| Mountain spleenwort | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Division: | Polypodiophyta |
| Class: | Polypodiopsida |
| Order: | Polypodiales |
| Suborder: | Aspleniineae |
| Family: | Aspleniaceae |
| Genus: | Asplenium |
| Species: | A. montanum |
| Binomial name | |
| Asplenium montanum | |
| Synonyms | |
Asplenium montanum, commonly known as themountain spleenwort, is a small fern endemic to the eastern United States. It is found primarily in theAppalachian Mountains fromVermont toAlabama, with a few isolated populations in theOzarks and in theOhio Valley. It grows in small crevices insandstone cliffs with highly acid soil, where it is usually the onlyvascular plant occupying thatecological niche. It can be recognized by its tufts of dark blue-green, highly divided leaves. The species was firstdescribed in 1810 by the botanistCarl Ludwig Willdenow. No subspecies have been described, although a discolored and highly dissectedform was reported from theShawangunk Mountains in 1974.Asplenium montanum is a diploid member of the "AppalachianAsplenium complex," a group of spleenwort species and hybrids which have formed byreticulate evolution. Members of the complex descended fromA. montanum are among the few other vascular plants that can tolerate its typical habitat.
Asplenium montanum is a small,evergreen fern which grows in tufts.[2] The leaves are bluish-green and highly divided, proceeding from a long and often drooping stalk.A. montanum is monomorphic, with no difference in form between sterile and fertile fronds.[2]
The horizontalrhizomes, which are about 1 millimeter across,[3] may curve upward. They are not branched, but as new plants can form at root tips, a tightly packed cluster of stems may give the appearance of branching. The rhizomes are covered in dark brown, narrowlydeltate (triangular) scales, from 2 to 4 millimeters (0.1 to 0.2 in) long and from 0.2 to 0.4 millimeters across, with untoothed edges.[4] They are strongly clathrate (bearing a lattice-like pattern).[3]
Thestipe (the stalk of the leaf, below the blade) is dark brown to purplish-black and shiny at the base, gradually turning dull green as it ascends to the leaf blade. The stipe is from 2 to 11 centimeters (0.8 to 4.3 in) long, and may be from 0.5 to 1.5 times the length of the blade. Dark, narrowlylanceolate scales and tiny hairs are present only at the very base of the stipe,[4] which is slender and fragile,[2] and lacks wings.[3]
The leaf blade is thick and hairless,[4] and of a dark blue-green color;[5] therachis (leaf axis), like the stipe, is a dull green, with occasional hairs.[4] The blade is deltate or lanceolate, with a squared-off or slightly rounded base and a pointed tip. It ranges from 2 to 11 centimeters (0.8 to 4.3 in) long and from 1 to 7 centimeters (0.4 to 2.8 in) wide, occasionally as wide as 10 centimeters (3.9 in).[4][6] The blade varies from pinnate-pinnatifid to bipinnate-pinnatifid; that is, it is cut into lobed pinnae, and sometimes the pinnae themselves are cut into lobed pinnules. There are four to ten pairs of widely spaced pinnae per leaf, each of which is deltate to lanceolate, with coarse incisions in the edges,[4] which cut them into pinnules or deep lobes,[2] and a rounded to angled base.[4] The pinnules are indented, but not further cut.[2] The longest pinnae are those nearest the base of the leaf, which range from 6 to 35 millimeters (0.2 to 1.4 in) long and from 4 to 20 millimeters (0.2 to 0.8 in) across. The veins in the leaf do not form a meshwork, and are obscure.[4]
On fertile fronds, from 1 to 15 elliptical or narrowsori can be found on the underside of each pinna.[2][4] They are 0.5 to 1.5 millimeters long, covered by translucent, pale tanindusia with somewhat jagged edges.[7] Eachsporangium holds 64spores. The species has a chromosome number of 2n = 72 in thesporophyte; it is a diploid.[4]
The dark bluish-green color and the widely spaced, deeply cut and indented pinnae differentiateA. montanum from most related species. The pinnae of Bradley's spleenwort (A. bradleyi) are toothed and less deeply cut, and the dark color of the stipe continues partway up the rachis in that species.[8] Wall-rue (A. ruta-muraria) has a green stipe,[2] and its pinnae have longer stalks and are broadest near the tip.[8] Wherry's spleenwort (A. × wherryi), a hybrid between Bradley's spleenwort and mountain spleenwort, is intermediate between its parents. When compared with mountain spleenwort, the blade of Wherry's spleenwort is lance-shaped, rather than triangular; the upper parts of the blade are not as deeply cut; and the dark color of the stipe extends to the beginning of the rachis.[9]
