94. MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
specimens kept in spirit, it offers sufficient resistance to the 
knife to allow of its being cut through by a transverse section, 
like the cell-wall, and will then be found in the form of a more 
or less wavy ring in the cavity of the cell (fig. 1. from Pinus 
sylvestris). Not unfrequently the primordial utricle lies quite 
loose in the cell, and hence often falls out of a delicate trans- 
verse section. If dilute sulphuric acid is applied to a prepara- 
tion which has been treated with iodine, the previously un- 
coloured cell-walls swell up and are stained blue, while the 
primordial utricle does not alter in form, but acquires a darker 
yellowish-brown colour. In concentrated sulphuric acid this 
membrane loses its continuity. 
The foregoing observations might lead to the conclusion that 
Hartig was acquainted with the primordial utricle and had de- 
scribed it under the name of ptychode, but it will presently be 
shown that by this name Hartig meant something totally different. 
He describes the ptychode as the most internal membrane of full- 
grown cells and vessels, while in these all traces of the primary 
utricle have long since disappeared. For if we follow this out in 
internodes of medium growth, itis evident that it undergoes the 
most essential modifications, with the increased age of the cells. 
These changes commence simultaneously or at shortly succeeding 
periods, in various portions of the stem, in the innermost layer of 
the wood, in the cells of the middle layers of the bark, and in the 
centre of the medulla, extending with the increasing age of the 
plant from these points to the rest of its structure. These changes 
are more easily traced in parenchymatous than in prosenchy- 
matous cells or dotted ducts, both on account of the greater size 
of the cells and of the slighter thickness and greater transpa- 
rency of their walls. In general they consist in this: as the 
cell grows older and its wall becomes thickened by the deposi- 
tion of secondary layers, the primordial utricle becomes propor- 
tionately thinner and proceeds towards its dissolution. Two 
modifications of the way in which this takes place present them- 
selves. We either find, as in the older cells, the primordial 
utricle adhering more firmly to the cell-walls, and applied over 
the inner surface of the cell in the form of a thin coating, in 
which case it may be detected by the yellow colour it assumes 
with iodine and by its finely granular structure; or, as in the 
younger cells, it is detached from the cell-membrane, no longer 
however in the form of a closed cell, but as an irregular web 
