Rock fern, polypody. Notice the underground stem giving off roots from its lower surface, and leaves (C), (S), from its upper surface.
Species are grouped together in a larger group called a genus. For example, many kinds of peas—the wild beach peas, the sweet peas, and many others—are all grouped in one genus (called Lathyrus, or vetchling) because they have certain structural characteristics in common.
Plant and animal genera are brought together in still larger groups, the classification based on general likenesses in structure. Such groups are called, as they become successively larger, Family, Order, and Class. Thus both the plant and animal kingdoms are grouped into divisions, the smallest of which contains individuals very much alike; and the largest of which contains very many groups of individuals, the groups having some characters in common. This is called a system of classification.
Classification of the Plant Kingdom.—The entire plant kingdom has been divided into four sub-kingdoms by botanists:—
| 1. Spermatophytes. | Angiosperms, true flowering plants. Gymnosperms, the pines and their allies. |
| 2. Pteridophytes. | The fern plants and their allies. |
| 3. Bryophytes. | The moss plants and their allies. |
| 4. Thallophytes. | The Thallophytes form two groups: the Algæ and the Fungi; the algæ being green, while the fungi have no chlorophyll. |
Rockweed, a brown algæ, showing its distribution on rocks below highwater mark.
The extent of the plant kingdom can only be hinted at; each year new species are added to the lists. There are about 110,000 species of flowering plants and nearly as many flowerless plants. The latter consist of over 3500 species of fernlike plants, some 16,500 species of mosses, over 5600 lichens (plants consisting of a partnership between algæ and fungi), approximately 55,000 species of fungi, and about 16,000 species of algæ.
A moss plant. G, the moss body; S, the spore-bearing stalk (fruiting body).