HOW TO KNOW THE FRESH-WATER ALGAE 



400b Spores absent; trichomes embedded in hard mucilage to form 

 globular thalli which may coalesce, thus producing a continuous 

 stratum; trichomes parallel and densely compacted or radiate. 

 Fig. 288 RIVULARIA 



Fig. 288. Rivularia sp. a, diagram of 

 portion of attached colony to show 

 arrangement of filaments; b, one fila- 

 ment showing basal heterocyst. 



There are several species of this 

 genus reported from the United States. 

 They may be differentiated from Gioeo- 

 trichia by the hardness of the mucilage, 

 the compactness of arrangement of the 

 trichomes, and the absence of akinetes. 

 Small globular balls or expanded mass- 

 es of dark green mucilage occur on 

 rocks or submerged logs, etc. Some 

 species grow in moist aerial surfaces, 

 especially marine forms. Rivularia is 

 not found free-floating, whereas Gioeo- 

 See notes under the latter genus. 



Figure 288 



trichia (Fig. 287) usually is. 



401a (398) Filaments branching freely; the basal portion of the branches 

 lying within the sheath of the main filament for some distance, 

 then diverging. Fig. 289 DICHOTHR1X 



Fig. 289. Dichothrix gypsophila 

 (Kuetz.) Bor. & Flah. 



In this genus the tapering tri- 

 chomes are enclosed 2 or 3 to- 

 gether within branching sheaths. 

 Brush-like tufts are produced by 

 their habit of growth and these 

 sometimes attain macroscopic pro- 

 portions. The species are differ- 

 entiated by size and sheath char- 

 acters. They are customarily 



found intermingled with miscellaneous algae; are sometimes attached 



or at least adherent to aquatic plants. 



Figure 289 



401b Filaments not branched (or seldom so); if branched, the branches 



not inclosed in the sheath of the main filament. See Fig. 275. 



CALOTHRIX 



167 



