THE CELL.
If we make a thin slice across the stem of a rapidly growing
plant,--_e.g._ geranium, begonia, celery,--mount it in water, and
examine it microscopically, it will be found to be made up of numerous
cavities or chambers separated by delicate partitions. Often these
cavities are of sufficient size to be visible to the naked eye, and
examined with a hand lens the section appears like a piece of fine
lace, each mesh being one of the chambers visible when more strongly
magnified. These chambers are known as "cells," and of them the whole
plant is built up.
[Illustration: FIG. 1.--A single cell from a hair on the stamen of the
common spiderwort (_Tradescantia_), × 150. _pr._ protoplasm; _w_, cell
wall; _n_, nucleus.]
In order to study the structure of the cell more exactly we will
select such as may be examined without cutting them. A good example
is furnished by the common spiderwort (Fig. 1). Attached to the base
of the stamens (Fig. 85, _B_) are delicate hairs composed of chains
of cells, which may be examined alive by carefully removing a stamen
and placing it in a drop of water under a cover glass. Each cell
(Fig. 1) is an oblong sac, with a delicate colorless wall which
chemical tests show to be composed of cellulose, a substance closely
resembling starch. Within this sac, and forming a lining to it, is
a thin layer of colorless matter containing many fine granules.
Bands and threads of the same substance traverse the cavity of the
cell, which is filled with a deep purple homogeneous fluid. This
fluid, which in most cells is colorless, is called the cell sap, and
is composed mainly of water. Imbedded in the granular lining of the
sac is a roundish body (_n_), which itself has a definite membrane,
and usually shows one or more roundish bodies within, besides an
indistinctly granular appearance. This body is called the nucleus of
the cell, and the small one within it, the nucleolus.
The membrane surrounding the cell is known as the cell wall, and in
young plant cells is always composed of cellulose.
The granular substance lining the cell wall (Fig. 1, _pr._) is
called "protoplasm," and with the nucleus constitutes the living
part of the cell. If sufficiently magnified, the granules within the
protoplasm will be seen to be in active streaming motion. This
movement, which is very evident here, is not often so conspicuous,
but still may often be detected without difficulty.
[Illustration: FIG. 2.--An _Amœba_. A cell without a cell wall. _n_,
nucleus; _v_, vacuoles, × 300.]
The cell may be regarded as the unit of organic structure, and of
cells are built up all of the complicated structures of which the
bodies of the highest plants and animals are composed. We shall find
that the cells may become very much modified for various purposes, but
at first they are almost identical in structure, and essentially the
same as the one we have just considered.
[Illustration: FIG. 3.--Hairs from the leaf stalk of a wild geranium.
_A_, single-celled hair. _B_ and _C_, hairs consisting of a row of
cells. The terminal rounded cell secretes a peculiar scented oil that
gives the plant its characteristic odor. _B_, × 50; _C_, × 150.]
Very many of the lower forms of life consist of but a single cell
which may occasionally be destitute of a cell wall. Such a form is
shown in Figure 2. Here we have a mass of protoplasm with a nucleus
(_n_) and cavities (vacuoles, _v_) filled with cell sap, but no cell
wall. The protoplasm is in constant movement, and by extensions of a
portion of the mass and contraction of other parts, the whole creeps
slowly along. Other naked cells (Fig. 12, _B_; Fig. 16, _C_) are
provided with delicate thread-like processes of protoplasm called
"cilia" (sing. _cilium_), which are in active vibration, and propel
the cell through the water.
[Illustration: FIG. 4.--_A_, cross section. _B_, longitudinal section
of the leaf stalk of wild geranium, showing its cellular structure.
_Ep._ epidermis. _h_, a hair, × 50. _C_, a cell from the prothallium
(young plant) of a fern, × _150_. The contents of the cell contracted
by the action of a solution of sugar.]
On placing a cell into a fluid denser than the cell sap (_e.g._ a
ten-per-cent solution of sugar in water), a portion of the water
will be extracted from the cell, and we shall then see the
protoplasm receding from the wall (Fig. 4, _C_), showing that it is
normally in a state of tension due to pressure from within of the
cell sap. The cell wall shows the same thing though in a less
degree, owing to its being much more rigid than the protoplasmic
lining. It is owing to the partial collapsing of the cells,
consequent on loss of water, that plants wither when the supply of
water is cut off.
As cells grow, new ones are formed in various ways. If the new cells
remain together, cell aggregates, called tissues, are produced, and
of these tissues are built up the various organs of the higher plants.
The simplest tissues are rows of cells, such as form the hairs
covering the surface of the organs of many flowering plants (Fig. 3),
and are due to a division of the cells in a single direction. If the
divisions take place in three planes, masses of cells, such as make up
the stems, etc., of the higher plants, result (Fig. 4, _A_, _B_).