In 1820 this generic name was applied by Sternberg[656] to some specimens of branches bearing verticils of linear leaves. In 1828 Brongniart[657] thus defined the genus Annularia:—“Slender stem, articulated, with opposite branches arising above the leaves. Leaves verticillate, flat, frequently obtuse, traversed by a single vein, fused basally and of unequal length.”
In the works of earlier writers we find frequent illustrations of specimens of Annularia, which are compared with Asters and other recent flowering plants. Lehmann[658] contributed a paper to the Royal Academy of Berlin in 1756, in which he referred to certain fossil plants as probable examples of flowers, among them being a specimen of Annularia. He refers to the occurrence of fossil ferns and other plants, and asks why we do not find flowers of the rose or tulip; his object being “not to acquire vain glory, but to give occasion for others to look into the matter more clearly.”
The general habit of the fossils which are now included under Annularia agrees closely with that of Calamocladus. There is the same spreading form and a similar foliage in the two genera, but in Annularia the members of a whorl are always fused into a basal sheath, and the segments are not of equal length. We may thus summarise the characteristic features of the genus:—
Opposite branches are given off in one plane from the nodes of a main axis; the leaves are in the form of narrow sheaths divided into numerous and unequal linear or narrow lanceolate segments, each with a median vein. The segments in each whorl appear to be spread out in one plane very oblique to the axis of a branch, instead of spreading radially in all directions; the lateral segments are usually longer than the upper and lower members of a whorl. The vegetative branches possess the same type of structure as Calamites.
A comparison of Annularia and Phyllotheca has already been made in Chapter IX. (p. 282). Potonié[659] has recently given a detailed account of Annularian leaves; he compares them with those of Equisetum, and describes the occurrence on the lamina of each leaf-segment of a broad central band or midrib, with a groove, probably containing stomata, on either side. He shows that in well-preserved specimens of Annularia, it is possible to recognise certain minute surface-features, such as the presence of hairs and stomata, which enable one to detect a close resemblance between the leaves of Calamite stems and those of Annularian shoots.
It is not always easy to distinguish between Annularia and Calamocladus; the collar-like basal sheath in the leaves of the former is a characteristic feature, but that cannot always be recognised. On the other hand, the leaves of Calamocladus may sometimes be flattened out on the surface of the rock and simulate the deeply cut sheaths of Annularia. It is difficult to decide how far the manner of occurrence of Annularian leaves in one plane, which is commonly insisted on as a generic character, is an original feature, or how far it is the result of compression in fossilisation. Probably the leaves of a living Annularia were spread out at right angles to the axis, as in the ‘verticils’ of such a plant as Galium.
Dawson[660] has described some fossils from the Devonian rocks of Canada as species of Asterophyllites; the figures bear a closer resemblance to the genus Annularia. The same author figures some irregularly whorled impressions as Protannularia, which appear to be identical with a fossil described by Nicholson[661] from the Skiddaw slates (Ordovician) of Cumberland as Buthotrephis radiata, but the specimens are too imperfect to admit of accurate determination.
Annularia stellata (Schloth.). Fig. 88.
| 1820. | Casuarinites stellatus, Schlotheim[662]. |
| 1826. | Bornia stellata, Sternberg[663]. |
| 1828. | Annularia longifolia, Brongniart[664]. |
| 1834. | Asterophyllites equisetiformis, Lindley and Hutton[665]. |
| 1868. | Asterophyllites longifolius, Binney[666]. |
| 1887. | Annularia Geinitzi, Stur[667]. |
| 1887. | Annularia westphalica, Stur. |
This species was figured by Scheuchzer[668] in his Herbarium Diluvianum, and compared by him with a species of Galium (Bedstraw). Brongniart first made use of the generic name Annularia for this common Coal-Measure species, which may be defined as follows:—