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Asplenium rhizophyllum

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Asplenium rhizophyllum, the (American) walking fern, is a frequently-occurring fern native to North America. It is a close relative of Asplenium ruprechtiiTemplate:Sfn (syn: Camptosorus sibiricus) which is found in East Asia and also goes by the common name of "walking fern".<ref>Encyclopædia Britannica Online</ref>

Description

Asplenium rhizophyllum is a small fern whose undivided, evergreen leaves and long, narrow leaf tips, sometimes curving back and rooting, give it a highly distinctive appearance. It grows in tufts, often surrounded by child plants formed from the leaf tips. The leaves of younger plants tend to lie flat to the ground, while older plants have leaves more erect or arching.Template:Sfn

Roots and stipes

It does not spread and form new plants via the roots. Its rhizomes (underground stems) are upright or nearly so,Template:SfnTemplate:Sfn short,Template:Sfn about Template:Convert in diameter,Template:Sfn and generally unbranched.Template:SfnTemplate:Sfn They bear dark brownTemplate:Sfn or blackish,Template:Sfn narrowly triangularTemplate:Sfn or lance-shapedTemplate:Sfn scales which are strongly clathrate (bearing a lattice-like pattern).Template:Sfn The scales are Template:Convert long and Template:Convert wide (occasionally as narrow as Template:Convert) with untoothed margins.Template:Sfn The stipe (the stalk of the leaf, below the blade) is Template:Convert longTemplate:Sfn (occasionally up to Template:Convert long), and ranges from one-tenth to one and one-half times the length of the blade. The stipe is reddish-brown and sometimes shiny at the base, becoming green above,Template:Sfn and narrowly winged.Template:Sfn Scales like those of the rhizome are present at the stipe base, changing to tiny club-shaped hairs above.Template:Sfn

Leaves

Asplenium rhizophyllum plantlet sprouting from the leaf apex of its parent plant

The leaf blades are not subdivided, as in most other ferns, but are narrowly triangularTemplate:Sfn to linear or lance-shaped. Their shape can be quite variable, even on the same plant.Template:Sfn They measure from Template:Convert long and from Template:Convert across and have a leathery texture with sparse hairs, more abundant below than above. The rachis (leaf axis) is dull green in color and almost devoid of hairs. On the underside of the blade, the veins are difficult to see and anastomose (split and rejoin each other), forming a series of areoles (the small areas enclosed by the veins) near the rachis. Fertile fronds are usually larger than sterile fronds, but their shape is otherwise the same. The base of the blade is typically heart-shaped (with the stipe protruding from the cleft);Template:SfnTemplate:Sfn the bulges on either side of the cleft are frequently enlarged into auricles (rounded lobes), or occasionally into sharply-pointed, tapering lobes.Template:SfnTemplate:Sfn The leaf tips may be rounded but are typically very long and attenuate (drawn out); the attenuate tips are capable of sprouting roots and growing into a new plant when the tip touches a surface suitable for growth. On rare occasions, the auricles at the leaf base will also take on an attenuate shape and form roots at the tip.Template:Sfn The ability of the leaf tips to root and form a new plant at some distance from the parent gives the species its common name.Template:Sfn The young leaves forming from a bud at the leaf tip are round to pointed at their apex, not yet having developed the long-attenuate shape.Template:Sfn

Specimens of A. rhizophyllum with forked blades have been found in Arkansas and Missouri. The fork usually occurs in the tip, perhaps due to growth after insect damage, but one specimen was found forking from the upper part of the stipe.Template:Sfn

Sori and spores

Fertile fronds bear a large number of sori underneath, Template:Convert long,Template:Sfn which are not arranged in any particular order.Template:Sfn The sori are often fused where veins join,Template:Sfn and may curve to follow the vein to which they are attached.Template:Sfn The sori are covered by inconspicuousTemplate:Sfn thin, white indusia with untoothed edges.Template:Sfn Each sporangium in a sorus carries 64 spores. The diploid sporophyte has a chromosome number of 72.Template:Sfn

