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



proposing my theory, and which up to this time have their full 

 value to me, since I know of no anatomical facts which are in 

 contradiction to this theory, or would render any other explana- 

 tion half so probable. Prof. Harting mentions two circumstances, 

 the first of which he considers makes it unlikely that the secon- 

 dary layer is deposited upon the inner side of the primary mem- 

 brane, while the second affords him a positive proof of the deposi- 

 tion of the secondary layers upon the outer side. The first cir- 

 cumstance is the direct correspondence* of the dot in contiguous 

 cells, which it is very difficult to comprehend when we adopt my 

 theory. I confess distinctly that I do not at all comprehend it ; 

 I know only that it is so. We do not generally understand the 

 reason of a special organization, because we know nothing of the 

 nature of the power producing organization ; thus we do not com- 

 prehend, for instance, how it happens that in the putamen of the 

 cocoa-nut the embryo inclosed in albumen is situated opposite 

 an eye. We see the object of this arrangement, but do not under- 

 stand how it is that it becomes developed at this point of the 

 pericarp rather than in any other situation. 



The second circumstance, which Harting brings forward as a 

 proof of the deposition of secondary layers taking place upon the 

 outer side of the primary cell-wall, is somewhat complicated. 

 From a large number of micrometrical measurements (worthy of 

 all praise) which he made on yearling stems of dicotyledons in 

 the course of development f 9 Harting draws the conclusion that 

 in the internode of a dicotyledonous plant no multiplication of 

 cells takes place in a radial direction after it has passed beyond 

 the condition of bud, but that in the course of the first year 

 the resulting thickening of the internode is to be ascribed to the 

 expansion of the cells already existing in the bud. 



In reference to this he distinguishes two periods ; in the first, 

 which precedes the thickening of the walls of the ligneous cells, 

 this expansion proceeds in a similar proportion in all the layers 



• At the same time it must not be forgotten that this apposition is pecu- 

 liarly circumstanced. It is true that roundish dots correspond accurately 

 in position and generally in form; elongated and obliquely placed dots, on 

 the contrary, come into apposition only at their middles, and no longer cor- 

 respond in their form, since they cross ; finally, slits (much-elongated dots) 

 which run between spiral or annular fibres, &c. are usually without any 

 relation of position to each other, in contiguous cells. The connexion there- 

 fore is not so simple as Harting appears to have conceived, when he assumes 

 that openings from one cell into another occur in the primary membrane 

 which directly correspond ; of these however it would be as difficult to ex- 

 plain why they form of exactly the same size and in corresponding situa- 

 tions in the two cells, as it is easy to explain the origin of the dot when my 

 view of the structure of cell-membrane is admitted. 



f Tijdschrift voor naturlijke Geschiedenis, 1844. 



