II. VOX MOIIL ON CELLULOSE. 99 



potash, and the same phenomenon is seen in the wood-cells when 

 the solution of the infiltrated substance is imperfect. 



While the above-described action of potash on the cuticular 

 layer of epidermal cells is taking place, a very thin pellicle 

 becomes detached from the outer surface of the epidermis, either 

 in strips, or, when the epidermal cells separate from each other, 

 a piece of this coat remains adherent upon the outer side of each 

 of them. This pellicle is not coloured blue by iodine, but always 

 yellow. When the above treatment is applied to the epidermis 

 of organs in which the outer wall of the epidermal cells is not 

 much thicker than their side walls, and in which iodine and sul- 

 phuric acid demonstrate only a very thin cuticle, for instance, 

 of the leaves of Iris fimbriata, of stems of Epiphyllum truncatum, 

 of the petiole of Musa, &c., a thin yellow pellicle remains here 

 also on the outside of epidermal cells coloured blue by iodine. 

 If the epidermis is boiled with solution of potash, this pellicle 

 shrinks together and by longer boiling is completely dissolved, 

 while the epidermal cells only swell up and acquire a beautiful 

 blue colour with iodine. This pellicle, which occurs under all 

 circumstances upon the epidermis, whether it be a portion of its 

 cells transformed into cuticle or not, consists, judging from its 

 different behaviour with potash, of a substance essentially dif- 

 ferent from the cell-membrane, and is doubtless the same mem- 

 brane which Ad. Brongniart separated from leaves by maceration, 

 and denominated cuticle. It has been confounded, by myself 

 and others, with that portion of the w r alls of the epidermal cells 

 which is coloured yellow by sulphuric acid and iodine, under the 

 name of cuticle, because the methods of investigation hitherto in 

 use afforded no means of separating these two different parts 

 distinctly from each other. But it is evident that this membrane 

 must be distinguished from the subjacent cells ; I therefore pro- 

 pose to restrict the name of cuticle to it alone, and to apply the 

 term cuticular layer to that part of the epidermal cells which 

 becomes coloured yellow by sulphuric acid and iodine. The 

 cuticle exists on all cells exposed to the air, without exception ; 

 if any one choose to ascribe it to a secretion of the epidermal 

 cells, I have no objection to offer to the notion ; but it would 

 be difficult to bring forward any proof of its correctness ; and 

 perhaps we ought to regard the circumstance that this cuticle 



7*' 



