have been reproduced by means of seeds. The cases in which such evidence is decisive are but few, namely, Lyginodendron Pteridospermeae. oldhamium, Neuropteris heterophylla, Pecopteris Pluckeneti, Aneimites fertilis and Aneimites tenuifolius. In the first-named plant the structure, both of the vegetative and reproductive organs, is known, and the evidence, from comparison and association, is sufficiently strong. In the other cases there is direct proof of continuity between seed and plant, but only the external characters are known. In a great number of forms, amounting to a majority of the Palaeozoic plants of fern-like habit, the indirect evidence is in favour of their having possessed seeds. We will begin with the Lyginodendreae, a group in which the anatomical characters indicated a systematic position between Ferns and Cycads, long before the reproductive organs were discovered.
| |
| (After Williamson. Scott, Studies.) | |
|
Fig. 19.—Heterangium Grievii. Restoration of Stem, shown partly in transverse and longitudinal section, partly in surface view. | |
| x, | Primary wood. |
| x2, | Secondary wood. |
| p.c. | Phloem and pericycle. |
| c, | Cortex. |
| hy, | Hypoderma. |
| l.t, l.t, | Leaf-traces. |
| r, | Adventitious root. Several leaf-bases are shown. |
|
| (After Stur. Scott, Studies.) |
|
Fig. 20.—Sphenopteris elegans (foliage of Heterangium Grievii). Part of frond. (⅔ nat. size.) |
 | |
| (Scott, Studies.) | |
|
Fig. 21.—Heterangium Grievii. Part of the stele of the stem in transverse section, showing a primary xylem-strand and adjacent tissues (× 135.) | |
| px, | Protoxylem of strand. |
| x, | Centripetal. |
| x1, | Centrifugal primary wood. |
| mx, | Part of the internal wood. |
| c.p. | Conjunctive tissue. |
| x2, | Secondary wood. |
| cb, | Cambium. |
| ph2, | Phloem. |
Lyginodendreae.—Of the genus Heterangium, which still stands very near the true Ferns, several species are known, the oldest being H. Grievii, of Williamson, from the Lower Carboniferous of Scotland. This plant had a long, somewhat slender, ridged stem, the ridges corresponding to the recurrent bases of the spirally arranged leaves (fig. 19). The specimens on which the genus was founded are petrified, showing structure rather than habit, but conclusive evidence has now been obtained that the foliage of H. Grievii was of the type of Sphenopteris (Diplotmema) elegans (fig. 20), and was thus in appearance altogether that of a Fern, with somewhat the habit of an Asplenium. The stem has a single stele, resembling in general primary structure that of one of the simpler species of Gleichenia; there is no pith, the wood extending to the centre of the stele. The leaf-traces, where they traverse the cortex, have the structure of the foliar bundles in Cycads, for they are of the collateral type, and their xylem is mesarch, the spiral elements lying in the interior of the ligneous strand. The leaf-traces can be distinguished as distinct strands at the periphery of the stele, as shown in fig. 21. Most of the specimens had formed a zone of secondary wood and phloem resembling the corresponding tissues in a recent Cycad; the similarity extended to minute histological details, as is shown especially in H. tiliaeoides, a Coal Measures species, where the preservation is remarkably perfect. The cortex was strongly constructed mechanically; in addition to the strands of fibres at the periphery, horizontal plates of stone-cells were present in the inner cortex, giving both stem and petiole a transversely striated appearance, which has served to identify the different parts of the plant, even in the carbonized condition (cf. figs. 19 and 20). The single vascular bundle which traversed the petiole and its branches was concentric, the leaves resembling those of Ferns in structure as well as in habit. Heterangium shows, on the whole, a decided preponderance of Filicinean vegetative characters, though in the leaf-traces and the secondary tissues the Cycads are approached. The organs of reproduction are not yet known, though there is a probability that an associated seed allied to Lagenostoma (see below) belonged to Heterangium. In the Coal Measure genus Megaloxylon, of Seward, which in structure bears a general resemblance to Heterangium, the primary wood consists for the most part of short wide tracheides; probably, as the secondary tissues increased, it had become superfluous for conducting purposes, and was adapted rather for water-storage. In the genus Lyginodendron, of which L. oldhamium, from the
