Bertrand compares the stele of S. elongata with that of the type of Lepidodendron represented by the Burntisland species named by Williamson L. brevifolium ([fig. 186]) and now usually referred to L. Veltheimianum; the chief distinguishing features are the greater prominence in the French species of the surface-ridges or teeth of the primary xylem, a feature which occurs in L. Wünschianum, and the detachment of the leaf-traces from the bottom of each sinus ([fig. 202], C, lt) instead of from the sides of the sinus. It is, however, not clear how far this latter distinction is a real one; in Lepidodendron Wünschianum the leaf-traces appear to arise, as in Sigillaria, from the middle of each sinus.

Other types of ribbed Sigillaria stems have been briefly described by Scott[529], Kidston[530], and more recently, by Arber and Thomas[531].

The specimen described by Scott agrees in the main with S. elegans of Kidston and with S. elongata of Bertrand.

Kidston’s sections of S. scutellata show a continuous primary xylem cylinder with a slightly and irregularly crenulate outer margin. It would seem that one important diagnostic character in Sigillarian stems is afforded by the degree and form of the crenulations on the outer surface of the primary xylem. S. scutellata has been described also by Arber and Thomas; these authors were the first to demonstrate the presence of a ligule and ligular pit on the leaf-base in a petrified stem, and they also contribute the important fact that the leaf-traces in passing through the phelloderm bifurcate and enter the leaf as two distinct vascular strands. This double bundle has been referred to in the description of Sigillaria leaves. ([page 214].)

Although our knowledge of the anatomy of Sigillaria has been considerably extended since Williamson[532] drew attention to our comparative ignorance of the subject, there are several points on which information is either lacking or very meagre. As regards the stele, it is in all types so far investigated, of the medullated type and constructed on the same plan as that of Lepidodendron Wünschianum, L. Veltheimianum, and other species. Secondary xylem was developed at an early stage of growth, and its relation to the primary xylem, from which as Kidston points out in his description of S. elegans, it may be separated by a few parenchymatous elements, is like that in Lepidodendron. The tendency of the outer face of the secondary xylem to present a crenulate appearance in transverse sections may, as Scott thinks[533], be a feature of some diagnostic importance, but this is not a constant character in the genus. In origin and in their mesarch structure, the leaf-traces closely resemble those of Lepidodendron. The earlier account of the structure of the leaf-traces of Sigillaria, which were described as possessing both centrifugal and centripetal wood, led Mettenius[534] to draw attention to an important anatomical resemblance between this genus and modern Cycads. This comparison was, however, based on a misconception; the Cycadean leaf-trace, consisting solely of primary wood, is not strictly comparable with those of some species of Sigillaria, in which one part of the xylem is primary and another secondary. The occasional presence of secondary xylem in Sigillarian leaf-traces is matched in some Lepidodendra[535], and cannot be accepted as a distinguishing feature.

The origin of the leaf-traces from the middle of the sinuses on the edge of the primary xylem is regarded as a difference; in Lepidodendron the leaf-traces are said to arise in some species from the sides of the crenulations; but, as already pointed out, this is a distinction of doubtful value. The division of the primary xylem into separate strands in some stems of Sigillaria of the Clathrarian and Leiodermarian forms is a characteristic peculiarity; but S. spinulosa forms a connecting link between this type and the continuous arrangement of the xylem in S. elongata and S. elegans. Kidston[536] has shown that the discontinuous primary xylem occurs in Lower Permian species, a fact consistent with the view that the greater abundance of the centripetally developed wood, characteristic of the older species, represents a more primitive feature. This is not merely a conclusion drawn from a consideration of geological age, but it is in harmony with the view expressed by Scott[537] that as plants achieved greater success in producing secondary centrifugal wood, the retention of any considerable quantity of primary xylem became superfluous. As yet we know very little of the structure of the perixylic tissues of Sigillaria, but there is no sufficient reason for supposing that these differ in essentials from those in Lepidodendron. The middle and outer cortical tissues are practically identical in the two genera. The parichnos is of the same type, except that in Sigillaria it reached greater dimensions in the outer part of its course.

v. Sigillaria Brardi[538] Brongniart.
Figs. [196], A–C; [200]; [203].

Fig. 203. Sigillaria Brardi Brongn. (¾ nat. size). From a photograph of a specimen in Dr Kidston’s collection, from the Upper Transition Series of Staffordshire. Published by Kidston (02) Pl. LIX. fig. 1.