In a section of Ankyropteris corrugata in the Williamson Collection the radial arrangement of the more external metaxylem elements suggests the addition of secondary tracheae[1166]. This suggestion of secondary thickening, a point which requires much more thorough investigation, is interesting in relation to a new type of stem named by Scott Botrychioxylon[1167], but not yet fully described. This generic name has been given to a stem stele which closely resembles that of Ankyropteris corrugata except in the regular radial arrangement of the peripheral xylem elements. The name Botrychioxylon was chosen by Scott because of the secondary xylem characteristic of the recent genus Botrychium ([fig. 247], p. 322).
Fig. 316. Ankyropteris corrugata. Petiole. a, narrow xylem loop; b, spaces in cortex. From a section in the Cambridge Botany School Collection. (× 10.)
In the petiolar vascular strand represented in [fig. 315] the narrow band of tracheae which forms a loop external to the antennae is clearly seen, also the small-celled parenchyma between the loops and the larger metaxylem elements of the antennae. The crushed tissue lying on the outer face of each of the loops probably represents the phloem and pericycle; the thin-walled elements above and below the horizontal band of metaxylem are probably sieve-tubes.
Fig. 316 shows a transverse section of a petiole of this species: the loops, a, of small tracheae are seen bending round the outer edge of the antennae. The inner and more delicate cortical tissue is partially preserved and spaces, b, have been formed in it as the result of contraction previous to petrifaction. In the petiole represented in [fig. 317] the tracheae of the horizontal band are considerably crushed; the section is, however, of interest because of the presence of Lyginodendron roots, l, in the space originally occupied by the inner cortex.
Fig. 317. Ankyropteris corrugata. From a section in the Cambridge Botany School Collection. (× 9.)
In a paper on the tyloses of Rachiopteris corrugata, Weiss[1168] draws attention to the fact that similar inclusions have not been found in the tracheae of recent ferns. The occurrence of thin-walled parenchymatous cells in the large tracheae of Ankyropteris corrugata petioles and of other species is a striking feature. Williamson[1169] compared these cells with the tyloses in the vessels of recent flowering plants, and in a later paper[1170] he suggested that the included cells may belong to saprophytic or parasitic fungi. It is, as Weiss points out, difficult to explain the occurrence of tyloses in tracheae not immediately in contact with living parenchyma. It may be that the pits in the tracheae of Ankyropteris were open spaces as in the xylem of recent ferns described by Gwynne-Vaughan, and if so this would facilitate the invasion of the conducting elements by growing cells. A comparison is made by Weiss between certain cell-groups found by him in the tracheae of Ankyropteris and by Miss Jordan[1171] in the vessels of the recent dicotyledon Cucumis sativus. In a more recent paper on tyloses Miss McNicol[1172] expresses the opinion that pseudoparenchyma in the tracheae of the fossil petioles owes its origin to fungal hyphae.
Williamson compared the petiole bundles of Ankyropteris corrugata with those of recent Osmundaceae, a comparison based on the structure of the leaf-trace before its separation from the stem and its assumption of the H-form. It is noteworthy, however, that this comparison has acquired a greater significance as the result of recent work. The stele of Ankyropteris bears a fairly close resemblance to that of Zalesskya described by Kidston and Gwynne-Vaughan; in both types the xylem is represented by two kinds of tracheal tissue. In the Permian Osmundaceous genus the centre of the stele consists of short storage tracheids, while in Ankyropteris we may regard the central parenchyma and scattered tracheae as derivatives of the solid xylem core of some ancestral type. Moreover, the appearance and arrangement of the phloem and the tangentially elongated elements external to it ([fig. 314]) remind one of the extra-xylem zone in recent Osmundaceae. That the Osmundaceae and Zygoptereae are closely related groups there can be little doubt; of this affinity and common origin[1173] Ankyropteris corrugata affords striking evidence.
The difference between the steles of Ankyropteris Grayi and A. scandens (figs. [310], D; [311]) and that of Ankyropteris corrugata is comparatively small. In the two former species the cylindrical form has become stellate owing to the radial extension of the xylem arms. It may be that this more elaborate style of vascular construction is connected with the climbing habit of A. scandens and possibly A. Grayi. The radial extension of the xylem and the consequent alternation of the yielding parenchymatous cortex and the more rigid tracheal arms would probably render the water-conducting elements less liable to injury in a twisting axis[1174]. In Anachoropteris Decaisnii[1175], described by Renault, and more especially in Asterochlaena laxa[1176] Stenzel, a Lower Permian type from Saxony ([fig. 324]), the xylem of the stele is much more deeply lobed than in Ankyropteris Grayi or A. scandens.