HOW TO KNOW THE FRESH-WATER ALGAE 



404b Filaments not forming a definitely shaped gelatinous thallus, but 

 spreading irregularly 405 



405a Filaments with more than 1 series of cells within a wide gelatinous 

 sheath; heterocysts small, cut off laterally from the vegetative 

 cells. Fig. 292 ST1GONEMA 



Fig. 292. a. Stigonema muscicola Borzi 

 (Fischerella muscicola (Thur.) Gom.); b, 

 S. turfaceum (Berk.) Cook. 



Although there are several species re- 

 ported from the United States, S. furfaceum, 

 and S. oceliafum (Dillw.) Thur. are by 

 far the most common. The latter is one 

 which frequently does not show the mul- 

 tiseriate arrangement of cells. The sheath 

 is wide and mucilaginous and in some 

 species is distinctly lamellate. The hetero- 

 cyst typically is cut off laterally from a 

 vegetative cell. S. ocellatum shows inter- 

 cellular connections. These species form 

 brownish or blue-green growths on submerged reed stems, on exposed 

 roots, and on other aquatic vegetation, or they may occur as velvety 

 growths on moist soil, rocks, or concrete. Stigonema ocellatum invari- 

 ably is found in acid water (desmid) habitats. 



Figure 292 



405b Filaments with cells in 1 series within a narrow sheath; hetero- 

 cysts within the series of vegetative cells (intercalary). Fig. 293. 

 HAPALOSIPHON 



Fig. 293. Hapalosiphon hibernicus West & 

 West. 



This genus is differentiated from Stigo- 

 nema (Fig. 292) principally by the fact that 

 the cells are always arranged in a single 

 series with the heterocyst (usually some- 

 what rectangular) intercalary rather than 

 lateral. Species are differentiated mostly 

 by size and by habit of growth, some 

 sprawling irregularly over a substrate, oth- 

 ers branching more regularly and erect. 

 Whereas Sfigonema cells are usually 

 rounded or oval and sometimes show in- 

 tercellular connections, those of Hapalosi- 

 phon are ordinarily quadrate or quadran- 



gular-globcse and more compactly arranged within a close, thinner 



sheath. 



Figure 293 



169 



