyll, and is attended by a 

 setting free of oxygen precisely as in Pteris. Nitrogen is prob- 

 ably derived from nitrates or ammoniacal compounds, minute 



