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



less completely the reaction of cellulose, which this in its normal 

 condition exhibits towards iodine and sulphuric acid. Such 

 an infiltration might perhaps occur without visible thickening 

 of the layer, either if it were not in very great abundance, or if 

 the growth of the membrane in a lateral direction connected 

 with development of the cell were to afford space for the deposit 

 of a considerable quantity of a foreign compound. In these cases, 

 the possibility of which in the first place certainly no one will call 

 in question, a layer would indeed be formed altogether new in a 

 chemical aspect, but no alteration in anatomical relations would 

 appear ; and from this subsequently-resulting chemical transfor- 

 mation no conclusion should be drawn as to the order in which 

 the different layers of the cell-membrane originate, since these 

 metamorphoses may take place quite as readily in the last as in the 

 first formed layer*. If we admit the possibility of such a meta- 

 morphosis in particular layers, it must also be admitted that the 

 chemical reaction of a certain layer affords no sure means by 

 which it may be recognised as a peculiar anatomical layer, since 

 it may easily be imagined, that in different cells, the layers cor- 

 responding to each other in an anatomical point of view may ex- 

 hibit a great distinction in regard to their chemical transforma- 

 tions. Until well-grounded experience has taught us which of 

 the cases, which have here been mentioned as possible, really 

 occurs in nature, we can only allow ourselves to be guided in 

 the recognition of the different layers and the determination of 

 the order in which they make their appearance by their anato- 

 mical relations ; and although in very many cases the influence of 

 chemical reagents affords an excellent means by which we are 

 enabled to distinguish the individual layers of cell-membrane, 

 which without this assistance it would be difficult or impossible 

 to recognise, yet in availing ourselves of this assistance we must 

 keep the anatomical relations constantly in view. 



The consideration of these relations leads one, I believe, to a 

 result diametrically opposed to that maintained by Mulder and 

 Harting. 



In the next place will come conveniently the question, whether 

 the outer wood-membrane is produced out of a cellulose mem- 

 brane, or is deposited on the outside of an already formed cell. 

 This membrane exhibits the most striking contrast to the mem- 

 brane composed of cellulose ; if it can be proved to owe its origin 

 to the transformation of a cellulose membrane, the much slighter 



* This is no mere guess, as in the parenchyma-cells of some Fern stems, 

 especially of PoJypodium incanum, P. nitidum, the inmost layer of the cell, 

 an analogue of their primary membrane, is far richer in a substance coloured 

 yellow by iodine than the intermediate layer, and requires a much stronger 

 action of sulphuric acid for the production of a blue colour, 



