Figure 101 



HOW TO KNOW THE FRESH-WATER ALGAE 



138a With a circle of folds or creases at the base of the 'semicell/ with 



a tooth on each fold. Fig. 101 DOCIDIUM 



Fig. 101. Docidium undula- 

 tion Bailey. 



This genus is scarcely to 

 be separated from PJeurofae- 

 nium (Fig. 102). In cells that 

 are living the density of the 

 chloroplast makes obscure the characteristic creases in the wall where 

 the two semicells are adjoined. One needs to focus carefully to see 

 these folds which produce teeth-like projections at the very outer mar- 

 gin of the bases of the semicell, especially in individuals which have 

 a granule on the fold. Another species, D. Baculum Breb., less com- 

 mon than the species illustrated, has smooth lateral walls. 



138b Without a circle of creases at the base of the 'semicell.' Fig. 102. 

 PLEUROTAENIUM 



Fig. 102. a, Pleurotaenium trabe- 

 cule! (Ehr.) Naegeli; b, P. nodo- 

 sum Bailey. 



There are more species in this 

 genus than in Docidium (Fig. 101) 

 and they are more widely dis- 

 tributed. Although all of them are 

 elongate and usually have sub- 

 parallel margins, there is considerable variation in details of the shape 

 and in decoration of the wall. In some species the margins are nodose 

 or undulate; some spiny. Usually there is a circle of granules around 

 the poles of the cell. Some species are not so restricted in their distri- 

 bution as most desmids, and may occur in basic or slightly alkaline 

 waters as well as in acid or soft water habitats. 



139a Cells with 2 star-shaped chloroplasts, one in each 'semicell.' Fig. 

 103 CYLINDROCYSTIS 



Fig. 103. Cylindrocystis Brebissonii Menegh. 



Although some of the half-dozen species of 

 this genus have slightly constricted cells, they 

 have a wall composed of one piece and have 

 no wall pores, which thus identifies them as 

 saccoderm (not true) desmids. There is one 

 star-shaped chloroplast with a large pyrenoid 

 in each half of the cell. This genus is not at 

 all confined to acid habitats; may occur in alkaline bogs or among 

 mosses, or may form gelatinous masses on wet stones in alpine 

 situations. 



139b Cell with other types of chloroplasts 140 



Figure 102 



Figure 103 



71 



