HOW TO KNOW THE FRESH-WATER ALGAE 



385a (382) Filaments bearing false branches (a branch formed by pro- 

 liferation of a broken trichome which pushes off to one side of the 

 main axis; not branching by lateral division of a cell in the main 

 axis); branching often sparse. Fig. 276 PLECTONEMA 



Fig. 276. Plectonema Wollei 

 Farlow. 



The false habit of branching 



places this genus in the Scy- 



tonemataceae, but unlike other 



Figure 276 members of the family there 



are no heterocysts. The spe- 

 cies illustrated is a common one, occurring in brownish green or black, 

 cottony masses at or near the surface of the water; is a relatively large 

 species being up to 50 /x in diameter. 



385b Filaments not branched 386 



386a Trichomes without a sheath 387 



386b Trichomes with a sheath 388 



387a Trichomes solitary or intermingled, not lying in parallel bundles, 

 sometimes tapered slightly at the anterior end, or with the apical 

 cell swollen (capitate). Fig. 277 OSCILLATOR1A 



Figure 277 



Fig. 277. a, Oscillatoria rubescens DeCand.; b, O. splendida Grev. 



The chief characteristic of species in this genus is their lack of a 

 sheath and another one is their active, oscillating movement. A mass 

 of the plants left in a laboratory dish will creep up the sides of the con- 

 tainer. There are numerous species, differentiated on the basis of size 

 and cell proportions and also upon the morphology of the apex of the 

 trichome, sometimes being straight and not tapering, sometimes taper- 

 ing and possessing a swollen apical cell. 



161 



