Fossil plants, Vol. 3

Before describing other genera represented by petrified vegetative organs exhibiting in their anatomical features points of contact with the Medulloseae, a short account is intercalated of some imperfectly known seed-bearing fronds and seeds belonging to the Pteridosperms but which do not afford sufficient data to admit of their reference to a more precise position in a natural classification.

Pecopteris Pluckeneti (Schlotheim).

In the section in Volume ii. devoted to the genus Pecopteris reference was made to the species P. Pluckeneti[439], further treatment being deferred until other Pteridosperms had been described. The fern-like fronds originally described by Schlotheim as Filicites Pluckeneti[440] and afterwards transferred by Brongniart and other authors to Pecopteris[441] are now recognised as the leaves of a Pteridosperm. Some doubt has been expressed as to the specific identity of the specimens figured by Schlotheim and Brongniart respectively, but Potonié’s examination of the type-specimen of the earlier author convinced him that Brongniart’s leaves were correctly named. The large fronds of Pecopteris Pluckeneti are characterised by the bifurcation of the principal axis which bears opposite pairs of bi- or tri-pinnate branches and in the angles of the bifurcations of the rachis undeveloped buds occur on prolongations of the axis, a habit recalling recent species of Gleichenia[442] (figs. 225 and 226, vol. ii.). The variation in the form of the pinnules is shown in fig. 442, A, which represents both the apical portion and parts of pinnae 10 cm. lower on the rachis of a large leaf from the Coal Measures of Radstock. The species is characteristic of the Upper Coal Measures and is recorded also from Permian strata. In 1883 Sterzel adopted for this species the generic name Dicksonites because of the occurrence of shallow circular cups at the end of the lowest lateral vein on some of the pinnules which he believed to be sori of the Cyatheaceous type. The cups have an involute margin and occasionally a small scar in the centre (fig. 442, B). Stur[443] declined to accept Sterzel’s evidence as satisfactory and suggested a fungal origin for the sorus-like impressions, a view, as Sterzel objected, that is rendered improbable by the constant position of the single cups on several pinnules. The nature of Sterzel’s ‘sori’ has not been demonstrated: it is suggested by Grand’Eury[444] that they mark the position of microsporangia. Some seed-bearing specimens in Dr Kidston’s collection show cups, like those figured by Sterzel, on pinnules from which the seeds have fallen, and it is not improbable that they are the scars of seeds. In 1905 Grand’Eury published a description and photographs of specimens of P. Pluckeneti from the St Étienne coal-field showing hundreds of well-preserved seeds, many of them attached to pinnules characterised by a very slightly reduced lamina. Some fronds were found to be entirely fertile, while others bore both sterile and fertile pinnae. The smallest seeds, 5 mm. long and 3 mm. broad, were found at the tips of unexpanded leaves: the mature seeds, only slightly larger, agree in their broadly oval form and narrow marginal ‘wing’ with small examples of Samaropsis[445]. The seeds were figured by Grand’Eury[446] in an earlier work as Carpolithes granulatus. They are believed to have hung free from the lamina, a conclusion based on the position of the seeds relative to the plane of the pinnule in well-preserved examples. Prof. Zeiller informs me that he is by no means certain that Grand’Eury’s seed-bearing fronds should not be referred to Pecopteris Sterzeli; but as that species and P. Pluckeneti are very closely allied forms and may well have borne the same general type of fructification, the question of specific difference does not affect the significance of Grand’Eury’s discovery. A statement was made in vol. ii.[447], quoted from Grand’Eury, that the fronds of P. Sterzeli were borne on a Psaronius stem, but Prof. Zeiller told me that in his opinion the fronds and stem are merely in association and not in organic contact. It is probable that the Psaronius stem bore fronds of some species of Pecopteris with sori of the Asterotheca or Scolecopteris type and not seeds. Pecopteris Pluckeneti and P. Sterzeli are no doubt the fronds of a Pteridosperm[448], but apart from the seed-impressions there is no evidence as to the nature of the reproductive organs or stem. The form of the seeds with a fairly thick sarcotesta, which gives them a winged appearance, suggests a member of the Medulloseae rather than a plant with seeds like those of Lyginopteris and Heterangium.

