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Mr. Berkeley states that he has been unable to find any account of 

 the chemical constituents of Nostoc. The chemical condition of such 

 species as he had been enabled to examine, under the influence of 

 iodine and sulphuric acid, seems to vary not only in the different spe- 

 cies, but in individual specimens, and even in parts of the same spe- 

 cimen. 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 there- 

 fore 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 protein contents of 

 the cells and the blue cellulose of which their wall is formed. In the 

 Arctic specimens, and in English Nostoc commune, the bassoiin tint 

 prevails, while in specimens from Thibet (probably. Nostoc salsum, 

 Kutz.), gathered by Dr. Thomson, in pools of water where the soil is 

 covered with an efflorescence of carbonate of soda, cellulose is indi- 

 cated, but with every intermediate shade. Mr. Berkeley has, how- 

 ever, 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 pos- 

 sibly 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 very con- 

 stant chemical characters 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 assimilated. The 

 habit of the Arctic species is exactly that of Nostoc commune, and 

 Mr. Berkeley would not hesitate to regard it as identical, 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 sec- 

 tion. At a later period, when the chains are ready to break up at the 

 connecting joints, no trace of this envelope is to be detected, and the 

 plant then exhibits the true characters 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, 

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