Nostocacece 327 



There are about 15 British species of Nostoc, some of which are much 

 more abundant than others. N. piscinale Kiitz., N. cceruleum Lyngbye 

 (fig. 149 C) and N. pruniforme Ag. are widely distributed in ponds and 

 ditches, generally occurring as free-floating masses. N. commune Vauch. 

 prefers damp ground which is frequently inundated. N. muscorum Ag., 

 JV. humifusum Carm. and N. macrosporum Menegh. are found principally 

 among mosses on wet rocks. N. sphcericum Vauch. and N. verrucosum Vauch. 

 commonly occur attached to the rocks and stones in the beds of rapid streams 

 and rivers. The cells of N. humifusum are only 2 2'5 p in diameter, whereas 

 those of N. macrosporum reach a diameter of 8 9 /x. 



Genus Anabaena Bory, 1822. [Sphterozyga Ag., 1827 ; Tri- 

 chormus Allman, 1842; Dolichospermum Thwaites, 1850.] The 

 trichomes are straight or circinate, often destitute of all traces of 

 a sheath, and are either free-floating or aggregated to form a thin 

 mucous stratum. The cells are generally of equal size, but some- 

 times there is a slight attenuation of the trichome, and the apical 

 cell may be conical. The cell-contents may be either homogeneous 

 or granular. The heterocysts are numerous and intercalary. The 

 spores are variously disposed ; in some they are solitary, in others 

 there are two spores one on each side of a heterocyst, and rarely 

 they are in short concatenate series. In most species they arise 

 centrifugally with regard to the heterocysts, but in some they are 

 developed centripetally. The latter were formerly placed in another 

 genus Sphcerozyga. 



As a rule, Anabcenas cannot be kept alive very long after 

 collection, unless placed in a large volume of water. One or two 

 days in a small glass vessel is usually sufficient to cause a 

 disarticulation of the filaments of most of the species, and under 

 these conditions they often lose their specific characters, develop- 

 ing forms which are quite unknown in their natural state. 



Hansgirg at one time suggested the union of this genus with 

 the genus Nostoc, but there are many good reasons for keeping the 

 genera separate. Species of Anabaena even when found in a 

 mucous stratum, which is but rarely, never approach in consistency 

 the tough thallus of a Nostoc', with one or two exceptions the 

 trichomes are rigid and fragile, and they are never at any time so 

 contorted as those of a Nostoc', the spores are generally much more 

 elongate than those produced in the latter genus, and the habits 

 of the two genera are mostly quite different. I doubt if the merest 

 novice at the study of freshwater Algae would confound an Anabcena 

 with a Nostoc. He certainly would not had he collected them 



