PSILOTITES, ETC.
In 1842 Münster[52] instituted the genus Psilotites for a small impression of a slender branched axis from Jurassic rocks near Mannheim in Germany which he named Psilotites filiformis; Schimper[53] spoke of the specimens as too doubtful for determination, an opinion with which every botanist would cordially agree. Goldenberg’s species Psilotites lithanthracis[54] from the Saarbrücken coal-field is founded on impressions of axes: some of these are dichotomously branched and bear small oval projections, which may be rudimentary leaves or possibly leaf-scars. More recently Kidston[55] described specimens of branched axes from the Lanarkshire coal-field bearing a row of lateral thorn-like projections under the title Psilotites unilateralis; but these fragments, as Dr Kidston himself admits, are of no botanical value.
In a paper on fossil Salvinias, Hollick[56] mentions Salvinia reticulata, originally described by Heer and by Ettingshausen and S. Alleni Lesq.[57] a Tertiary species, and calls attention to their very close resemblance in form, nervation, and apex to the leaves of the genus Tmesipteris: he refers both species to that genus. The drawings reproduced by Hollick represent leaves with a midrib and numerous anastomosing lateral veins, whereas in Tmesipteris the lamina of the leaf has a midrib without lateral branches. An enlarged drawing of the outlines of the epidermal cells would correspond closely with the small reticulations in the fossil leaves and it may be that there has been some confusion between veins and cell-outlines. In any case there would seem to be no reason for the use of the recent generic name[58].
Among other fossils assigned to the Psilotales we have Marion’s genus Gomphostrobus from the Permian of France and Germany[59]. Marion placed this plant in the Coniferales on the strength of its resemblance to Walchia and Araucaria, but Potonié[60] is inclined to recognise in the leaves and monospermic sporophylls characters suggestive of Lycopodiaceous affinity.
The latter author in 1891[61], in ignorance of Marion’s proposal to adopt the name Gomphostrobus, instituted a genus Psilotiphyllum for the sporophylls of a species originally described by Geinitz[62] as Sigillariostrobus bifidus, but he subsequently adopted Marion’s designation and with some hesitation included the French and German specimens in the Psilotales. As stated elsewhere[63], Potonié’s arguments in favour of his view hardly carry conviction, and it is probably more in accordance with truth to deal with Gomphostrobus in the chapter devoted to the Coniferales.
Psilophyton.
The generic title Psilophyton, instituted by the late Sir William Dawson[64], has become familiar to geologists as that of a Pre-Carboniferous plant characteristic of Devonian and Silurian rocks in Canada, the United States of America, and Europe. From the botanist’s point of view the name stands for miscellaneous remains of plants of different types and in many cases unworthy of record. The genus was founded on impressions of branched axes from the Devonian strata of New Brunswick resembling the rachis and portions of lateral pinnae of ferns or the forked slender twigs of a Lycopod. The type-species Psilophyton princeps Daws. as represented on somewhat slender evidence in Dawson’s restoration, which accompanies the original description of the genus and has since been copied by several authors, is characterised by the possession of a horizontal rhizome bearing numerous rootlets and giving off dichotomously branched aerial shoots with spinous appendages, compared with rudimentary leaves, and terminating in slender branchlets bearing pendulous oval “spore-cases” from their tips. Some of the branchlets exhibit a fern-like vernation. The plant is spoken of by Dawson as apparently a generalised type[65], resembling in habit and in its rudimentary leaves the recent genus Psilotum and presenting points of contact with ferns. Specimens were found in an imperfectly petrified state showing a central cylinder of scalariform tracheae surrounded by a broad cortical zone of parenchyma and fibrous tissue.
Among other species described by the author of the genus we need only mention Psilophyton robustius, characterised by vegetative shoots and “spore-cases” similar to those of the type-species; but, as Solms-Laubach[66] has pointed out, the petrified sections referred by Dawson to P. robustius are of an entirely different anatomical type from that of P. princeps[67].
British fossils from the Old Red Sandstone from the north of Scotland, Orkney and Caithness, originally figured by Hugh Miller and compared by him with algae but more especially with recent Lycopods, were subsequently placed by Carruthers[68] in the genus Psilophyton as P. Dechianum, the specific designation being chosen on the ground that the Scotch specimens are specifically identical with fossils described by Goeppert[69] as Haliserites Dechianus.
Various opinions have been expressed in regard to the nature of the Devonian species Haliserites Dechianus Goepp. with which Carruthers[70] identified Miller’s Old Red Sandstone plant: reference may be made to a paper by White[71] containing figures of dichotomously branched impressions described as species of Thamnocladus which he includes among the algae.