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



acknowledge as accurate the observations upon which they are 

 founded, it is otherwise with the objections which those ob- 

 servers have brought forward in a chemical point of view, since 

 I do not indeed differ from them as to the facts they mention, 

 but cannot agree with the conclusions they have thence drawn. 



Although Harting and Mulder are not themselves always of 

 the same opinion in reference to the chemical constitution of the 

 compounds found in the cell-wall, yet in regard to the history of 

 development of the cell-wall they draw similar conclusions from 

 their joint investigations, so that I can here take their objections 

 together. The most important points coming under consideration 

 are the following : — 



The wall of young cells consists in general of cellulose alone, it 

 being coloured blue by iodine and sulphuric acid ; in older cells 

 on the contrary, which possess thickened walls, distinct layers may 

 usually be distinguished, differing chemically. In the wood-cells, 

 bark-cells and milk-vessels, the outermost layer {external wood- 

 membrane of Mulder ; cuticle of the wood-cell of Harting) consists 

 of a substance wholly insoluble in sulphuric acid. That this 

 membrane is produced after that which is composed of cellulose, 

 is evident from the circumstance that the young wood- cells ac- 

 quire the blue colour in every part ; the outer membrane is there- 

 fore considered by Harting and Mulder as a layer deposited on 

 the outside of the membrane composed of cellulose. From the 

 relation of this outer membrane to the first-formed pores, Hart- 

 ing derives the variations of the canals of the dots : when the 

 outer membrane is produced in proportionately more abundant 

 quantity and spreads itself between two cells, over their whole 

 surface, the pores become closed ; if, on the contrary, this mem- 

 brane be only deposited in the same proportion as the cells in- 

 crease in breadth, the pores remain open ; if, lastly, its develop- 

 ment do not keep pace with the expansion of the cell, a cavity 

 is produced between the dots. From the circumstance that 

 in the full-grown cell the layer of cell-membrane surrounded 

 by this outer membrane is usually no longer coloured blue by 

 iodine and sulphuric acid, but this colour, even when it appears 

 at all, is only to be found in the inmost layer bordering the ca- 

 vity of the cell, while the remaining portion is coloured either 

 yellow or green, it is further deduced, that these intermediate 

 layers of the cell (Mulder's intermediate ligneous substance), which 

 take a yellow colour with the reagents mentioned and are soluble 

 in stronger sulphuric acid, have been deposited, at the same time 

 as the outermost layer, in the direction from within outwards. 

 Mulder's and Harting' s views however do not wholly agree in 

 reference to the formation of this layer. The former assumes, 

 that either the cellulose is wholly absorbed and becomes replaced 



