Hymenophyllum tunbrigense (Tunbridge filmy fern)
Tunbridge filmy fern is a diminutive plant with an almost worldwide distribution in temperate rainforests and mountain cloud forests.
About this species
Delicate, sensitive and engagingly beautiful, this tiny fern is found in moist woodlands, temperate rainforests and mountain cloud forests, where it occurs on the vertical surfaces of acidic rocks and sandstone, or sometimes as an epiphyte on mossy branches.
The scientific name Hymenophyllum (literally, ‘membranous leaf’) refers to the many delicate fronds, the beauty of which is most apparent under a magnifying glass or hand lens. The specific epithet tunbrigense was presumably chosen to refer to the town of Tunbridge Wells, where some of the earliest British specimens were found.
Geography & Distribution
Tunbridge filmy fern has a cosmopolitan, but discontinuous, distribution. It occurs along the Atlantic fringe of Europe, with small populations in Germany and the Mediterranean. Within Africa it is found in Kenya, South Africa, Madagascar and Mauritius. It is also found in New Zealand, Central and South America, Jamaica and in a single locality (in South Carolina) in North America.
It usually occurs on steep rock faces, more rarely on tree trunks, in moist woodlands, temperate rainforests and mountain cloud forests, where humidity is consistently high.
In the UK, Tunbridge filmy fern is confined mainly to western areas, with an outlier population in West Sussex. It occurs at up to 500 m above sea level in the UK.
Overview: Often described as resembling the ruffled plumage of a bird, this small fern has many membranous, translucent fronds that are only 2–10 cm long.
Fronds: Each frond is almost as wide as it is long, is blue-green in colour and glistens with moisture under ideal conditions. Thin petioles (leaf stalks) or ‘stipes’ hold each frond above dark, thread-like, wiry rhizomes that creep across the surface, forming dense masses over time. Fronds live for several years, and can shrivel up in dry spells, only to be revived in the next rains. Frond growth usually occurs during the wettest months in winter.
Life-cycle: In common with all ferns, filmy ferns have life-cycles with two phases. The more visible and recognisable membranous fronds produce spores, from which the much smaller, threadlike gametophyte or prothallus develops, which produces male and female reproductive cells, and after fertilisation produces the next generation of ferns.
Spores: The minute spores are enclosed in a purse-like covering (indusium) with a toothed edge; this feature can be used to identify Tunbridge filmy fern, as the spore cases of the other species of Hymenophyllum occurring in the UK, Wilson’s filmy fern (Hymenophyllum wilsonii), are smooth.
Threats & Conservation
Tunbridge filmy fern is listed as Least Concern (LC) globally according to IUCN Red List criteria.
However, in southeastern England up to 20% of its sites have been lost since 1950, mainly through woodland loss, plant collection and shading by Rhododendron ponticum. By 1995 a 72% decline was reported in the number of surviving colonies in southeastern England. Since that date, several organisations, including RBG Kew, have been involved independently in protecting habitat sites or propagating plants in order to re-establish colonies.
In 2011, The National Trust Nymans Estate in West Sussex lost a large community of Tunbridge filmy ferns hundreds of years old to an unscrupulous plant collector who virtually destroyed the site in their desire to possess the plant. A five-year project at the estate, to improve and protect the fern’s habitat, had just reached completion when the theft occurred.
This species at Kew
Tunbridge filmy fern can be seen growing in its natural habitat within the Francis Rose Reserve at Wakehurst Place, one of only 12 locations in the High Weald where it survives.
Micropropagation of Tunbridge filmy fern at Kew
Hymenophyllum tunbrigense on a sandstone chip (Image: Margaret Ramsay, RBG Kew)
Hymenophyllum tunbrigense is cultivated using plant tissue culture and micropropagation techniques at Kew.
Tiny sections of plant material were grown on in petri dishes of alginate and subsequently weaned onto small chips of sandstone with the ultimate aim of attaching those rock pieces to the surfaces of sandstone outcrops in-situ at new habitat sites in the wild, and in particular, at Kew’s Francis Rose Reserve.
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