IV. SPERMATOPHYTES should probably be di vided into two primary groups, although at pres ent they are treated as one. The two great di visions, gymnosperms and angiosperms, differ more in essential features from one another than does the former group from pteridophytes, but they are held together at present by the common character of seed production. The existing gymnosperms are grouped as follows: Gnetales.
Gymnosperm te=cfgeroaaress ies). Cycadales (cycads).
The angiosperms comprise a vast assemblage of forms that are easily separated into two great series, monocotyledons and dicotyledons, but whose further division is at present in a some what chaotic state. Most of the subdivisions heretofore suggested are confessedly artificial, and probably as far as one may go safely with natural groups is as follows: 1 Dien tyled ones svmpetairc.
A' re rehichlamyde.
Monocotyledones The unit of classification used by taxonomists is the species, a group very difficult to define, but understood in a general way. The species always bears two names, as Quercus alba, the systema tist's name for white oak, 'alba' indicating the species, and `Quercus' the genus to which the species belongs. In some cases forms of a spe cies may be distinct enough to be characterized, and are called varieties, being designated by adding a third name to the species binomial. The next higher taxonomic group is the genus, which comprises one or more species. For ex ample, `Quercus' is the oak genus, containing one or many species. The next higher taxonomic category is the family, which comprises one or more genera, and is indicated except in a few ex ceptional cases by the common termination `nem,' as `Rosaceie,' the rose family. The next higher category is the order, the form of whose designation is not so fixed as that of the family, but which is increasingly indicated by the ter mination 'ales,' as 'Coniferales.' Still higher categories are often employed, but the usage is so variable that nothing definite can be stated in reference to them. All of these categories have their intermediate subdivisions, which general usage has in the main established. For example, a genus may have its species grouped into sub genera; a family may be broken up into tribes, each containing its own genera; and an order often has its The literature of taxonomic botany is vast in extent, and a complete list of even the most im portant works cannot be cited. No work as yet
contains a systematic presentation of all the known species of plants. The most extensive current works are as follows: Engler and Prana, Die natiirlichen I'flurnzcufaniiliea (Leipzig), completed in four sections, each containing nu merous parts, and including all known genera; Bentham and Hooker, (hnera Plantarum (Lon don, 1863-83), including all known genera of flowering plants; Engler, Das Pflanzenreich (Leipzig), a few parts of which have appeared, and which is planned to be completed in twenty years, including descriptions of all known species of plants.
Each country has its own manuals containing descriptions of its flora. The current manuals dealing with the flowering plants and in some cases the fern plants of the United States are as follows: Gray, Manual of Botany (6th edition, revised by Watson and Coulter, New York, 1890), including Northeastern United States; Britton and Brown, Illustrated Flora of the Northern United States and Canada- (ib., 1896 98) ; Britton, Manual (ib., 1901), including the same range; Chapman, Flora of the Southern United States (3d edition, Cambridge, Mass., 1897) ; Stuatig Flora of the Southeastern States (New York, 1903) ; Coulter, Manual of Rocky Mountain Botany (New York, 1885) ; id., Botany of Western Texas (Washington, Brew er, Watson, and Gray, Botany of California (Cambridge, Mass., 1876 and 1880) ; Greene, Manual of the Bay-Region, Botany (San Fran cisco, 1894), including region south of Maryland and Kentucky west to 100th meridian; Howell, A Flora of Northwest America, not yet complete (Portland, Oregon, 1897—) ; Rydberg, Flora of Montana (New York, 1900). The only at tempt to include in a single work the whole flora of North America is Gray's Synoytieal Flora of North America. incomplete (1878-95). For pteridophytes (fern-plants) of the United States, consult: Underwood, Our Native Ferns and Their Allies (New York, 1888) ; Clute, Our Ferns in Their Haunts (New York, 1901). For mosses of the United States, see Lesquereux and James, Mosses of North America (1884; out of print) ; Barnes, Keys to North American Mosses (Madison, Wis., 1896) ; Grout, Mosses with a Hand Lens (Brooklyn, 1900; revised and en larged, 1903). For the important taxonomic lit erature of alga and fungi, see articles on the principal groups.