HOW TO KNOW THE FRESH-WATER ALGAE 



402a (397) Trichomes branching by lateral division of cells of the main 



axis (true branching) 403 



402b Tichomes unbranched, or with false branches (sections of tri- 

 chome developing a series of cells to one side of a break in the 



main axis) 406 



403a Individual trichome sheath not apparent; colony of trichomes in- 

 vested by a mucilage; heterocysts usually on the ends of short 



branches. Fig. 290 . NOSTOCHOPSIS 



Fig. 290. Nostochopsis lobatus 

 Wood, a, diagram of filament 

 arrangement in colony; b-d, fila- 

 ments and lateral heterocysts. 



This is the only species reported 

 from the United States. The fila- 

 ments are composed of Anabaena- 

 like cells and bear true branches. 

 The heterocysts are borne lateral- 

 ly along the filaments or on the 

 ends of short branches (rarely in- 

 tercalary also). The filaments are 

 colonial and are inclosed in a firm 

 mucilage in the form of cylinders 

 or hollow tubes which extend ver- 

 tically from the bottom in quiet 

 Figure 290 water, or lie horizontally in cur- 



rents. In shallow water the gelatinous tubes may reach the surface 

 and then broaden and flatten horizontally. 



403b Individual trichome sheaths evident; heterocysts in the same ser- 

 ies with the main axis, or cut off laterally from them but not on 



the ends of branches 404 



404a Filaments closely aggregated, forming an attached, gelatinous 

 thallus 1-2 mm. in diameter. Fig. 291 CAPSOSIRA 



Fig. 291. Capsosira Brebissonii Kuetz. a, habit of at- 

 tached colony; b, portion of filament showing lat- 

 eral heterocysts. The sheath is thin and soft with- 

 out a definite limiting membrane. 



This species builds small bulbous mucilaginous 

 colonies attached to the substrate. The individual 

 trichomes which bear true branches are surrounded 

 by a definite sheath which' is yellowish. Heterocysts 

 are usually lateral, but may be intercalary in the 

 trichomes which do not taper toward the apices. There 

 is but 1 species reported from widely separated sta- 

 tions in the United States. 



168 



