HOW TO KNOW THE FRESH-WATER ALGAE 



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Figure 104 



140a Cells short-cylindric, or subcylindrical; chloroplasts 1 in each 'sem- 

 cell' (rarely 3 or 4 chloroplasts forming transverse zones in the 

 cell); no vacuoles with moving granules in the poles of the cell. 

 Fig. 104 PENIUM 



Fig. 104. Penium margaritaceum (Ehr.) 

 Breb. 



Some species of this genus are shaped 

 like those of Cylindrocystis (Fig. 103) 

 but have a wall of two pieces that ad- 

 join in the midregion, and the wall usu- 

 ally shows punctations or granulations. 

 In general, Penium cells are more cylin- 

 drical than Cylindrocystis and because 



new wall sections are built in when the cells divide, they may become 



as long as some small Pleurotaenium (Fig. 102). 



140b Cells slightly attenuated at the apices; chloroplasts with several 

 pyrenoids; vacuoles with moving granules in the poles of the cell. 

 See Fig. 86 CLOSTERIUM 



141a (135) Chloroplasts spiral ribbons 142 



141b Chloroplasts some other shape 143 



142a Cells "cigar"-shaped, poles rounded. Fig. 105 SP1ROTAEN1A 



Figure 105 



Fig. 105. Spirotaenia condensata Breb. 



This "cigar"-shaped cell is usually straight but may be slightly 

 curved. Although one species in the United States has an axial chloro- 

 plast, the others have a characteristic spirally twisted one in each cell. 

 The cell is never constricted in the midregion and the wall is composed 

 of 1 piece as in other saccoderm desmids. 



142b Cells cylindrical with truncate ends. Fig. 106 GENICULARIA 



Fig. 106. Genicularia e 1 e g a n s 

 West, a, Single cell; b, fila- 

 mentous arrangement. 

 The cells may be solitary or 

 occur in filaments. Although the 

 chloroplasts are spirally twisted 

 and show a superficial resemblance to Spirogya (Fig. 147) this genus 

 is usually identifiable by the cells being slightly enlarged at the poles. 

 Genicularia is classified in the Gonatozygonaceae but is closely related 

 to the desmids and is found associated with them in nature. 



b C 



Figure 106 



72 



