i6 



CYANOPHYCEAE 



containing numerous intertwined and contorted filaments with 

 individual hyaline or coloured sheaths which may be absent, 

 indistinct or conspicuous. The heterocysts are terminal or inter- 

 calary and are arranged singly or in series. Reproduction is by 



( D 



Fig. lo. Rivularia atra. A, plants on stones ( x f). B, transverse section of 

 thallus ( X 9). C, transverse section of thallus ( x 45). D, single trichome in 

 sheath ( x 300). (A-C, after Newton; D, original.) 



means of hormogones or spores, the latter arising midway between 

 the heterocysts and developing centrifugally. N. commune forms 

 gelatinous masses and is fairly common on damp soils. 



The closely related genus Anahaena only diifers from Nostoc in 

 that no firm colony is formed. Some species are often symbiotic 

 (cf. p. 297), whilst both Anahaena and Nostoc are apparently capable 

 of fixing nitrogen from the atmosphere (cf. p. 304). 



