DOM II.

ALGAE.[3]

[3] Algae (sing. _alga_).

In the second sub-kingdom of plants is embraced an enormous assemblage
of plants, differing widely in size and complexity, and yet showing a
sufficiently complete gradation from the lowest to the highest as to
make it impracticable to make more than one sub-kingdom to include
them. They are nearly all aquatic forms, although many of them will
survive long periods of drying, such forms occurring on moist earth,
rocks, or the trunks of trees, but only growing when there is a
plentiful supply of water.

All of them possess chlorophyll, which, however, in many forms, is
hidden by the presence of a brown or red pigment. They are ordinarily
divided into three classes--I. The Green Algae (_Chlorophyceae_);
II. Brown Algae (_Phaeophyceae_); III. Red Algae (_Rhodophyceae_).

CLASS I.--GREEN ALGAE.

The green algae are to be found almost everywhere where there is
moisture, but are especially abundant in sluggish or stagnant fresh
water, being much less common in salt water. They are for the most
part plants of simple structure, many being unicellular, and very few
of them plants of large size.

We may recognize five well-marked orders of the green algae--I. Green
slimes (_Protococcaceae_); II. _Confervaceae_; III. Pond scums
(_Conjugatae_); IV. _Siphoneae_; V. Stone-worts (_Characeae_).

ORDER I.--_Protococcaceae_.

The members of this order are minute unicellular plants, growing
either in water or on the damp surfaces of stones, tree trunks, etc.
The plants sometimes grow isolated, but usually the cells are united
more or less regularly into colonies.

A common representative of the order is the common green slime,
_Protococcus_ (Fig. 11, _A_, _C_), which forms a dark green slimy
coating over stones, tree trunks, flower pots, etc. Owing to their
minute size the structure can only be made out with the microscope.

[Illustration: FIG. 11.--_Protococcaceae._ _A_, _C_, Protococcus. _A_,
single cells. _B_, cells dividing by fission. _C_, successive steps in
the process of internal cell division. In _C_ iv, the young cells have
mostly become free. _D_, a full-grown colony of _Pediastrum_. _E_, a
young colony still surrounded by the membrane of the mother cell. _F_,
_Scenedesmus_. All, x 300. _G_, small portion of a young colony of the
water net (_Hydrodictyon_), x 150.]

Scraping off a little of the material mentioned into a drop of water
upon a slide, and carefu