British ferns (Filicopsida)
Sporophyte. Plants floating and duckweed-like, not rhizomatous (the fragile, zig-zag stems 1–10 cm long, slender, scale-less, with pendulous roots and abundant branches from the lower halves of the nodes).
Leaves to in pairs at the upper halves of the nodes, 0.05–0.25 cm long (2-ranked, closely overlapping); not circinnate (green early in the season, but becoming bright red in autumn); unconventionally compound (the members of each pair bi-lobed, the dorsal lobes floating, hyaline-margined, rendered unwettable by a covering of unicellular hairs, and containing a cavity inhabited by the nitrogen-fixing cyanobacterium Anabaena azollae, the ventral lobes thin and submerged).
Heterosporous. The sporangia in paired sporocarps borne on the submerged ventral lobes of the first leaves of the branches, each sporocarp covered by a flange of the upper lobe, containing one sorus on a cylindrical receptacle and the sori of each pair either both containing megasporangia, or both with one microsporangium and one megasporangium (the former with 64 tiny microspores, the latter with a single large megaspore); aggregated in sori. The sori remaining discrete at maturity; with a true indusium. The sporangial wall without an annulus (the megasporangium, which contains only one spore, sinks to the bottom and the female prothallus is released from it by decay of the indusium).
Distribution and habitat. Aquatic; floating. On ponds, canals and dykes, often covering the water surface along with Lemna. Naturalized. Introduced from tropical America; central and southern Britain north to mid-west Yorks, Man, E S Ireland, Channel Islands.
Vice-county records. Britain: West Cornwall, East Cornwall, South Devon, North Devon, South Somerset, North Somerset, North Wiltshire, South Wiltshire, Dorset, Isle of Wight, South Hampshire, North Hampshire, West Sussex, East Sussex, East Kent, West Kent, Surrey, South Essex, North Essex, Hertfordshire, Middlesex, Berkshire, Oxfordshire, Buckinghamshire, East Suffolk, West Suffolk, East Norfolk, West Norfolk, Cambridgeshire, Bedfordshire, Huntingdonshire, Northamptonshire, East Gloucestershire, West Gloucestershire, Monmouthshire, Herefordshire, Worcestershire, Warwickshire, Staffordshire, Shropshire, Glamorgan, Carmarthenshire, Pembrokeshire, Cardiganshire, Montgomeryshire, Merionethshire, Caernarvonshire, Denbighshire, Flintshire, Anglesey, South Lincolnshire, North Lincolnshire, Leicestershire, Nottinghamshire, Derbyshire, Cheshire, South Lancashire, West Lancashire, South-east Yorkshire, North-east Yorkshire, South-west Yorkshire, Mid-west Yorkshire, Durham, South Northumberland, Westmorland, Cumberland, Isle of Man, Ayrshire, Lanarkshire, Midlothian, West Lothian, Fifeshire, Stirlingshire, Dunbartonshire, Clyde Isles, Channel Islands. Ireland: Mid Cork, East Cork, Limerick, Kilkenny, Wexford, Carlow, Leix, West Galway, Kildare, Wicklow, Dublin, Meath, Louth, Tyrone, Armagh, Down, Antrim.
Classification. Family Azollaceae (C.T.W.); Azollaceae (Swale and Hassler); Azollaceae (Stace). Order Marsileales (Swale and Hassler).
Illustrations. • A. filiculoides: details from Goebel (1905).
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’.