Page:EB1911 - Volume 20.djvu/587

PALAEOZOIC] Coal Measures, is now the best-known of all Palaeozoic plants, the central wood has disappeared altogether and is replaced by pith; the primary wood is only represented in the leaf-trace strands, which form a ring of distinct collateral bundles around the pith; thus the “medullate-monostelic” structure characteristic of the higher plants was already attained. The individual bundles, however, have the same structure as in Heterangium, and agree closely with the foliar bundles of Cycads. The secondary tissues, which are highly developed, are also of a Cycadean character (fig. 22, Plate). The vegetative organs of the plant are very completely known; the foliage has proved to be that of a Sphenopteris, identical with the species long known under the name of S. Höninghausi, Apart from the important advance shown in the anatomy of the stem, Lyginodendron agrees structurally with Heterangium. There is reason to believe that Lyginodendron oldhamium was a climbing plant comparable in some respects to such recent Ferns as Davallia aculeata. The roots were at first like those of Marattiaceae but grew in thickness like the roots of Gymnosperms.

The first definite evidence of the mode of reproduction of Lyginodendron oldhamium was due to F. W. Oliver, who in 1903 identified the seed, Lagenostoma Lomaxii, by means of the glands on its cupule, which agree exactly with those on the associated leaves and stems of the plant (cf. figs. 24 and 25). No similar glands are known on any other Palaeozoic plant. Lagenostoma Lomaxii is a small barrel-shaped seed (5.5 by 4.25 mm. when mature) enclosed in a husk or cupule, which completely enveloped it when young, but was ultimately open (figs. 23 and 26 and fig. 27 from another species). The seed was stalked, and there is an exact agreement in structure between the vascular strands of the stalk and cupule of the seed, and those of the rachis and leaflets of Lyginodendron, thus confirming the evidence from the glands. The seed itself is of a Cycadean type, and radially symmetrical. The single integument is united to the nucellus, except at the top, and is traversed by about nine vascular strands. In the apex of the nucellus, as in most Palaeozoic seeds and in recent Cycads, a pollen-chamber, for the reception of the pollen-grains or micro spores, is excavated (fig. 26). In Lagenostoma the pollen-chamber has a peculiar structure, a solid column of tissue rising up in the middle, leaving only a narrow annular crevice, in which pollen-grains are found. The neck of the flask-shaped pollen-chamber projected a little from the micropyle and no doubt received the pollen directly. The seed, which need not be described in further detail, was a highly organized structure, showing little trace of the cryptogamic megasporangium from which we must suppose it to have been derived. From the structure of the seed-bearing stalk, and from the analogy of the similar form Lagenostoma Sinclairi (fig. 27) it appears that the seed was borne on a leaf, or part of a leaf, reduced to a branched rachis.

The male organs of Lyginodendron were discovered by Kidston, a year or two after the seeds were identified. They are of the type known as Crossotheca, formerly regarded as a Marattiaceous fructification. The genus is characterized by the arrangement of the sporangia, which hang down from the lower surface of the little oval fertile leaflets, the whole resembling an epaulet with its fringe (fig. 15, F; fig. 28). In the case of Lyginodendron the Crossotheca occurs in connexion with the vegetative parts of the frond. Each fertile pinnule bore six, or rarely seven fusiform microsporangia, described as bilocular; not improbably each may represent a synangium. The microspores are tetrahedral. This is the first case in which the pollen-bearing organs of a Pteridosperm have been identified with certainty.

It will be seen that, while the seeds of Lyginodendron were of an