THE GREEN ALGAE OF NORTH AMERICA 133 



a red stigma, one chromatophore occupying nearly the whole 

 cell, with one to many pyrenoids ; gonidial cells developing 

 asexually to form new colonies, by repeated divisions ; sexual 

 reproduction unknown. 



A genus comprising two species, closely related to Eudorina, 

 and until sexual reproduction has been discovered, there must 

 be some question whether it is not a stage in the life history of 

 the older genus. 



KEY TO THE SPECIES OF PI.EODORINA. 

 i. Gonidial cells 2-3 times larger than the vegetative cells, of about the 



same number. i. P. californica. 



i. Gonidial cells somewhat larger than the four vegetative cells. 



2. P. illinoisensis. 



1. P. CALIFORNICA Shaw, 1894, p. 279, PI. XXVII. Col- 

 ony up to 300 /A diarn., containing 64 or 128 cells, about equally 

 divided between vegetative and gonidial ; gonidial cells about 

 2-3 times larger diameter than vegetative cells. In warm shal- 

 low fresh water, Cal., Ind., 111. Ceylon. 



2. P. ILLINOISENSIS Kofoid, 1898, p. 274, PI. XXXVI, 

 XXXVII. Colony 46 ; 38 - - 200 ; 175 ^, containing 32, 

 rarely 16 or 64 cells ; vegetative cells always 4 ; gonidial cells 

 from slightly larger to twice the diam. of the vegetative cells. 

 Fig. 1 8. In warm shallow water, 111. Germany. 



The ellipsoid form of this species, with the 4 vegetative cells 

 at the forward end, distinguishes it from other species of Volvo- 

 caceae. The colony moves forwards in the direction of the long 

 axis, rotating at the same time about this axis ; but the rota- 

 tion is not uniform in direction, changing frequently, like the 

 screw of a steamer when the engine is reversed ; the movement 

 of the colony through the water is not unlike that of the 

 steamer's screw, except that reversing the direction of revolu- 

 tion has no effect on the forward motion, which goes on the 

 same, the end with the vegetative cells being always at the 

 front. Another differentiation is that the red stigmata in the 

 cells are brighter, as the cells are located nearer to the forward 

 end. 



4. PLATYDORINA Kofoid, 1899, p. 419. 



Colony much flattened, cells in one layer, but alternate!}' 

 pointing in opposite directions ; cells differently arranged in the 

 anterior and posterior ends ; cells all similar, biciliate, with red 



