106 MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
Plate II. some of these cells are represented as they appear in a 
transverse section lying in water, and they apparently show an 
inner membrane quite distinctly. The application of a weak 
acid to the preparation, particularly if it is allowed to act for some 
time, say twenty-four hours, is a convenient mode of detecting 
this error; for, after the cell-membranes have acquired an equal 
transparency, and their stratification has become more distinct, 
it is perceived not merely that no inner membrane runs across 
the dots, but that the innermost layer terminates at them, and is 
similarly circumstanced to that of the innermost lamellz of the 
secondary membrane of all other cells; and now the outer cel- 
lular membrane likewise appears, which in most cases had not 
previously been visible, and it is perceived that these cells, ap- 
parently so anomalous, are constructed quite according to the 
general rule (fig. 23. Plate II.). 
The foregoing shows that the positive decision as to whether 
the cells are clothed by a special membrane or not, is opposed 
by no mean difficulties, since both optical illusion and a slight 
modification of the substance of the innermost cell-layers, quite 
possible in intermediate layers, may lead to the belief that we 
have found the supposed membrane. Perhaps further improve- 
ments of the microscope and other methods of investigation, may 
in time give rise to another opinion, but for the present I must 
decidedly assert, that in the majority of cells no inner membrane, 
distinct from the secondary layers, occurs. In this respect I 
believe that the medullary cells of Tax odium distichum, on which 
Hartig lays such great stress, form no exception. An inner 
brown coating, investing the secondary layer, does indeed occur 
in these cells, but that this is not a membrane belonging to the 
cell appears sufficiently clear from the facts that it is unequal in 
thickness, in many cells filling the whole cavity, and that in 
young cells starch granules are mixed up with it, &c. 
Hartig believes that the secondary cell-layers are secreted by 
the ptychode; he does not say whether he considers the inner 
or the cuter lamellz the older. In my opinion, there can be no 
doubt that the exterior are the older; the mechanical relation of 
the different layers to each other, require positively the adoption 
of this opinion. In this respect two modifications occur. In 
the first, the layers of the secondary membrane do not run par- 
allel to the outer walls of the cell, but exhibit an arched curve 
directed towards the interior of the cell, for instance in Junger- 
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