_Thamnopteris_. _Thamnopteris Schlechtendalii_ (Eich.). Figs. 250, 312, A, Frontispiece. In 1849 Brongniart[777] proposed the name _Thamnopteris_ for a species of fern from the Upper Permian of Russia originally described by Eichwald as _Anomopteris Schlechtendalii_. A new name was employed by Brongniart on the ground that the fossil was not generically identical with the species previously named by him _Anomopteris Mougeotii_[778]. Eichwald’s specimen has been thoroughly investigated by Kidston and Gwynne-Vaughan[779]. The stem (Frontispiece) agrees in habit with those of _Zalesskya_ and recent Osmundaceae; on the exposed leaf-bases the action of the weather has etched out the horse-shoe form of the vascular strands and laid bare numerous branched roots boring their way through the petiole stumps. The centre of the stem is occupied by a protostele 13 mm. in diameter consisting of solid xylem separated by a parenchymatous sheath from a cylinder of phloem. The xylem is composed mainly of an axial column of short and broad reticulately pitted tracheae (fig. 250, _b_ and Frontispiece), distinguished from the sharply contrasted peripheral zone of normal scalariform elements, _a_, by their thinner walls and more irregular shape. The protoxylem, _px_, is represented by groups of narrower elements rather deeply immersed in the peripheral part of the metaxylem. A many-layered pericycle, _per_, and traces of an endodermis, _en_, succeed the phloem, _ph_, which is characterised by several rows of large contiguous sieve-tubes; beyond the endodermis is a broad thin-walled inner cortex. The leaf-traces arise as in _Zalesskya_, but the protoxylem in _Thamnopteris_ is at first central; as the trace passes outwards a group of parenchyma appears immediately internal to the protoxylem elements and gradually assumes the form of a bay of thin-walled tissue on the inner concave face of the curved xylem. The next stage is the repeated division of the protoxylem strand until, in the sclerotic outer cortex, the traces acquire the Osmundaceous structure (fig. 312, A, p. 453). The petiole bases have stipular wings as in _Todea_ and _Osmunda_. [Illustration: FIG. 250. _Thamnopteris Schlechtendalii_ (Eich.). Part of stele: _a_, outer xylem; _b_, inner xylem. (After Kidston and Gwynne-Vaughan. × 13.)] [Sidenote: OSMUNDACEAE]