110 II. VON MOIIL ON CELLULOSE. 



only are the inner layers of the cells dissolved, but also the 

 outer, which before this treatment with nitric acid were inso- 

 luble in sulphuric acid, and on the boundary-lines between the 

 adjacent cells, as in the parenchyma-cells, remains a pellicle of 

 the utmost delicacy coated with fine granules. Since in these 

 cells also, when very thin sections are coloured only pale blue by 

 a small quantity of iodine, it is impossible to detect by a yellow 

 colour this pellicle insoluble in sulphuric acid, I consider it pro- 

 bable that this possesses a blue colour so long as it is not ex- 

 posed to the action of sulphuric acid. The presence of this 

 outer thin membrane, and the fact that it is only yellow under 

 the simultaneous action of sulphuric acid and iodine, and blue 

 with iodine alone, may perhaps be still more clearly proved by 

 macerating a vascular bundle of the black palm-wood in dilute 

 nitric acid (which however may require six to twelve months), 

 or boiling it in this acid until the liber-cells are separable by a 

 slight pressure. In this case isolated pieces of variable size of 

 the outer membrane may often be found among the separate 

 liber-cells, and we can then convince ourselves that they are 

 coloured blue by iodine, and only assume a yellow colour when 

 sulphuric acid is added. These are the observations which 

 principally led me to the opinion that the outer membrane of 

 parenchyma- and prosenchyma-cells contains cellulose, since the 

 impossibility of seeing the yellow colour in this membrane in 

 the cross section appeared not perfectly conclusive, on account 

 of its very slight thickness, though at the same time it must be 

 admitted that this circumstance is also of great importance. 



Passing to the prosenchymatous cells of the wood of Dicoty- 

 ledonous plants, it is well known that cellulose may be demon- 

 strated in their internal layer by means of sulphuric acid and 

 iodine, but that these layers do not usually, it is true, assume a 

 pure blue colour with these reagents, mostly acquiring only a 

 green tint, which leads to the conclusion that cellulose does 

 really exist in them, but that its reaction is more or less obscured 

 by the presence of a yellow infiltrated substance. Even when, 

 as in the wood of Taxus^ the resistance to sulphuric acid is very 

 considerable, the presence of cellulose may be shown by applying 

 a very strong acid, which completely destroys the texture of the 

 cell-wall, and then, by adding tincture of iodine diluted with a 



