Fig. 311. Ankyropteris Grayi. Stele. (From a section in Dr Kidston’s Collection, × 18.)
Ankyropteris scandens Stenzel[1153]. Fig. 310, D.
This Lower Permian species is very similar to or perhaps identical with Ankyropteris Grayi (Williamson). The stem of
A. scandens was found in association with the roots of a Psaronius stem evidently petrified in situ as it burrowed, like Tmesipteris, tropical aroids, and other recent plants, among the living roots of the tree-fern. The stem, 10–11 mm. in diameter, bore fronds with an H-shaped vascular strand, small scale-leaves, and adventitious roots. The stele consists of a five-angled cylinder of scalariform tracheae surrounding an axial strand of parenchyma containing scattered tracheae of smaller diameter. This axial tissue extends as a narrow strip into each of the short and obtusely truncated arms (cf. [fig. 311]). A striking feature is the production of a shoot in the axil of the foliage-leaves ([fig. 310], D), a manner of branching characteristic of Trichomanes (see [page 365]).
Ankyropteris Grayi (Will.). Fig. 311.
In describing this species, Williamson wrote—“That no classification of these fossil ferns based solely upon transverse sections of the petiolar bundles is or can be of much value, is clearly shown when tested amongst those living ferns the classification of which is chiefly based upon the sporangial reproductive organs[1154].” This is a view entirely opposed to that which inspires P. Bertrand’s recent monograph. Whether the value attached to the vascular structure of petioles as a basis of classification is upheld or not, it is noteworthy that since Williamson expressed his opinion, our knowledge of the anatomy of ferns and of the value of anatomical evidence has enormously increased. The slender stem[1155] of this Lower Coal-Measures species agrees closely with that of A. scandens; it bore spirally disposed fronds, scale-leaves, and roots. The stele has the form of an irregular five-rayed star ([fig. 311]) in which the relative length of the arms varies in different sections owing to the separation of the distal ends to form leaf-traces. The axial region is composed of parenchyma and associated narrow tracheae, as in A. scandens. The xylem, with protoxylem elements at the ends and especially at the angles of the arms, is completely surrounded by phloem. The cortex consists internally of parenchyma which becomes thicker-walled towards the periphery and bears multicellular epidermal hairs. A leaf-trace is detached in the form of a triangular strand and is formed by the tangential extension of the distal end of an arm of the stele. The trace, on its way through the cortex, divides into two; the outer branch gradually changes from a slightly curved band to an H-shaped meristele; the inner branch, which supplied an axillary shoot, is similar to the stele of the stem, but smaller. Scott[1156] has recently recorded the occurrence of scale-leaves (aphlebiae) in this species like those described by Stenzel in A. scandens.
Fig. 312.
- Thamnopteris Schlechtendalii. Leaf-trace: px, protoxylem; s, island of parenchyma. (After Kidston and Gwynne-Vaughan.)
- Ankyropteris corrugata. Single trachea with tyloses.
- A. bibractensis. Part of foliar strand. (After P. Bertrand.)