Eremopteris artemisaefolia Steinberg with Samaropsis acuta Lindley and Hutton.

The generic name Eremopteris was instituted by Schimper for a type of frond from the Coal Measures of Newcastle described by Sternberg as Sphenopteris artemisaefolia. He included also a second species, E. Neesii, from the Permian of Bohemia: this was removed by Zeiller to Callipteris. The type-species of Eremopteris is included in this chapter on the ground that the almost constant association with the fronds of seeds comparable with those described by White as Aneimites (Wardia) fertilis affords a strong argument in favour of assigning Eremopteris artemisaefolia to the Pteridosperms.

The large compound fronds of this species[449] are characterised by the regular dichotomy of the main branches, a feature frequently met with in Palaeozoic fern-like leaves: the cuneate or oval-cuneiform pinnules (fig. 443, A, C) vary considerably in breadth from the typical cuneate type of segment as figured by Brongniart[450] to narrow, almost linear, leaflets like those of Sphenopteris crithmifolia. Several spreading veins traverse the lamina. Lindley and Hutton, while admitting a very close resemblance between their species and S. artemisaefolia, adopted a distinctive name. The only evidence so far obtained as to the stem of the plant is furnished by some specimens in the Hutton collection (Newcastle-upon-Tyne) one of which shows a piece of rhizome bearing several petioles (fig. 443, B): there are no pinnules attached to the rachises but some occur in close association. Brongniart noticed the frequent association of Eremopteris fronds with small seeds, but he regarded it as accidental. Dr Kidston[451] has recently drawn attention to a note by Prof. Duns published in 1872 on the juxtaposition of seeds and fronds, and Mr Howse[452] in his Catalogue of the Hutton plants considers that the seeds were borne on the Eremopteris leaves; in his synonymy of E. artemisaefolia he includes Cardiocarpon acutum Lind. and Hutt. as the ‘spore-cases or sporangia.’ The Eremopteris seeds are of the platyspermic (Samaropsis) type, broadly oval and about 7 mm. long with an obtuse base and two slightly divergent acute processes at the apex (fig. 444). Some specimens in Dr Kidston’s collection from the Lower Coal Measures of Midlothian, which were associated with Eremopteris fronds, are preserved as mummified cuticular membranes and on microscopical examination they show clearly the presence of a pollen-chamber. The seeds are of the Samaropsis type. The drawings reproduced in fig. 444 were made for me by Dr Kidston from two specimens, in his collection, of exceptionally well-preserved seeds from Midlothian: the seeds of this species vary considerably in size and form; some are almost orbicular and show no distinction between nucule and border (fig. 444, A) while in others (B) the impression of the flattened and longitudinally striated sarcotesta is clearly distinguished. Kidston is of opinion that in younger seeds there is a single apical point replaced in a later stage of development by two cusps, as seen in figs. A and B, formed by the opening of the micropylar tube. A ridge in the middle of the flattened surface indicates the position of the vascular bundles in the principal plane as in Cardiocarpus. The fully developed seeds are 8–9 mm. long. The correlation of the seeds represented in figs. 443 and 444 with Eremopteris fronds furnishes an additional illustration of the impossibility of trusting to external form as a criterion of affinity, for it is known that seeds of the Samaropsis type were produced by Pteridosperms with foliage represented by Eremopteris artemisaefolia and Pecopteris Pluckeneti, also by some members of the Cordaitales (e.g. fig. 480). Dr Arber[453] has recently proposed a new generic name Cornucarpus for Cardiocarpon acutum, but the drawings that he gives of seeds from the Kent coal-field referred to this species suggest a type distinct from that of Lindley and Hutton. In the absence of specimens showing actual attachment it is impossible to say how the seeds were borne, but the analogy of Wardia fertilis and Pecopteris Pluckeneti lends support to the view that the seeds were attached to pinnules with a reduced lamina. Eremopteris artemisaefolia occurs in the Lower and Middle Coal Measures of England: a species recorded by Kidston from the Calciferous sandstone of Scotland as E. Macconochii[454] is now believed by that author to be generically distinct[455]. With the exception of the unsatisfactory specimen reproduced in fig. 443, B, we have no information with regard to the habit of the stem to which the Eremopteris leaves were attached.

