ells with thick walls showing evident stratification.
At intervals branches are given off, which may in turn branch,
giving rise to a complicated branching system. These branches begin
as little protuberances of the cell wall at the top of the cell.
They increase rapidly in length, and becoming slightly contracted at
the base, a wall is formed across at this point, shutting it off
from the mother cell.

The protoplasm lines the wall of the cell, and extends in the form
of thin plates across the cavity of the cell, dividing it up into a
number of irregular chambers. Imbedded in the protoplasm are
numerous flattened chloroplasts, which are so close together as to
make the protoplasm appear almost uniformly green. Within the
chloroplasts are globular, glistening bodies, called "pyrenoids."
The cell has several nuclei, but they are scarcely evident in the
living cell. By placing the cells for a few hours in a one per cent
watery solution of chromic acid, then washing thoroughly and
staining with borax carmine, the nuclei will be made very evident
(Fig. 13, _B_). Such preparations may be kept permanently in dilute
glycerine.

[Illustration: FIG. 13.--_Cladophora._ _A_, a fragment of a plant,
x 50. _B_, a single cell treated with chromic acid, and stained with
alum cochineal. _n_, nucleus. _py._ pyrenoid, x 150. _C_, three stages
in the division of a cell. i, 1.45 p.m.; ii, 2.55 p.m.; iii,
4.15 p.m., x 150. _D_, a zooespore x 350.]

If a mass of actively growing filaments is examined, some of the
cells will probably be found in process of fission. The process is
very simple, and may be easily followed (Fig. 13, _C_). A ridge of
cellulose is formed around the cell wall, projecting inward, and
pushing in the protoplasm as it grows. The process is continued
until the ring closes in the middle, cutting the protoplasmic body
completely in two, and forms a firm membrane across the middle of
the cell. The protoplasm at this stage (_C_ iii.) is somewhat
contracted, but soon becomes closely applied to the new wall. The
whole process lasts, at ordinary temperatures (20 deg.-25 deg. C.), from
three to four hours.

At certain times, but unfortunately not often to be met with, the
contents of some of the cells form, by internal division, a large
number of small, naked cells (zooespores) (Fig. 13, _D_), which
escape and swim about actively for a time, and afterwards bec