22 NATURE OF THE CELL AND ITS CONTENTS. 
then, by first describing the nature of the cell and its contents ; 
and then pass on to a more detailed exaniination of its various 
forms, sizes, and structure. 
I. NatTuRE OF THE CELL AND ITs ContENTs.—In the very 
earliest stage of a plant’s existence—in, for example. the ger- 
minal vesicle (oosphere) of the higher plants—the cell consists. 
only of a naked mass of a semifluid substance to which the name 
of protoplasm has been given. Ina few cases the cell remains in 
this condition, and is then termed a primordial cell. But asa 
general rule this protoplasm very shortly surrounds itself on the 
outside with a thin transparent skin of cellulose—the cell wall— 
and in this condition three distinct parts can be observed in the 
cell (fig. 87): (1) the cell wall, a; (2) the internal protoplasm 
above mentioned, b ; .and (3) the nucleus, c, which is a rounded, 
denser portion lying in the midst of the protoplasm, At first 
the protoplasm completely fills the cavity, but as the cell grows 
larger, cavities (vacuoles) containing a clear watery fluid (which 
in the very young cells is generally diffused), called the cell-sap 
(figs. 38 and 39, s’, s’), make their appearance in it, and the 
nucleus, k’, is then suspended in the cell and connected to the 
protoplasm lining its inner wall by slender threads or bands of 
the same substance (fig. 39, p’, p’). (De Vries has stated recently 
that these vacuoles are enclosed by a distinct membrane, and he 
regards this vacuole membrane as a special organ to which he 
has given the name of tonoblast, and which has for its function 
the production of turgidity in the cell.) As the cell continues to 
enlarge, these vacuoles coalesce and form a single central sap- 
cavity (fig. 39, s, s), and the protoplasm is then confined to a 
thin layer lining the interior of the cell-wall—the primordial 
utricle, p, with the nucleus, k, k, showing asa denser mass in an 
enlargement of the protoplasm on one side. In this perfect cell, 
as it may be termed, we distinguish, (1) the cell-wall, (2) the pro- 
toplasm, (3) the nucleus, (4) the cell-sap. These structures may 
be well seen in Vallisneria. 
Such is the nature of cells so long as they retain their active 
vital state, but after a time the protoplasm with its contained 
nucleus disappears, leaving the cell filled with air alone or water. 
Those cells only which contain protoplasm can grow, form 
chemical combinations, and produce new cells ; while all others, 
as the cells of the wood and bark, are of use only in virtue of 
their physical properties, as, for example, giving firmness, and 
acting as protecting envelopes to the living cells beneath, and in 
other ways. We must now describe the parts of the cell in the 
order as placed above. 
1. THe CeLi-waLt (figs. 37, a, and 39, h).—We have just 
seen that the original cell, from the after divisions of which the 
future structure is built up, consists of protoplasm alone—-that, 
in other words, it has no cell-wall. Very shortly, however, this 
condition of things disappears; for the protoplasm, having 
