46 KINDS OF CELLS. 
to produce a branched or netted appearance (fig. 89) ; in which 
case the cells are termed reticulated. It is also by no means an 
uncommon circumstance to find in the same cell intermediate 
conditions of all these forms. 
The fibres in most cases are wound from left to right, 
although instances occur where they have a contrary direction. 
The turns of the fibre, or the rings, may be nearly in contact, 
or more or less separated by intervals of cell-wall ; this latter 
appearance is probably due to the growth of the cell-wall after 
the deposition of the fibre. The turns of the fibre, or of the 
rings, again, may be either intimately attached to the cell-wall, 
or but slightly adherent, or altogether free. As a general rule, 
the less the cell-wall grows after the deposition of the fibre, 
the more firmly is it attached to it. 
In some cases, again, as in the Yew (jig. 91), we find a 
spiral fibre or fibres developed in addition to the pits; such 
cells have been sometimes termed tracheides. 
These different kinds of fibrous cells are connected by a 
number of intermediate forms (fig. 90) with the pitted cells 
already treated of, but all are formed on the same plan. That 
is, by the living protoplasm secreting the cellulose out of its 
own substance, and depositing it upon its external surface in 
different parts in varying thicknesses. 
Section 2. Or THE KINDS oF CELLS, AND THEIR CONNEXION 
WITH ONE ANOTHER, 
WE have already seen (page 39), that if the cells are of such 
forms that when combined together they merely come in con- 
tact with one another without perceptibly overlapping, they are 
called parenchymatous ; but that when elongated and pointed at 
their ends, so that in combination they overlap one another, 
they are termed prosenchymatous. We have also seen that such 
extreme forms are connected by all sorts of transitional ones. 
But, besides these elongated prosenchymatous cells, other 
lengthened tubular organs are also found in plants, which are 
termed vessels (see Vessels, page 51). Formerly, all these elon- 
gated organs were supposed to have an entirely distinct origin 
from the ordinary parenchymatous cells, and were described 
under the names of Woody Fibres, and Vessels or Ducts ; but 
it is now known that they are all derived originally from such 
cells, and owe their peculiar appearances either to various 
modifications in form, which the latter undergo in the course of 
growth, or to their combination and union with one another. 
This common origin of the Woody Fibres of old authors and of 
the Vessels with the parenchymatous cells, is proved by the fact, 
that gradual transitional forms from the one to the other may 
be commonly observed ; and also by tracing their development, 
