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



version of the cambium-cell into a wood- cell depends on the de- 

 position, of secondary layers upon the outside of the cell; or 

 rather, as was shown above, the cavity of the cell must enlarge 

 in the radial direction in consequence of this external addition 

 of secondary membranes. If we compare with this the size of 

 our intermediate wood-cell, the hypothesis cannot be brought into 

 agreement with its dimensions, for the cavity of the cell lying in 

 the outermost circle of wood diminishes from ■£$ to T y ¥ of a mil- 

 limetre, while the total diameter of the cell increases to 7 y. These 

 calculations prove beyond a doubt, that in the conversion of a 

 cambium -cell into a wood-cell the cavity is far from remaining of 

 the same size or enlarging ; on the contrary, it becomes very ma- 

 nifestly smaller : this can only be accounted for by a deposition of 

 secondary layers on the inside of the primary membrane, or by 

 the assumption of the occurrence of an external compression of 

 the cell-membrane on every side, causing it to occupy a smaller 

 space, for which process no analogy is to be found throughout all 

 vegetable anatomy. That the total diameter of the cell distinctly 

 increases (from ■§■$ to 7 j millim.), while at the same time the ca- 

 vity becomes smaller, is not in the least an objection to the hy- 

 pothesis that a deposition of secondary layers takes place in the 

 interior of the cell, because there .is no reason to prevent our as- 

 suming that an elementary organ may increase in breadth, by the 

 intus-susception of new organic matter between the molecules of 

 which its membranes consist, during the deposition of secondary 

 membranes. That such a growth is possible and actually does 

 take place, convincing proof is offered by the spiral vessels situated 

 in the interior of the vascular bundle, the spiral fibre of which 

 every one certainly considers as a secondary deposit. This en- 

 largement of the whole cell does not yet attain its maximum 

 while it lies in the most external row of the wood-cells ; the above 

 measurements show that in the wood-cells of the second circle the 

 total diameter had increased from 7 j to y T , the cavity from T y^ 

 to T ^ of a millimetre. As seen by these numbers, the total 

 diameter of the cell has increased in a greater proportion than 

 the diameter of the cavity, whence the inference, that simulta- 

 neously with the enlargement of the cell, a thickening of its 

 walls takes place, which however is not quite sufficient to hinder 

 the enlargement of the cavity of the cell, by the expansion of the 

 cell- wall. 



If from this refutation of the reasons adduced by Harting in 

 favour of the external addition of the secondary layers, founded 

 on micrometrical measurements, we pass to an anatomical exa- 



curacy, the thickness of the walls parallel with the wood of the cambium- 

 cells of* Hoya amounted to at all events not more than ^Vtf of a millimetre, 

 if anything less. 



