Popular Science Monthly/Volume 20/April 1882/Fossil Seeds
| FOSSIL SEEDS.[1] |
By STANISLAS MEUNIER.
THE attention of Adolphe Brongniart was for a long time given to the silicified fossil seeds which are inclosed in the beds of Autun and Saint-Étienne. The illustrious naturalist found the study a virgin domain, and an appropriate complement to his great labors on fossil plants. Although our knowledge in this department is still very far from complete, Brongniart was able to lay the foundation of a classification of these interesting remains; and it gives us pleasure, as much in the historical point of view as in its bearing on botany, to give a succinct idea of it here.
The seeds which Brongniart examined are divided into two principal groups:
A. Seeds with a binary symmetry, more or less flattened and bi-carinated. This natural group includes the genera Cardiocarpus (Fig. 1, 1), Rhabdocarpus (Fig. 1, 2), Diplotesta, Sarcotaxus, Taxospermum, and Leptocaryon, analogous to genera of the existing family of the Taxineæ.
B. Seeds with a symmetrical radiation around an axis, in which the number of the divisions varies from three, as in Pachytesta, Trigonocarpus (Fig. 1, 3), Tripterospermum, to six, as in Ptychotesta, Hexapterospermum, Polypterospermum (Fig. 1, 5), Polylophospermum or eight, as in Eriotesta, Codonospermum (Fig. 1, 6), or the section of which is circular, as in Stephanospermum and Ætheotesta. These seeds appear to represent the Sigillariæ and the calamites, and some genera of the cycads and conifers. 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
Fig. 1.—Specimens of Fossil Seeds examined by Adolphe Brongniart.—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 necessarily supposing the complete exclusion of the two others." The reign of Acrogens was manifest during the Carboniferous and Permian periods; the reign of the Gymnosperms during the Vosgian and Jurassic; and the reign of the Angiosperms during the Cretaceous and Tertiary periods.
Proceeding to the study of the fossil seeds found silicified in the coal-beds of Saint-Étienne, M. Brongniart makes a comparative review
Fig. 2.—Cardiocarpus Sclerotesta, Longitudinal Section passing through the Micropyle, the Chalaza, and the Principal Plane of the Seed (magnified three times). a, albumen, well preserved, inclosed in the embryonary sac; b, hardly visible remains of the embryonary sac; c, envelope of the kernel reduced to its epidermis; co, the two corpuscles, placed symmetrically in the plane of the seed, below the pollinical chamber; m, the pollinical chamber, but slightly developed in this group of seeds; d, endotesta; mi, micropylary canal of the testa, leading to the pollinical chamber; ch, chalazian region of the seed.
of the structure of the seed and ovule of the cycads, and of different silicified seeds of the coal-beds, and announces one of the most remarkable discoveries in fossil botany, the value of which consists in the light which the study of fossils is made to cast upon the interior anatomy of existing forms. A singular feature was observed in the organization of a considerable number of these seeds, in that there existed, near the summit of the kernel, and in the corresponding part of the micropyle of the testa (or outer integument), a cavity in the cellular tissue, containing nearly always granules or free vesicles, which could only be regarded as grains of pollen; and from the presence of which M. Brongniart was led to designate the cavity as the pollinical chamber. Nothing of the kind is known in existing gymnosperms. The cycads, however, had been previously indicated as presenting analogies with the Palæozoic plants under study; and M. Brongniart's views upon this point have received a striking confirmation from the observations of the gardener of the museum and of M. Renault. The published volume of the "Lectures on Fossil Botany" of the latter gentleman contains a carefully copied plate, showing a similar pollinical chamber in the Ceratozamia Mexicana. Figs. 2 and 3 show the grains very plainly in Cardlo carpus sclerotesta. The work, of which we have given a brief summary, "Recherches sur les Graines Fossiles Silicifiées," consists of twenty-one plates, giving accurate and exactly colored representations of the seeds examined, with careful explanations. The completion of the text was interrupted
Fig. 3.—Part of the Preceding Figure, magnified Ten Times.
by the death of the author, but the plates are finished, the details in them that were left lacking having been supplied by M. Renault, after comparison with the identical specimens. The whole work, in its present form, constitutes a real monument erected by pious hands to the memory of the illustrious founder of the science of fossil botany.
- ↑ Translated for "The Popular Science Monthly" from "La Nature."