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 im 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. xii. p. 268). 
The examples already cited (vol. xiii. p. 423, figs. 45 & 49) of 
the perfectly independent construction of contiguous endogenous 
cells are not fayourable to the last-mentioned conception of the 
