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_. [Illustration: FIG. 246. _Ophioglossum vulgatum._ Transverse section of petiole and single bundle: _p_, phloem; _px_, endarch protoxylem.] [Illustration: 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. CHAPTER XXI. FOSSIL FERNS. =Osmundaceae.=