M. Mohl on the Growth of Cell-Membrane. 263 



mination of the wood-cells themselves, their structure affords 

 decided proof, that secondary layers are deposited upon the 

 inside of the primary membrane. The analogy between the 

 structure of the wood-cells and that of parenchymatous cells, 

 as for instance the cells of horny albumen, the dotted, thick- 

 walled medulla- and bark-cells of Hoya carnosa, in which Hart- 

 ing himself does not deny an internal growth, at once offer rea- 

 sons not to be disregarded, for the assumption of an analogous 

 process of development in the two kinds of cells. Where the 

 anatomical relations of the individual layers are so perfectly 

 analogous, it would require very clearly-ascertained facts to in- 

 duce us to assume that nature follows a different law of formation 

 in the wood-cells from that which obtains in the parenchymatous 

 cells, and of such facts I have no knowledge. On the other hand, 

 the history of the development of prosenchymatous cells affords in 

 my opinion very certain evidence of the contrary. In relation to 

 this perhaps there is nothing so instructive as the examination of 

 the cells of the Conifer -a, and I believe that a conclusion deduced 

 from these elementary organs will hold good in reference to the 

 wood-cells of Dicotyledons, since spiral fibres on the inner sur- 

 face of the cell, together with a bordered dot, resembling those 

 occurring in Taxus, are also found in many wood-cells, as for in- 

 stance in Viburnum Lantana. Now the examination of young 

 shoots of Pinus sylvestris (and exactly in a similar manner also, 

 the examination of young dotted vessels of dicotyledonous wood) 

 affords evidence that the cavity which subsequently forms the 

 border of the dot, and which is situated between the outer closed 

 membranes of two contiguous cells, appears very early, while the 

 cell-membrane is yet very thin, and is in every case already per- 

 fectly formed at a period when no trace can be seen of the dot, 

 leading to the cavity, situated in the inner layer of the cell. It 

 does not admit of the slightest doubt therefore, that the outer 

 closed membrane of the cell is the primary, and that the inner 

 layers which are perforated by the canals of the dots are subse- 

 quently deposited upon the inner surface of the primary mem- 

 brane. 



It is not here meant to be denied that deposits do occur upon 

 the outer side of the primary membrane in many cases, for in- 

 stance in this very wood of Pinus sylvestris. This takes place in 

 the intercellular passages which are found between the cells while 

 their walls are still thin, m which an intercellular substance is 

 deposited ; but this has nothing to do with the thickening or 

 growth of the cell-membrane. 



Although in the foregoing remarks, I have been forced, in the 

 defence of my theory, to repel many of the objections advanced 

 by Harting and Mulder on anatomical grounds, because I cannot 



