The most striking and unusual feature of the fern is its simple, undivided fronds. The leaves' supposed resemblance to the tongue of a hart (an archaic term for a male red deer) gave rise to the common name "hart's-tongue fern".
The Latin specific epithet scolopendrium is derived from the Greek skolopendra, meaning a centipede or millipede; this is due to the sori pattern being reminiscent of a myriapod's legs. The leaves are 10–60 cm long and 3–6 cm broad, with sori arranged in rows perpendicular to the rachis.
A global phylogeny of Asplenium published in 2020 divided the genus into eleven clades, which were given informal names pending further taxonomic study. A. scolopendrium belongs to the "Phyllitis subclade of the "Phyllitis clade. Members of the Phyllitis clade have undivided or pinnatifid leaf blades with a thick, leathery texture, persistent scales on their stalk, and often possess anastomosing veins. Members of the Phyllitis subclade have undivided leaves with freely branching veins and single or paired sori. They are widely distributed through the Northern Hemisphere. Its closest relative within the subclade is A. komarovii, with which it forms a clade (the former segregate genus Phyllitis), sister to A. sagittatum.
Three variations or subspecies are currently known:
- A. scolopendrium var. scolopendrium, the most common variation, is native to Eurasia and North Africa;
- A. scolopendrium var. americanum, a threatened variation, is native to the United States and Canada;
- A. scolopendrium var. lindenii, the least-studied variation, is native to Mexico and Hispaniola.
Morphological differences between the variations are minor, but the North American variations (i.e. americanum and lindenii) are tetraploid, while the Old World variation (i.e. scolopendrium) is diploid. Some taxonomists group lindenii with americanum, leaving just two variations.
Asplenium scolopendrium is a common species in the Old World:
- The variation scolopendrium has been found throughout Europe (including the Caucasus and the British Isles), the Middle East, East Asia (in Japan, China and South Korea), and North Africa (in Morocco, Algeria, Tunisia and Libya, as well as the Canary Islands); it has also been introduced to the Falkland Islands of South America. Specimens of this variation found in North America (such as in New Brunswick and Ontario in Canada, and Maryland in the United States) are considered naturalized descendants of cultivated plants.
In North America, it occurs in rare, widely scattered populations located in different locales:
- The variation lindenii grows in the Mexican states of Chiapas, Coahuila, Nuevo León, and Oaxaca, as well as the Caribbean island of Hispaniola.
- The variation americanum mostly grows along the Onondaga Limestone and Niagara Escarpment geological formations in Central New York (present in 2 counties), southern Ontario (present in 7 counties), and the eastern Upper Peninsula in Michigan (present in 2 counties). Exceptions are disjunct populations that exist in Alabama (in Fern Cave National Wildlife Refuge, a wildlife refuge centered around an off-limits cave in Jackson County, Alabama, where it has declined heavily due to illegal plant collecting, and an undisclosed pit in Morgan County that is also off-limits and protected), and Tennessee (in just a single county); these southern populations are at dire risk of extirpation. An introduced population descended from New York plants is found in New Jersey; it is a remnant of a 1936 effort to practice ex-situ conservation of populations in New York.
- In 2020, a new population of hart's-tongue ferns was discovered inside of a cave with basaltic lava flows in El Malpais National Monument, Cibola County, New Mexico; this represents the first confirmed population of the species in North America west of the Mississippi. Genetic analyses and surveys are currently being performed to determine the population's variation and overall health.
The plants grow on neutral, calcium-rich, and/or lime-rich substrates under deciduous hardwood canopies (usually sugar maples in eastern North America), including moist soil and damp crevices in old walls; they are found most commonly in shaded areas. Plants in full sun are usually stunted and yellowish in colour, while those in full shade are dark green and healthy. The disjunct populations of the North American variation in the southeastern US are found exclusively in sinkhole pits or limestone caves. These populations may be relics of cooler Pleistocene climates.
In the United States, A. scolopendrium var. americanum was declared endangered by the U.S. Fish and Wildlife Service in 1989. The reasons for its rarity are currently being researched, with reintroduction programs in New York and elsewhere also in development.
