British ferns (Filicopsida)


L. Watson and M. J. Dallwitz

Asplenium onopteris L.

“Irish Spleenwort”.

~ Asplenium adiantum-nigrum subsp. onopteris (L.) Heufl.

Sporophyte. The rhizomes short; creeping (or decumbent); when young, bearing scales. Plants with no clear distinction into fertile and sterile leaves.

Leaves aggregated terminally; to 10–50 cm long; persistent; compound; complexly divided; bipinnate with more or less undivided pinnules to tripinnate with undivided ultimate pinnules (the pinnae varying from lobed to pinnate). Pinnae very long-acuminate, 6–17 on each side of the leaf. The petioles longer than the blades, or about as long as the blades to longer than the blades (reddish brown or blackish, scaly at the extreme base); vascularised via a single strand (representing fusion of a pair of leaf traces). Leaf blades in outline very long-acuminate, ovate-triangular, or lanceolate-triangular; somewhat leathery to ‘herbaceous’. The longest pinnae near the base of the blade; 2–6 cm long (decreasing markedly in length upwards from the second pair). The venation of the lamina open.

The sporangia superficial; initially aggregated in sori. The sori linear elongated (1–2 mm long, two to several per segment, borne obliquely, occupying the greater part of the length of the lateral veins but nearer the indistinct segment midrib than its margin); becoming confluent when mature (finally occupying the greater part of the segment); with a true indusium. The indusia attached on one side along a vein; whitish, entire or sinuate. Paraphyses absent. The mature spores monolete; with a perispore.

Distribution and habitat. On base-rich substrates (mostly on limestone). On dry banks and rock faces near the sea. Southern and SW Ireland, very local.

Vice-county records. Britain: West Cornwall, Caernarvonshire. Ireland: South Kerry, North Kerry, West Cork, Mid Cork, East Cork, Waterford, Limerick, North Tipperary, Kilkenny, Wexford, West Galway, Wicklow, Dublin, Roscommon, West Mayo, Tyrone, Armagh, Down, Antrim.

Classification. Family Polypodiaceae (C.T.W.); Aspleniaceae (Swale and Hassler). Order Aspleniales (Swale and Hassler).

Adiantum x ticinense D.E. Mey. = A. adiantum-nigrum s. lat. x A. onopteris.

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Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2004 onwards. British ferns (Filicopsida). Version: 4th January 2012.’.