Similar species

The leaf shape and proliferating tips easily distinguish A. rhizophyllum from most other ferns. Its hybrid descendants share the long-attenuate leaf tip, but are more deeply lobed. An artificial backcross between A. rhizophyllum and A. tutwilerae was closer to A. rhizophyllum in morphology, but still remained some lobes in the basal part of the blade, had a shallowly undulating, rather than smoothly curved, leaf edge in the apical part, showed a maroon color in the stipe up to the base of the leaf blade, and possessed the abortive spores of a sterile hybrid.Template:Sfn A. ruprechtii, the Asian walking fern, also possesses attenuate, proliferating tips, but has a lanceolate leaf blade, which tapers to a wedge at the base rather than forming a heart shape.Template:Sfn A. scolopendrium, the hart's-tongue fern, has larger, longer leaves that are glossy with a rounded tip.Template:Sfn

Taxonomy

This species is commonly known as North American walking fernTemplate:Sfn or simply walking fern,Template:Sfn because the growth of new plants at the leaf tip allows it to "walk" across surfaces over several generations.Template:Sfn The specific epithet "rhizophyllum", meaning "root leaf", also reflects this characteristic.Template:Sfn

Linnaeus first gave the walking fern the binomial Asplenium rhizophyllum in his Species Plantarum of 1753.Template:Sfn In 1833, J.H.F. Link placed the species in a segregate genus, Camptosorus, because of the irregular arrangement of its sori (in contrast to the rest of Asplenium, where the sori are confined to the edge of veins).Template:Sfn John Smith did not feel that this character was sufficient to segregate it from the rest of Asplenium, but placed it in the genus Antigramma, another Asplenium segregate, on the basis of its reticulate venation, to the convolutions of which he attributed the soral arrangement.Template:Sfn It was commonly placed either in Asplenium and Camptosorus by later authors,Template:Sfn but phylogenetic studies have shown that Camptosorus is nested within Asplenium and its species should be treated as part of that genus.Template:SfnTemplate:Sfn

The name Asplenium rhizophyllum has also been applied to two other species; in current botanical practice, these are illegitimate later homonyms of Linnaeus' name of 1753. The first of these homonyms was created by Linnaeus himself in 1763, when he accidentally used the name twice, applying it first to his original taxon and again to a species from the West Indies which also proliferates at the leaf tips.Template:Sfn He had referred to the West Indian species as A. radicans in 1759,Template:Sfn the name by which it is known today.Template:Sfn In 1834, Gustav Kunze transferred the species Caenopteris rhizophylla to Asplenium without changing the epithet;Template:Sfn George Proctor identified this species, based on a specimen from Dominica, with A. conquisitum,Template:Sfn now synonymized with A. rutaceum.Template:Sfn

Hybrids

Walking fern is one of the three parental species of the "Appalachian Asplenium complex", a group of Asplenium hybrids and their progenitors known from eastern North America. Hybridization between walking fern and mountain spleenwort (A. montanum) has given rise through chromosome doubling to a new, fertile, species, lobed spleenwort (A. pinnatifidum). The sterile hybrid between walking fern and ebony spleenwort (A. platyneuron), known as Scott's spleenwort ( A. × ebenoides), may be found where the two parents are in contact; at one locality, in Havana Glen, Alabama, A. × ebenoides has undergone chromosome doubling to produce a fertile species, Tutwiler's spleenwort (A. tutwilerae).Template:Sfn

Much more rarely, walking fern hybridizes with two other common spleenworts of eastern North America. The hybrid between walking fern and wall-rue (A. ruta-muraria), known as unexpected spleenwort (A. × inexpectatum), is known from a single specimen collected on dolomite in Adams County, Ohio.Template:Sfn The hybrid between walking fern and maidenhair spleenwort, (A. trichomanes ssp. trichomanes), Shawnee spleenwort (A. × shawneense), is known from one collection on sandstone in the Shawnee Hills of Illinois.Template:Sfn