Ontario, Canada has the highest population numbers of A. scolopendrium var. americanum of any region in the variation's distribution, with around 80% of all subpopulations and around 94% of all individuals. The fern was reported at more than 100 sites across the province, with around 75 still believed to be existing. Despite this, A. scolopendrium var. americanum was listed as a species of Special Concern under the Committee on the Status of Species at Risk in Ontario in May 2017, due to its extremely specific habitat requirements, relatively small distribution, and some subpopulations consisting of too little individuals.
In spite of being much more common in Europe than in North America (and therefore present in more protected areas), A. scolopendrium is still declining in certain areas of the continent. The fern was listed as "Vulnerable" in the National Red Lists for Albania in 2014 and Norway in 2010 (under Criterion D1); considered "critically threatened and rare" in Czechia's 2012 plant Red List; and "Endangered" in Sweden's 2010 Red List. However, it was not considered threatened in Germany's 1996 Red List of Threatened Plants. A. scolopendrium is protected by law in the Netherlands since 1998.
Asplenium scolopendrium is often grown as an ornamental plant, with several cultivars selected with varying frond form, including with frilled frond margins, forked fronds and cristate forms. The species has gained the Royal Horticultural Society's Award of Garden Merit, as has the cultivar 'Angustatum'.
The American variation is reputed to be difficult to cultivate (making conservation efforts for it even more troublesome); due to this, most, if not all, cultivated individuals are derived from the Old World variation.
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- NatureServe. 2011. Asplenium scolopendrium var. americanum, American Hart's-tongue Fern. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.135863/Asplenium_scolopendrium_var_americanum. Accessed 11 November 2021.
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- Harrison, Lorraine (2012). RHS Latin for Gardeners. United Kingdom: Mitchell Beazley. ISBN 978-1845337315.
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- Xu et al. 2020, p. 41.
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- Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.
- "Tropicos | Name - Asplenium scolopendrium L".
- Short, John W.; Spaulding, Daniel D. (2012). Ferns of Alabama. Tuscaloosa, AL: University of Alabama Press. ISBN 9780817356477.
- Snyder, D.B. 1990. Botanist, New Jersey Natural Heritage Program. Personal communication with Wayne Ostlie, MRO, The Nature Conservancy.
- Currie, Robert R. (September 1993). American hart's-tongue recovery plan (PDF) (Report). Atlanta, Georgia: U.S. Fish and Wildlife Service. Retrieved 2010-02-28.
- "American Hart's Tongue Fern".
- Zoological Society of London. 2014. National Red Lists Database: a focal point for national red lists and action plans. London Available at: http://www.nationalredlist.org. (Accessed: February 2014).
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- "RHS Plant Selector - Asplenium scolopendrium". Retrieved 12 February 2020.
- "Asplenium scolopendrium 'Angustatum'". RHS. Retrieved 20 February 2020.
- Mickel, John T. (2003). Ferns for American Gardens. Timber Press. ISBN 9780881925982. This book is a reprinting of Mickel, John T. (1994). Ferns for American Gardens. MacMillan. ISBN 9780025844919.
- Hill, John (1812). The family herbal: or An account of all those English plants, which are remarkable for their virtues, and of the drugs which are produced by vegetables of other countries; with their descriptions and their uses, as proved by experience. C. Brightly and T. Kinnersley. p. 162.
- Xu, Ke-Wang; Zhang, Liang; Rothfels, Carl J.; Smith, Alan R.; Viane, Ronald; Lorence, David; Wood, Kenneth R.; Cheng, Cheng-Wei; Knapp, Ralf; Zhou, Lin; Lu, Ngan Thi; Zhou, Xin-Mao; Wei, Hong-Jin; Fan, Qiang; Chen, Su-Fang; Cicuzza, Daniele; Gao, Xin-Fen; Li, Wen-Bo; Zhang, Li-Bing (2020). "A global plastid phylogeny of the fern genus Asplenium (Aspleniaceae)". Cladistics. 36 (1): 22–71. doi:10.1111/cla.12384. PMID 34618950. S2CID 201197385.
- Hyde, H. A., Wade, A. E., & Harrison, S. G. (1978). Welsh Ferns. National Museum of Wales. ISBN 0-7200-0210-9.
- Parker, Rosemarie (December 2009). "A Real Rarity". Finger Lakes Native Plant Society. A popular article on hart's tongue fern that includes several references and a discussion of cultivation possibilities for the European and American varieties. The article strongly discourages collection and or cultivation of the North American variety.
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