Fig. 320. Stauropteris oldhamia. (After Tansley. From a section in Dr Scott’s Collection. × 60.)
It is practically certain that the fronds described by Grand’Eury[1182] as Schizopteris pinnata ([fig. 309], E) and Schizostachys frondosus represent respectively the sterile and fertile leaves of Etapteris. Zeiller[1183] gives expression to this by substituting the generic name Zygopteris for Schizopteris, and we may now speak of the leaves as Etapteris. Dr White[1184] has referred to a new genus, Brittsia, some impressions of pinnate fronds from the Coal-Measures of Missouri which, as he points out, bear a close resemblance to Schizopteris pinnata Grand’Eury ([fig. 309], E). No sporangia have been found; it is, however, probable that Brittsia problematica represents fragments of a leaf borne by a plant closely allied to Etapteris (Zygopteris). The broad rachis bears crowded pinnae given off at a wide angle; the small pinnules are rather deeply lobed or pinnatifid (3–10 mm. long by 1·5–3 mm. broad). The lamina is traversed by irregularly lobed and occasionally anastomosing veins. In the fertile pinnae the segments have no lamina but bear bundles of pedicellate sporangia.
It should be noticed that the sporangia described by Renault and by other authors as those of Zygopteris (fig. 319, A–C) have not been found in organic continuity with a frond showing a well-preserved vascular strand. It is, however, certain that this characteristic annulate sporangium, borne on branched and slender pedicels, was produced on fronds with a much reduced lamina belonging to some species of the Zygoptereae, Etapteris and probably also Ankyropteris.
Stauropteris.
This genus was instituted by Binney for petioles from the Lower Coal-Measures of Oldham (Lancashire).
Stauropteris oldhamia Binney[1185] is characterised by a stele (figs. [308], E–G; [310], C; [320]; [321]) composed of four groups of xylem which Bertrand regards as homologous with the antennae of Diplolabis, Ankyropteris, and Etapteris, the horizontal cross-piece of these genera being absent in Stauropteris. Williamson spoke of this species as “one of the most beautiful and also one of the most perplexing of the plants of the Coal-Measures”; he discussed its possible affinity with both Lycopods and ferns, deciding in favour of the latter group[1186]. In transverse section the petiolar vascular axis is approximately square, the xylem groups forming the ends of the diagonals; the tracheal groups are separated by phloem and the centre of the stele in the primary rachis is also occupied by that tissue, which is connected by four narrow strips with the external phloem. The structure of the petiolar vascular axis is very clearly shown in the drawing by Mrs Tansley reproduced in [fig. 320]. Protoxylem elements occur close to the surface of each of the four arms of the xylem; the bays between the two lateral and the two lower xylem groups contain large sieve-tubes. Portions of the inner cortex are seen in places abutting on the small-celled pericyclic tissue.
The right and left halves of the stele are not absolutely identical ([fig. 320]; [fig. 308], E); this is due to the fact that secondary branches are given off in four rows, two alternately from the right and left sides. The preparation for the departure of the lateral strands alters the configuration of the stelar xylem groups. The protoxylem groups are not external but separated from the surface by one or two layers of metaxylem. In [fig. 308], E, the occurrence of two protoxylem strands in the right-hand groups of metaxylem marks an early stage in the detachment of branches. These two protoxylems are the result of division of single protoxylem strands like those in the left-hand half of the stele. At a later stage the petiolar stele assumes the form shown in [fig. 308], F, and two small bundles are detached to supply aphlebiae: this is followed by the stage shown in fig. G, where two four-armed strands are passing out to a pair of branches of the leaf axis. The separation of these two meristeles leaves the right-hand half of the stele in the condition seen on the left-hand side of fig. E. The diagrammatic sketch represented in [fig. 310], C, shows one pair of branches in organic connexion with the rachis, and each of these arms contains an obliquely cut vascular strand like those in [fig. 308], G.
The cortex consists for the most part of fairly thick-walled parenchyma ([fig. 321]) which in the hypodermal region is replaced by a zone of thin-walled lacunar tissue. A few stomata have been recognised in the epidermis[1187]. The lower left-hand branch seen in [fig. 310], C, has been shaved by the cutting wheel so that the aerenchymatous tissue, l, is shown in surface-view: a portion of this tissue is enlarged in fig. C′. The same delicate chlorophyllous tissue forms a folded and shrivelled layer with an uneven margin on the surface of the rachis and lateral branches. This hypha-like tissue, which was discovered by Scott[1188] and figured by Bertrand[1189], doubtless represents the much reduced lamina of the highly compound leaves; it may be compared with the green outer cortex of Psilotum shoots and with the lacunar tissue in the capsule of the common moss, Funaria hygrometrica.
