In habit the stem of Zalesskya resembles that of an Osmunda or a Todea, but it differs in the possession of a stele composed of a continuous cylinder or solid column of xylem surrounded by phloem, and by the differentiation of the xylem into two concentric zones. The leaves are represented by petiole-bases only; the sporangia are unknown. The stem and leaf-base anatomy fully justifies the inclusion of Zalesskya in the Osmundaceae.
Zalesskya gracilis (Eichwald). [Fig. 248].
The type-specimen is a partially decorticated stem, from Upper Permian beds in Russia, provided with a single stele, 13 mm. in diameter, surrounded by a broad thin-walled inner cortex containing numerous leaf-traces and occasional roots: this was doubtless succeeded by a sclerotic outer cortex. In its main features Zalesskya gracilis agrees closely with Z. diploxylon represented in [fig. 249]. The stele consists of a continuous cylinder of xylem exhibiting a fairly distinct differentiation into two zones, (i) a broader outer zone of narrower scalariform tracheae (x ii, [fig. 248]) in which 20 to 25 protoxylem strands (px) occur just within the edge, (ii) an inner zone of broader and shorter tracheae ([fig. 248], x i). The protoxylem elements (px, [fig. 248]) are characterised by a single series of scalariform pits, while the metaxylem elements have multiseriate pits like those on the water-conducting elements of recent Osmundaceae. The tracheae show an interesting histological character in the absence of the middle substance of their walls, a feature recognised by Gwynne-Vaughan[775] in many recent ferns. External to the xylem and separated from it by a parenchymatous sheath is a ring of phloem, ph, composed of large sieve-tubes and parenchyma separated from the inner cortex by a pericycle 4 to 5 layers in breadth. The occurrence of a few sclerotic cells beyond the broad inner cortex points to the former existence of a thick-walled outer cortex. The leaf-traces are given off as mesarch strands from the edge of the xylem; they begin as prominences opposite the protoxylem and become gradually detached as xylem bundles, at first oblong in transverse section, then assuming a slightly crescentic and reniform shape, while the mesarch protoxylem strand takes up an endarch position. As a trace passes further out the curvature increases and the protoxylem strands undergo repeated bifurcation; it assumes in fact the form and general type of structure met with in the leaf-traces of Todea and Osmunda. Numerous diarch roots, given off from the stele at points just below the outgoing leaf-traces, pass outwards in a sinuous horizontal course through the cortex of the stem.
Fig. 248. Zalesskya gracilis (Eich.). Transverse section of part of the stele: ph, phloem; x i, x ii, xylem; px, protoxylem. (After Kidston and Gwynne-Vaughan. × 20.)
Fig. 249. Zalesskya diploxylon. Kidston and Gwynne-Vaughan. Transverse section of stem. ph, phloem. (After Kidston and Gwynne-Vaughan. × 2½.)
In Zalesskya gracilis the xylem cylinder was probably wider in the living plant than in the petrified stem. In Zalesskya diploxylon[776], in all probability from the same Russian locality, there can be little doubt that the xylem was originally solid to the centre ([fig. 249]). In this species also the phloem forms a continuous band (ph, [fig. 249]) consisting of four to six layers of sieve-tubes.
Thamnopteris.
Thamnopteris Schlechtendalii (Eich.). Figs. [250], [312], A, Frontispiece.