Hymenophyllum tunbrigense

Tunbridge filmy fern, Englischer Hautfarn
Conservation status
Not evaluated (IUCN 3.1)
Scientific classification
Kingdom: Plantae
Division: Pteridophyta
Class: Polypodiopsida
Order: Hymenophyllales
Family: Hymenophyllaceae
Genus: Hymenophyllum
Species: H. tunbrigense
Binomial name
Hymenophyllum tunbrigense
Hook

Hymenophyllum tunbrigense (Tunbridge filmy fern) is a small, fragile perennial leptosporangiate fern which forms large dense colonies of overlapping leaves from creeping rhizomes. The common name derives from the leaves which are very thin, only a single cell thick, and translucent, giving the appearance of a wet film. The evergreen fronds are bipinnatifid, deeply and irregularly dissected, about 3 to 6 cm long, 2 cm across with dark winged stipes. In contrast to the similar H. wilsonii the fronds are more divided, flattened, appressed to the substrate and tend to have a bluish tint.

The fronds are monomorphic and produce sori along the frond segments close to the rachis. Up to 5-10 purse-shaped sori are produced per frond, each covered by two strongly convex, flattened indusial valves. The valve margins are jagged and used to distinguish H. tunbrigense from H. wilsonii, where the edges are entire.

In common with all ferns, H. tunbrigense exhibits a gametophyte stage in its life cycle (alternation of generations) and develops a haploid reproductive prothallus as an independent plant. Information about the gametophyte is scarce but it is likely to be inconspicuous with a narrow ribbon-like thallus.[1] The gametophyte may be able to reproduce itself vegetatively by gemmae in the absence of the sporophyte. Gametophytes of the related Killarney fern (Trichomanes speciosum) have been found outside the geographical range of the sporophyte[2] and this may prove to be true of Hymenophyllum tunbrigense.

Distribution

Apparently worldwide, but discontinuous, in distribution but most frequent in western European oceanic regions like the British Isles. There is uncertainty whether similar plants found in Mexico, the West Indies, Central America, South America, South Africa and Asia should also be included in this species. H. tunbrigense has a similar though distinct distribution to H. wilsonii and can also be found in North America. Its habitat preference may mean that it has been under-recorded in many less-well populated parts of the world.

In continental Europe it occurs much further east than H. wilsonii but these sites are very disjunct and may represent relict populations from a different climatic period. Many of the continental populations are declining or have disappeared in recent times.

The specific name is derived from Royal Tunbridge Wells in Kent, England which was one of its disjunct eastern localities in Britain.

Ecology and Conservation

Filmy ferns (Hymenophyllum species, Trichomanes species) are very vulnerable to desiccation which limits the habitats in which they can survive. Hymenophyllum species are dependent as much, or more, upon the microclimate of a site as the macroclimate. H. tunbrigense is usually associated with rock outcrops, especially when deep fissures or crevaces are present.

H. tunbrigense appears not to reproduce effectively from spores under present environmental conditions. Gametophytes have not been identified in the field although there is circumstantial evidence that sexual reproduction does take place.[3]

References

  1. ↑ Stokey, A.G. (1940). Spore Germination and Vegetative Stages of the Gametophytes of Hymenophyllum and Trichomanes. Bot Gaz 101: 759
  2. ↑ Rumsey, F.J., Sheffield, E., Farrar, D.R (1990) British Filmy-fern gametophytes. Pteridologist 2: pp. 40-42
  3. ↑ Richards, P.W., Evans, G.B. (1972). Biological Flora of the British Isles. No. 126. Hymenophyllum tunbrigense (L.) Sm. (pp. 245-258), Hymenophyllum wilsonii Hooker (258-268). Journal of Ecology 60.

External links

Further reading

Page, C.N. (1997). The ferns of Britain and Ireland. 2nd Ed. Cambridge University Press. Cambridge.

Proctor, M.C.F. (2003). Comparative Ecophysiological Measurements on the Light Responses, Water Relations and Desiccation Tolerance of the Filmy Ferns Hymenophyllum wilsonii Hook. and H. tunbrigense (L.) Smith Ann Bot 91 (6).

Richards, P.W., Evans, G.B. (1972). Biological Flora of the British Isles. No. 126. Hymenophyllum tunbrigense (L.) Sm. (pp. 245–258), Hymenophyllum wilsonii Hooker (258-268). Journal of Ecology 60.