II. VON MOHL ON CELLULOSE. 107 



that this outermost membrane of the parenchyma-cells also is 

 composed of cellulose and is coloured blue with iodine, but that 

 nitric acid is incapable of extracting the infiltrated matters com- 

 pletely, whence results the insolubility of this membrane in 

 sulphuric acid, and the yellow colour which it acquires from this 

 acid. In this respect this membrane would bear a resemblance 

 to the cuticular layer of the epidermal cells, in which caustic 

 potash, like 'nitric acid here, is capable of freeing the cellulose 

 from the influence of the infiltrated matters so far that it reacts 

 with iodine, but at the same time does not extract these matters 

 perfectly, and render the membrane saturated with them soluble 

 in sulphuric acid. 



Among all the parenchyma-cells which I have investigated, 

 those which form the outer part of the pith of a shoot, several 

 years old, of Clematis Vitalba, are perhaps the most interesting 

 in regard to the structure of their walls. These cells have very 

 thick walls, and their membrane is composed of a tolerable 

 number of layers which may be easily distinguished. It is co- 

 loured yellow by iodine. Sulphuric acid causes the inner layers 

 to swell up, and at the same time they acquire a green colour : 

 under this operation an outer layer which is on an average y^r tn 

 of a line in thickness, remains wholly unchanged. This layer 

 therefore displays the character of Mulder's " outer cell-mem- 

 brane." When strong objectives are employed, a delicate line is 

 seen running through the middle of this layer, indicating the 

 boundaries of the contiguous cells. In a cross- section of these 

 cells boiled with nitric acid, the inner layers are coloured deep 

 blue by iodine, and the outer layer just described assumes, ac- 

 cording to the amount of action exerted by the acid, a yellow, 

 green, or blue colour. When such a preparation is wetted with 

 dilute sulphuric acid, the inner layers swell up strongly, they are 

 bleached and by degrees dissolved ; the outer layer also swells 

 to some extent, but very slightly, and is bleached, remaining other- 

 wise unaltered. If the preparation is placed in stronger acid, 

 this outer layer is also dissolved, leaving behind an immeasurably 

 thin pellicle (with adherent granules) which lie in the middle of 

 it. It is therefore evident that the outer membrane of these 

 cells, which at the first hasty glance might be taken for the 

 outermost coat, is also composed of cellulose, but that it is in- 



