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.
Fig. 250. Thamnopteris Schlechtendalii (Eich.). Part of stele: a, outer xylem; b, inner xylem. (After Kidston and Gwynne-Vaughan. × 13.)
OSMUNDACEAE
The striking feature exhibited by these Permian plants is the structure of the protostele, which in Thamnopteris and probably in Zalesskya diploxylon consists of solid xylem surrounded by phloem: this may be regarded as the primitive form of the Osmundaceous stele. In Osmunda regalis and in other recent species of the genus the xylem cylinder has the form of a lattice-work; in other words, the departure of each leaf-trace makes a gap in the xylem and the overlapping of the foliar-gaps results in the separation of the xylem into a number of distinct bundles. In Zalesskya gracilis the continuity of the xylem is not broken by overlapping gaps; in this it agrees with Lepidodendron. In Thamnopteris the centre of the stele was occupied by a peculiar form of xylem obviously ill-adapted for conduction, but probably serving for water-storage and comparable with the short and broad tracheae in Megaloxylon[780]. There is clearly a well-marked difference in stelar anatomy between these two Permian genera and Todea and Osmunda: this difference appears less when viewed in the light of the facts revealed by a study of the Jurassic species Osmundites Dunlopi.
Fig. 251. Lonchopteris virginiensis. (After Fontaine. ½ nat. size.)
As possible examples of Triassic Osmundaceae reference may be made to some species included in Stur’s genus Speirocarpus[781]. S. virginiensis was originally described by Fontaine[782] from the Upper Triassic rocks of Virginia as Lonchopteris virginiensis ([fig. 251]) and has recently been figured by Leuthardt[783] from the Keuper of Basel. The sporangia, which are scattered over the lower surface of the pinnules, are described as globose-elliptical and as having a rudimentary apical annulus; no figures have been published. In habit the frond agrees with Todites Williamsoni, but the lateral veins form an anastomosing system like that in the Palaeozoic genus Lonchopteris ([fig. 290], B). There would seem to be an a priori probability of this species being a representative of the Osmundaceae and not, as Stur believed, of the Marattiaceae. Seeing that Lonchopteris is a designation of a purely provisional kind, it would be convenient to institute a new generic name for Triassic species having the Lonchopteris venation, which there are good reasons for regarding as Osmundaceous ferns.
Similarly Speirocarpus tenuifolius (Emmons) (= Acrostichites tenuifolius Font.), which resembles Todites Williamsoni (see p. 339) not only in habit and in the distribution of the sporangia but also in the venation, is probably an Osmundaceous species.