42 F. E. FEITSCH. 



fuscescentibus vel hyalinis, non lamellatis ; cellulis parvis, globosis vel interdum 

 doliformibus, arete vel laxe dispositis, contentu dilute serugineo, paulo granuloso ; 

 heterocystis plerumque non in vaginis inclusis, globosis aut interdum modice elongatis, 

 etiam ellipticis finibus paulo acutis, diametro articuli vegetativi circiter duplo latioribus, 

 plerumque singulis ; sporis ignotis. 



Diam. cell. veg. = 3-4 ju, ; diam. heterocyst. = 5-7/u. ; long, heterocyst. = 6-7 • 5 /a. 



Hob. — Gap pond, Winter Harbour. 



This characteristic species was very obvious, owing to the dark colour of even 

 relatively small colonies. The latter were often completely spherical (fig. 139), 

 although young colonies of an elongated shape were not uncommon (figs. 143, 144) ; 

 in the older colonies, owing to the central part becoming hollow, the thallus often 

 appears flattened and may even become torn open and rather irregular. As no spores 

 were observed, it is conceivable that the thallus may attain even larger dimensions 

 than those given in the diagnosis. 



The most striking feature lies in the highly developed sheaths of the trichomes 

 (figs. 140-142). They are excellently developed in the young colonies, but are 

 generally quite distinct also in the outer parts of the older colonies. The sheaths are 

 sharply differentiated into two regions, an outer one, which is difiluent and hyaline or 

 pale yellowish-brown, and an inner one, which is denser, sharply demarcated from the 

 outer and of a brown or deep blackish -brown colour. This inner sheath shows very 

 obvious constriction between the cells, often amounting to complete septation (figs. 139, 

 141, 142). In the inner parts of older colonies there is no differentiation in the 

 sheath. 



Although no spores were observed, young stages of this species were not 

 uncommon. They consist of a rather dense tangle of filaments, already provided with 

 well-marked brown sheaths (figs. 139, 144). Quite small colonies already possess 

 an investment. 



L. Gain has recently (" Deux especes nouvelles de Nostoc provenant de la region 

 antarctique sud-ame'ricain," Comptes Rendus, clii. (1911), pp. 1693-4) described a new 

 species of Nostoc under the name of N. pachyderniaticum, based on a single specimen. 

 It is possible that N. fuscescensi is identical with Gain's species, but there are certain 

 striking diff"erences. Thus Gain speaks of " articulis ellipticis, 2 -5-3 /a crassis, 

 4-5 ju, longis," whereas the cells of N. fuscescens are nearly or quite spherical ; the 

 sheaths are described as " amplis, mesentericis, fuscescentibus, 13-16 )u- crassis," but 

 there is no mention of their characteristic diff'erentiation which, as Gain evidently had 

 a very small colony before him, would have been very obvious. For these reasons I 

 scarcely think that the two forms can be identical, and have preferred to describe the 

 specimens from the Gap pond as a distinct species. 



N. fuscescens will probably find its place in Bornet and Flahault's fourth section 

 Intricata, most of the species of which are spherical at first and have coloured sheaths, 

 although none show the striking diff'erentiation characteristic of the Antarctic species. 



