British ferns (Filicopsida)
Least Adders Tongue.
Sporophyte. The rhizomes subterranean, very short, erect (with new plants arising from adventitious buds on the roots); naked. Plants with no clear distinction into fertile and sterile leaves (but hard to interpret as such: usually bearing only a single leaf comprising two components, the lower sterile and laminate, the upper a fertile, non-laminate spike - see below), or bearing markedly different fertile and sterile leaves (the fertile leaf sometimes accompanied by one or two sterile blades).
Leaves to 2–6(–10) cm long; dying in the autumn; not circinnate; compound (when fertile, see above, though the green blade is simple), or simple and compound (when sterile blades occur). The fertile leaves having the appearance of a stem with a terminal fertile portion and a single lateral leaf, but actually comprising two pinnae, the upper fertile, stalked and non-laminate, the lower sterile, sessile, and laminate. The sterile green blades 0.6–3 cm long (linear to narrowly elliptic); simple and entire; green blades without free vein endings within the network. The fertile spikes 0.3–1.5 cm long; unbranched. The petioles vascularised via a single strand (this representing a single leaf trace arising from the lower angle of the leaf gap). The venation of the lamina reticulate (i.e., in the sterile blade, which by contrast with that of O. vulgatum does not exhibit free vein endings within the network).
Eusporangiate. The sporangia all alike, relatively massive, sessile and coalescing in two rows on the spikelike fertile component of the leaf (3–8 on euther side of the spike); marginal (in origin); exposed; not aggregated into sori. The sporangial wall of several cell layers; without an annulus. The sporangia dehiscing via median slits transverse to the axis bearing them (i.e., transversely to the axis of the spike). The mature spores trilete.
Prothallus. Prothalli non-green, subterranean, mycorrhizal.
Distribution and habitat. In short turf by the sea. Local in Guernsey and the Scillies.
Vice-county records. Britain: West Cornwall, Channel Islands.
Classification. Family Ophioglossaceae (C.T.W.); Ophioglossaceae (Swale and Hassler); Ophioglossaceae (Stace). Order Psilotales (Swale and Hassler).
O. azoricum C. Presl may derive from O. vulgatum x O. lusitanicum.
Illustrations. • O. lusitanicum: Eng. Bot. 1836 (1886). • O. lusitanicum: Sowerby and Johnson (1859). • Botrychium and Ophioglossum (inter alia).
To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2004 onwards. British ferns (Filicopsida). Version: 4th January 2012. http://delta-intkey.com’.