th. Each is a colorless, spiral thread with about three coils, one
end being somewhat dilated with a few granules; the other more
pointed, and bearing two extremely long and delicate cilia (_K_). To
see the cilia it is necessary to kill the spermatozoids with iodine
or some other reagent.

After fertilization the outer cells of the ooegonium become very
hard, and the whole falls off, germinating after a sufficient period
of rest.

According to the accounts of Pringsheim and others, the young plant
consists at first of a row of elongated cells, upon which a bud is
formed that develops into the perfect plant.

There are two families of the _Characeae_, the _Chareae_, of which
_Chara_ is the type, and the _Nitelleae_, represented by various
species of _Nitella_ and _Tolypella_. The second family have the
internodes without any cortex--that is, consisting of a single long
cell; and the crown at the top of the ooegonium is composed of ten
cells instead of five. They are also destitute of the limy coating of
the _Chareae_.

Both as regards the structure of the plant itself, as well as the
reproductive organs, especially the very complex antheridium, the
_Characeae_ are very widely separated from any other group of plants,
either above or below them.

CHAPTER VI.

THE BROWN ALGAE (_Phaeophyceae_).

[Illustration: FIG. 24.--Forms of diatoms. _A_, _Pinnularia_. i, seen
from above; ii, from the side. _B_, _Fragillaria_ (?). _C_,
_Navicula_. _D_, _F_, _Eunotia_. _E_, _Gomphonema_. _G_, _Cocconeis_.
_H_, _Diatoma_. All x 300.]

These plants are all characterized by the presence of a brown pigment,
in addition to the chlorophyll, which almost entirely conceals the
latter, giving the plants a brownish color, ranging from a light
yellowish brown to nearly black. One order of plants that possibly
belongs here (_Diatomaceae_) are single celled, but the others are for
the most part large seaweeds. The diatoms, which are placed in this
class simply on account of the color, are probably not closely related
to the other brown algae, but just where they should be placed is
difficult to say. In some respects they approach quite closely the
desmids, and are not infrequently regarded as related to them. They
are among the commonest of organisms occurring everywhere in stagnant
and running water, both fresh and salt, forming usually, slimy,
yellowish coatings on stones, mud, aquatic plants, etc. Like the
desmids