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



&c, as the liber-cells may easily be detached from each other if 

 the vascular bundles have been kept for some time in dilute ni- 

 tric acid, by which means the outer membranes of the contiguous 

 cells are not separated from each other, but from the secondary 

 membranes, and may be obtained isolated in large pieces. With 

 iodine and sulphuric acid of a degree of concentration which 

 does not dissolve the secondary membranes, but colours them 

 bright blue, this outer membrane behaves exactly like the outer 

 membrane of the wood-cells of dicotyledons, that is, it does not 

 swell up, but acquires a dark yellow colour. If we apply stronger 

 sulphuric acid, capable of completely dissolving the secondary 

 layers, the outer membrane, without any perceptible expansion, 

 acquires either an intense greenish or tolerably pure blue colour. 

 This contains cellulose also, but in what I may call a much more 

 strongly combined condition than is the case in the secondary 

 layers, so that not only is a far stronger acid necessary to bring- 

 out the blue colour, but the cellulose present in this membrane 

 is also protected from solution. This greater resistance to the 

 action of sulphuric acid clearly can only depend upon the pre- 

 sence of the substance which acquires the yellow colour with 

 iodine and sulphuric acid. This resistance however has a certain 

 limit,. since this membrane is soluble in more concentrated sul- 

 phuric acid. It differs therefore in reference to this last circum- 

 stance from the outer membrane of the wood-cells of dicotyledons, 

 which resists the action even of the more concentrated sul- 

 phuric acid. To try therefore whether cellulose might not be 

 discovered in the latter by the action of a stronger acid, I sub- 

 mitted the wood-cells of various Conifer a, particularly of Pinus 

 sylvestris, to a similar treatment with nitric acid, &c. The ex- 

 periment succeeded but imperfectly. After the action of a strong 

 acid, the outer membrane exhibited throughout a greenish co- 

 lour, but the development of the blue colour was so weak, that 

 I remained in doubt whether it was actually situated in the outer 

 membrane itself, or whether possibly it was not to be ascribed to 

 a thin layer of adhering cellulose. I place no weight therefore 

 on this experiment, and mention it here chiefly to invite others 

 to direct their attention also to this point. 



The following observations made on Ferns appear to me to 

 bear more importantly upon the theory of the development of 

 cell-membrane : — The brown cells which in Ferns form the layer 

 by which the vascular bundles are surrounded, withstand the ac- 

 tion of sulphuric acid as obstinately as perhaps any other vege- 

 table tissue. In many Ferns all the walls of these cells do not 

 possess a brown colour, but merely those portions of the walls 

 lying upon the vascular bundle, or these and the side walls, 

 while the side turned away from the vascular bundle is unco- 