WARDIA. White.

Wardia fertilis (White). Another example of a fern-like frond bearing seeds is afforded by specimens from the Lower Pottsville series of Virginia (correlated with the Millstone grit of British geologists) described by Mr David White[456] as Aneimites (Wardia) fertilis. The compound fronds usually referred to the genus Adiantides or Adiantites[457] are characterised by cuneate pinnules with a thin lamina and forked, slightly divergent veins (fig. 445, A, D). White discards the name Adiantides in favour of Dawson’s genus Aneimites on the ground that Goeppert[458], who instituted the former term, applied it in the first instance to leaves of Ginkgo which he identified as simple Fern fronds. In spite of this misapplication of the name it has been constantly used and is well established. The discovery of seeds is, however, a reason for the adoption of a new generic name, and as White proposed Wardia for the seeds it may appropriately be extended to the fronds in place of the provisional term Aneimites. The seeds which occur at the apices of slender pedicels on pinnae bearing relatively small pinnules with a reduced lamina (fig. 445, B, C) are rhomboidal in shape, 4·5 mm. long and 2·5 mm. broad. The bilaterally symmetrical seeds were probably enclosed, as White suggests, in a fleshy integument which on pressure became laterally extended as a wing-like border. In some of the seeds there is an indication of a ‘slight collapse within the apex of the nutlet,’ which may mean the presence of a pollen-chamber; but while the preservation is too imperfect to afford any decisive evidence as to anatomical features, there is no reason to doubt the conclusion as to the seed-nature of the organs described by White. Nothing is known of the stem, though the opinion may be hazarded that Wardia is a member of the Medulloseae.

Adiantites bellidulus Heer and Lagenospermum Arberi Nathorst.

Reference is made to the genus Lagenospermum in the account of Lagenostoma[459]. The species Lagenospermum Arberi has recently been founded by Dr Nathorst[460] on some seeds obtained from Lower Carboniferous rocks in Spitzbergen: a brief description is intercalated here because it is probable that they were borne on fronds of the Adiantites type similar to those on which White found the seeds described by him as Wardia. The seeds of L. Arberi, 14–18 × 5 mm., are spindle-shaped with an obtuse apex and longitudinally ribbed with a stalk at least 7 mm. long. Nathorst considers that a cupule was probably present: the specimens do not convey the impression of naked seeds and in some examples there are indications of an investing envelope, though this may be the result of tearing of the testa. Nathorst regards the Spitzbergen seeds as probably specifically identical with a specimen described by Schmalhausen[461] from Carboniferous rocks in the Urals as Rhabdocarpus orientalis Eich., a species which agrees closely with Lagenospermum nitidulum as described by Heer[462] (under the name Carpolithes nitidulus) and Nathorst[463] from Spitzbergen. It is also possible that Kidston’s Rhabdocarpus elongatus[464], from the Lanarkshire coal-field and elsewhere, recently transferred by Arber[465] to Platyspermum, is an example of the same species. The chief interest of Lagenospermum lies in the fact, assuming Nathorst’s correlation of the seeds with Adiantites bellidulus Heer to be correct, that it is a typical radiospermic seed, while Wardia, borne on foliage of the same general type, is an equally typical platyspermic seed.