Gleicheniaceae.

The application by Goeppert[844] and other earlier writers of the generic name Gleichenites to examples of Palaeozoic ferns was not justified by any satisfactory evidence. One of Goeppert’s species, Gleichenites neuropteroides, is identical with Neuropteris heterophylla[845], a plant now included in the Pteridosperms.

The resemblance of sporangia and sori, whether preserved as carbonised impressions or as petrified material, from Carboniferous rocks, to those of recent species of Gleicheniaceae is in many cases at least the result of misinterpretation of deceptive appearances. Williamson[846] drew attention to the Gleichenia-like structure of some sections of sporangia from the English Coal-Measures, but he did not realise the ease with which sections of Marattiaceous sporangia in different planes may be mistaken for those of annulate (leptosporangiate) sporangia. In the regular dichotomous habit of Carboniferous fronds described as species of Diplothmema (Stur) and Mariopteris (Zeiller)[847] we have a close correspondence with the leaves of Gleichenia, but the common occurrence of dichotomous branching among ferns is sufficient reason for regarding this feature as an untrustworthy criterion of relationship. It is, however, interesting to find that in addition to the existence of some Upper Carboniferous ferns with sori like those of recent Gleichenias, the type of stelar anatomy illustrated by Gleichenia dicarpa ([fig. 237], C, p. 310) and other species is characteristic of the primary structure of the stem of the Pteridosperm Heterangium. We find in Carboniferous types undoubted indications of anatomical and other features which in succeeding ages became the marks of Gleicheniaceae.

Some Carboniferous fronds with short and small pinnules of the Pecopteris type, bearing sori composed of a small number of sporangia, have been assigned by Grand’Eury and other authors to the Gleicheniaceae; the same form of sorus is met with also on fronds with Sphenopteroid segments. The former is illustrated by Oligocarpia Gutbieri[848] and the latter by O. Brongniarti described by Stur and by Zeiller[849]. Zeiller has described the circular sori of Oligocarpia ([fig. 270], B) as consisting of three to ten pyriform sporangia borne at the ends of lateral veins and possessing a complete transverse annulus, but Stur[850] believes that the annulus-like appearance is due to the manner of preservation of exannulate sporangia. In this opinion Stur is supported by Solms-Laubach[851] and by Schenk[852]. Despite an agreement between Oligocarpia and Gleichenia, as regards the form of the sori and the number of sporangia, it is not certain that the existence of a typical Gleicheniaceous annulus has been proved to occur in any Palaeozoic sporangia[853].

From Upper Triassic beds of Virginia, Fontaine has figured several fronds for which he instituted the genus Mertensides[854]. The habit, as he points out, is not dichotomous, but the sori are circular and are said to be composed in some species of four to six sporangia. No satisfactory evidence is brought forward in support of the use of a designation implying a close relationship with recent Gleichenias (sect. Mertensia). One of the species described by Fontaine was originally named by Bunbury Pecopteris bullatus[855], the imperfect type-specimen of which is now in the Museum of the Cambridge Botany School. In the form of the frond, the thick rachis, and in the pinnules this Triassic species resembles Todites Williamsoni, but the resemblance does not extend to the sori. Two of Fontaine’s species are recorded by Stur from Austria[856], but he places them in the genus Oligocarpia and includes them in the Marattiaceae.

Leuthardt[857] figures what appears to be a Gleicheniaceous fern from the Upper Triassic beds of Basel as Gleichenites gracilis (Heer) showing sori composed of five sporangia ([fig. 265], C) with a horizontal annulus. A Rhaetic species Gleichenites microphyllus Schenk[858] from Franconia agrees in the form of its small rounded pinnules with Gleichenia, but no sporangia have so far been found.

An impression of a frond from Jurassic rocks of northern Italy figured by Zigno as Gleichenites elegans[859] closely resembles in habit recent species of Gleichenia; though no sporangia have been found, the habit of the frond gives probability to Zigno’s determination.

A Jurassic species from Poland, Gleichenites Rostafinskii, referred by Raciborski[860] to Gleichenia, exhibits a close agreement in habit and in the form of the soral impressions to some recent species of Gleichenia.

As we pass upwards to Wealden and more recent rocks it becomes clear that the Gleicheniaceae were prominent members of late Mesozoic floras in north Europe and reached as far north as Disco Island. In English Wealden beds portions of sterile fronds have been found which were assigned to a new genus Leckenbya[861], but it is probable that these specimens would be more correctly referred to Gleichenites. Similarly fragments of Gleichenia-like pinnae with very small rounded pinnules occur in the Wealden rocks of Bernissart, Belgium[862], in north Germany[863], and elsewhere. Conclusive evidence has been obtained by Prof. Bommer of the existence of Gleichenites in Wealden beds near Brussels, where many plant remains have been found in a wonderful state of preservation. The specimens, which I had an opportunity of seeing some years ago, might easily be mistaken for rather old and brown pieces of recent plants. Some of the Belgian fragments, of which Prof. Bommer has kindly sent me drawings and photographs, are characterised by an arrangement of vascular tissue identical with that in the petioles and rhizomes of some protostelic Gleichenias. The stele of one of the Belgian rhizomes appears to be identical with that of Gleichenia dicarpa ([fig. 237], C. p. 310).

Fig. 262.

Gleichenites longipennis Heer.
G. delicatula Heer.
G. Nordenskioldi Heer.
G. Zippei. (Corda.)

(After Heer; A, B, D, very slightly reduced.)

Gleichenites Zippei (Corda). Fig. 262, D.

This species, originally described by Corda as Pecopteris Zippei[864] and afterwards figured by Heer[865] as Gleichenia Zippei ([fig. 262], D) from Urgonian rocks of Greenland, affords a striking example of a Mesozoic member of the Gleicheniaceae. It is characterised by the dichotomous branching of the frond and by the occurrence of arrested buds in the forks. The long and slender pinnae, reaching a length of 9 cm. and a breadth of 6–8 mm., bear small crowded pinnules occasionally with circular sori which are described by Heer as consisting of a small number of sporangia (cf. [fig. 262], C). Several other Lower Cretaceous species are recorded by Heer from Greenland, some of which are probably unnecessarily separated from Gleichenites Zippei. Examples of these are represented in [fig. 262], A, B, C.

A Gleicheniaceous species described by Debey and Ettingshausen from Lower Cretaceous rocks of Aix-la-Chapelle as Didymosorus comptonifolius[866] is very similar in habit to some of Heer’s Greenland species: this should probably be referred to the genus Gleichenites.

Gleichenites hantonensis, Wank. Fig. 263.

From the Eocene beds of Bournemouth, Gardner and Ettingshausen[867] have described under the name Gleichenia hantonensis what is in all probability a true Gleichenia ([fig. 263]). This species, originally recorded by Wanklyn[868], is characterised by a slender forked rachis showing what may be traces of arrested buds between the arms of the branches, by circular sori of six or eight sporangia and by the presence of peculiar tendril-like appendages on the pinnae. If the description of the tendrils is correct, this British species affords one of the few instances of ferns adapted for climbing and may be compared with the recent species Davallia aculeata ([fig. 232], p. 299).