303 Linnaan Society. 



specimen. In some the gelatinous matter and the chains of 

 spores assume a more or less deep tint of violet, indicating that the 

 greater portion consists of cellulose, perhaps in some cases partially 

 changed to dextrine by the action of the sulphuric acid ; while in 

 other cases the prevailing tint is yellow-brown, indicating rather 

 bassorin. No purple tint occurs where merely iodine is used, and 

 the change therefore is not due to the presence of amylum. In fresh 

 specimens of Nostoc commune, the spores assume a beautiful green 

 tint, which is probably due to the combined tint of the yellow pro- 

 tein contents of the cells and the blue cellulose of which their wall 

 is formed. In the Arctic specimens and in English Nostoc commune 

 the bassorin tint prevails, while in specimens from Thibet (probably 

 Nostoc salsum, Kiitz.), gathered by Dr. Thomson, in pools of water 

 where the soil is covered with an efflorescence of carbonate of soda, 

 cellulose is indicated, but with every intermediate shade. Mr. Berke- 

 ley has, however, found that in woody fibres which in bleaching 

 have been exposed to salt water, a deeper purple tint is assumed than 

 when they have been bleached by rain water, so that something may 

 possibly be due to the peculiar place of growth of the Thibetan 

 species. In Nostoc edule the yellow-brown tint is stronger than in 

 any other specimen examined ; but it is scarcely probable that any 

 ver}' constant chemical chai-acters will be found to prevail in the 

 different species. In either case there would be a very nutritious 

 food, and one from its gelatinous condition probably easily assimi- 

 lated. The habit of the Arctic species is exactly that of Nostoc 

 commune, and Mr, Berkeley would not hesitate to regard it as iden- 

 tical, if there were no other difference than a little increase in the 

 relative size of the threads of spores ; but in parts of the fronds the 

 chains are surrounded by a distinct gelatinous envelope, presenting 

 an appearance somewhat similar to that of toad-spawn, which is very 

 visible in a transverse section. At a later period, when the chains 

 are ready to break up at the connecting joints, no trace of this en- 

 velope is to be detected, and the plant then exhibits the true cha- 

 racters of Nostoc. It appears indeed, from the remarks of Thuret, 

 that when the threads of Nostoc are first generated from the large 

 connecting bodies, there is really such an envelope ; but this exists 

 in Nostoc, as far as is at present known, merely in the infant state ; 

 and consequently if the genus Hormosiphon is to be retained, the 

 Arctic species must be regarded as belonging to it, for no such ap- 

 pearance has been detected by Mr. Berkeley either in dried or freshly- 

 gathered specimens of Nostoc commune. It is possible that more 

 extended observation may show that this character is not of the 

 consequence attributed to it by Kiitzing ; but in the mean time Mr. 

 Berkeley characterizes these specimens as — 



Hormosiphon arcticus, foliaceo-plicatiis viridis vel fuscescens, filis de- 

 mum (gelatina difTiisa) liberis. 



Fronds foliaceous, variously plicate, sometimes contracted into a 

 little ball. Gelatinous envelope at length effused ; connecting cells 

 at first solitary, then three together ; threads (which are nearly twice 



