M. Mohl on the Growth of Cell- Membrane. 151 



of the stem ; in the second period, on the contrary, in which the 

 wood-cells become thickened, they expand in a greater propor- 

 tion than the remainder of the cells, and, indeed, in such a man- 

 ner that the expansion of their cell-cavities is in proportion to 

 the expansion of the cells which do not become thickened, and 

 that besides this, the radial diameter of the ligneous cells becomes 

 increased by the thickening of their wall. From the circum- 

 stance that by the occurrence of a deposition of secondary layers 

 in the cavity, this latter must necessarily be contracted, — that 

 however such a contraction is not indicated by the micrometrical 

 measurement of this cell, the cavity of the cell enlarging to 

 the same size as where no thickening of the wall takes place, — 

 Harting draws the conclusion that the deposition of layers of 

 increment takes place upon the outside of the cell. 



Let us examine these assertions as to the ligneous cells some- 

 what more closely. First, it is stated that in the wood of dico- 

 tyledons no multiplication, but only an expansion of cells takes 

 place. Here Harting rests, not so much on the direct counting 

 of the cells lying in the said direction in the woody bundle, as 

 on the estimate depending on the measurement of the cavities 

 and the thickness of the walls of certain of these cells. I wholly 

 disregard the question, whether, from the different magnitude of 

 the ligneous cells, of which those lying in the outer part of the 

 wood are mostly much smaller, while the larger are situated 

 toward the interior, this method of investigation is adapted to 

 furnish an accurate result, and whether Harting has proceeded 

 with the necessary regard to all circumstances in carrying it 

 into effect, since distinct facts exist which demonstrate the view 

 that the ligneous cells do not multiply in a radial direction to be 

 completely erroneous. There is, to go no further, evidence of 

 this in the direct calculation of the ligneous cells which lie in 

 the radial direction in different internodes of the same yearling 

 shoot. 



The following calculations were instituted on transverse sec- 

 tions, always from the middle of the internodes, of twigs which 

 were cut off in January, in which therefore all the woody cells 

 of the first year's ring were fully developed. The internodes are 

 indicated from below upward by the numbers 1, 2, 3; that 

 marked 1, however, not being always the lowest internode of the 

 twig. The number of cells refers to the perfect wood-cells lying 

 in the direction of a radius between the pith and the cambium 

 layer. They were counted in those places where no vessels, or 

 as few as possible, lay in the direction of the radius ; when how- 

 ever, as is unavoidable in the thicker internodes, one or more 

 vessels were situated in the row of cells, the calculation of the 



