=928. Aquatic plant societies.=—In general we might distinguish three kinds, 1st. _Fresh-water plant societies_, with floating algæ like spirogyra, œdogonium, etc., the floating duck-meats, riccias; the plants of the lily type with roots and stems attached to the bottom and leaves floating on the surface, like the water-lily and certain pondweeds, and finally the completely submerged ones like certain pondweeds, the bassweed (Chara), etc. 2d. _Marine plant societies_, which are made up mostly of the red and brown algæ or “seaweeds,” though some green algæ and flowering plants also occur. 3d. _The salt marshes_ where the water is brackish and there is usually a luxuriant growth of marsh-grasses. FOOTNOTES: [47] For a fuller discussion of this subject by the author see Chapters XLVI-LVII of his “College Text-book of Botany” (Henry Holt & Co.). [48] =οῖκος= = house, and =λόγος= = discourse. [49] Term used by Schimper. [50] See the author’s “College Text-book of Botany.” Chapter XLIX. [51] =ἔδαφος= = ground. [52] For a full discussion of forest societies see Chapter L in the author’s “College Text-book of Botany.” [53] See Chapter LIV of the author’s “College Text-book of Botany.” CHAPTER XLVII. CLASSIFICATION OF THE ANGIOSPERMS. Relation of Species, Genus, Family, Order, etc. =929. Species.=—It is not necessary for one to be a botanist in order to recognize, during a stroll in the woods where the trillium is flowering, that there are many individual plants very like each other. They may vary in size, and the parts may differ a little in form. When the flowers first open they are usually white, and in age they generally become pinkish. In some individuals they are pinkish when they first open. Even with these variations, which are trifling in comparison with the points of close agreement, we recognize the individuals to be of the _same kind_, just as we recognize the corn plants, grown from the seed of an ear of corn, as of the same kind. Individuals of the same kind, in this sense, form a _species_. The white wake-robin, then, is a species. But there are other trilliums which differ greatly from this one. The purple trillium (T. erectum) shown in fig. 495 is very different from it. So are a number of others. But the purple trillium is a species. It is made up of individuals variable, yet very like one another, more so than any one of them is like the white wake-robin.