l the fundamental vital properties can be traced? We will first
glance at the structure of the cell as it was understood by the earlier
microscopists. A typical cell is shown in Fig. 9. It will be seen that
it consists of three quite distinct parts. There is first the _cell wall
(cw)_ which is a limiting membrane of varying thickness and shape. This
is in reality lifeless material, and is secreted by the rest of the
cell. Being thus produced by the other active parts of the cell, we will
speak of it as _formed_ material in distinction from the rest, which is
_active_ material. Inside this vesicle is contained a somewhat
transparent semifluid material which has received various names, but
which for the present we will call _cell substance_ (Fig. 9, _pr_). It
may be abundant or scanty, and has a widely varying consistency from a
very liquid mass to a decidedly thick jellylike substance. Lying within
the cell substance is a small body, usually more or less spherical in
shape, which is called the _nucleus_ (Fig. 9, _n_). It appears to the
microscope similar to the cell substance in character, and has
frequently been described as a bit of the cell substance more dense than
the remainder. Lying within the nucleus there are usually to be seen one
or more smaller rounded bodies which have been called _nucleoli_. From
the very earliest period that cells have been studied, these three
parts, cell wall, cell substance, and nucleus have been recognized, but
as to their relations to each other and to the general activities of the
cell there has been the widest variety of opinion.

[Illustration: FIG. 9.--A cell; _cw_ is the cell wall; _pr_, the cell
substance; _n_, the nucleus.]

==Cellular Structure of Organisms==.--It will be well to notice next just
what is meant by saying that all living bodies are composed of cells.
This can best be understood by referring to the accompanying figures.
Figs. 10-14, for instance, show the microscopic appearance of several
plant tissues.

[Illustration: FIG. 10.--Cells at a root tip.]

[Illustration: FIG. 11.--Section of a leaf showing cells of different
shapes.]

At Fig. 10 will be seen the tip of a root, plainly made of cells quite
similar to the typical cell described. At Fig. 11 will be seen a bit of
a leaf showing the same general structure. At Fig. 12 is a bit of plant
tissue of which the cell walls are very thick, so that a very dense
structure is formed. At Fig. 13 is a bit of a potat