A triploid hybrid between walking fern and Tutwiler's spleenwort was accidentally produced in culture. A similar plant collected from limestone in Shepherdstown, West Virginia could have originated from the same parents, from an unreduced (diploid) gametophyte of Scott's spleenwort crossed with walking fern, or from an unreduced walking fern gametophyte crossed with ebony spleenwort.Template:Sfn

Infraspecific taxa

In 1813, Henry Muhlenberg listed lobed spleenwort as Asplenium rhizophyllum var. pinnatifidum, although he did not provide a description distinguishing the variety from the typical species.Template:Sfn It was described as the species A. pinnatifidum by Thomas Nuttall in 1818.Template:Sfn

A number of forms have been described, of limited taxonomic value. In 1883, J. C. Arthur described walking ferns from limestone cliffs in Muscatine County, Iowa that lacked auricles at the leaf base, with the blade abruptly tapering at the base instead. In this respect, the plants closely resembled A. ruprechtii, but the leaf shape of the Iowa plants was lanceolate (widest near the base) rather than ovate (widest in the middle), and the wide point of the leaf in the Iowa plants appeared slightly lobed. He named these plants Camptosorus rhizophyllus var. intermedius;Template:Sfn the variety was subsequently given the status of a form by Willard N. Clute.Template:Sfn In 1922, Ralph Hoffmann gave the name C. rhizophyllus f. auriculatus to specimens with proliferating auricles, based on material on limestone from New Marlborough, Massachusetts.Template:Sfn In 1924, Frederick W. Gray described as C. rhizophyllus f. angustatus material from a sandstone boulder in Monroe County, West Virginia. These had a short stipe, less than Template:Convert long, and narrow leaf blades, less than Template:Convert wide, with the sori almost at the margins. He argued that as they were found along with normal material, they were not solely due to sun exposure.Template:Sfn Finally, in 1935, Carl L. Wilson described C. rhizophyllus f. boycei based on material collected from the base of a limestone boulder in Highgate Springs, Vermont by Guy Boyce. These plants had deeply lobed auricles, and erose (jagged or indented) leaf margins with rounded edges.Template:Sfn

Distribution

Range map of Asplenium rhizophyllum

The principal range of A. rhizophyllum is in the Appalachian Mountains and the Ozarks. It can be found from southern Quebec and Ontario along the Appalachians and Piedmont southwestward to Mississippi and Alabama, along the Ohio Valley and into the Ozarks west to Nebraska and Oklahoma, and along the Mississippi Valley north to Wisconsin and Minnesota. It has become extinct in Maine and Delaware.Template:Sfn The distribution typically follows area of limy soil;Template:Sfn sometimes said to be rare, it is better described as locally abundant where conditions favor it.Template:Sfn

Ecology and conservation

Walking fern grows on shaded boulders, ledges and in crevices, usually covered with moss.Template:Sfn On rare occasions, it is found on fallen tree trunks,Template:Sfn as an epiphyte,Template:Sfn or on the ground.Template:Sfn It is usually found on limestone or other alkaline rocks, rarely on sandstone or other acidic rocks.Template:SfnTemplate:Sfn

While globally secure, it is endangered in some states and provinces at the edge of its range. NatureServe considers it critically imperiled (S1) in Mississippi, New Hampshire and Rhode Island, imperiled (S2) in Michigan and South Carolina, and vulnerable (S3) in North Carolina and Quebec.Template:Sfn

Cultivation

It was introduced into cultivation in England in 1680.Template:Sfn It prefers low to medium light levels, and a moist, basic potting mix,Template:SfnTemplate:Sfn or soil with added lime chips.Template:Sfn

Citations

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Bibliography

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External links

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