XXI 
By this system, plants are grouped into classes and orders ac¬ 
cording 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 sub¬ 
class, called Thalamiflorce, 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, 
Calycijiorce, consists of those having the stamens perigynous or 
epigynous, or placed upon the calyx or the ovary. The third, 
Corolliflorce, includes those with the petals united, and the stamens 
either hypogynous or joined to the petals. The fourth, Mono- 
chlamydece, 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 peri¬ 
anth, or floral envelope, more or less perfect, Floridese, and those 
whose flowers are enclosed only in scaly bracts or glumes, Glumiferse, 
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, Dictyogense; 
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 re- 
