H. VOX MOHL ON CELLULOSE. 103 



a dirty blue, so that it remains doubtful whether cellulose forms 

 any considerable part of their substance, or even whether it is 

 present at all, at least in some of the layers. These circum- 

 stances very readily explain why Mulder, who thought he had 

 an unerring and very delicate test for cellulose in the application 

 of sulphuric acid and iodine, was of opinion that the pith of 

 Sambucus nigra, for example, was composed of cellulose only in 

 the earlier conditions, and of a peculiar substance in the full- 

 grown state. But the matter turns out quite differently when 

 this pith is treated in the manner above described with boiling 

 nitric acid, for it then assumes a beautiful indigo-blue colour 

 with iodine. 



While iodine and sulphuric acid produce, if not a bright blue, 

 at least a green colour in old parenchyma-cells, and thus the 

 presence of cellulose is placed beyond doubt even by this method 

 of investigation, the brown cells which surround the vascular 

 bundles of Ferns usually resist sulphuric acid as obstinately as 

 even cuticle, and it is absolutely impossible to demonstrate the 

 presence of cellulose by its help. In the essay above referred 

 to, I endeavoured to show on anatomical grounds, that this mem- 

 brane in the Ferns is produced by the transformation of a cellu- 

 lose membrane, but was compelled to leave undecided whether 

 or not it still contained cellulose in its fully developed condition. 

 The application of nitric acid affords an easy means of deciding 

 this question, and furnishes the proof that this membrane is 

 only prevented from reacting with iodine by its combination 

 with some infiltrated substance. For instance, if the black 

 membrane surrounding the vascular bundles of the petiole of 

 Aspidium filix mas, is boiled in nitric acid until its dark brown 

 colour is changed into bright yellow, iodine imparts a beautiful 

 blue colour to the membrane of these cells, the texture of which 

 is not changed in the slightest degree. 



In many cases, in the Ferns, other portions of their cellular 

 tissue are so saturated with foreign compounds that they do not 

 react with iodine and sulphuric acid. Among these, for example, 

 are the outer layers of the dark brown petiole of Adiantum 

 pedatum, on the cells of which the said reagents do not act at 

 all at first ; only when the acid has been in contact with the cells 

 for twenty-four hours does the blue colour which shows itself at 



