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xxi By this system, plants are grouped into classes and orders according to certain structural characters, more or less definite, and of a similar nature to those upon which the primary divisions of genera and species are made. The plants thus connected by structure are usually found to possess a resemblance in general appearance or habit, and not unfrequently in properties; so that the term natural, commonly applied to this mode of arrangement, is, in a relative sense, not altogether inapplicable.

Flowering plants, by this plan of classification, are primarily divided into Exogens and Endogens,—a distinction founded upon peculiarities of the stem and seed which have been already explained. Exogens are grouped into four subclasses, according to the relative position of the parts of the flower. The first subclass, called Thalamifloræ, contains those groups of plants which have the petals distinct and the stamens hypogynous, or inserted beneath the pistil upon the thalamus or receptacle. The second, Calycifloræ, consists of those having the stamens perigynous or epigynous, or placed upon the calyx or the ovary. The third, Corollifloræ, includes those with the petals united, and the stamens either hypogynous or joined to the petals. The fourth, Monochlamydeæ, is formed of those exogenous plants having only one floral envelope, and of those having neither calyx nor corolla.

The Endogens have been variously divided by botanists; but they present at least two well-marked sections—those possessing a perianth, or floral envelope, more or less perfect, Florideæ, and those whose flowers are enclosed only in scaly bracts or glumes, Glumiferæ, a division including the two great families of Grasses and Carexes. Some botanists separate certain Endogens with net-veined leaves and wood of a peculiar structure into a third group, Dictyogenæ; these latter appear to form a connecting link between Exogens and Endogens.

These subclasses are divided into Natural Orders, composed of certain genera, or assemblages of species which bear a general