In the modern world we divide all vegetation into two great series, that of the Flowering Plants (Phænogams), which also produce true fruits and seeds, and that of the Flowerless Plants (Cryptogams), which produce minute spores instead of seeds. The latter is in every respect the lower group. This lower series is again divisible into three classes—first and lowest, that of the Seaweeds, Moulds, and Lichens (Thallophytes). Secondly, that of the Mosses and their allies (Anophytes). Thirdly, that of the Ferns, Equisetums and Club-mosses (Acrogens). In like manner the second, or higher series is divisible into three classes: that of the Pines and Cycads (Gymnosperms), having naked seeds not covered by true fruits, and woody tissue of simple structure; that of the Palms and Grasses and their allies (Endogens); and last and highest, that of the ordinary timber trees and other plants allied to them, with exogenous stems, netted-veined leaves, and a two-leaved embryo (Exogens). These last are in every respect the dominant plants on our present continents. Carrying with us this twofold division of the vegetable kingdom and its subdivisions, we shall be prepared to understand the relation of the more ancient floras to that now living.
Fig. 88.—Trunk of a Devonian Tree-fern (Caulopteris Lockwoodi, Dn.). Gilboa, New York. One-third natural size.
Fig. 89.—Frond of Archæopteris Jacksoni (Dn.). Devonian, of Maine.
Fig. 90.—Portion of a branch of Leptophleum rhombicum (Dn.). A Lycopodiaceous tree of the Devonian of Maine. Natural size.