Metaclepsydropsis duplex (Will.) [fig. 310], A[1146]. [= Rachiopteris duplex, Williamson 1874. Asterochlaena (Clepsydropsis) duplex, Stenzel 1889. Clepsydropsis, Renault 1896.]

The vascular axis of the main axis of the frond is characterised by the hour-glass shape of the xylem which consists entirely of tracheae, most of which are reticulately pitted. In a transverse section ([fig. 310], A) the two ends of the stele are dissimilar; at one end of the long axis is a small bay of thin-walled tissue (phloem) enclosed by a narrow band of xylem, and at the other the bay is open and has two protoxylem groups. The latter represents the earliest stage in the production of secondary bundles: at a later stage the bay is closed by the elongation of the edges, the enclosed group of phloem is vertically extended, and the protoxylem strands are more widely separated. The curved band of xylem becomes detached as a curved arc and divides into two ([fig. 310], A). In a single section of this species one often sees several strands of xylem enclosed in a common cortex with the main vascular axis; these are the xylem bundles of lateral pinnae. Metaclepsydropsis duplex shows the method of branching of the petiole vascular axis which has already been noticed in Diplolabis and Zygopteris. In reference to this feature, Williamson wrote in 1872—“I know of no recent fern in which the secondary branches of the petiole are thus given off in pairs, which pairs are distichously arranged on the primary axis, and each of which secondary petioles sustains ternary ones arranged distichously.” By slightly altering the primary stele of this type of frond, by narrowing of the constricted portion of the hour-glass and extending the lateral groups of xylem obliquely upwards, the form of stele shown in [fig. 310], A, would be converted into the Diplolabis type ([fig. 308], C).

Clepsydropsis.

Unger[1147] instituted this genus as a subdivision of Corda’s family Rhaciopterideae[1148], the name having reference to the hour-glass form of the vascular axis[1149]. The type-species C. antiqua ([fig. 308], A) is spoken of as the commonest fossil plant in the Devonian rocks of Thuringia. In some sections the xylem has the form seen in [fig. 308], A, in which an invagination of thin-walled tissue occurs at each end; in other sections ([fig. 308], A′) the bays become islands in the xylem. Solms-Laubach speaks of Unger’s species as Rachiopteris (Clepsydropsis) antiqua. P. Bertrand[1150], who has recently described Unger’s plant, while recognising that C. antiqua and Metaclepsydropsis duplex closely resemble one another, draws attention to certain differences in the structure of the xylem which he regards as sufficient to justify a generic separation. The leaf-traces of Clepsydropsis are described by Bertrand as almost circular closed rings of xylem instead of an arc as in Metaclepsydropsis.

Fig. 310.

[A, from a section in Dr Kidston’s Collection (Lower Carboniferous); B, C, from sections in the Cambridge Botany School; D, after Stenzel.]

Ankyropteris.

Stenzel adopted this name for a subdivision of Corda’s genus Zygopteris, applying it to a species described by Renault as Z. Brongniarti, to a Permian species described by himself as Z. (Ankyropteris) scandens, and to Z. Lacattii Ren.; Rachiopteris Grayi Will. and Rachiopteris corrugata Will. are also included in this genus. The characters emphasised by Stenzel[1151] are (i) the double anchor-like form of the H-shaped petiole strand in which the lateral arms (antennae) are curved like the flukes of an anchor, and (ii) the emission of four rows of branches instead of two. The latter distinguishing feature no longer holds good, as Z. primaria also gives off four rows of bundles and not two as Stenzel supposed. P. Bertrand has adopted Stenzel’s genus in a narrower sense[1152].