Psaronieae.
This family name, first suggested by Unger, may be conveniently adopted for the numerous species of petrified tree-fern stems characteristic of the Lower Permian and Upper Carboniferous strata. In his monograph Über die Staarsteine published in 1854, Stenzel[1053] referred to the Psaronieae as a special subdivision of the Filices most nearly allied to the Polypodiaceae. There is now a consensus of opinion in favour of including Psaronius in the Marattiales, or at least of regarding the genus as more closely allied to the Marattiaceae than to any other family. While admitting that the balance of evidence is in favour of this view, it is probably wiser to retain the distinctive term Psaronieae on the ground that species of Psaronius differ in several respects from any recent ferns, and because of our comparative ignorance in regard to the nature of the fructification.
Psaronius.
This generic name was proposed by Cotta in his classic work Die Dendrolithen[1054]. The stems so named, formerly included by Sprengel[1055] in the genus Endogenites, had long been familiar as petrified fossils. Most of the specimens described by the earlier writers were obtained from Lower Permian rocks in the neighbourhood of Chemnitz, Saxony. The mottled appearance presented by their polished surfaces is said to have given rise to the appellation Staarsteine (starling stones), a term expressing a resemblance, more or less remote, to a starling’s breast. It has been suggested that this word is a corruption of Stern Steine or star stones[1056], a descriptive term suggested by the stellate arrangement of the vascular strands in transverse sections of the roots. Parkinson[1057], in his Organic Remains of a former World, speaks of these stems as starry stones. The history of our knowledge prior to 1854 is summarised by Stenzel. At first compared with corals or the stems of sea-lilies, Psaronii were recognised by Sprengel, who first used a lens in the examination of the fossils, as fern stems most nearly allied to those of recent Cyatheaceae. By other authors, e.g. Schlotheim and Sternberg, they were referred to Palms, and by Brongniart considered to be the lower portions of Lycopodiaceous (Lepidodendron) stems. Corda and many subsequent authors selected the Marattiaceae as the most closely allied family among existing plants.
Psaronius is represented by specimens obtained from the Lower Permian of Saxony and Upper Carboniferous rocks in Central France, also from Bohemia, Brazil and North America. As yet a few fragments only have been found in the English Coal-Measures. The genus was recognised by Williamson[1058] who described the roots and a small piece of the vascular tissue of a stem which he called P. Renaulti, and this type has since been more fully described by Scott[1059]. The roots of another species have been described by Butterworth[1060] as P. Cromptonensis.
It was pointed out in the account of Lepidodendron that several generic names have been used for the same type of stem in different states of preservation; in Psaronius accidents of fossilisation have been responsible for a similar confusion in nomenclature. The name Psaronius is applied to petrified specimens which, as a rule, lack external features. Casts or impressions of Palaeozoic tree-fern stems provided with leaf-scars are described as species of Caulopteris, Megaphyton, and less commonly as Ptychopteris ([figs. 297–299]). The first name is applied to stems exhibiting spirally disposed leaf-scars like those of recent tree-ferns; in Megaphyton the scars are distichously arranged, in two rows, while Ptychopteris is applied to decorticated stems. These terms are used for stems belonging to one generic type and possessing the structure of Psaronius stems.
Fig. 295. Psaronius stem with roots. (Much reduced. After Grand’Eury.)
The researches of Grand’Eury[1061] led to the discovery that certain Psaronius stems bore fronds of the Pecopteris type some of which bore sori of the Asterotheca or Scolecopteris type. The same author[1062] has also contributed many interesting facts, obtained by an examination of the relation of Psaronius stems to the sediments of French Coal-fields in which they occur, in regard to habitat and manner of growth. The specimen represented in [fig. 295] shows a portion of a Psaronius stem, the upper part of which illustrates the Caulopteris state of preservation, while the lower part is covered by a mass of roots. It is probable, as Rudolph[1063] suggests, that this rich development of roots, which gives to an old Psaronius stem the appearance of an elongated cone, may have served an important mechanical purpose analogous to the secondary thickening in a Dicotyledon or a Conifer. A specimen of Psaronius Cottai in the Hofmuseum, Vienna, is cited in illustration of the enormous breadth of the root-system: the radii of the stem proper and of the encasing cylinder of roots bear the ratio 17 to 165. The comparatively frequent occurrence of a lacunar cortex in the roots points to the growth of the stems in swampy ground, a conclusion in harmony with the evidence afforded by the anatomical features of many other Palaeozoic genera.
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