Bertrand includes in Ankyropteris Renault’s species Zygopteris bibractensis[1157] and Williamson’s species Rachiopteris corrugata[1158]: the former he names A. bibractensis var. westphaliensis. The fossil described by Williamson as R. irregularis or inaequalis[1159] are the secondary branches of A. bibractensis.
Ankyropteris bibractensis, var. westphaliensis. Figs. [312], C; [313].
The rachis stele of this species, which is represented by portions of fronds only, has the form of a double anchor ([fig. 313]); the antennae are continued at the outer edge of their distal ends into a narrow band (“filament” of P. Bertrand) ([fig. 312], C, and 313, a) composed of smaller tracheae and separated from the xylem of the antennae by a strip of thin-walled tissue (phloem?). A group of protoxylem occurs at the junction of the filament and antennae. The whole of the xylem is surrounded by phloem.
Fig. 313. Ankyropteris bibractensis: s, stigmarian rootlet; a, narrow loop of xylem. (Cambridge Botany School; × 6).
The section reproduced in [fig. 313] shows the characteristic form of the petiolar vascular axis, consisting of a horizontal band of metaxylem with groups of much smaller tracheae on both the upper and lower margins. At the junction between the antennae, curved like the flukes of an anchor, and the horizontal band of xylem, the latter is only one trachea in breadth. The narrow loops of smaller xylem elements are shown on the outer edge, a ([fig. 313]), of the antennae separated from the arcs of larger tracheae by a dark line which represents a crushed band of delicate tissue. The spaces enclosed by the incurved antennae are largely occupied by parenchymatous ground-tissue. The cylinder of outer cortex consists internally of comparatively thin-walled parenchyma succeeded externally by a zone of dark and thicker-walled cells characterised by a fairly regular arrangement in radial series, as if formed by a secondary meristem; there is, however, no indication of a meristematic layer. Below the small-celled epidermis are a few layers of thinner-walled cells which are not arranged in radial series. The structure of the outer part of the cortex is similar to that in the petiole of recent species of Angiopteris ([fig. 243], p. 319) and Marattia, in which a more delicate hypoderm is succeeded by a band of mechanical tissue.
The rachis of this type of frond gives off two rows of lateral branches from the vascular axis, the plane of symmetry being at right angles to the primary rachis. Each pinna bore at its base two aphlebiae supplied with vascular strands from the leaf-traces.
We have no certain information in regard to the sporangia of this species, but Scott points out that “pear-shaped sporangia, with a very broad and extensive annulus, are commonly found associated with Zygopteris bibractensis and Z. corrugata in the petrifactions of the English Lower Coal-Measures[1160].”
Ankyropteris corrugata (Will.). Figs. [312], B; [314–317].
The stem of this type of Zygoptereae was described by Williamson from the Lower Coal-Measures of Lancashire as Rachiopteris corrugata and included by him in the sub-group Anachoropteroides. The stele ([fig. 314], B) is oval in transverse section; it consists of a cylinder of xylem tracheae enclosing a central region occupied by parenchymatous tissue and scattered narrow scalariform tracheae. The central tissue extends radially in the form of narrow arms which reach almost to the outer edge of the tracheal tissue and divide it up into 5–7 groups. A cylinder of thin-walled tissue encloses the xylem and in this occur groups of large sieve-tubes ([fig. 314], D, Sv).