the order in fresh water are those
belonging to the genus _Vaucheria_, but these are to be had almost
everywhere. They usually occur in shallow ditches and ponds, growing
on the bottom, or not infrequently becoming free, and floating where
the water is deeper. They form large, dark green, felted masses, and
are sometimes known as "green felts." Some species grow also on the
wet ground about springs. An examination of one of the masses shows it
to be made up of closely matted, hair-like threads, each of which is
an individual plant.

In transferring the plants to the slide for microscopic examination,
they must be handled very carefully, as they are very easily
injured. Each thread is a long tube, branching sometimes, but not
divided into cells as in _Spirogyra_ or _Cladophora_. If we follow
it to the tip, the contents here will be found to be denser, this
being the growing point. By careful focusing it is easy to show that
the protoplasm is confined to a thin layer lining the wall, the
central cavity of the tube being filled with cell sap. In the
protoplasm are numerous elongated chloroplasts (_cl._). and a larger
or smaller number of small, shining, globular bodies (_ol._). These
latter are drops of oil, and, when the filaments are injured,
sometimes run together, and form drops of large size. No nucleus can
be seen in the living plant, but by treatment with chromic acid and
staining, numerous very small nuclei may be demonstrated.

[Illustration: FIG. 21.--_A_, _C_, successive stages in the
development of the sexual organs of a green felt (_Vaucheria_). _an._
antheridium. _og._ ooegonium. _D_, a ripe ooegonium. _E_, the same after
it has opened. _o_, the egg cell. _F_, a ripe spore. _G_, a species in
which the sexual organs are borne separately on the main filament.
_A_, _F_, x 150. _G_, x 50. _cl._ chloroplasts. _ol._ oil.]

When the filaments are growing upon the ground, or at the bottom of
shallow water, the lower end is colorless, and forms a more or less
branching root-like structure, fastening it to the earth. These
rootlets, like the rest of the filament, are undivided by walls.

One of the commonest and at the same time most characteristic
species is _Vaucheria racemosa_ (Fig. 21, _A_, _F_). The plant
multiplies non-sexually by branches pinched off by a constriction at
the point where they join the main filament, or by the filament
itself becoming constricted a