Fig. 170 Rivularia atra. a, plants on stones ( x f). b, transverse 

 section of thallus ( x 9). c, single trichome in sheath ( x 300). (a, b, 



after Newton; c, original.) 



near the base of the trichomes, but they are diffluent farther up. 

 The heterocysts are basal and in the closely aUied genus Gloeotrichia 

 spores are produced next to them. 



* Nostocaceae: iV(?5^oc (used by Paracelsus). Fig. 171 



The gelatinous thallus is soUd or hollow, floating or attached, and 

 varies much in size and shape. The spheres usually break open 

 when mature and give rise to a flat expanse. There is a dense limit- 

 ing layer containing numerous intertwined and contorted monili- 

 form filaments with individual hyaUne or coloured sheaths, though 

 the sheath may sometimes be absent or very indistinct. Heterocysts 

 are terminal or intercalary and occur singly or in series. Reproduc- 

 tion is by means of hormogones or spores, the latter arising midway 

 between the heterocysts and developing centrifugally. The gelatin- 

 ous masses of iV. commune are fairly common on damp soils. 



The closely related genus Anabaena only differs from Nostoc in 

 that no firm colony is formed. Some species are symbiotic (cf. p. 

 428), whilst species of both genera are capable of fixing atmos- 

 pheric nitrogen (cf. p. 413). 



290 



