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.
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.
The next contribution to our knowledge of the anatomy of Sigillaria was made by Renault and Grand’Eury[519] who described the structure of Sigillaria spinulosa Germar[520], a species now recognised as the Leiodermarian condition of S. Brardi, and probably, therefore, not specially distinct from the specimen described by Brongniart in 1839 as S. elegans. In Brongniart’s fossil the leaf-cushions are in contact (Clathrarian form of S. Brardi: fig. 203, upper part) whereas in the specimen now under consideration the leaf-scars are further apart (Leiodermarian form of S. Brardi, fig. 203, lower part, and fig. 196, C). It may be, as Scott suggests, that these two specimens are not specifically identical but closely allied, an opinion based on certain anatomical differences[521]; we may, however, include both under the comprehensive name S. Brardi.
The primary xylem (fig. 200, B, x), is in some regions separated into distinct strands, in others it forms a continuous band equal in length to several of the separate groups. This type of stele, in which the primary xylem consists in part of separate strands and in part of a continuous cylinder, forms a transition between that represented in fig. 200, A, and the steles of Sigillaria elegans (fig. 202, A) and most species of Lepidodendron. The tendency of the primary xylem strands to become united laterally, forming broader bands, was first described by Solms-Laubach[522] in a French specimen of Sigillaria spinulosa in the Williamson collection. The leaf-traces arise from the middle of the concave outer face of the primary xylem groups. The inner cortex is composed of small parenchymatous cells as in Lepidodendron, and it is noteworthy that traces of partially disorganised tissue, described as large canals, in the region external to the secondary wood, bear a resemblance[523] to the secretory tissue of Lepidodendron.
Other interesting features are presented by the structure of the outer cortex and the parichnos. The outer cortex in the leaf-scar region is composed of parenchyma, but for the most part it consists of radially elongated groups of thin-walled parenchyma enclosed in a framework of thicker-walled and elongated elements (fig. 200, B, c3). This type of cortex, to which Brongniart applied the name Dictyoxylon, would produce a cast in the case of a partially decorticated stem characterised by a surface formed of irregularly oval and raised areas bounded by narrow grooves; the greater prominence of the former being due to the more rapid decay of the softer tissue, which would produce depressions on the exposed face of the dead stem. Casts of this type are not uncommon in Carboniferous rocks, and while some may belong to the Pteridosperm Lyginodendron, others may be those of Sigillarian stems.
The large parichnos-strands, produced as in Lepidodendron, by the forking of a single strand arising in the middle cortical region, consist in part of tissue containing secretory canals, a structure like that recently described by Miss Coward[524] in the large parichnos strands of Syringodendron stems.
An example of a decorticated specimen is described by Renault[525] as Sigillaria xylina. This stem is presumably referred to Sigillaria because the primary xylem consists of separate strands. It is characterised by the unusually large development of secondary wood and by the relatively small size of the pith. The xylem cylinder has a diameter of 4–5 cm. and the pith is only 4–5 mm. in breadth.
Another example of a petrified Sigillaria stem has been described by Kidston[526] as S. elegans Brongn.[527] (fig. 193, D), a species characterised by vertical rows of sub-hexagonal and contiguous leaf-scars and by the presence of verticils of cone-scars. Fig. 193, D, represents Kidston’s specimen in surface-view; one row of leaf-scars is shown, but most of the superficial tissues have been destroyed. The crushed stele, 13 mm. in its longest diameter, has a continuous cylinder of primary xylem, (fig. 202, A, x) characterised by a regularly crenulate outer margin with the smallest elements at the edge; the prominent ridges separating the sinuses are rounded. The leaf-traces arise from the bottom of each sinus; the leaf-bundles are mesarch, and consist exclusively of primary elements. The secondary xylem, x2, like that of the primary xylem, has a crenulate outer edge. The most interesting feature of the outer cortex is afforded by a tangential section which, in addition to the leaf-scars, cuts through a cone-scar showing a solid primary stele surrounded by the cortex of the cone-peduncle.
Another type of Sigillaria, probably S. elongata Brongn. (fig. 202, B, C), which is very similar to S. scutellata has been briefly described by Prof. Bertrand[528], to whom my thanks are due for the two photographs reproduced in fig. 202, B., C. His specimen, from the Pas de Calais Coal-field, shows a ribbed Rhytidolepis form of surface (fig. 202, B). The stele (fig. 202, C) agrees closely with that of S. elegans as described by Kidston, but the ridges on the fluted surface of the primary xylem are more pointed. “In the immediate neighbourhood of the origin of a leaf-trace, the spiral elements form a median band in the middle of a sinus” and from this the leaf-traces are given off. No secondary xylem was found in the leaf-traces at any part of their course.
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.
The aerial shoots of this species are occasionally branched dichotomously[539], the apical portions bearing short crowded leaves[540]; the surface of the bark is either completely covered with contiguous leaf-scars without definite leaf-cushions or with projecting cushions forming a narrow sloping surface surrounding each leaf-scar. Other parts of the plant may possess cushions similar in their kite-shaped form to those of Lepidodendron, but without a median vertical groove, or the leaf-scars may be spirally disposed at varying distances apart on a comparatively smooth and longitudinally wrinkled bark. The species exhibits striking instances of a transition between the Favularian, Clathrarian, and Leiodermarian forms of stems. The leaf-scars, which are hexagonal in outline,—the lateral angles pointed and transversely elongated, the upper and lower angles rounded,—bear three scars, the central leaf-trace and two straight or curved lateral parichnos scars; a ligular pit occurs immediately above the centre of the upper edge of the leaf-scar and occasionally circular elevations with a central pit occur singly or in pairs below a leaf-scar (fig. 196, A). The linear leaves, which may persist on shoots having a fairly large diameter[541], have a single median vein and two stomatal grooves on the lower surface[542] (fig. 200, D).
Partially decorticated and younger shoots are characterised by the occurrence of pairs of elliptical parichnos areas and a smaller median leaf-trace scar. The surface of older stems, which may show signs of longitudinal splitting (Syringodendron state), bears pairs of parichnos scars reaching a length of 2–2·5 cm. and a breadth of 10–13 mm. The regularity of the leaf-scar arrangement is interrupted at intervals by the occurrence of more or less regular verticils of scars marking the position of deciduous shoots. Grand’Eury[543] has figured cones which he believes to be those of this species, and Zeiller refers the large strobili, Sigillariostrobus major, to Sigillaria Brardi[544].
The subterranean axes were characterised by spirally disposed rootlet-scars like those of Stigmaria ficoides (figs. 204, 205) and by a cortical surface with the features of Stigmaria rimosa Gold.[545]
The anatomy of the stele and leaves has already been described (p. 219). The stele of the Stigmarian portion of the plant consists of a band of centripetal primary xylem and a cylinder of centrifugally formed secondary wood with medullary rays containing vascular bundles passing out to the rootlets[546].
Sigillaria Brardi occurs not uncommonly in Permian rocks; it is recorded from France[547], Germany[548], Pennsylvania[549], and elsewhere. It is found in the Upper, Middle, and Lower Coal-Measures of England[550] and in Permo-Carboniferous strata in Africa[551] and Brazil[552].