This genus was founded by Baily[1194] on fragments of a fern from Carboniferous rocks in County Limerick, Ireland, characterised by a peculiar type of fructification which he named Corynepteris stellata. More complete examples of the same genus have been described by Zeiller[1195] from the Coal-field of Valenciennes. The sporangia are large, ovoid, and sessile; the annulus ([fig. 309], D) has the form of a complete vertical band several cells in breadth: five to ten sporangia are grouped round a receptacle. Zeiller describes two species as Sphenopteris (Corynepteris) coralloides Gutb. and S. (Corynepteris) Essinghii And.; in both the fronds are quadripinnate and bear aphlebiae at the base of the pinnae. The former species is recorded by Kidston[1196] from the South Wales Coal-field. A single pinnule of C. coralloides is shown in [fig. 309], C. Potonié[1197] refers this frond to his genus Alloiopteris: the portion of a pinna represented in [fig. 354], G shows the characteristic modified pinnule next the rachis. Zeiller draws attention to the occurrence of two parallel lines on the rachis of a specimen of Corynepteris coralloides which he figures[1198], and suggests that these may indicate the existence of an H-shaped form of vascular strand like that of Etapteris and Ankyropteris. The sorus of Corynepteris is comparable with that of the Marattiaceae, but the broad annulus is a difference which suggests affinity to Etapteris. The sorus is similar to that in Diplolabis ([fig. 309], A), but in that genus the sporangia are exannulate.
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The vascular axis in the stems of different members of the Coenopterideae assumes a variety of types. In Botryopteris antiqua the xylem forms a solid protostele in which no protoxylem strands have been recognised; in other species, e.g. B. ramosa, the cylindrical stele is similar to that of Trichomanes radicans (Hymenophyllaceae) in the more or less central position of the protoxylem. In Botryopteris forensis the protostele is said to be exarch. The probability is that the central Botryopteris type is the endarch protostele, a form of vascular axis which may be regarded as primitive. The leaf-traces of the Lower Carboniferous Botryopteris antiqua are simple oval strands differing but slightly from the cylindrical stele of the stem. In the Upper Carboniferous British species the petiolar vascular strand has become more specialised and farther removed from that of the stem; in B. forensis the distinction between leaf and stem steles is still more pronounced. It is perhaps legitimate to regard these types as representing an ascending series, the more primitive of which are distinguished by the greater similarity between leaf and stem, organs differentiated from a primitive thallus[1199], that is from a vegetative body. Portions of this ultimately became specialised as lateral members or leaves, while a portion acquired the character of a radially constructed supporting axis or stem.
ANKYROPTERIS, ETC.
The vascular strand characteristic of the Zygoptereae is represented by the H-shaped form as seen in Ankyropteris corrugata or in a more complex form in A. bibractensis. This style of strand may be regarded as a development from the simple strands of Grammatopteris and Tubicaulis or Botryopteris antiqua along other lines than those followed by B. forensis. The extension of the xylem in two symmetrically placed arms at the ends of the cross-piece of the H is correlated with the habit of branching of the leaf-system which forms one of the striking peculiarities of many of the Zygoptereae. The solid type of stele characteristic of the Botryoptereae is closely matched by that in the Lower Carboniferous stem discovered by Mr Gordon[1200]. By the partial transformation of the central xylem region into parenchymatous tissue and the concentration of water-conducting elements in the peripheral region the style of Ankyropteris corrugata was developed. The vascular strand of the older plant, which is of the Diplolabis type, may be regarded as a more primitive style than that of the H-form of petiole strand represented by Ankyropteris corrugata. A further stage in evolution is seen in the stem stele of Ankyropteris Grayi and A. scandens, both of which have the H-form of meristele. This step in increasing complexity of stem stele, though probably connected with the increasing specialisation of the leaf-traces, as held by Mr Tansley, may also be associated with the development of a climbing habit. In Asterochlaena laxa Stenzel ([fig. 324]) and A. ramosa (Cotta)[1201] the tendency towards a stellate expansion of the originally cylindrical form of stele reaches a higher degree, with the result that a style is evolved which agrees closely with that of the conducting tissue of some existing Dicotyledonous Lianes.
Attention has already been drawn to the generalised features exhibited by the Coenopterideae both in the anatomy of the steles and in the structure of the sporangia. The conclusion arrived at is that while the Coenopterideae foreshadow in some of their characters more than one group of more recent ferns, some at least of their members afford convincing evidence of the correctness of the view—which is also that of Dr Kidston and Mr Gwynne-Vaughan—that the Osmundaceae and the Coenopterideae are offshoots of a common stock.
Fig. 324. Asterochlaena laxa: part of stem with petiole and a few roots. From Tansley, after Stenzel.