HOW TO KNOW THE FRESH-WATER ALGAE 



414b Cells globose to cylindric, or barrel-shaped, not compressed as 

 above. See Fig. 271 ANABAENA 



415a (413) Plant mass definite in shape, usually globular, bound by a 

 firm, gelatinous tegument; colonies microscopic or macroscopic. 

 Fig. 300 NOSTOC 



Fig. 300. a, Nostoc pruniforme Ag., 

 showing habit of colonies; b, N. 

 linckia (Roth) Bor. & Flah., with 

 two trichomes in detail showing 

 heterocyst and akinetes. 



Figure 300 



This genus is characterized chiefly 

 by the firm outer tegument of the 

 mucilaginous material in which nu- 

 merous bead-like filaments are in- 

 closed, thus giving the thallus a defi- 

 nite form. One species forms colon- 

 ies as large as a goose egg, whereas another, N. commune Vauch., 

 builds tough, membranous green or brown layers on the bottoms of 

 pools or in swampy places, sometimes in wet alpine meadows. N. 

 pruniforme is a very common species which forms marble- or acorn- 

 sized colonies on damp soil, often among grasses in marshy meadows. 

 The globular or oval thalli are frequently mistaken for turtle or some 

 other type of reptilian egg. N. amplissimum Gard. is known as Mare's 

 Eggs in the far West where it produces colonies 10 cm. in diameter. 



415b Plant mass not definite in shape; mucilage soft, not bounded by 

 a firm tegument 416 



416a Trichomes forming small bundles within a gelatinous sheath, eith- 

 er entangled or parallel. Fig. 301 AULOS1RA 



Fig. 301. Aulosira laxa Kirch. 



This species (the only one 

 of the genus found in the 

 United States) is much like 

 Microchaete (Fig. 294) and 

 the genus is sometimes clas- 

 sified with it. Some authori- 

 ties differentiate it on the 

 basis of the soft sheath, the intercalary heterocysts, and the akinetes 

 which occur intercalary and of the same size and proportions as the 

 heterocysts. 



416b Trichomes not forming bundles. See Fig. 271 ANABAENA 



Figure 301 



173 



