M. Mohl on the Growth of Cell-Membrane. 271 



possesses the chemical peculiarities of cuticle, and that half, on 

 the other hand, which is contiguous to the parenchyma of the 

 leaf, the characters of cellulose {e.g. in Hoya carnosa, Aloe obliqua, 

 margaritifera) . In all these cases cells present themselves to us, 

 the walls of which, either in certain situations or throughout their 

 whole extent, withstand sulphuric acid, and in which no cellulose 

 is to be discovered. The analogy which exists between these cells 

 and the above- described cells of Polypodium nitidum appears to 

 me to be of importance to the explanation of these latter circum- 

 stances. If it be certain in these last, that their membranes, not- 

 withstanding that no cellulose is any longer to be demonstrated 

 in them, nevertheless have their origin from a cellulose layer 

 which exhibits exactly the same organization and thickness as the 

 incrusting membrane, and in many cases still forms particular parts 

 of the membrane, not even then must the conclusion be drawn 

 in respect to the cuticle from its chemical constitution, that it is 

 a layer secreted upon the upper surface of the epidermis-cells, 

 until it can be demonstrated that this theory is in accordance 

 with the anatomical phenomena, and that the instances I have 

 given of a composition of cuticle from cell-membranes, and of the 

 occurrence of epidermis-cells with side walls, partly consisting of 

 cellulose and partly of the substance of cuticle, are founded upon 

 false observations. 



Whether now in these cases the cellulose is partly or wholly 

 absorbed and replaced by the incrusting matter, or whether its 

 reaction to iodine and sulphuric acid is merely prevented by the 

 latter, is uncertain. It appears however to me not improbable 

 that the latter is the true view, since the assumption that in- 

 crusting substance coloured yellow by iodine and sulphuric acid at 

 least to a certain degree interferes with the known reaction of cel- 

 lulose, supported not only by the above-mentioned behaviour of 

 the outer layer of the liber fibres of a Palm and of the wood-cells 

 of Pinus sylvestris, but also by the behaviour of the secondary 

 layers in almost all full-grown wood- and parenchyma-cells. 

 Young cells, for instance the pith of a young shoot of Sambucus 

 nigra, the cambium-cells of dicotyledons, &c, become coloured 

 bright blue by the application of a very dilute acid, while the 

 medulla-cells of a full-grown branch of Sambucus and the perfect 

 wood-cells, treated with the same acid, only develope a yellow 

 colour and require it much more concentrated, and then as deep 

 a blue colour is not produced, on account of the yellow colour of 

 the incrusting matter mixing with and rendering it green. A 

 bright and intense blue colour can usually only be obtained in 

 the secondary layers of full-grown wood-cells when so strong an 

 acid is employed that they do not merely swell up but are par- 



