On the Structure and Affinities of
Sigillaria, Calamites
, and
Calamodendron
Authors: Dawson J. W. 
Published: October 2009

Abstract

The object of this paper was to illustrate the structure and affinities of the genera above named, more especially with reference to the author's previous papers on the “Structures in Coal” and the “Conditions of Accumulation of Coal,” and to furnish new facts and conclusions as to the affinities of these plants. With reference to
Sigillaria
, a remarkably perfect specimen of the axis of a plant of this genus, from the Coal-field of Nova Scotia, was described as having a transversely laminated pith of the
Sternbergia
type, a cylinder of woody tissue, scalariform internally and reticulated or discigerous externally, the tissues much resembling those of Cycads. Medullary rays were apparent in this cylinder; and it was traversed by obliquely radiating bundles of scalariform vessels or fibres proceeding to the leaves. Other specimens were adduced to show that the species having this kind of axis had a thick outer bark of elongated or prosenchymatous cells. The author stated that Prof. Williamson had enabled him to examine stems found in the Lancashire Coal-field, of the type of Binney's
Sigillaria vascularis
, which differed in some important points of structure from his specimens; and that another specimen, externally marked like
Sigillaria
, had been shown by Mr. Carruthers to be more akin to
Lepidodendron
in structure. These specimens, as well as the
Sigillaria elegans
illustrated by Brongniart, probably represented other types of Sigillarioid trees; and it is not improbable that the genus Sigillaria, as usually understood, really includes several distinct generic forms. The author had recognized six generic forms in a previous paper and in his ‘Acadian Geology;’ but the type described in the present paper was that which appeared to predominate in the fossil Sigillarian forests of Nova Scotia, and also in the mineral charcoal of the coal-beds. This was illustrated by descriptions of structures occurring in erect and prostrate
Sigillariæ
, on the surface of
Sternbergia
-casts, and in the coal itself.

The erect Calamites of the coal formation of Nova Scotia illustrate in a remarkable manner the exterior surface of the stems of these plants, their foliage, their rhizomata, their roots, and their habit of growth. Their affinities were evidently with Equisetaceæ as Brongniart and others had maintained, and as Carruthers and Schimper had recently illustrated. The internal structure of these plants, as shown by some specimens collected by Mr. Butterworth, of Manchester, and soon to be published by Prof. Williamson, showed that the stems were more advanced in structure than those of modern
Equiseta
, and enabled the author to explain the various appearances presented by these plants, when the external surface is preserved, wholly or in part, and when a cast of the internal cavity alone remains. It was further shown that the leaves of the ordinary Calamites are linear, angular, and transversely wrinkled, and different from those of
Asterophyllites
properly so called, though some species, as
A. comosus
, Lindley, are leaves of Calamites.

The
Calamodendra
, as described by Cotta, Binney, and others, and further illustrated by specimens from Nova Scotia, and by several interesting and undescribed forms in the collection of Prof. Williamson, are similar in general plan of structure to the Calamites, but much more woody plants; and, if allied to Equisetaceæ, are greatly more advanced in the structure of the stem than the modern representatives of that order. Specimens in the collection of Prof. Williamson show forms intermediate between
Calamites
and
Calamadendron
, so that possibly both may be included in one family; but much farther information on this subject is required. The tissues of the higher
Calamodendra
are similar to those of Gymnospermous plants. The wood or vascular matter of the thin-walled Calamites consists of multiporous cells or vessels, in such species as have been examined.

In conclusion, a Table was exhibited showing the affinities of
Sigillariæ
, on the one hand (through
Clathraria
and
Syringodendron
) with Lycopodiaceæ, and on the other hand (through
Calamodendron
) with Equisetaceæ; while in another direction they presented links of connexion with Cycads and Conifers.