Neuropteris Scheuchzeri, Hoffmann. Figs. 354, F; 373.
Fragments of this well-known Coal-Measure species were figured by Scheuchzer in his Herbarium Diluvianum[1540] as Lithosmunda minor, and by Lhywd (Luidius[1541]) as Phyllites mineralis as early as 1760. Neuropteris Scheuchzeri, so named by Hoffmann in 1826, is a type which many authors have described under different names. Lesquereux[1542] figured it as N. hirsuta from the Coal-fields of Pennsylvania, and under the same name it is recorded by Fontaine and White[1543] from Permian rocks of Virginia. The oval patches on the surface of a pinnule described by these authors as sori are certainly not of that nature. The same species is described by Bunbury[1544] from Nova Scotia as N. cordata Brongn. var. angustifolia. For a full synonymy of the species reference should be made to lists published by Kidston[1545], White[1546], and Zeiller[1547].
The large tripinnate fronds are characterised by the long linear- or oval-lanceolate pinnules (fig. 373)[1548] with a pointed apex and numerous bristle-like hairs on the lamina; two much smaller Cyclopteroid segments occur at the base of the pinnae which are terminated by the linear leaflets (fig. 354, F, p. 535).
Neuropteris Scheuchzeri is characteristic of the Upper and Middle Coal-Measures of Britain and is recorded from several localities in North America and the Continent. Zalessky[1549] has recently recorded the species from the Coal-Measures of Donetz. The frequent occurrence of detached pinnules points to a caducous habit. Even single leaflets can, however, be identified by their large size, the pointed apex, and hairy lamina. The hairs are preserved as fine oblique lines simulating veins; they were so described by Roemer[1550] who took them for cross-connexions between the secondary veins and referred the pinnules to Gutbier’s genus Dictyopteris.
Another example of Neuropteris with hairy pinnules is described from the Commentry Coal-field by Renault and Zeiller as N. horrida[1551]. The oval-linear, bluntly rounded, pinnules are characterised by a median band of hairs on each surface and a narrower strip at the edge of the lamina.
Cyclopteris.
This generic name was created by Brongniart in 1828[1552] for specimens which he believed to be complete single leaves of orbicular or reniform shape similar to those of Trichomanes reniforme. The lamina is traversed by numerous dichotomously branched veins which spread from the centre of the base.
It was suspected by Lindley and Hutton[1553] that certain Cyclopteris leaves belonged to the frond of a species of Neuropteris, and some years later Lesquereux[1554] concluded that Brongniart’s genus was founded on orbicular leaflets of Neuropteris. In 1869 Roehl[1555] figured a specimen of Neuropteris bearing Cyclopteroid pinnules on its rachis. It is now universally admitted that Cyclopteris is not a distinct genus and that the specimens so named were borne as modified pinnules on the main rachis of Neuropteris and Odontopteris. It is, however, convenient to retain the name for detached leaflets which cannot be referred to the fronds on which they were borne. A specimen found by Mr Hemingway in the Upper Coal-Measures of Yorkshire and described in 1888[1556] affords a striking example of the large size attained by what was probably a frond of Neuropteris. The piece of main rachis reached a length of over 120 cm. and bore five pairs of Cyclopteris pinnules, some of which were 7 cm. long and 5 cm. broad. The complete frond must have reached a length of at least 4 metres. Fig. 370 shows some typical Cyclopteroid leaflets on the petiole of a Neuropteris frond.
Linopteris.
The Upper Palaeozoic fronds included in this genus are more familiar as species of Dictyopteris. Potonié[1557] has, however, pointed out that the creation of this name by Lamouroux in 1809 for a genus of Brown Algae which is still retained, makes it advisable to fall back upon the designation Linopteris. Gutbier[1558] proposed the genus Dictyopteris in 1835: Linopteris was first used by Presl[1559] in 1838. The fronds so named are identical with species of Neuropteris except in the anastomosis of the secondary veins; Linopteris bears to Neuropteris the same relation as Lonchopteris bears to Alethopteris. As in Neuropteris, Cyclopteroid pinnules occur on the petioles of Linopteris, but the veins form a fine reticulum. Grand’Eury[1560] records the association of Linopteris Brongniarti with seeds belonging to the genus Hexagonocarpon, a fact which points to the Pteridosperm nature of the foliage.
Some fertile pinnules of Linopteris Schutzei (Roemer) are described by Zeiller[1561] from Autun as bearing on the under surface of the lamina two rows of long and pointed sporangia, probably united in groups. The presumption is that these are microsporangia.
