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 _Amoeba_. A cell without a cell wall. _n_,
nucleus; _v_, vacuoles, x 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_, x 50; _C_, x 150.]

Ve