I. Botryoptereae.
Grammatopteris.
Renault instituted this genus for petrified stems from the Permo-Carboniferous beds of Autun. Grammatopteris Rigolloti[1115], the type-species, is represented by a fragment, 12–15 mm. in diameter, surrounded by crowded petioles characterised by a vascular strand in the form of a short and comparatively broad plate with the smallest tracheae at each end. The solid xylem of the stem stele (protostele) has peripheral groups of protoxylem. Nothing is known as to the form of the leaves, but sporangia similar to those of Etapteris (Zygopteris) were found in association with the stem. It is possible, as P. Bertrand suggests, that Renault’s species may be the stem of a Tubicaulis.
Tubicaulis.
Tubicaulis solenites (Sprengel)[1116]. Fig. 304.
This species from the Lower Permian of Saxony has been fully described by Stenzel[1117]. It is characterised by a very slender erect stem bearing numerous spirally disposed leaves associated with adventitious roots; the single stele (protostele) consists exclusively of tracheae, described as intermediate between the scalariform and reticulate type, surrounded by phloem. Leaf-traces are given off from the periphery of the stele where groups of smaller elements occur; these have the form of a wide-open U-shaped strand with the base of the U facing the axis of the stem. As the trace passes out towards the leaves, the ends of the U become more or less incurved. The stem is said to reach a metre in length and to bear compound fronds a metre long. The orientation of the leaf-trace with its concavity turned outwards is in striking contrast to the relation between leaf-trace and stem in recent ferns.
Fig. 304. Tubicaulis solenites. (From Tansley, after Stenzel.) Stem and petioles: the latter numbered in the order of their age.
Tubicaulis Sutcliffii, Stopes[1118].
In this species the vascular axis, 2 mm. in diameter, is almost cylindrical and of the protostelic type with the protoxylem “near to or at the edge”: the tracheae are scalariform or reticulate. The leaf-traces, when first separated from the edge of the stele, are oval and gradually assume the curved form seen in T. solenites ([fig. 304]) with the convex side towards the axis of the stem. The transition from the scalariform to the reticulate type of pitting on the tracheal walls referred to by Miss Stopes has also been noticed in some recent ferns (e.g. Helminthostachys) and in Sigillaria ([fig. 200], C, p. 212). The fact that the scalariform type of pitting is practically universal in the xylem of recent ferns, would seem to show that this character has been acquired in the course of evolution and retained in preference to the reticulate form characteristic of several Palaeozoic species. The distinction between the two methods of pitting is one of little phylogenetic importance.
Botryopteris.
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.
Fig. 305. Botryopteris cylindrica (× 30). From sections in the Cambridge Botany School.
Botryopteris ramosa (Williamson). Fig. 306.
This species, which bears a close resemblance to Botryopteris hirsuta, was originally described by Williamson from the Lower Coal-Measures of England as Rachiopteris ramosa[1128], the specific name being chosen on account of the numerous and crowded branches given off from the main axis. The section shown in [fig. 306], A, illustrates Williamson’s description of the stem as being “always surrounded [when seen in transverse sections] by a swarm of similar sections of the large and small branches, though of varying shapes and sizes.” The stele is composed of a solid and more or less cylindrical rod of xylem tracheae of the reticulate type surrounded by phloem (figs. A and D): one or more internal groups of smaller protoxylem elements occur in an approximately central position (fig. A, px). The stele is in fact endarch like those of Selaginella spinosa and Trichomanes reniforme, a feature which, as Tansley[1129] believes, probably entitles the vascular axis to be considered a primitive form of protostele. In the specimens represented in [fig. 306] the phloem and inner cortical tissues were almost completely destroyed before petrifaction. The thick-walled outer cortex bears at its periphery numerous multicellular hairs. Some of the xylem strands given off from the stele no doubt supplied adventitious roots, but in most cases the outgoing branches are leaf-traces and the numerous sections of axes of different sizes seen in fig. A point to a repeated subdivision of the crowded fronds. The structure of a petiole is shown in figs. C and D. As seen in fig. C, the oval vascular strand has three protoxylem groups, px, on its flatter side; a well-defined epidermal layer is shown at e in fig. C.
Fig. B shows at a a section of a leaf-axis in the act of branching and the row of branchlets at b represents a further stage in subdivision. At sp in fig. A the section has cut through a single sporangium characterised by a group of larger (“annulus”) cells on one side of the wall.
Fig. 306.
A–D. Botryopteris ramosa; stem and frond axes. (A × 7; B × 15; C × 26; D × 13. From sections in the Cambridge Botany School Collection.) px, protoxylem; sp, sporangium; e, epidermis.
This slender fern with its numerous repeatedly branched leaves may perhaps have lived epiphytically on more robust plants.
Botryopteris antiqua, Kidst. Fig. 307.
This species, recently described by Kidston[1130] from the Culm of Pettycur near Burntisland, is represented by sections of a small stem with a cylindrical stele 0·40 mm. in diameter composed entirely of scalariform tracheae without any recognisable protoxylem. The petioles are larger than the stem; the meristele ([fig. 307]) is oval with protoxylem elements on the slightly more rounded adaxial face. As Kidston suggests, this stem may belong to a scrambling plant which required support to bear its relatively large leaves. An interesting feature is the absence of projecting teeth in the leaf-trace, a character in marked contrast to the ω form assumed by the petioles of Botryopteris forensis ([fig. 319], G) and B. hirsuta. This leads Kidston to suggest that the vascular strand of the petiole tends “to become more simple ... as traced back in geological time.” The greater similarity in this species between the stele of the stem and that of the petiole is probably another mark of a more primitive type.
Fig. 307. Botryopteris antiqua: Petiolar vascular strand. (After Kidston: × 65.)
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In these three types, Grammatopteris, Tubicaulis, and Botryopteris, we have monostelic plants, for the most part of very small size, with leaf-traces varying in shape from the oblong band-form in Grammatopteris, and the oval form of Botryopteris antiqua, to the ω type represented in its most pronounced form by B. forensis. In several species the stem stele is endarch. Our knowledge of the leaves is very meagre: in B. forensis they were repeatedly branched and apparently bore small fleshy pinnules; the sporangia, though differing from those of recent ferns, may be compared with the spore-capsules of Osmundaceae as regards the structure of the annulus. The abundance of hairs on the stems and leaves of some species, the tracheal sheath in the sporangium described by Oliver[1131] as Tracheotheca (= Botryopteris?), and the apparent absence of a large well-developed lamina, may perhaps be regarded as evidence of xerophilous conditions.