MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 95 
with broader and narrower membranous appendages and threads 
covered with fine granules, similar to those mucous reticulations 
(which are coloured yellow by iodine), frequently met with distri- 
buted over inner walls of fresh cells. The first modification occurs 
in the medullary cells of Pinus sylvestris, the second in those of 
Asclepias syriaca (fig. 5). If we examine still older internodes 
the primordial utricle will have finally disappeared without leay- 
ing a trace of its existence; this happens at different periods in 
different plants, earlier in woody than in succulent; in which 
latter the primordial utricle will be found in perfection at a later 
period in the medullary, and particularly in the bark cells of 
Cactus and the fleshy Euphorbie (fig. 6). 
That the primordial utricle disappears in an analogous man- 
ner in the elementary organs of the wood, soon after the depo- 
sition of the secondary layer commences, is easily seen both in 
the vessels and ligneous cells of Dicotyledons and in the punc- 
tated tubes of the Conifere. 
As we trace the primordial utricle to its origin, the difficulty 
of observation and the uncertainty of its result increase in pro- 
portion as the cell is younger, because the question of its origin 
is complicated with that very difficult question, the origin of the 
cell. In the examination of the apex of the stems or the root of 
all plants, in the act of development, and of the cambium layer 
_ of Dicotyledons, we find a cellular tissue, which (setting aside 
the slight thickness of its walls) differs from the tissue of the 
more developed parts, by a very close application of its cells to 
each other so as to exclude entirely intercellular passages, and 
also by the unequal thickness of the cell-walls. On this last 
circumstance Unger has already very properly laid great stress ; 
it is especially striking in the cambium layer, in which the cell- 
walls lying parallel with the bark are thinner than those parallel 
with the medullary ray, as is shown in fig. 1, which exhibits a 
transverse section of the cambium layer of Pinus sylvestris, 
where however the distinction between the different cell-walls 
is much slighter than in many Dicotyledons. Bearing some 
though not very striking analogy to this, the young organs in 
parenchymatous cells also not unfrequently present thinner trans- 
verse walls, especially in the green bark, in the cells of which 
the thin transverse walls generally have a direction perpendicu- 
lar to the surface of the bark. 
Since we see then, in these instances, and especially in the 
