British ferns (Filicopsida)
Tunbridge Filmy Fern.
Sporophyte. The rhizomes slender (filiform); creeping; glabrous, naked.
Leaves persistent for some years; to 2.5–8(–12) cm long; simple (usually described as pinnate, but the combination of abnormally thin lamina and narrowly winged rachis and rachillae blurs the conventional definitions); more or less flat, conspicuously, pinnately lobed (and with the pinnae divided more or less dichotomously but irregularly into oblong, flat segments, the segments sharply but remotely serrulate). The petioles shorter than the blades to about as long as the blades (about a third to a half of the leaf length, wiry, wingless, naked or with a few hairs); vascularised via a single strand (representing a single leaf trace). Leaf blades in outline oblong to elliptic, or ovate; very thin and translucent - only one cell thick, lacking stomata. The venation of the lamina open (the lobes one-veined, the vein falling slightly short of the apex).
The sporangia marginal; aggregated in sori. The sori remaining discrete at maturity; ambiguously protected by true and false indusia combined (these combined to constitute the ostensible indusium). The indusia projecting from the margins of the very thin lamina and enveloping the sori from their bases; by contrast with those of Trichomanes, of two valves, without a protruding bristle; valves orbicular and toothed. The sporangia developing sequentially within a sorus, or not developing sequentially (? - without the elongated, gradate receptacle characterizing Trichomanes (q.v.)); with an oblique annulus.
Prothallus. Prothalli green, flat and strap-shaped.
Distribution and habitat. In humid conditions, on shaded damp rock faces and tree trunks, often with H. wilsonii. Local in Ireland and Western Britain from Cornwall to N. Ebudes, also in E. Sussex.
Vice-county records. Britain: West Cornwall, East Cornwall, South Devon, North Devon, South Somerset, West Sussex, East Sussex, West Kent, West Gloucestershire, Monmouthshire, Herefordshire, Staffordshire, Glamorgan, Breconshire, Carmarthenshire, Pembrokeshire, Cardiganshire, Montgomeryshire, Merionethshire, Caernarvonshire, Anglesey, South Lancashire, West Lancashire, North-east Yorkshire, South-west Yorkshire, Mid-west Yorkshire, Durham, South Northumberland, North Northumberland, Cumberland, Dumfriesshire, Ayrshire, Renfrewshire, Stirlingshire, West Inverness-shire, Argyll Main, Dunbartonshire, Clyde Isles, Kintyre, South Ebudes, Mid Ebudes, North Ebudes, West Ross, East Ross. Ireland: South Kerry, North Kerry, West Cork, Mid Cork, East Cork, Waterford, South Tipperary, Limerick, North Tipperary, Kilkenny, Wexford, Carlow, West Galway, Wicklow, Longford, Roscommon, East Mayo, West Mayo, Sligo, Leitrim, Cavan, Louth, Monaghan, Fermanagh, East Donegal, West Donegal, Tyrone, Armagh, Down, Antrim, Londonderry.
Classification. Family Hymenophyllaceae (C.T.W.); Hymenophyllaceae (Swale and Hassler); Hymenophyllaceae (Stace). Order Hymenophyllales (Swale and Hassler).
Illustrations. • H. tunbrigense: as H. tunbridgene, Eng. Bot. 1840 (1886). • H. tunbrigense: Sowerby and Johnson (1859). • H. tunbrigense: details. • Hymenophyllum and Trichomanes (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’.