Fig. 374 is a reproduction of a careful drawing, originally published by Zeiller[1562], of a pinnule of the type-specimen of Gutbier’s species Linopteris neuropteroides. This species differs from Linopteris obliqua, instituted by Bunbury[1563] for specimens obtained by Lyell[1564] from the Coal-Measures of Nova Scotia, in the smaller size of the meshes. Linopteris obliqua occurs in the Upper and Middle Coal-Measures of Britain; it is recorded by Zeiller from Asia Minor, by Lesquereux[1565] from Pennsylvania, and by other authors from several European localities. The pinnules frequently occur detached from the frond and like those of some species of Neuropteris were caducous. Linopteris is rare in British strata.
Alethopteris.
The name Alethopteris, instituted by Sternberg[1566], is applied to compound fronds often reaching a considerable size, exhibiting the following features:
The linear pinnules are attached by the whole breadth of the base, with the lower edge of the lamina decurrent and usually continuous with that of the next pinnule (figs. 290, A, p. 399; 375). The ultimate segments are entire, with an acute or rounded apex and often characterised by a fairly thick lamina convex on the upper surface. From a prominent midrib, continued to the apex of the pinnule, numerous simple and forked secondary veins are given off at a wide angle, the decurrent portion of the lamina being supplied by veins direct from the axis of the pinna. In the upper part of a frond or primary pinna the pinnules may be replaced by a continuous, lobed, or entire simple lamina. The main rachis occasionally exhibits dichotomous branching, but the fronds are for the most part constructed on the pinnate plan. Single Cyclopteroid pinnules[1567] occur on the petiole of some species of the genus.
In certain species of Alethopteris the pinnules appear to have been deciduous as in Didymochlaena among recent ferns[1568]. A piece of cuticle from the upper surface of a pinnule of Alethopteris Grandini (Brongn.) figured by Zeiller[1569] shows very clearly the polygonal form and straight walls of most of the epidermal cells, those above the veins being almost rectangular. The position of the sunken stomata is revealed by small circular spaces surrounded by a circle of cells.
The absence of fertile specimens of this common genus of Upper Carboniferous plants led Stur[1570] to exclude it from the ferns. Although no seeds have so far been found in organic connexion with an Alethopteris frond, it is certain that some species, probably all, represent the foliage of Pteridosperms. Renault was the first to describe petrified specimens of Alethopteris fronds exhibiting the anatomical structure of Myeloxylon (leaf-axis of Medullosa). The calcareous nodules from English Coal-seams contain numerous fragments of the Myeloxylon type of rachis bearing Alethopteroid pinnules.
The constant association of the fronds of Alethopteris lonchitica and Trigonocarpon seeds noticed by Mr Hemingway in the Coal-Measures of Yorkshire led him to regard the species as seed-bearing: it has since been recognised as the foliage of the Pteridosperm Medullosa anglica[1571].
Grand’Eury[1572] has recorded the association in French Coal-fields of species of Alethopteris with Trigonocarpon and Pachytesta seeds.
Alethopteris lonchitica (Schlotheim)[1573]. Figs. 364, A; 290, A.
This species, described by Schlotheim in 1820 as Filicites lonchiticus and previously figured by Scheuchzer[1574], is abundant in the Middle and Lower Coal-Measures of Britain[1575]. It is characterised by large tripinnate fronds, probably quadripinnate in the lower part, bearing primary pinnae of a more or less triangular form divided into pinnate branches replaced in the apical region by linear segments. The pinnules, 8–30 mm. long and 3–5 broad, are linear- or oval-lanceolate with an obtuse apex; the upper margin of the lamina is slightly contracted at the base, while the lower edge is decurrent.
Alethopteris Serlii (Brongniart)[1576]. Fig. 375.
This species, figured by Parkinson in 1811, closely resembles A. lonchitica, but is distinguished by the more crowded and relatively longer pinnules which are joined to one another by a narrow connecting lamina (Fig. 375). The secondary veins in A. Serlii are rather finer and more numerous. Grand’Eury[1577] records the association of the seed Pachytesta with fronds of this species in the Coal-Measures of St Étienne.
A. Serlii is very abundant in the Upper Coal-Measures but rare in the Middle Coal-Measures of Britain[1578].
Lonchopteris.
This name was proposed by Brongniart[1579] for sterile fronds from Upper Carboniferous rocks which are practically identical with species of Alethopteris, but differ in the reticulate venation of the pinnules. It has been pointed out in a previous chapter[1580] that Lonchopteris is usually used for Palaeozoic species, the Wealden leaves, which were placed in this genus by Brongniart, being transferred to Weichselia.
There can be little doubt as to the close relationship of Lonchopteris with Alethopteris: both may be referred to the Pteridosperms. Lonchopteris rugosa Brongn.[1581] (fig. 290, B, p. 399) and L. Bricei Brongn., both British species, are fairly common in Upper Carboniferous strata. In L. rugosa, a Middle Coal-Measures species, the anastomosing secondary veins form polygonal meshes (fig. 290, B, p. 399) smaller than those of L. Bricei.
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.
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.