Copeland has recently described a Marattiaceous leaf which he makes the type of a new genus, Macroglossum alidae. The sori are nearer the margin than in Angiopteris and are said to consist of a greater number of sporangia. The photograph[758] of a single pinna which accompanies the brief description hardly affords satisfactory evidence in support of the creation of a new genus. The structure of a petiole which I have had an opportunity of examining, through the kindness of Mr Hewitt of Sarawak, shows no distinctive features.
III. Ophioglossales. (Isosporous and Eusporangiate.)
The three genera, Ophioglossum, Botrychium, and Helminthostachys, are characterised by the division of the leaves into a sterile and a fertile lobe. The fertile lobe in Ophioglossum bears two rows of spherical sporangia sunk in its tissue; in Botrychium and Helminthostachys the spores are contained in large sporangia with a stout wall[759]. The prothallus is subterranean and without chlorophyll. In the British species of Ophioglossum, O. vulgatum (the adder’s tongue fern), an almost cosmopolitan species, the sterile part of the frond is of oval form and has reticulate venation. In O. pendulum and O. palmatum the lamina is deeply lobed. In the genus Botrychium, represented in Britain by B. Lunaria, both sterile and fertile branches of the frond are pinnately divided, while in Helminthostachys the sporangia are borne on sporangiophores given off from the margin of the fertile branch of a frond similar in habit to a leaf of Helleborus.
Fig. 246. Ophioglossum vulgatum. Transverse section of petiole and single bundle: p, phloem; px, endarch protoxylem.
Fig. 247. Botrychium virginianum: e, endodermis; c, cambium; x, xylem.
A, diagrammatic section of stem; B, portion of the stele and endodermis enlarged.
(A, after Campbell; B, after Jeffrey.)
The stem of Ophioglossum is characterised by a dictyostele of collateral bundles with endarch protoxylem: the vascular system of the leaf-stalk is also composed of several separate strands ([fig. 246]). In Botrychium the stele is a cylinder of xylem surrounded externally by phloem. This genus affords the only instance among ferns of a plant in which the addition of secondary tracheae occurs on a scale large enough to produce a well-defined cylinder of secondary xylem traversed by radial rows of medullary-ray cells[760] ([fig. 247]). The unsatisfactory nature of the evidence in regard to the past history of the Ophioglossales renders superfluous a fuller treatment of the recent species.