Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/342

 2. On the Structure and Affinities of Sigillaria and allied Genera. By W. Carruthers.

[Plate X.]

The genus Sigillaria is one of the most abundant and important among the plants of the Carboniferous period. But, except that it was a large tree with a fluted and scarred stem, and had immense succulent roots, little is certainly known regarding it. The structure of the root is well known, having frequently formed the subject of investigation. Numerous specimens of stems have been published, as examples of the genus ; but with the exception of that described by Brongniart under the name of Sigillaria elegans, they belong rather to the genus Lepidodendron ; and even S. elegans does not represent the most characteristic form of the genus. The stem was most probably simple, though in some species it was certainly branched. The foliage in some forms was composed of long linear leaves ; and Goldenberg has figured organs of reproduction in the form of small round sporangia, several of which were borne on the somewhat dilated bases of the leaves at the termination of a branch.

The affinities of the genus have chiefly been deduced from the structure of the stem and roots ; and in accordance with the views taken by the different interpreters has been the place they have given it. Some, like Corda and Martius, have held it to be a polypetalous dicotyledon, while, on the other hand, Dr. Hooker, with a little hesitancy, places it among Lycopodiaceoe beside Lepidodrendron. The generally received opinion is somewhat intermediate, that it is a gymnosperm belonging to an extinct type. Brongniart first enunciated this view ; and it has been subsequently maintained by Goppert, Unger, &c., and recently by Dawson in an important and elaborate paper on the Coal Formation of Nova Scotia, published in the Journal of this Society in 1866, and two years later in his 'Acadian Geology.'

I propose to investigate shortly the grounds upon which these eminent palaeontologists have given this position to the genus, and how far their interpretations agree with the actual structure of the plant.

I shall first consider the structure of the root, the relation of which to the stem has been established beyond question. The investigations of Mr. Binney on the external form and of Prof. King and Dr. Hooker on the internal structure make it unnecessary that I should describe at any length the root so well known by the name of Stigmaria. The general structure may be given in a few words. It was a thick root having a medulla of considerable size, surrounded by a cylinder of scalariform vessels ; and this again was invested by a large mass of cortical parenchyma. From numerous pits on the outer surface of this cortical layer proceeded the long cellular rootlets, each furnished with a small bundle of firm scalariform vessels that had its origin in the vascular cylinder. In the arrangement of the parts, and in the general aspect of a transverse section of the stem, it agrees with the structure of the stems of Cycadeoe, and of the fleshy Euphorbiaceoe and Cacteoe. It is unnecessary to say that