HOW TO KNOW THE FRESH-WATER ALGAE 



172a Cells associated to form an extended stratum on trees, wood, 

 stones. See Fig. 66 PROTOCOCCUS (PLEUROCOCCUS) 



172b Cells not forming such a stratum 173 



173a Chloroplast a thin layer along the wall; pyrenoid usually lack- 

 ing; free-living or in tissues of animals (sponges, etc.), reproduc- 

 ing by autospores (replicas of the adult cell). See Fig. 91 



CHLORELLA (ZOOCHLORELLA) 



173b Chloroplast a massive cup with a pyrenoid; on damp aerial sub- 

 strates; reproducing by zoospores. See Fig. 42 



CHLOflOCOCCl/M 



174a (167) Cells oval, ovate, or irregular globose, with thick-layered 

 walls bearing knobs and protrusions 175 



174b Cells without thick walls and knob-like protrusions 176 



175a Chloroplasts numerous, parietal, cone-shaped. Fig. 126 



EXCENTROSPHAERA 



Fig. 126. Excentrosphaera viridis G. 

 T. Moore. 



This is the only species reported 

 for the genus. It is found in the 

 water or in very wet soil and is 

 identified by its irregular shape pro- 

 duced by a lamellated thickening of 

 Fl9ure 126 the wall in one or more places. The 



chloroplasts are cone-shaped and all 

 are directed inwardly from their parietal position along the wall. 



83 



