[3] Algæ (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 Algæ (_Chlorophyceæ_); II. Brown Algæ (_Phæophyceæ_); III. Red Algæ (_Rhodophyceæ_). CLASS I.--GREEN ALGÆ. The green algæ 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 algæ--I. Green slimes (_Protococcaceæ_); II. _Confervaceæ_; III. Pond scums (_Conjugatæ_); IV. _Siphoneæ_; V. Stone-worts (_Characeæ_). ORDER I.--_Protococcaceæ_. 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.--_Protococcaceæ._ _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, × 300. _G_, small portion of a young colony of the water net (_Hydrodictyon_), × 150.]