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.