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Among the vegetable remains of the Lias, several species of Zamia have been found at Lyme Regis, and the remains of coniferous plants at Whitby. M. Ad. Brongniart enumerates forty-seven liassic acrogens, most of them ferns; and fifty gymnosperms, of which thirty-nine are cycads, and eleven conifers.
It belongs to the genus MARSILLEA, order MARSILLEACEAE, and that class of sexual or flowerless plants called Acrogens, which have distinguishable stems and leaves, in contra-distinction to THALLOGENS, in which stems and leaves are indistinguishable, as sea-weeds, fungi, and lichens.
Angiosperms are again divided into the monocotyledons, as the palms, and dicotyledons, which include most European trees. Thus: Adolphe Brongniart termed the coal era the "Age of Acrogens," because, as we shall see, of the great predominance in those times of vascular cryptogamic plants, known in Dr Lindley's nomenclature as "Acrogens." Spenophyllum cuneifolium.
Cryptogamous acrogens: Acrogens: Mosses, equisetums, ferns, lycopodiums Lepidodendra. Dicotyledonous gymnosperms: Gymnogens: Conifers and Cycads. Dicotyledonous Angiosperms: Exogens: Compositae, leguminosae, umbelliferae, cruciferae, heaths, etc. All native European trees except conifers. Monocotyledons: Endogens.
In the vegetal kingdom we pass from the Thallogens, consisting of mere masses of similar cells, to the Acrogens, in which the cells are not similar throughout the whole mass; but are here aggregated into a structure serving as leaf and there into a structure serving as root; thus forming a whole in which there is a certain subdivision of functions among the units, and therefore a certain mutual dependence.
He does not pretend to trace an exact historical series from the sea-weed to the fern, or from the fern again to the conifers and cycads, and lastly from those families to the palms and oaks, but he, nevertheless, points out that the cryptogamic forms, especially the acrogens, predominate among the fossils of the primary formations, the Carboniferous especially, while the gymnosperms or coniferous and cycadeous plants abound in all the strata, from the Trias to the Wealden inclusive; and lastly, the more highly developed angiosperms, both monocotyledonous and dicotyledonous, do not become abundant until the Tertiary period.
The coal-period was called by Adolphe Brongniart the age of Acrogens, so great appears to have been the numerical preponderance of flowerless or cryptogamic plants of the families of ferns, club-mosses, and horse-tails.
Naturally, too, we need not expect them in plants having modes of growth which early produce an outer practically dead part, that effectually shields the inner actively living part of the stem from the influence of the medium long-lived acrogens such as tree-ferns and long-lived endogens such as palms.
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