288 



THE ALGAE 



being very delicate or more frequently absent. The apical cell is 

 sometimes provided with a cap or calyptra. The name comes from 

 the rhythmic motion of the threads which results in slow forward 

 progression (cf p. 281). There are a number of very common 

 species which occur in a wide range of habitats. 



Oscillatoriaceae: Lyngbya (after H. C. Lyngbye, a Danish algo- 

 logist). Fig. 168 

 This genus differs from Oscillatoria in the presence of a sheath 

 of variable thickness and colour, the character of which is largely 

 dependent upon the environment. The plants are either attached or 

 free floating, and are often aggregated into a mucous blue-green 

 mat on the soil. When the hormogones escape from the sheath it is 

 almost impossible to determine whether they belong to this or other 

 allied genera. The species occur in the same types of habitat as 

 Oscillatoria. 



Fig. 168 Lyngbya aestuarii. A, apex. B, C, portions of threads. 



(After Chapman.) 



Scytonemataceae: Scytonema {scyto, leather; nema, thread). Fig. 

 169 

 The trichomes differ from those of the preceding genus in the 

 presence of heterocysts. The filaments (trichome and sheath) have 

 distinct basal and apical regions forming little erect tufts. Branch- 

 ing is of the type known as false, the branches arising either be- 



