THE 
NATURAL ORDERS OF PLANTS. 
Class I. EXOGENS ; or DICOTYLEDONS. 
Dicotyledones, Juss. Gen. 70 . ( 1789 ) ; Desf. Mem. Inst. 1 . 478 . ( 1796 ). — ExoRHizEi® 
and SynorhizejE, Rich. Anal. ( 1808 .) — Dicotyledones or Exogens, DC. Theor. 
p. 209 . ( 1813 .) — Phanerocotyledones or Seminifers, A^ardh. Aph. 74 . ( 1821 ). 
Essential Character. Elementary organs consisting of both cellular and vascular 
tissue, a portion of the latter being elastic spiral vessels. Cuticle with stomates. Trunk 
more or less conical, consisting of three parts, one within the other ; viz. bark, wood, and 
pith, of which the wood is enclosed between the two others ; increasing by an annual deposit 
of new wood and cortical matter between the wood and bark. Leaves always articulated 
with the stem, often opposite, their veins, if present, composed in part at least of spiral 
vessels, and branching and reticulated. Flowers, if with a distinct calyx, often having a 
quinary division. Propagation effected by the agency of stamens and pistils, which are 
analogous to the sexes of animals. Ovules always enclosed before fertilization in a pericarp, 
and fertilized by the action of pollen upon a stigma ; finally becoming seeds, containing an 
embryo with two or more opposite cotyledons, which often become green and leaf-like after 
germination ; radicle naked, i. e. elongating into a root without penetrating any external 
case. 
The plants belonging to this class constitute by far the most considerable 
portion of the Vegetable Kingdom ; and they may be considered to be in some 
respects the most highly developed ; not that they possess any organs which 
are not found elsewhere, but because of the much greater diversity of combi- 
nations into which their organs enter, and because of the more complicated 
nature of their woody and venous systems. While Gymnosperms have no 
vasiform cellular tissue {See Introduction to Botany, 2d Edit. p. 15), only a small 
supply of spiral vessels, and but an imperfectly constructed sexual apparatus. 
Exogens have an abundance of both these elementary organs, and their parts of 
reproduction are in the most complete condition. Endogens on the other hand, 
in which the latter are equally perfect, have their woody system arranged in a 
confused manner, and not disposed in the symmetrical way which is characte- 
ristic of Exogens. As to Rhizanths, the very imperfect state of their vas- 
cular system and reproductive organs places them lower down in the scale of 
structure than either of the others ; while the total absence of sexes, the 
general want of a vascular system, except in such highly developed orders as 
the Filical and Lycopodal fiances, the gradual loss in the lowest tribes of 
even symmetrical form, till at last in the simplest forms of Algacese and Fun- 
gacese the very elementary parts are disintegrated, reduce Acrogens to little 
more than a mere vesicular state of existence, and place the vegetable king- 
dom not only in contact with the microscopic animalcules of the animal world, 
but even bring it to the limits of inorganic matter. 
Practically their reticulated leaves distinctly articulated wdth the stem, 
usually distinguish Exogens from Endogens, from which they are also known 
by the following points ; Exogens have a distinct deposition of pith, wood, 
and bark ; Endogens have aU these parts confounded : Exogens, if trees, are 
conical and branched (as an Oak); Endogens are cylindrical and simple- 
stemmed (as a Palm). Besides which, the following characters deserve atten- 
tion : Exogens in germination protrude their radicle at once ; while in Endo- 
gens it is contained within the substance of the embrvo, through which it 
B 
