The first use of this name was by Artis[1083], who gave it to a long flattened cast, Megaphyton frondosum, found in Carboniferous strata in Yorkshire, characterised by two vertical rows of large scars and by impressions of sinuous roots. Kidston records the genus from the Middle and Upper Coal-Measures of Britain. A good example of this type of cast is afforded by M. McLayi Lesq.[1084] from the Coal-Measures of North America, which has been recognised in European Carboniferous rocks. The leaf-scars are rounded or oval, broader than high; the vascular impression has the form of a closed ring (5–8 × 3–6 cm.), more or less circular and with a tendency to a rectangular outline, characterised by a deep inverted U-shaped sinus in the middle of the lower surface and by a W-shaped impression of an internal strand ([fig. 298], B)[1085].
iii. Ptychopteris.
This generic name, instituted by Corda[1086], is applied to decorticated stems of Psaronius, the surface of which is that of the vascular region on which the form of the leaf-scars is more or less clearly defined. The scar-areas are limited by an impression of the sclerenchymatous sheath enclosing the leaf-meristele, and internal to this is the impression of the leaf-trace. In some specimens a layer of coaly material which represents the carbonised cortex and adventitious roots covers the Ptychopteris cast. The Ptychopteris cast represented in [fig. 299] shows the decorticated surface of part of a long stem on which the leaf-scars are arranged as in Megaphyton. An example of Ptychopteris is figured by Fontaine and White[1087] from Virginia as Caulopteris gigantea.
Fig. 299. Ptychopteris. ⅙ nat. size. From the Middle Coal-Measures of Lancashire. (The Manchester Museum.)
Fig. 300. Dicksonia antarctica (half of stem in transverse section): st, stele; s, sclerenchyma.
Position of Psaronius.
A comparison of Psaronius with the Marattiaceae and other recent ferns leads to the conclusion that, on the whole, the evidence is in favour of the view usually held, namely that this genus is more closely related to the Marattiaceae than to any other recent ferns. It is, however, important not to overlook the differences between Psaronius and recent genera of Marattiaceae, or the resemblances between the extinct genus and the Cyatheaceae. In habit Psaronius agrees closely with recent tree-ferns; in the vascular system and in the sequence of events connected with the production of leaf-traces, there are striking resemblances between Psaronius and the Cyatheaceous fern Saccoloma adiantoides (= Dicksonia Plumieri Hook.) as described by Mettenius[1088]. The piece of stem of Dicksonia antarctica represented in [fig. 300] exhibits a fairly close agreement with species of Psaronius, e.g. P. infarctus ([fig. 296], A, B). Moreover, the peripheral steles, which Zeiller has shown are confined to the stem and play an important part in the production of the roots and in the anastomoses with leaf-traces, are not represented in any Marattiaceous fern; on the other hand, they are comparable with the accessory strands met with in stems of recent Cyatheaceous tree-ferns[1089] (cf. [fig. 240]). The complex system of concentric dictyosteles is a feature more closely matched in Angiopteris (Marattiaceae) than in any Cyatheaceous genus, the chief difference being in the more band-like form of the steles in Psaronius, though in a stem of Angiopteris figured by Mettenius we see a close approach to the extinct type. The position of the protoxylem has unfortunately not been clearly defined in Psaronius stems, but in P. Renaulti it is stated by Scott[1090] to be endarch, a position which some of the protoxylem strands occupy in Angiopteris[1091]. The occurrence of large sieve-tubes described by Scott in P. Renaultii is another feature shared by recent Marattiaceae. In many of the continental species of Psaronius the phloem has not been preserved, and our knowledge of this tissue is comparatively meagre. In the Marattiaceae the roots arise mainly from the inner portions of the stele, while in Psaronius they are usually formed from the external vascular bands. The formation of secondary cortical tissue is a peculiarity of Psaronius; on the other hand, if Butterworth[1092] is correct in referring to that genus the roots with secondary xylem, which he describes as P. Cromptonensis, a comparison may be made with the occurrence of secondary tracheae in the stem steles of Angiopteris[1093].
The absence of mechanical tissue in the stem of Angiopteris is in contrast with its occurrence in the fossil stems and in recent tree-ferns; but this is a character of secondary importance and one which can be readily explained by the difference in habit between Angiopteris and Psaronius.