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



by micrometrical measurement, if it were possible to examine 

 one and the same cell at different periods of its development. 

 Since this is not possible, we are obliged to compare the older 

 and younger cells of the same internode with each other ; here 

 however the unequal size which different wood-cells attain brings 

 us into a difficulty which is almost insurmountable, since not 

 only would a vast number of measurements, robbing us of much 

 time, be involved in order to obtain a value moderately approxi- 

 mating the truth if all the cells of the woody mass were deve- 

 loped in a tolerably similar manner, but the detection of alter- 

 ations which the size of the wood-cells undergoes in the course 

 of time is rendered particularly difficult, by the fact that cells 

 lying in different layers of the ring of wood annually produced 

 attain a different size. However, before I give certain measure- 

 ments which I made in relation to this, there is a point to be con- 

 sidered which Harting appears to have totally overlooked. The 

 cambium- cells evidently become thickened on all sides in their 

 transition into wood-cells ; the cells, which form the innermost 

 layer of the cambium, before becoming thickened, have already 

 pushed forward their walls so as to be in immediate contact with 

 each other laterally, and thus form a circle which straitly incloses 

 the outermost circle of thickened perfect wood-cells. Let us 

 assume with Harting, that during the transition of cambium- 

 cells into wood-cells their cavity is not lessened in size, but that 

 the increment of their walls is referable to the application of new 

 layers upon the outer side of their walls. In this case it would 

 necessarily follow, from mechanical reasons, that the cavity of 

 the cambium-cell inclosed by the primary walls, under these re- 

 lations, as the side walls would be thickened by each new deposit, 

 would be compressed laterally and the cell must become ex- 

 tended in the direction of the radius, since otherwise the ring 

 formed from the cambium layer must, in consequence of the 

 production of deposits between the side walls of each cell, be- 

 come expanded to a much more considerable size than they pre- 

 viously possessed, and be torn away from the outer circle of the 

 wood-cells. Since the latter evidently does not happen, we must 

 assume'that if the surface of the transverse section of the cell- 

 cavity does not become enlarged in the conversion of the cam- 

 bium-cells into wood-cells, yet in any case an alteration of form 

 and an expansion of the cell-cavity in the radial direction must 

 take place. Now to prove whether this is really the case or not, 

 I selected a twig of Hoya carnosa, which plant appears to me to 

 be especially suited for these investigations, because its wood-cells 

 are of tolerably equal size, and because during the development 

 of its cylinder of wood, the limit between the wood and the cam- 

 bium shows itself very distinctly. That I might not be exposed 



