MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 111 
which the inner layer is very distinctly developed, but because 
it was accidentally in them that I ratified my previous obser- 
vations of this appearance, before writing the present remarks. 
It is far more uncommon for the inner layer to be coated by 
spiral fibres, as occurs in Viburnum Lantana (fig. 15, 16), Euo- 
nymus atropurpureus, Sambucus nigra. 
Passing to the outer cellular membrane, we must first con- 
sider a remark of Hartig’s (founded indeed on a perfectly correct 
observation), namely the assertion that the outer membrane of 
contiguous cells is to be regarded as a layer common to both, 
without any line of separation in the middle, and that there- 
fore phytotomical drawings hitherto published which exhibit 
such a line of separation are incorrect. This assertion is fully 
warranted as far as relates to the cells and vessels of Phanero- 
gamia, but in the Algz, in which a more or less abundant inter- 
cellular substance is present, the outer membrane of each cell is 
separated from that of the neighbouring cell by a layer of inter- 
cellular substance (fig. 19). Is it probable, as Hartig believes, 
that the outer membrane lying between two cells is common to 
both, or does it appear to us merely as a simple membrane be- 
cause the line of separation escapes our optical instruments ? 
This question is difficult of solution by the direct observation of 
Phanerogamia, but it cannot be doubted that the membrane 
forms no homogeneous layer between the contiguous cells, as 
Hartig assumes, but that they are either composed of two mem- 
branes intimately coherent, as would appear from the cells of 
the Alge, or it is at least possible that a division into two mem- 
branes takes place in consequence of organic processes (which 
really has been effected by nature in many cases); in favour of 
which view may be instanced the separation of the cells of Zyg- 
nema, and the formation of intercellular passages in Phaneroga- 
mia. That this membrane is the youngest, and is separated in 
a fluid state from the already existing inner cell-membranes, as 
Hartig assumes, is wholly improbable, from the firm coherence 
which the cells exhibit at every period of their existence. Lea- 
ving out of the question that not one fact is known which speaks 
in favour of this view (for Hartig’s assertion that the outer mem- 
brane is present only in the intercellular passages, in many gela- 
tinous cells, is founded on imperfect observation), a complete 
proof that the outer cellular membrane is not the youngest but 
the oldest, exists principally in two circumstances; in the first 
