Miadesmia membranacea, Bertrand. Fig. 219, A–D.

Fig. 219.

The slender stem, characterised by unequal dichotomy, has a single protostele composed of scalariform tracheids with 3–6 peripheral protoxylem groups. A zone of delicate tissue surrounds the xylem; this is described as phloem, but it is not clear whether the designation is based on histological characters or primarily on its position. The cortex consists of an inner lacunar tissue and an outer region limited by a small-celled superficial layer sharply contrasted with the underlying layers of larger cells. The stem of Miadesmia is not uncommon in sections of the Lancashire calcareous nodules, and may be recognised by the delicate crushed tissue of which it mainly consists and by large hypodermal parenchyma. The spirally disposed leaves bear a conspicuous and relatively large ligule, 3 mm. long, in a deep pit ([fig. 219], B and C) roofed over by a few layers of tissue corresponding to the velum in Isoetes (cf. [fig. 133], E, v). The fairly thick central region of the lamina is expanded laterally into thin wings, which in the living state probably bore delicate hairs. These delicate leaves, apparently without stomata, were attached to the stem at an acute angle, and Miss Benson suggests that their form and arrangement may have enabled them to hold water by surface-tension. As seen in [fig. 219], B, C, which represents part of a transverse section near the leaf-base, the ligule is a very characteristic feature, and the size of the single vein is in keeping with the almost filmy nature of the lamina.

In addition to the sections in British collections, I have been enabled by the kindness of Prof. Bertrand to see photomicrographs of the sections on which he founded the genus. One of these sections, transverse to the stem and leaves, illustrates in a striking manner the relatively large size of the leaves and ligules in proportion to the delicate axis of the shoot.

The megasporangiate cone has an axis which agrees in its structure with that of the vegetative stem and bears several megasporophylls approximately at right-angles. As in the foliage leaves, the ligule is prominent and large, and lies in a groove which contains also the megasporangium; both ligule, l, and sporangium, s, as seen in the transverse section represented in [fig. 219], D, are covered by an integument or velum which arises in the proximal part of the leaf and leaves a circular micropylar opening at the beak-like apex of the sporangium. The circular micropyle is surrounded by numerous hairs borne on the integument and which presumably played the part of a feathery stigma. A single megaspore with a thin membrane, m, abuts on the fairly strong sporangial wall, s; in some cases the sporangium and megaspore walls may be indistinguishable, a feature suggesting comparison with seed-structure. Some megaspores have been found filled with a prothallus. The longitudinal section shown in [fig. 219], A, illustrates the characteristic horizontal position of the megasporophyll, as also the relation of the ligule, l, to the sporophyll with its single vascular bundle, and to the hairy integument, which overarches both sporangium and ligule; the line m shows the position of the megaspore-membrane, detached from the sporangial wall on the upper side but in contact with it below. The microsporophyll shown in 219, E, was originally referred to Miadesmia but has since been recognised by Watson[673] as that of a Bothrostrobus.

Miadesmia affords an example of a Palaeozoic plant comparable with Isoetes and Selaginella; it agrees also with Lepidocarpon in possessing true seeds, and with Watson’s Bothrodendron cone in the shape of the sporangia, which are more like those of Selaginella than the radially elongated sporangia of Lepidostrobus. Miadesmia agrees with Selaginella, e.g. S. spinosa, in its stelar structure, in the form of the sporangia, and in the presence of a ligule. It is distinguished by having only one instead of four megaspores in a sporangium, in the possession of an integument which formed a close investment to the spore and served as a stigma (comparable with the stigma-like integument of the male flower of Welwitschia), and in the shedding of the megasporophylls, which have been aptly compared with winged seeds.

LEPIDOCARPON

On the ground of their general anatomical features Lepidocarpon and Miadesmia are clearly entitled to be included among extinct representatives of the Pteridophyta. These plants had, however, crossed what it has been customary to regard as the boundary between Pteridophytes and Phanerogams: they possessed megasporangia with the attributes of seeds. It has been suggested by Lester Ward[674] that Pteridophytic seed-bearing plants shall be recognised as a distinct phylum for which he proposes the name Pteridospermaphyta, a designation implying exclusion from the Spermatophyta as usually understood. For seed-bearing Lycopodiaceous genera he suggests the name Lepidospermae. As knowledge of the Palaeozoic seed-plants increases revision of existing classifications and group names will become necessary, but as yet we are hardly in a position to draw up a satisfactory scheme of grouping; we know little of Lepidocarpon as a whole and it would be premature to commit ourselves, even provisionally, to a classification which is based on such meagre evidence as we possess. Moreover the value to be attached to the seed-habit as a basis of classification can hardly be estimated until fuller information is obtained.