MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 103 
to be avoided, as the first does not act and the second exerts too 
strong a disintegrating power on the membrane, under neither 
of which circumstances can the layers be made out. It is like- 
wise necessary to employ an acid when we wish clearly to un- 
derstand the structure of the membrane in those cells where the 
stratification is visible without its application, as it frequently 
happens that before it is applied, only a few thick layers are vi- 
sible, and the acid demonstrates the presence of a great number 
of very delicate layers, for instance, in the thick-walled medul- 
lary cells of Hoya carnosa, of which fig. 11. Plate I. represents a 
fresh cell, and fig. 12 a part of the same swollen by the action 
_ of sulphuric acid. 
When an acid acts strongly enough to render the stratifica- 
tion of a membrane distinct, it always causes it to swell more or 
less. Under these circumstances parenchymatous cells are ex- 
tended in every direction; prosenchymatous cells and vessels, 
on the contrary, chiefly in thickness and breadth, and very 
slightly in length, as we also observe when they are moistened 
with water. The degree of distention, however, which a mem- 
brane undergoes in water, bears not the slightest relation to the 
tumefaction it exhibits in acid, for cellular membranes which swell 
up very much in water, for instance, the gelatinous cells situated 
beneath the epidermis, increase in size very little more in acid, 
while the wood- and liber-cells on which water exerts very slight 
influence, swell up exceedingly in acid, and the softer cells be- 
neath the ligneous cells in the inner part of the annual ring, in 
a greater degree than the firmer ones situated more externally. 
Very frequently the thick-walled wood- and liber-cells which 
have not been treated with acid, exhibit alternate broader and 
darker, narrower and paler layers; for instance, the liber-cells 
of Cocos botryophora (fig. 8. Plate I.),and in a less marked degree 
the liber-cells of Calamus (figs. 25,26. Plate II.). This circum- 
stance indicates a certain unconformity in the substance of the 
cell-wall, which is doubtless in consequence of a successive depo- 
sition of the layers. Another circumstance here presents itself to 
which at first one might be inclined to attach no importance, but 
to which I must call attention on account of the subsequent consi- 
deration of the wood-cells of Dicotyledons. The above-described 
layers are so stratified, that the most internal is always a thin and 
paler one. If we allow the liber-cells of Cocos botryophora to swell 
up in dilute sulphuric acid, the individual layers separate very 
