British ferns (Filicopsida)

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L. Watson and M. J. Dallwitz

Hymenophyllum wilsonii Hook.

“Wilson’s Filmy Fern”.

H. peltatum auct., H. unilaterale auct.

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); conspicuously, pinnately lobed (but differing from the flat fronds of H. tunbridgense in having the “pinnae” bent back from the rachis, and their segments fewer and more unilateral). 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 narrowly elliptic, or lanceolate; very thin and translucent - only one cell thick, lacking stomata. The venation of the lamina open (the lobes one-veined, the vein reaching 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 ovate and entire. The sporangia developing sequentially within a sorus, or not developing sequentially (? - without the conspicuously elongated, “gradate”, bristle-tipped 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. tunbrigense. Local in Ireland and western Britain, with a similar distribution there but commoner than H. tunbrigense, and north to Shetland, but not in E. Sussex.

Vice-county records. Britain: West Cornwall, East Cornwall, South Devon, North Devon, West Gloucestershire, Herefordshire, Worcestershire, Staffordshire, Shropshire, Glamorgan, Breconshire, Radnorshire, Carmarthenshire, Pembrokeshire, Cardiganshire, Montgomeryshire, Merionethshire, Caernarvonshire, Denbighshire, Anglesey, South Lancashire, West Lancashire, North-east Yorkshire, South-west Yorkshire, Mid-west Yorkshire, North-west Yorkshire, Durham, South Northumberland, North Northumberland, Westmorland, Cumberland, Isle of Man, Dumfriesshire, Kirkcudbrightshire, Wigtownshire, Ayrshire, Renfrewshire, Peeblesshire, Selkirkshire, Roxburghshire, East Lothian, Fifeshire, Stirlingshire, West Perthshire, Mid Perthshire, East Perthshire, Angus, South Aberdeenshire, East Inverness-shire, West Inverness-shire, Argyll Main, Dunbartonshire, Clyde Isles, Kintyre, South Ebudes, Mid Ebudes, North Ebudes, West Ross, East Ross, East Sutherland, West Sutherland, Caithness, Outer Hebrides, Orkney islands, Shetland. Ireland: South Kerry, North Kerry, West Cork, Mid Cork, Waterford, South Tipperary, Limerick, Clare, Wexford, Carlow, Leix, South-east Galway, West Galway, Offaly, Wicklow, Dublin, 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. wilsonii: as H. unilaterale, Eng. Bot. 1841 (1886). • H. wilsonii: Sowerby and Johnson (1859). • 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’.

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