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. [Illustration: 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.)] [Illustration: 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.