MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 113 
I have never succeeded in separating the primary membrane 
into two layers by the application of acid, in the Phanerogamia, 
even by boiling in nitric acid. It swells somewhat in the acid. 
In gelatinous or horn-like cells, it generally extends in breadth 
in the same proportion as the secondary layers; in lignified cells, 
on the contrary, its expansion in breadth is usually very slight, 
so that it is consequently torn by the violent swelling of the 
secondary layers. Its increase of thickness is never remark~ 
able; in some cases where this resulted to a greater extent than 
usual, it showed that the primary membrane consisted of many 
superposed layers; this was especially the case in the liber-cells 
of Cocos botryophora and in the parenchymatous cells of the 
stem of Polypodium incanum. An outer layer presented itself 
in both, which in Cocos (fig. 9) met and came in contact at one 
point, at the corner of the cells, with the corner of the corre- 
sponding layers of the other cell-walls ; in Polypodium (fig. 33), 
on the contrary, a triangular interval was left, either empty or 
filled with intercellular substance. Joining these decided facts 
concerning the stratification of primary membrane with the cir- 
cumstance that the septa which form in the cambium layer are 
usually perceptibly thinner than the primary membranes of full- 
grown cells, and that a similar thickening of the primary mem- 
brane is visible on comparing an old and young leaf in mosses 
(figs. 2, 3), we must conclude that the primary membrane in- 
creases in thickness during the development of the cell, and that 
this, analogous to the increase of thickness of secondary mem- 
brane, consists not in an extension in thickness of the original 
membrane, but rather in the deposition of lamelle upon it. 
EXPLANATION OF THE PLATES. 
PLATE I. 
Fig. 1. Transverse section of the cambium layer of Pinus sylvestris. 
The primordial utricles have separated from the cell-wall in conse- 
quence of the action of spirit of wine, and are coloured by icdine. 
Fig. 2. Cell of a full-grown leaf of Jungermannia Taylori. The 
primordial utricle is detached from the cell-wall. 
Fig. 3. Cell of a young leaf of Jungermannia Taylori. 
Fig. 4. Cell of a leaf of Vallisneria spiralis. The primordial utricle 
is coloured by iodine. 
Fig. 5. Medullary cell of Asclepias syriaca. The primordial utricle 
is in the act of dissolution. 
Fig.G. Bark cells of Euphorbia canariensis. The primordial utricles 
are coloured by iodine. 
VOL, IV. PART XIII. I 
