Fig. 201. Sigillariostrobus.

Sigillariostrobus rhombibracteatus Kidston[513]. Fig. 201, A, C.

Kidston described this species from the Middle Coal-Measures of England: it is similar in habit and in the form of the sporophylls to S. Tieghemi, but rather smaller, and the more definitely rhomboidal sporophylls have a ciliate margin. The cone was probably heterosporous, but megaspores alone have so far been discovered. The sporophylls bear a close resemblance to those of Lycopodium cernuum ([fig. 126], C). In some of the illustrations of this type given by Kidston the naked cone-axis with its numerous sporophyll-scars is clearly shown, reminding one of the naked axes of the cones of the Silver Fir (Abies pectinata) or Cedar after the fall of the scales.

Our knowledge of Sigillarian cones is too incomplete to admit of a detailed comparison with the strobili of Lepidodendron or with those of recent Pteridophytes. There can, however, be little doubt that Goldenberg[514] was correct in his selection of Isoetes as the most nearly allied recent plant so far as the fertile leaves are concerned. It would seem that the sporangia were comparatively delicate structures which have left no clearly defined remains of their walls in the carbonised specimens; Kidston, indeed, speaks of the hollow bases of the sporophylls as holding the spores, but this is hardly likely to have been the case. Our knowledge of the anatomy of Sigillariostrobus is practically nil, but in one specimen of a Sigillaria elegans stem Kidston[515] describes the structure of the tissues as seen in a transverse section of a scar of a fertile shoot; from this we learn that the stele was composed exclusively of primary tracheids forming a solid strand without a pith. It is probable that the cones of Sigillaria were heterosporous, but in no instance have undoubted microspores been discovered; the megaspores in each megasporangium were fairly numerous as in Isoetes ([fig. 133], E). In one species, Sigillariostrobus major (Germar), from Permian rocks of France and Germany, Zeiller[516] states that the whole of a single cone bore megaspores (0·8–1 mm. in diameter) only; this is, however, not opposed to the idea of heterospory, as we find instances in Selaginella of strobili bearing one kind of spore only (cf. p. 56).

In a few instances, it has been possible to correlate cones with certain species of Sigillaria, but in most cases the strobili occur as isolated fossils.

iv. The structure of Sigillarian stems.

The first account of the anatomy of Sigillaria we owe to Brongniart[517] who published a description of the internal structure of an agatised stem, about 4 cm. in diameter, from Autun, which he referred to Sigillaria elegans. It has, however, been shown by Zeiller[518] and by Renault that this petrified fragment belongs to Brongniart’s species S. Menardi, which is probably a young form of S. Brardi. Brongniart’s specimen, now preserved in the Paris Natural History Museum, is a very beautiful example of a silicified plant: on part of the surface are preserved the hexagonal contiguous leaf-scars, like those shown in [fig. 193], A, and on the polished transverse section is seen a relatively large stele consisting of a ring of secondary xylem surrounding a series of crescentic groups of primary xylem ([fig. 200], A) enclosing a wide pith occupied by concentric layers of silica. A portion of the outer cortex is preserved, and this is separated from the stele by a broad space filled with siliceous rock. The main features of this type may be described in a few words. The primary xylem differs from that of such Lepidodendron stems as have been described in being made up of groups of scalariform and occasionally reticulate ([fig. 200], C) tracheae, having a plano-convex or more or less crescentic form as seen in transverse section. These primary strands, in contact with one another laterally, have their narrowest elements on the outer edge. The leaf-traces are given off from the middle of the abaxial face of each xylem strand ([fig. 202], C, lt); these pass obliquely outwards through medullary rays and then, as in Lepidodendron, turn sharply upwards before bending outwards again on their way to the leaves. Each leaf-trace consists of a group of primary tracheae to which a few secondary tracheae are added during the passage through the secondary wood. The secondary xylem forms a continuous cylinder of tracheae with scalariform bands on both radial and tangential walls; the medullary rays are numerous and consist of long and narrow series, usually one cell broad, of parenchymatous cells with occasional short rays one or more cells in depth.

The slightly greater breadth of the rays between each primary xylem strand tends to divide the secondary wood into bundles corresponding in breadth to the primary groups. The outer cortex closely resembles that of Lepidodendron; it consists internally of radial series of secondary, elongated and rather stout, elements abutting on the parenchymatous tissue of the leaf-cushions.