Page:Popular Science Monthly Volume 20.djvu/801

Rh Before entering upon the account of his studies, Brongniart describes, in an introductory chapter, the periods of vegetation and the different floras that have succeeded each other on the earth's surface. "We may consider," he says, "as having been deposited during a same epoch of the creation of the vegetable kingdom, and as belonging to

 —1, Cardiocarpus drupaceus. Section through the plane of the keel. a, thick testa; b, chalaza; c, kernel and albumen; d, micropylar extremity of the kernel (natural size). 2, Rhabdocarpus conicus. Grand' Eury. General form from a specimen that was broken before preparation (natural size). 3, Trigonocarpus pusilus (natural size). 4, longitudinal section passing through the chalaza and the micropyle (magnified five times); a, testa; b, chalaza: c, micropyle; d, kernel; e, cavity on the summit of the kernel, with grains of pollen. 5, Polypterospermum Renaultii, transverse section (natural size). 6. Codonospermum anomolum, external view, as it appears in the broken rock.

a same ancient flora, the different beds in which we find the same collection of species, and during the deposition of which some at least of these species have persisted from the beginning to the end of the local phenomena.

"This is what constitutes an epoch in the geological study of vegetable fossils; but several of these epochs often succeed one another, all preserving a considerable number of common characters in the nature and relative proportion of the principal families that belong to them; and this succession of analogous epochs forms a period in the history of the successive development of the vegetable kingdom."

The successive creation of different vegetable forms is thus divided into three long periods, called the reign of the Acrogens, the reign of the Gymnosperms, and the reign of the Angiosperms—"expressions indicating only the successive predominance of one or another of these three grand divisions of the vegetable kingdom, without