Pecopteris.
Reference has already been made to this genus in the chapter on Marattiales, so far as regards certain species of fertile fronds the sporangia of which resemble those of recent Marattiaceae. It is, however, by no means safe to assume that such Pecopteris fronds were borne on stems having the anatomical characters of ferns. The sporangia in some at least of the species may have contained microspores. In one Upper Carboniferous species usually referred to Pecopteris, P. Pluckeneti, Schlot., Grand’Eury[1582] has recorded the occurrence of seeds on the pinnules of the ordinary fronds. This species will be referred to in Volume III. The substitution of such generic names as Ptychocarpus, Asterotheca, Hawlea, Dactylotheca and others for the purely provisional designation Pecopteris indicates a step towards a conclusion as to natural affinity. The probability is that Pecopteris, as applied to Palaeozoic species, in many cases stands for the compound fronds of true ferns, but the possibility of the inclusion of those of Pteridosperms in the same category is by no means excluded. The designation Pecopteris may conveniently be retained for sterile bipinnate, tripinnate, or quadripinnate fronds bearing pinnules having the following characteristics:
Lamina short, attached to the rachis by the whole of the base and at a wide angle, with the edges parallel or slightly converging towards the usually blunt apex; adjacent pinnules may be continuous basally by a narrow lamina. A well-marked midrib extends to the apex and gives off simple or forked lateral veins almost at right angles ([fig. 352], D, p. 529).
Hydathodes like those on the leaflets of Polypodium vulgare and other recent ferns[1583] are occasionally seen at the ends of the lateral veins of Pecopteris pinnules.
In addition to the examples of Palaeozoic fronds with the Pecopteris form of pinnule referred to in chapter XXII., the species Pecopteris arborescens may be briefly described.
Pecopteris arborescens (Schlotheim)[1584]. Figs. [352], D: [376].
The species named by Schlotheim Filicites arborescens in 1804 is characteristic of the Upper Coal-Measures and is recorded also from Permian strata[1585].
Fronds large; the rachis, which may reach a breadth of 3 cm.[1586], gives off long ovoid-lanceolate pinnae in two alternate rows ([fig. 376]); pinnules small, 1·5–4mm. long and 1–2mm. broad, contiguous, with rounded apex, attached approximately at right angles; the upper surface of the lamina is slightly convex and may be hairy[1587]. The fertile pinnules, identical in shape with the sterile, bear groups of ovoid exannulate sporangia (synangia). The midrib extends to the apex of the pinnule and gives off simple veins at a wide angle ([fig. 352], D).
Our knowledge of the reproductive organs is very meagre. Grand’Eury described the synangia as consisting of 3–5 sporangia borne on a central receptacle; sporangia have been described also by Stur[1588], Renault and Zeiller[1589], and Potonié[1590], but no fertile British specimens are recorded. Stur places this species in the genus Scolecopteris, and Potonié regards the sporangia found by him on Permian fronds, which may be identical with Pecopteris arborescens, as conforming to those of the Asterotheca type. It is impossible to decide on the evidence available whether this species is a Pteridosperm or a fern, but there is a natural inclination in doubtful cases to give preference to the first of these two choices.
Fig. 376. Pecopteris arborescens (Schloth.). From the Upper Coal-Measures of Radstock. From a photograph by Dr Kidston. Reduced.
The numerous fronds from Carboniferous and Permian rocks described as species of Pecopteris exhibit a considerable range of variation in the form of the pinnules. In many species the pinnules are of the type represented in [fig. 352], D; in others the lamina of the ultimate segments is slightly contracted at the base and the secondary veins are given off at a more acute angle, as in Pecopteris polymorpha, Brongn.[1591] In Pecopteris unita, Brongn., already described as Ptychocarpus unita[1592], the pinnules are joined together except in the apical region. Some fronds included in Pecopteris possess pinnules in which Pecopteroid and Sphenopteroid features are combined; P. Sterzeli, Zeill.[1593] and P. Pluckeneti, Schlot. are examples of fronds in which the pinnules are lobed as in Sphenopteris, but the base of the lamina is only slightly contracted and the venation is not that of typical Sphenopteris species.
The species to which Potonié has applied the generic name Alloiopteris[1594] also illustrates the impossibility of drawing a sharp line between Pecopteris and Sphenopteris. The fronds already described in chapter XXV. under the designation Corynepteris bear pinnules with a contracted base; in some species the lamina is lobed, but in others ([fig. 354], G) it is entire with a midrib nearer one edge than the other. The species which Potonié assigns to Alloiopteris, like many other Sphenopteroid and Pecopteroid fronds, are characterised by the occurrence of an abnormal pinnule (aphlebia) at the base of each pinna ([fig. 354], G, p. 535). Young fronds of Pecopteris are occasionally met with showing very clearly the circinate vernation of the pinnae as in the leaves of Cycas and Angiopteris represented in [fig. 220], p. 283. The genus Spiropteris was created by Schimper[1595] for coiled unexpanded fronds of fossil ferns; it is however superfluous to apply a distinctive term to specimens of this kind.
The designation Pecopteris is employed chiefly for leaves of Palaeozoic age which are unknown in the fertile state, or do not afford sufficient evidence as to the nature of the sporangia to justify the substitution of a special generic name. Many Mesozoic species have also been referred to Pecopteris, but most of these are more appropriately included in Brongniart’s later genus Cladophlebis. The pinnules of Cladophlebis, as Brongniart pointed out, are intermediate between Pecopteris and Neuropteris; they are usually attached by the whole breadth of the base, as in Pecopteris, but the more acute origin, more arched form, and more frequent dichotomy of the lateral veins are features shared by Neuropteris. As a rule, Mesozoic sterile fronds with straight or folded, entire or dentate pinnules are of the Cladophlebis type: this genus is especially characteristic of Rhaetic and Jurassic floras. Examples of Cladophlebis pinnules are shown in figs. [256], [257] (pp. 340, 342). It is to be regretted that authors do not make more use of the generic name Cladophlebis in describing sterile fronds, instead of following the misleading and unscientific practice of employing such genera as Pteris, Asplenites, and others on wholly insufficient grounds.