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



One is less readily exposed to the risk of an illusion in exa- 

 mining the medulla-cells of full-grown yearling shoots of Sy- 

 ringa, JEsculus, or Sophora japonica, than in the young cells 

 previously mentioned ; but here also, and especially in Sambucus 

 and Sophora, when the cell-membrane has acquired a deep blue 

 tinge, we must be cautious, since the contrast between the dark 

 blue of the thick portion of the membrane and the brighter co- 

 lour of the thin allows the membrane closing the dot to be easily 

 overlooked. If, on the contrary, we expose the specimen from 

 twenty-four to forty-eight hours to the air, till part of the iodine 

 has again evaporated and the cell-membrane has acquired a 

 bright violet colour, we can readily make out the thin and also 

 violet-coloured membrane. When, as not unfrequently happens 

 in the full-grown medulla-cells of Syringa, &c, the outer cell- 

 membrane is coloured yellow, and the inner, in which canals of 

 the dots lie, blue, the membrane stretched over the dot appears 

 yellow, in which case also a delusion as to its presence is not 

 easily possible. 



The presence of dots on medulla-cells of buds of Sambucus 

 shows that we have here no longer to do with a simple mem- 

 brane ; in other cases, e. g. in the buds of Asclepias syriaca, on 

 the contrary, I found the membrane quite homogeneous and 

 without any trace of dots. 



I believe these observations to be decisive, and consider my- 

 self entitled to persevere in the view, that the primary cell-mem- 

 brane is closed. 



Another question is, whether the outermost cell-layer, as I 

 believe, or the innermost, as Harting and Mulder assume, is the 

 oldest. 



Before I enter upon the action which chemical agents exercise 

 upon the different layers of cell-membrane and the consequences 

 deduced from the appearances observed under such circumstances, 

 I may be permitted briefly to state the reasons which, on ana- 

 tomical grounds, induce me to declare the most external mem- 

 brane to be the oldest. 



It is a universal phenomenon, that the membrane of young 

 cells and vessels is smooth and thin ; that, on the other hand, 

 when the membrane has become thickened in the course of time, 

 two principal layers may be distinguished in it ; one exterior, 

 thin, and imperforate, and an interior, of greater or less thick- 

 ness, pierced with slits and holes. If the holes are small, the 

 inner layer appears as a continuous membrane, pierced like a 

 sieve with holes ; if they are large, or elongated into slits and 

 approximated together, it appears as a deposit of fibres, which 

 are sometimes combined in a reticulated manner, sometimes run- 

 ning spirally, sometimes annular, &c. In many cases, e. g. in 



