Fig. 309.

(A, B, after Renault; C, D, after Zeiller; E, after Renault and Zeiller.)

The vascular strand of the rachis of Zygopteris primaria ([fig. 308], D) is simpler than that of most of the Zygoptereae and exhibits a close resemblance to the type of strand described by Renault as Diplolabis ([fig. 308], C).

Diplolabis.

Renault[1136] instituted this genus for two species from the Culm beds and Coal-Measures of France based on the structure of the petioles. The stems are unknown. The main rachis has a stele similar to that of Zygopteris primaria, but distinguished by its greater similarity, in transverse section, to an X rather than to the letter H: the long transverse bar in Zygopteris is here much reduced in size. The petiole of Diplolabis forensis[1137] Ren. ([fig. 308], C) has a diameter of 1·5–2 cm. From the antennae a pair of small bundles is given off alternately from the right and left side, as in Zygopteris; the members of each pair coalesce after leaving the antennae and then separate to pass into the lateral branches of the frond. The position of the protoxylem and the formation of the lateral xylem strands previous to their separation are shown in [fig. 308], C. On the side of the vascular strand shown in fig. C, 2, the two lateral extensions of the antennae are converging towards one another previous to their separation and subsequent union. The ovoid sporangia occur in groups of three to six and are coalescent below with a central receptacle; they have no annulus, but the cells on the side next the receptacle are smaller than those on the external wall ([fig. 309], A). The synangial form of the sorus suggests comparison with Marattiaceae.

The species described by Renault from the Culm of Esnost is regarded by P. Bertrand as identical with that described by Solms, from the Culm of Falkenberg, as Zygopteris Roemeri[1138]. Diplolabis is compared by P. Bertrand with Metaclepsydropsis, the generic name given to the Lower Carboniferous petiole described by Williamson as Rachiopteris duplex[1139].

Mr Gordon has recently described in a preliminary note a new type of stem stele under the name Zygopteris pettycurensis from the Lower Carboniferous plant bed of Pettycur[1140]: he regards the petioles attached to the stem as identical with Zygopteris Roemeri Solms-Laubach[1141]. This species, founded by Solms-Laubach on petioles only, is placed by Bertrand[1142] in the genus Diplolabis and regarded as identical with D. esnostensis Ren. The stele found by Mr Gordon may therefore be assigned to the genus Diplolabis: it includes two regions composed exclusively of tracheae and is cylindrical in transverse section. The inner xylem zone consists of short, square-ended, reticulately pitted elements and the outer zone is composed of long and pointed conducting tracheae. The scalariform protoxylem elements are situated between the two metaxylem zones. As Mr Gordon says: this type of stem occupies a position “in the Zygopteroid alliance” corresponding to that which Thamnopteris Schlechtendalii (p. 329) occupies in the Osmundaceous series. The discovery of this stem supplies another link between the two fern groups, Osmundaceae and Coenopterideae. Pelourde[1143] has described an imperfectly preserved vascular strand from a locality near Autun as the type of a new genus Flicheia esnostensis. Mr Gordon has pointed out to me that this is a partially rotted petiole of Diplolabis esnostensis (= Zygopteris Roemeri).

In their recent account of fossil Osmundaceous genera, Kidston and Gwynne-Vaughan[1144] speak of the central parenchyma of the existing medullated stele as being derived from tracheal tissue. They add that if the Zygopteroid line of descent is at all close to the Osmundaceous, we must be prepared for the existence of a Zygopteris with a solid xylem like that of Thamnopteris: “such a discovery, in fact, we hopefully anticipate[1145].” The new Pettycur stem amply justifies this prophecy. It is noteworthy that Mr Gordon’s stem affords an instance of the occurrence of a type of stele, similar in its cylindrical form and in the absence of parenchyma to that of Botryopteris, in a plant bearing leaves characterised by the Zygopteris type of vascular strand.