104 H. VON MOHL ON CELLULOSE. 



the circumference of the preparation lead to the recognition of 

 the presence of cellulose in those cells, while the membranes 

 themselves remain yellowish brown. Here again a short boiling 

 in nitric acid suffices to render the membranes capable of taking 

 a very beautiful blue colour. 



In particular parts of the cellular tissue of Polypodium 

 percussum, the outer coat of the parenchymatous cells acquires 

 a yellow colour with iodine and sulphuric acid, while the inner 

 layers swell up and become blue; in a word, they behave in this 

 respect like the outer coat of wood-cells. In preparations boiled 

 with nitric acid the cells are coloured blue throughout; therefore 

 in these also cellulose is the basis of the outer layer, resisting 

 sulphuric acid. 



Such cells, resisting the action of sulphuric acid, are more 

 common than would be supposed from what has generally been 

 stated; for many thick-walled parenchyma-cells, in the same way 

 as many wood-cells, assume only a yellow or at most a greenish 

 tint with the said reagents, as is the case in the parenchyma- 

 cells of many Palm-stems, e.y, of Calamus, of Cocos botryophora, 

 in the thick-walled pitted cells of the pith and rind of Hoya 

 carnostty in the stony cells of the winter pear, &c. All these 

 cells assume a bright blue colour with iodine after they have 

 been boiled with nitric acid ; the statement of Mulder, that the 

 thick-walled pith-cells of Hoya contain no cellulose, is conse- 

 quently without foundation. 



Since nitric acid is capable of rendering the cellulose ac- 

 cessible to the reaction of iodine in cells which more or less 

 obstinately withstand sulphuric acid, it may readily be imagined 

 that this acid never fails us in common parenchyma-cells, in 

 which sulphuric acid and iodine readily produce a blue, when 

 we desire to impart the blue colour to such cells by means of 

 iodine. This always presents itself in the greatest purity, and 

 without requiring the continuance of the boiling long enough to 

 alter the texture of the cell-membrane in the shghtest degree. 

 When it is desired to facilitate the anatomical investigation of 

 cells by the production of this blue colour, for instance to exa- 

 mine minutely their pits, which always appear far more distinct 

 in the blue-coloured cells, this method is far preferable to the 

 application of sulphuric acid, from the very fact that it does not 



