ry 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, x 50. _C_, a cell from the prothallium
(young plant) of a fern, x _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_).

CHAPTER III.

CLASSIFICATION OF PLANTS.--PROTOPHYTES.

For the sake of convenience it is desirable to collect into groups
such plants as are evidently related; but as our knowledge of many
forms is still very imperfect, any classification we may adopt must be
to a great extent only provisional, and subject to change at any time,