194 Prof. H. Karsten on the Vegetable Cell. 



development of the porous membranes) by the similar aspect of 

 the above-mentioned seed-coats &c. The band visible between 

 them is the membrane of the secondary cell thickened internally 

 in ridges between the small vesicles adherent to it ; here it 

 separates readily in a spiral direction from the annular bodies, 

 which now and then detach themselves singly, because the ori- 

 ginal external lamina from which it grew, and to which the now 

 annularly thickened vesicles adhered, is almost entirely ab- 

 sorbed. 



The structure of these vascular walls differs from that of the 

 membranes of seeds and pollen- corpuscles in this respect, — that 

 in the latter the cells are immediately contiguous, and form a 

 continuous tissue, whilst in the former the spherical or expanded 

 vesicles are either completely separated or are in contact in one 

 direction only, though at times an actual coalescence takes place 

 between them. 



Since the profound researches of Mohl into the structure of 

 the cell-membrane, it has been known that even heterogeneous 

 layers of deposit occur upon the membranes composing one 

 tissue-cell. This remarkable phenomenon may be simply ex- 

 plained by the fact that in such tissue-cells the heterogeneous 

 membranes of different nested cells are closely approximated. 



Moreover the external primary cell-membrane (if we leave 

 out of consideration the spiral texture, which is certainly very 

 prevalent) appears almost constantly to be homogeneous, whilst 

 the membrane of the secondary cell very frequently has a pecu- 

 liar structure ; but the tertiary cell, where it attains the dimen- 

 sions of the secondary one, is likewise structureless. 



The cause of these well-known facts, as also of the parallel 

 occurrence of organized structures in one of the endogenous 

 cells, whilst in others there is only fluid, has not hitherto been 

 recognized. 



The frequent and almost normal absence of organized bodies 

 in the contents of the primary cell, and of peculiar forms of 

 thickening of its membrane, throws us back upon its develop- 

 mental history in order to decide whether this homogeneous 

 external membrane of the vegetable cell is the membrane of the 

 primary cell of the cell-system (as which I regard it) or only 

 the first structureless layer of deposit of the second inner cell- 

 membrane, which subsequently becomes thickened in another 

 form. The latter might then be regarded, with respect to the 

 former, as a primordial sac, if Mohl had not established a dif- 

 ferent conception of this designation (vol. xiii. p. 268). 



The examples already cited (vol. xiii. p. 423, figs. 45 & 49) of 

 the perfectly independent construction of contiguous endogenous 

 cells are not favourable to the last-mentioned conception of the 



