This genus, founded by Renault on a specimen from Autun, is represented in the Lower Coal-Measures of England by Botryopteris hirsuta (= Rachiopteris hirsuta Will.), B. ramosa (= R. ramosa Will.[1119]) ([fig. 306]) and B. cylindrica ([fig. 305]), also by B. antiqua ([fig. 307]) from the Culm of Pettycur, Scotland.
An important characteristic of the genus is the solid stele of the stem which agrees with that of Tubicaulis and Grammatopteris, except in the central or peripheral position of the smallest tracheae.
Botryopteris forensis Renault[1120]. Figs. [309], B; [319], D–G.
The stem of this species from the Upper Carboniferous of St Étienne is 1·7 cm. x 7·5 mm. in diameter. The solid stele consists of reticulate tracheae with the smallest elements on the outer edge. The comparatively broad cortex of the type-specimen is traversed by a leaf-trace in an almost vertical course and by vascular strands passing horizontally to roots. The petioles are circular in section and their vascular strand has the form of an ω in transverse section ([fig. 319], G), the three projecting arms pointing to the axis of the stem. Both stem and leaves bore large multicellular hairs, spoken of by Renault as equisetiform because of the finely toothed sheaths of which they are composed. The compound fronds had fleshy lobed pinnules with dichotomously branched veins ([fig. 309], B); stomata are said to be confined to the upper surface, an observation which leads Renault to describe the plant as aquatic on evidence which is hardly convincing.
The pyriform and pedicellate sporangia are borne in groups of two to six on the ultimate divisions of the frond; the wall is composed of two layers of cells and on one side of the sporangium is an annulus several cells in breadth ([fig. 319], D, F). An interesting type of sporangium described by Oliver[1121] from Grand’Croix in France may, as he suggests, belong to Botryopteris forensis; the differences between Renault’s and Oliver’s specimens being the result of the more perfect preservation of the tissues in the latter. The sporangium described by the English author is circular in section and measures 0·65 × 0·53 mm.; the wall is in part composed of a single layer of cells and in part of two to three layers, a character recalling the “annulate” sporangia of Botryopteris. Between the spore-mass and the wall is an interrupted ring of short tracheal elements similar to the xylem-mantle which occurs at the periphery of the nucellus of certain Palaeozoic gymnospermous seeds. In the absence of proof of a connexion between this sporangium and Botryopteris it is convenient to use the generic name Tracheotheca subsequently proposed by Oliver[1122]. In the recent ferns Helminthostachys and Botrychium, and, as Oliver notices, in the microsporangia of the Australian Cycad Bowenia spectabilis, vascular strands extend almost to the sporogenous tissue, but the fossil sporangium is unique in having a tracheal layer in immediate contact with the spores. These xylem elements may, as Oliver suggests, have served the purpose of conveying water to the ripening spores.
Botryopteris hirsuta (Will.)[1123].
This English species has a slender axis bearing numerous leaves with petioles equal in diameter to the stem. The surface of the vegetative organs bears large multicellular hairs. The leaf-traces resemble those of B. forensis, but the projecting teeth which terminate in protoxylem elements are less prominent than in the French species; the petioles were named by Felix Rachiopteris tridentata[1124]. As a leaf-trace passes into the stele of the stem the three protoxylem strands unite and take up an internal position in the solid stele. The stele may, therefore, be described as endarch. The small tracheae at the edge of the stele supply the xylem strands of adventitious roots.
Sporangia similar to those of B. forensis have been found in association with the English species.
Botryopteris cylindrica (Will.). Fig. 305.
A plant originally described by Williamson[1125] from the Lower Coal-Measures of England as Rachiopteris cylindrica ([fig. 305]) and afterwards more fully dealt with by Hick[1126], has a slender stem with a cylindrical stele characterised by well-defined central protoxylem elements in one or two groups. The leaf-traces are semi-lunar in section with the protoxylem on the flatter side. The stele of Botryopteris cylindrica ([fig. 305], A) is more cylindrical in section than that of B. ramosa ([fig. 306]) and shows more clearly the differentiation into smaller central and larger peripheral tracheae. In the section reproduced in [fig. 305], B the stele is giving off a branch almost identical in structure with the main vascular axis. Scott[1127], in referring to the inclusion of this type in the genus Botryopteris, expresses the opinion that its habit must have been very different from that of other species, and he suggests the institution of a new genus.