Aphlebia.

This name was proposed by Presl[1389] for large leaf-like impressions having a pinnate or pinnatifid form and characterised by a confused irregular type of venation, or by a fine superficial striation or wrinkling which simulates veins. Gutbier had previously described similar fossils as Fucoides, and other authors have described Aphlebiae as species of Rhacophyllum, Schizopteris, and other genera[1390]. The term Aphlebia is retained, not as denoting a distinct genus but (i) as a descriptive name for detached leafy structures similar to those figured by Presl, which are now recognised as laminar appendages of the petioles of ferns or fern-like fronds, and (ii) as an epithet for highly modified pinnules which frequently occur at the base of the primary pinnae of Pecopteroid and Sphenopteroid fronds (e.g. Dactylotheca plumosa, [fig. 293])[1391].

Modified pinnules, similar in their reduced and deeply dissected lamina to those represented in [fig. 293], are frequently found at the base of the primary pinnae of Palaeozoic species of Sphenopteris and other genera of Pteridosperms or ferns, including members of the Coenopterideae. Potonié[1392] gives a list of various types of Aphlebiae in his paper on these organs. A striking case has recently been described by Zeiller in a French Upper Carboniferous species, Sphenopteris Matheti[1393]. It would seem that the larger examples of Aphlebiae are more frequently associated with the compound leaves of Pteridosperms than with those of Ferns[1394].

As examples of the larger types of Aphlebiae reference may be made to Aphlebia crispa (Gutb.)[1395], which reaches a length of nearly 60 cm. and has the form of a more or less triangular pinnate leaf divided into decurrent deeply lobed segments, to a similar species represented by A. Germari (= Schizopteris lactuca Germ.)[1396] which simulates the leaves of endive (Cichorium endivia L.), and to some large forms figured by Grand’Eury[1397] as species of Schizopteris.

Aphlebiae such as that figured by Kidston[1398] as Rhacophyllum crispum, with narrow ultimate segments, might easily be mistaken for the impressions of an alga.

The term Aphlebia may be applied also to the Cyclopteroid pinnules on the petioles of some species of Neuropteris, Odontopteris and Archaeopteris. Goebel[1399] has referred to the application by Potonié and other authors of the term Aphlebioid to the pinnules which serve as bud-protecting organs in recent fronds of Gleichenia ([fig. 226], p. 290); he expresses the opinion that it is superfluous and misleading to make use of a special designation for structures which are undoubtedly modified pinnules. In the case of fossils it is, however, convenient to employ the term Aphlebia as a descriptive name for modified pinnules or stipular structures which cannot be connected with definite species of fronds. It is clear that some Aphlebiod leaflets, such as those of Dactylotheca, served as protective organs for the unexpanded pinnae[1400], and in all probability the large Aphlebiae served the same purpose as the fleshy stipules of Angiopteris and Marattia which cover the uncoiled fronds. The pinnatifid scale-leaves of considerable size ([fig. 351]) which occur in the leaf-axils or as ochrea-like stipules on the fronds of Gunnera (a tropical and subtropical Dicotyledonous genus) bear a very close resemblance to some Palaeozoic Aphlebiae, e.g. Aphlebia crispa (Gutb.). The recent and fossil scale-leaves may be regarded as similar in function as in form; moreover the delicate coiled fronds of Palaeozoic Pteridosperms or ferns, like those of some recent flowering plants, may have been kept moist by a secretion of mucilage. The pinnatifid stipules of Marattia fraxinea ([fig. 241], B, p. 317) resemble certain fossil Aphlebiae, and the wrinkled surface of the recent stipules presents an appearance similar to that which in some fossil forms has been erroneously described as veining. It is not improbable that mantle-leaves of such recent ferns as Polypodium quercifolium ([fig. 234], M, p. 303) are comparable with some fossil Aphlebiae which may have served as humus-collectors for Palaeozoic epiphytes.

Fig. 351. Scale-leaf of Gunnera manicata. (Slightly reduced. M.S.)

The filiform appendages on the petioles of the recent fern Hemitelia capensis ([fig. 235], p. 304) have often been compared with the aphlebioid leaflets of fossil fronds.

Potonié who has discussed the nature of Aphlebiae regards them as vestiges of a once continuous lamina, which formed a winged border to the branched axes of more primitive forms of fronds. It is possible that the pinnules between the pinnae on the rachis of Archaeopteris and the Cyclopteroid leaflets of Neuropteris and Odontopteris may have the morphological significance attributed to them by Potonié. In some cases it is probable that the Aphlebiae, whether vestiges or not, served the purpose of protecting either the whole frond or individual pinnae. Aphlebiae, though especially characteristic of Palaeozoic leaves, are occasionally met with in the form of modified pinnules at the base of the primary pinnae on Mesozoic ferns, e.g. in Coniopteris hymenophylloides[1401].

In some fern fronds the lowest pinnule of each pinna differs in shape or size from the normal ultimate segments, but it would be almost affectation to extend the use of the term Aphlebia to such pinnules. The Jurassic species Cladophlebis lobifolia (Phill.) is a case in point[1402]. In this fern, which some authors speak of, without sufficient reason, as Dicksonia lobifolia[1403], the lowest pinnule is large and different in shape from the others.

Fig. 352.