MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 107 
mannia Taylori (fig. 2. Plate 1.), Schotia (fig. 17). Ifin such cases 
we compare the young with the full-grown cell (viz. in Junger- 
mannia Taylori, fig. 3 with fig. 2), we see clearly that in the former 
the narrowest layers alone, situated most externally, are yet in 
existence. The second modification occurs in the cases where the 
dots are small; in these the lamelle of the secondary membrane 
run parallel to the outer wall of the cell, as in Hoya carnosa 
(fig.11). In this case, indeed, the mechanical conditions of the 
layers present no grounds for declaring the inner to be the 
younger, but disregarding the analogy drawn from the foregoing 
case, the history of the development of these cells, in very many 
cases, does not allow the slightest doubt to be entertained but 
that here also the inner layers are the younger, since these cells 
frequently attain their full size in the condition of thin-walled 
cells, and then first commences the deposition of secondary layers 
within them. 
In most cells no distinction is perceived between the different 
secondary layers, save the above described difference of colour. 
To this however there are some remarkable exceptions. The 
ligneous cells of some of the Ficoidee I have examined exhibit a 
striking peculiarity in this respect, particularly part of the lig- 
neous cells of Ff. carica, whose secondary membrane consists of 
two layers, of which the outer and firmer is coloured yellowish- 
brown by iodine, and the inner and softer violet (fig. 21. Plate II.). 
Exactly similar appearances are presented by the liber cells of 
a yearling shoot of Rhus typhinum. 
While in this case the general layers are only distinguished 
by their different consistence, in other cases we find moreover a 
difference of structure. This occurs, for instance, in many plants 
in the cells of the seed membrane and the achenium containing a 
spiral fibre and swelling in water, in which three layers are gene- 
rally to be distinguished, the primary outer membrane, a mucila- 
ginous layer swelling in water, and the spiral fibre. The last 
may lie within the mucilaginous layer, as in Collomia, Senecio 
vulgaris, &c., or as in Ruellia strepens (fig. 32), surround the mu- 
cilaginous layer and adhere to the outer membrane. The muci- 
laginous layers of these cells are by no means to be considered 
as unorganized mucilage, for in certain cases, as in Ruellia, they 
appear to be composed of distinct superposed lamelle. 
In my opinion the ligneous cells of Taxus and the allied forma- 
tions also belong to this division of cells, having two distinct 
