110 MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
mefied cellular membrane might be taken as an evidence of the 
correctness of this theory; but, on the other hand, this disrup- 
tion into lamellz may also be considered as a dissolution of a 
hitherto coherent membrane of a distinctly fibrous structure. 
I believe the history of the development is in favour of the latter 
view, at least I have never been able to see fibres in the interior 
of the cells of Pinus, during the progress of their growth, which 
were not yet attached toa membrane. Be this as it may, the 
firm coherence which the outer layer of secondary membrane of 
the said plants retains in acid, shows as clearly as does the 
roundish form of the canals of the dots in this layer, that it pos- 
sesses a structure different from the inner layer. We have here 
therefure a relation similar to that in Taxus, with the difference 
only that the spiral layer is here uncommonly developed, while 
in Taxus it is extremely thin, and as respects the outer layer, the 
contrary is the case. It is certain that in Pinus the outer layer 
is developed before the inner. 
The wood of other Coniferee I have examined does not exhibit 
such a striking distinction between the outer and inner layer of 
the secondary membrane; one difference however may be per- 
ceived, which is, that the cavity of the cell is surrounded by a 
band of light with tolerably distinct outlines. That this band is 
not occasioned by an optical illusion is evident, from the inner- 
most layer of the cell swelling much less strongly than the outer 
in sulphuric acid, in consequence of which it tears away in one 
or more places (fig. 13), upon which the parts of the outer layer 
corresponding to the torn places expand very much in breadth ; 
or the outer membrane detaches itself from the inner in particu- 
lar places on account of the more considerable swelling (fig. 14), 
for example in Juniperus virginiana, Sabina, Abies pectinata, 
Ginyko biloba. No peculiarity of structure is to be perceived in 
this inner layer. 
The presence of an inner layer is not peculiar to the ligneous 
cells of Coniferze, but occurs equally in the prosenchymatous 
cells of Dicotyledons, and the same modifications we found in 
Taxus and Torreya, and in Juniperus, Gingko, &c., are repeated 
in these. The latter is the usual modification in Dicotyledons, 
as instances of which I may name Quercus Robur, Piscidia 
Erythrina (fig. 28, before tumefaction; fig. 27, the same swollen), 
Myrtus acris, Cobea scandens, Aralia arborea, Laurus nobilis, 
which plants however I do not bring forward as instances in 
