60 THE MEDALS OF CREATION. Cuap. IV. 
processes ; while endogens do not possess such a structure, 
having no central pith. In exogens the veins (venation) 
of the leaves, are disposed in meshes, like net-work, but in 
endogens the veins run parallel to each other. 
The number of parts in the flower of an exogenous plant 
is usually five, or its multiples: in the endogens it is com- 
monly three, or its multiples. In the germination, the 
young root of exogens is a mere extension of the radicle ; but 
in endogens it is protruded from within. 
Thus, in the flowering or vascular plants, we have two 
groups distinct from each other in their germination, the 
structure of their stems and leaves, their mode of growth, 
the arrangement of the parts of the flower, and in the 
structure of the embryo. 
The vegetable kingdom is thus separated into three 
natural classes,—1l, the Hxogens, 2, the Endogens, 3, the 
Acrogens ; but there are likewise other divisions, a know- 
ledge of which is of great importance in the study of 
fossil botany ; the sub-class termed GyMNOSPERMS especially 
requires notice. 
In the strictly exogenous and endogenous plants, the 
fertilizing principle is communicated to the young seeds 
through the medium of a stigma and style, that terminate 
the case or pericarp in which the seeds are enclosed: but 
in another important group of the vegetable kingdom, the 
pollen is directly applied to the ovule, without the interven- 
tion of any pericarpial apparatus ; hence these are termed 
Gymnosperms, signifying naked seeds. These plants have 
the same relation to the other exogens, as frogs and analogous 
reptiles bear to the other orders of their Class; they com- 
prise the two natural orders Conifer, and Cycadacee. 
The Gymnosperms also possess peculiarities of a subor- 
dinate nature: thus, many kinds have more than two 
cotyledons, and are therefore termed polycotyledons ; again, 
the radicle usually adheres to the albumen in which the 
