This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
The Plant Kingdom is composed of four subkingdoms, divisions or primary groups:
1. Thallophyta, the Algae, Fungi and Lichens.
2. Bryophyta, the Mosses and Moss-allies.
3. Pteridophyta, the Ferns and Fern-allies.
4. Spermatophyta, the Seed-bearing plants.
In addition to these main ranks, subordinate ones are sometimes employed, when closer grouping is desirable: thus a Class may be separated into Subclasses, as the Class Angio-spermae into the Sublasses Monocotyledones and Dicotyledones; Families may be separated into Tribes, as in the treatment of Gramineae in the following pages; Genera are often separated into Subgenera; Species into Subspecies.
Critical field observations of plants in the wild state, supplemented by the cultivation side by side of species supposed to be distinct and by the lessons learned from experimental plant breeding, have developed the theory that many species, perhaps all, are composed of a greater or lesser number of races, differing from each other too little to cause them to be regarded as species, notwithstanding the fact that they may breed true from seed to such slight or trivial differentiations. It also seems to have been proved, by DeVries and others, that such differentiations may originate abruptly from seed, in a single generation, and remain constant for at least several generations thereafter if. so isolated from their relatives as to prevent cross-pollination. These recently ascertained phenomena of mutation are most suggestive, and experimentation and observation concerning them are now occupying the attention of many students.
In the present edition of "Illustrated Flora," the view is taken that the races composing many species are often too numerous and too slightly characterized to be described so as to be recognized; many of them have been described as species and many more as varieties, and varieties of different degrees of differentation have been suggested. We here regard species alone as entitled to distinct botanical appelation; it has been suggested that races may be indicated numerically.
Other than the omission of descriptions of varieties, the general system of classification used in the first edition has been maintained in the second. A few new family groups and a number of genera have been separated or distinguished from their congeners.
The grouping of Races into Species, of Species into Genera, and of Genera into Families, though based upon natural characters and relationships, is not governed by any definite rule that can be drawn from nature for determining just what characters shall be sufficient to constitute a Species, a Genus or a Family. These groups are, therefore, necessarily more or less arbitrary and depend upon the judgment of scientific experts, in which natural characters and affinities, as the most important and fundamental factors, do not necessarily exclude considerations of scientific convenience. The practice among the most approved authors has accordingly been various. Some have made the number of genera and families as few as possible. This results in associating under one name species or genera that present marked differences among themselves. The present tendency of expert opinion is to separate more freely into convenient natural groups, according to similarity of structure, habit, form or appearance. While this somewhat increases the number of these divisions, it has the distinct advantage of decreasing the size of the groups, and thus materially facilitates their study. This view has been taken in the present work, following in most instances, but not in all, the arrangement adopted by Engler and Prantl in their great work, " Naturliche Pflanzen-familien," in which nearly all known genera are described.