Renault[504] has shown that the leaf-traces of Sigillaria spinulosa (= S. Brardi) are accompanied in the outer cortical region of the stem by a fairly large amount of secondary xylem; in sections of the free lamina which he figures the secondary elements are much less obvious and represented by a few tracheae only. Similarly, in the leaf-base of S. Brardi ([fig. 200], E) the xylem consists of both primary and secondary elements (x, x2), but in the lamina the latter is poorly if at all represented. In the lamina of the leaves of S. Brardi the primary xylem forms a narrow slightly curved band with two lateral groups of narrower, presumably protoxylem elements; this is surrounded by delicate parenchyma styled by Renault, on very slender evidence, phloem (“liber”). Some dark cells below the xylem are described as sclerous tissue, and surrounding the bundle is a sheath of transfusion tracheae (dotted area in [fig. 200], E). It is possible that the elements spoken of with hesitation by Renault as secondary xylem are transfusion tracheae.

There has probably been some confusion in the minds of authors between sclerous tissue and dark secretory tissue in Sigillarian leaves; the crescentic band, a, shown in [fig. 142], B, which corresponds in position with the sclerous tissue of Renault in S. Brardi leaves, appears to be of the nature of secretory tissue.

The diagram shown in [fig. 142], B, illustrates a type of leaf very like those already described, except that there are two xylem strands, x. The difference between the double strand and the single bundle seen in figs. [142], A, C and [200] E, is comparatively small, but it is a real distinction. This type of leaf ([fig. 142], B) was originally described by Renault[505] under the generic title Sigillariopsis. The genus was founded on a French petrified specimen consisting of part of a ribbed stem possessing a stele of the Sigillarian type and characterised by separate primary xylem strands, like those of S. Brardi described by Brongniart in 1839. Renault considered the presence of two xylem strands in the leaf a sufficient reason for the institution of a new genus and named the specimen Sigillariopsis Decaisnei. Prof. Bertrand of Lille kindly photographed for me Renault’s type-specimen and sent several prints with explanatory notes. The transverse section of the leaves shows very clearly the two xylem strands; each strand consists of a triangular group of primary tracheae with the protoxylem apex pointing towards the lower surface of the lamina. Below each primary strand of centripetal xylem is an arc composed of a few small tracheae which Renault and Bertrand describe as secondary xylem; it is, however, not clear from the photomicrographs that these are of secondary origin, their position and appearance reminding one of the primary centrifugal xylem of a cycadean foliar bundle. Below this centrifugal xylem is another arc of imperfectly preserved elements described by Renault as a protective sheath and by Bertrand as glandular tissue; the latter term is probably the more correct as the tissue may well correspond to the secretory-zone tissue of Lepidodendron stems. Fairly large groups of transfusion tracheids occur on the flanks of the xylem. Prof. Bertrand points out that one of his sections, cut nearer the apex of a leaf than that figured by Renault with a single xylem strand, contains a double strand and thus shows the latter’s description to be an incorrect interpretation of the imperfectly preserved tissues.

The Sigillariopsis type of leaf was recognised by Scott[506] in English material on which he founded the species Sigillariopsis sulcata. In a section which he has recently figured[507] a lacuna below the two xylem strands is described as “representing secretory tissue”; a band of transfusion tracheae almost encircles the pair of bundles.

In a note published in 1907, Kidston[508] demonstrated the association of Sigillariopsis leaves with an undoubted Sigillarian stem of the Rhytidolepis type and expressed his conviction that Renault’s genus is identical with Sigillaria. The correctness of Kidston’s conclusion has been proved by Arber and Thomas[509] who found that the leaf-traces of Sigillaria scutellata bifurcate during their course through the outer region of the cortex and enter the leaf as two distinct strands of primary xylem. In the section from Dr Kidston’s collection shown in [fig. 142], B, the lamina, 4 mm. wide, consists mainly of thin-walled assimilating tissue composed of radially elongated cells abutting at the periphery on hypodermal mechanical tissue, except at the edges of the stomatal grooves which are bounded by the small-celled epidermis. A broad sheath of thicker-walled elements, s, surrounds numerous scattered transfusion tracheae, t, and below the two xylem strands, x, which are embedded in delicate parenchyma there is a crescentic band of dark tissue, a, resembling the smaller strand, a, in [fig. 142], A′, and the secretory zone tissue of a Lepidodendron stem.

iii. Fertile shoots of Sigillaria.

Reference has already been made to the manner of occurrence of strobili on Sigillarian stems; it remains to describe the structure of these reproductive shoots. Sigillariostrobus, the name given to Sigillarian strobili, may be defined in general terms as follows:

Cylindrical cones, rarely dichotomously branched[510] as in species of Lycopodium and Selaginella, which may reach a length of 30 cm. (e.g. Sigillariostrobus nobilis Zeill.[511]) and a diameter 2–5 cm.; peduncle long and slender, sometimes bearing acicular bracts or, after leaf-fall, characterised by leaf-cushions and leaf-scars like those on vegetative shoots ([fig. 201], E). The stalked cones are borne in irregular verticils and in some species in vertical series, the fertile zones being separated by comparatively long sterile portions of the stem ([fig. 199]). The cones were deciduous and, in certain cases if not in all, the individual sporophylls became detached from the cone-axis on maturity. The slender axis bore spiral or verticillate imbricate sporophylls attached at right angles or more or less obliquely. The basal rhomboidal portion bore spores on its upper surface ([fig. 201], F), presumably enclosed in a somewhat radially elongated sporangium (fig. B) and was prolonged distally into a narrow lanceolate free portion, in some species with a ciliate border (fig. D). The sporangia probably produced megaspores and microspores, but such spores as have been recognised appear to belong to the former category. The designation Triletes is applied to isolated spores of Sigillaria or to those of Lepidodendron.

Sigillariostrobus Tieghemi Zeiller[512] ([figs. 201], E, F). In this species, from the Coal-field of Valenciennes, the pedicel bore acicular leaves or bracts attached to the upper portion of leaf-cushions arranged in vertical series (fig. E). The cones reached a length of 16 cm. and a breadth of 2·5–5 cm.; the sporophylls are borne in alternating verticils with 8–10 in each whorl. Several megaspores (2 mm. in diameter) appear to have been produced in tetrads in each sporangium.