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



foreign matter in a quantity very small in proportion to the cel- 

 lulose. 



I thought it necessary to enter more minutely into the de- 

 scription of these cells, because they offer the clearest evidence 

 that the presence of a compound differing chemically from cel- 

 lulose in a thickened cell-wall, even when traces of cellulose can 

 no longer be detected in the membrane by iodine and sulphuric 

 acid, affords no sufficient ground for the assumption that the 

 thickening of the wall depends on the deposition of an incrust- 

 ing substance, and that we have to regard those portions of the 

 cell-wall formed of this substance as produced subsequently to 

 the portions which are composed of cellulose. Were the in- 

 crusting substance, situated at particular points, to penetrate 

 through the whole cell-wall (primary and secondary membrane) 

 in these cells, the extent to which it spread would include the 

 outer layer of the cells, so that this layer would possess all the 

 peculiarities of the outer wood-membrane, and it would thus ex- 

 actly fit all the conclusions respecting this membrane which 

 Mulder and Harting have drawn ; on the other hand, it is not 

 necessary to indicate more minutely how false would be the as- 

 sumption of its originating subsequently. 



The organization of the above-described cells of Polypodium 

 nitidum appears to me to be of importance in so far as it is capable 

 of warranting our conclusions as to the structure of epidermis- cells 

 and cuticle, which corresponds with it exactly in an anatomical 

 point of view. Some years since* I stated the anatomical grounds 

 which prevented my regarding the cuticle as a layer secreted upon 

 the outside of the epidermis-cells, and which testified that it con- 

 sists of the thickened outer walls, and partly also of the side walls 

 of the epidermis-cells, the substance of which has become capable 

 of resisting sulphuric acid in consequence of a peculiar metamor- 

 phosis. This explanation does not appear to have met with a fa- 

 vourable reception, but renewed researches have caused me to per- 

 severe in iry view, and it appears to me to be especially proved by 

 such cases as where the cuticle of canals of dots is continued out 

 from the cavity of the epidermis-cell (as in the leaves of Hakea 

 gibbosa), or where the side walls of the epidermis-cells are dotted 

 and possess the same chemical peculiarities as the cuticle {e.g. in 

 Hakea gibbosa, H. pachyphylla, Hoy a carnosa), where also un- 

 doubted primary and secondary membranes in a similar manner 

 exhibit the chemical characters of the outer wood-membrane; 

 lastly, such cases as where the primary membrane of the side wall 

 in that half which is directed toward the upper surface of the leaf 



* Linnaea, 1. 10, Verm. Schriften, 260. 



