Page:EB1911 - Volume 20.djvu/583

PALAEOZOIC] In one great division of the genus—the Eusigillariae—the stems are ribbed, each rib bearing a vertical row of leaf-scars; the ribbed Sigillariae were formerly divided into two sub-genera—Rhytidolepis, with the scars on each rib rather widely spaced, and Favularia, where they are approximated and separated by transverse furrows, each rib thus consisting of a series of contiguous leaf-bases. This distinction, however, has proved to have no constant taxonomic value, for both arrangements may occur on different parts of the same specimen. The species without ribs—Subsigillariae—were in like manner grouped under the two sub-genera Clathraria and Leiodermaria; in the former each scar is seated on a prominent cushion, while in the latter the surface of the stem (as in Bothrodendron) is perfectly smooth. Here also the distinction has proved not to hold good, S. Brardi, for example, showing both conditions on the same stem. All these names, however, are still in use as descriptive terms. Generally, the Eusigillariae are characteristic of the older Carboniferous strata, the Subsigillariae of the Upper Coal Measures and Permian. The leaf scars throughout the genus show essentially the same prints as in Lepidodendron, differing only in details, and here also a ligule was present (fig. 14).

The anatomy of Sigillaria is not so well known as that of Lepidodendron, for specimens showing structure are comparatively rare, a fact which may be correlated with the infrequency of branching in the genus. The structure of a Clathrarian Sigillaria (S. Menardi), from the Permian of Autun, was accurately described by Brongniart as long ago as 1839, and a similar species, S. spinulosa (= S. Brardi) was investigated by Renault in 1875, but it was long before we had any trustworthy data for the anatomy of the ribbed forms. This gap in our knowledge has now been filled up, owing to Bertrand's investigation of a specimen referred by him to S. elongata, followed by the detailed researches of Kidston and Arber on Sigillaria elegans, scutellata and mamillaris. The structure of the ribbed Sigillariae, as at present known, essentially resembles that of a medullate Lepidodendron, though the ring of primary wood is narrower. Its outer margin is crenelated, the leaf-traces being given off from the middle of each bay. Secondary wood was formed in abundance, precisely as in most species of Lepidodendron. In the Subsigillarian species S. Menardi the primary wood is broken up into distinct bundles, while in S. spinulosa their separation is sometimes incomplete. The secondary cortex or periderm attained a great development, and in some cases shows considerable differentiation. On the whole, the anatomy of Sigillaria is closely related to that of the preceding group, and in fact a continuous series can be traced from the anatomically simplest species of Lepidodendron to the most modified Sigillariae. The leaves of Sigillaria are in some cases almost identical in structure with those of Lepidodendron,

but in certain species (S. scutellata and S. mamillaris) there is evidence that they were of the Sigillariopsis type, the leaf being traversed by two parallel vascular strands, derived from the bifurcation of the leaf-trace.

The nature of the fructification of Sigillaria was first satisfactorily determined in 1884 by Zeiller, who found the characteristic Sigillarian leaf-scars on the peduncles of certain large strobili (Sigillariostrobus). The cones, of which several species have been described, bear a strong general resemblance to Lepidostrobus, differing somewhat in the form of the sporophylls and some other details. The megaspores (reaching 2 mm. or more in diameter) were found lying loose on the sporophylls by Zeiller; the sporangia containing them were first observed by Kidston, in a species from the Coal Measures of Yorkshire. That the cones were heterosporous there can be no doubt, though little is known as yet of the microsporangia. The discovery of Sigillariostrobus, which was the fructification of Subsigillariae as well as of the ribbed species, has finally determined the question of the affinities of the genus, once keenly discussed; Sigillaria is now clearly proved to have been a genus of heterosporous Lycopods, with the closest affinities to Lepidodendron.

Stigmaria.—On present evidence there is no satisfactory distinction to be drawn between the subterranean organs of Sigillaria and those of Lepidodendron and its immediate allies, though some progress in the identification of special forms of Stigmaria has recently been made. These organs, to which the name Stigmaria was given by Brongniart, have been found in connexion with the upright stems both of Sigillaria and Lepidodendron. In the Coal Measures they commonly occur in the under clay beneath the coal-seams. Complete specimens of the stumps show that from the base of the aerial stem four Stigmarian branches were given off, which took a horizontal or obliquely descending course, forking at least twice. These main Stigmarian axes may be 2 to 3 ft. in diameter at the base, and 30 or 40 ft. in length. Their surface is studded with the characteristic scars of their appendages or rootlets, which radiated in all directions into the mud. Petrified specimens of the main Stigmaria are frequent, and those of its rootlets extraordinarily abundant. The two parts are very different in structure: in the main axis, as shown in the common Coal Measure form Stigmaria ficoides, the centre was occupied by the pith, which was surrounded by a zone of wood, centrifugally developed throughout. In other species, however, the centripetal primary xylem is represented. Phloem, surrounding the wood, is recognizable in good specimens; in the cortex the main feature is the great development of periderm. The rootlets, which branched by dichotomy, contain a slender monarch stele exactly like that in the roots of Isoëtes and some Selaginellae at the present day; they possessed, however, a complex absorptive apparatus, consisting of lateral strands of xylem, connecting the stele with tracheal plates in the outer cortex. The morphology of Stigmaria has been much discussed; possibly the main axes, which do not agree perfectly either with rhizomes or roots, may best be regarded as comparable with the rhizophores of Selaginellae; they have also been compared with the embryonic stem, or protocorm, of certain species of Lycopodium; the homologies of the appendages with the roots of recent Lycopods appear manifest. It has been maintained by some palaeobotanists that the aerial stems of Sigillaria arose as buds on a creeping rhizome, but the evidence for this conclusion is as yet unconvincing.

Lycopoditeae.—Under this name are included the fossil Lycopods of herbaceous habit, which occur occasionally, from the Devonian onwards. One such plant, Miadesmia, has already been referred to, as one of the seed-bearing Lycopods. In some Lycopoditeae the leaves were all of one kind, while others were heterophyllous, like most species of Selaginella. The genus Selaginellites, Zeiller, is now used to include those forms in which the fructification has proved to be heterosporous. In Selaginellites Suissei there was a definite strobilus bearing both micro- and megasporangia; in each of the latter from 16 to 24 megaspores were contained; in Selaginellites primaevus, however, the number of megaspores was only 4, and the resemblance to a recent Selaginella was thus complete. Selaginellites elongates, another heterosporous species, is remarkable for having no differentiated strobilus, a condition not known in the recent genus. The antiquity of the Selaginella type indicates that this group had no direct connexion with the Lepidodendreae, but sprang from a distinct and equally ancient herbaceous stock. There is, however, some evidence that Isoëtes, which in several respects agrees more nearly with the Lepidodendreae, may actually represent their last degenerate survivors (see Pleuromeia, in § II., ). No homosporous Lycopoditeae have as yet been recognized.

IV. Filicales.—Of all Vascular Cryptogams the Ferns have best maintained their position down to the present day. Until recently it has been supposed that the class was well represented in the Palaeozoic period, and, indeed, that it was relatively, and perhaps absolutely far richer in species even than in the recent flora. Within the last few years, however, the position has completely changed, and the majority of the supposed Palaeozoic