The vegetable kingdom is thus separated into three natural classes,—1, the Exogens, 2, the Endogens, 3, the Acrogens; but there are likewise other divisions, a knowledge 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 intervention 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 comprise the two natural orders Coniferæ, and Cycadaceæ.
The Gymnosperms also possess peculiarities of a subordinate 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 embryo lies, hence they are sometimes named Synorhiza. The veins of the leaves (in those whose leaves are veined), are either simple or forked; in which respect they approach the endogens on the one hand, and the acrogens on the other.
This concise definition of the natural divisions of the vegetable kingdom will enable the reader to comprehend the botanical principles which must guide him in his attempt to explore the ancient floras, whose fossil remains are generally found in a very fragmentary condition.
I need only add that M. Ad. Brongniart, in his great work on Fossil Plants, arranges the vegetable kingdom into five classes, viz.:—
1. Cellular Cryptogamia,[36] or Amphigens.
2. Vascular Cryptogamia,[37] or Acrogens.
3. Monocotyledons.[38]
4. Gymnospermous Dicotyledons.[39]
5. Angiospermous Dicotyledons.[40]