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.
Fig. 375. Alethopteris Serlii (Brongn.). From a specimen in the York Museum. ¾ nat. size.
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].