M. Mohl on the Growth of Cell-Membrane. 269 



loured, and reacts like cellulose with iodine and sulphuric acid. 

 The brown-coloured walls are usually much thicker than those 

 consisting of cellulose. Leaving the brown colour out of view, 

 these cells correspond exactly, in respect to form and their be- 

 haviour toward iodine and sulphuric acid, with the epidermis- 

 cells of many leaves. Similar cells occur in the parenchyma of 

 the stem of Polypodium nitidum, Kaulf, some isolated, some in 

 groups of three or four, scattered among the parenchyma-cells, 

 which are usually composed of cellulose ; in these cells also one 

 wall is generally thinner and formed of cellulose, while the re- 

 maining walls are very thick and brown, and withstand sulphuric 

 acid. All sides of these cells are finely dotted, as is also the 

 case in the cells of the brown coat inclosing the vascular bun- 

 dle ; the dots penetrate as well in the thickened brown as in the 

 thin walls, from within outward to the thin outer and imperfo- 

 rate membrane, which membrane possesses the same chemical 

 peculiarities as the secondary layers lying behind it ; that is to say, 

 it consists sometimes of cellulose, at others of a substance with- 

 standing sulphuric acid. Now I found, both among the cells 

 scattered in the parenchyma and in the brown layer inclosing 

 the vascular bundle, particular cells, which certainly, in reference 

 to their form, though not in regard to their chemical characters, 

 wholly agreed with neighbouring brown cells, in which therefore 

 one wall was also thin and the rest considerably thickened. In 

 some parts all the walls of these cells, both thick and thin, con- 

 sisted of cellulose ; in other parts the thickened walls were only 

 composed of the brown substance in one point, while the remain- 

 ing portion, transversely through the whole thickness of the cell- 

 wall, consisted of cellulose ; the line of demarcation between the 

 brown and the uncoloured portions was riot distinctly defined. 

 From the piecemeal composition of the cell- walls of tracts formed 

 of cellulose, and others consisting of brown substance, it clearly 

 results that the greater thickness which the brown walls of these 

 cells usually possess, compared with the walls consisting of cel- 

 lulose, is neither to be ascribed to the deposition of membranes 

 upon the outside of the young cellulose membrane, nor to the 

 interposition of a considerable mass of brown substance between 

 the molecules of the cellulose, since if the formation of the thick- 

 ened brown walls depended on these causes, the portions con- 

 sisting of cellulose could not have exhibited the same thickness 

 and form as the coloured portions in the only partially brown- 

 coloured cell-walls. The reason of the brown colour therefore, 

 and of the altered chemical behaviour, must be looked for in a 

 transformation of the whole substance leaving the form and or- 

 ganization of the cell-wall unchanged, or in the infiltration of a 



