106 II. VON MOHL ON CELLULOSE. 



In all the cases which I examined, the parenchyma-cells as- 

 sumed a pure blue colour through the entire thickness of their 

 membrane, and no yellow-coloured outer coat could be detected 

 at the boundary between the cross- sections of the walls of two 

 contiguous cells. In like manner the membrane which closed the 

 pits always exhibited a pure blue, or, in cells which were dried 

 and then assumed a violet colour, a bright violet, and here also no 

 trace of a yellow membrane lying between the cells could be seen, 

 as may be very distinctly observed in the cells of the petiole of 

 Cycas revoluta, especially on account of the large size of the pits. 

 When such cells, coloured blue by iodine, for instance those of 

 the pith of the elder, of the medullary rays of Buxus, the paren- 

 chyma-cells of the stem of Calamus, the cells of the petiole of 

 Cycas, &c., are placed in dilute sulphuric acid, their membranes 

 swell up strongly and finally dissolve entirely, becoming more or 

 less completely bleached in the process. Under this treatment 

 an extremely delicate yellow pellicle comes to light at the boun- 

 laries between the cells, to which pellicles small yellow-coloured 

 granules (or drops of a fluid substance ?) are in most cases ad- 

 herent. We are here reminded of an analogous structure de- 

 scribed by Mulder and Harting under the name of the "outer 

 cell-membrane," and the "cuticle of the wood-cells." The 

 question then arises whether this pellicle possessed the yellow 

 colour at the time the cells were coloured blue with iodine, or 

 was at first coloured blue like the inner layers of the cell-walls, 

 the yellow colour having been only produced subsequently by 

 the combined action of sulphuric acid and iodine. I regard it 

 as more probable that the latter was the case; for if that pellicle 

 possessed a yellow colour before the operation of the sulphuric 

 acid, we should have an indication of its presence in the trans- 

 verse slice of the walls of two contiguous cells, in spite of its 

 very slight thickness, and its yellow colour would cause a greenish 

 discoloration of the thin bright blue membrane which closes the 

 pits. Yet all my efforts to discover even a trace of such a yellow 

 membrane, with the application of the strongest objectives, which 

 gave a perfectly faultless image with a great amount of light, 

 were without the least success. Both this circumstance, and 

 above all the observations on the outer membrane of many 

 parenchyma-cells, to be mentioned below, lead me to conclude 



