HOW TO KNOW THE FRESH-WATER ALGAE 



105b Cells elliptic, lemon-shaped, or nearly cylindrical, 1 to several 

 generations of mother-cell walls inclosing daughter cells. Fig. 85. 



OOCYSTIS 



Fig. 85. a, Oocystis Eremosphaeiia 

 G. M. Smith; b, O. Borgei Snow. 



There are several species of this 

 genus common in both the euplank- 

 ton and the tychoplankton. The vari- 

 ous forms are differentiated by the 

 presence or absence of nodules at 

 the poles and by the number of 

 chloroplasts. Two or 3 generations 

 of cell walls may be inclosed within an original mother-cell wall which 

 enlarges so that it often appears as a gelatinous sheath and is, there- 

 fore, misleading as a differentiating genus character. 



106a Cells spherical, occurring as evenly distributed clumps within the 

 gelatinous sheaths which sometimes are lacking; chloroplasts sev- 

 eral angular plates. See Fig. 55 PLANKTOSPHAERIA 



106b Cells variously shaped but not distributed in clumps as above; 

 often densely aggregated; chloroplast 1, parietal 107 



107a Cells spherical or angular from mutual compression when oc- 

 curring in clumps; subaerial 108 



107b Cells fusiform or needle-shaped, aguatic. See Fig. 68 



ANKISTRODESMUS 



108a Cells spherical, clustered but not adjoined, sometimes solitary. 

 See Fig. 42 CHLOROCOCCUM 



108b Cells in dense clumps, forming a film or a layer on moist sub- 

 aerial substrates; cells spherical or angular from mutual com- 

 pression. See Fig. 66 PROTOCOCCUS 



(PLEUROCOCCUS) 



109a (36) Cells crescent-shaped, or sickle-shaped, with sharply pointed 

 or narrowly rounded and tapering apices 110 



109b Cells some other shape 117 



62 