This fern was at first identified byAndré Michaux, in 1803, as black spleenwort (Asplenium adiantum-nigrum).[10]Carl Ludwig Willdenow recognized anddescribed it as a separate species, which he namedAsplenium montanum, in 1810.[11] In 1901,John A. Shafer attempted to transfer it to the genusAthyrium asAthyrium montanum,[12] but this name is illegitimate as a laterhomonym ofAthyrium montanum (Lam.) Röhl. ex Spreng. The species was segregated fromAsplenium asChamaefilix montana byOliver Atkins Farwell in 1931.[13] The change was not widely accepted and current authorities do not recognize this segregate genus.[4]
A global phylogeny ofAsplenium published in 2020 divided the genus into eleven clades,[14] which were given informal names pending further taxonomic study.A. montanum belongs to the "Onopteris subclade" of the "Pleurosorus clade".[15] ThePleurosorus clade has a worldwide distribution; members are generally small and occur on hillsides, often sheltering among rocks in exposed habitats. TheOnopteris subclade hasAspidium-typegametophytes.[16] The closest relatives ofA. montanum within the subclade areA. onopteris and its allopolyploid descendant,A. adiantum-nigrum.[15]
In 1974,Timothy Reeves described an unusual population ofA. montanum from theShawangunk Mountains. Having usedchromatography to show that it was not ahybrid, he interpreted it as a new form,Asplenium montanum formashawangunkense. In this form, as contrasted with the usual formamontanum, the leaf blade is yellow-green, the fronds continue highly dissected to the apex and do not come to a pointed tip, the fronds are shorter and more highly dissected than usual, and all fronds are sterile.[5]
Asplenium montanum readily formshybrids with a number of other species in the "AppalachianAsplenium complex". In 1925,Edgar T. Wherry noted the similarities betweenA. montanum, lobed spleenwort (A. pinnatifidum), and Trudell's spleenwort (A. × trudellii),[17] and in 1936 concluded that Trudell's spleenwort was a hybrid between the first two.[18] In 1951,Herb Wagner, while reviewingIrene Manton'sProblems of Cytology and Evolution in the Pteridophyta, suggested in passing thatA. pinnatifidum itself might represent a hybrid betweenA. montanum and the American walking fern,Camptosorus rhizophyllus (nowA. rhizophyllum).[19]
In 1953, he reported preliminary cytological studies on theAspleniums and suggested thatA. montanum had crossed with ebony spleenwort (A. platyneuron) to yield Bradley's spleenwort (A. bradleyi), noting thatD. C. Eaton andW. N. Clute had already made tentative suggestions along those lines. He also made chromosome counts ofA. × trudellii, which had been classified by some simply as a variety ofA. pinnatifidum. AsA. pinnatifidum proved to be atetraploid whileA. montanum was adiploid, a hybrid between them would be atriploid, and Wagner showed that this was in fact the case forA. × trudellii.[20] His further experiments, published the following year, strongly suggested that bothA. bradleyi andA. pinnatifidum wereallotetraploids, the product of hybridization betweenA. montanum and anotherAsplenium to form a sterile diploid, followed by chromosome doubling that restored fertility.[21]
These cytotaxonomic findings were supported by subsequentchromatographic studies.A. montanum was shown to produce a pattern of seven substances chromatographically distinct from those produced by the other diploid members of the AppalachianAsplenium complex. These substances were present in the chromatograms of all tested hybrids believed to descend fromA. montanum at one or more removes:A. bradleyi,A. × gravesii,A. × kentuckiense,A. pinnatifidum,A. × trudellii, andA. × wherryi.[22] Four of the compounds present in the chromatograms ofA. montanum and its descendants,fluorescing gold-orange underultraviolet light, were subsequently identified as thexanthonoidsmangiferin,isomangiferin, and theirO-glucosides.[23] The other two were identified askaempferol derivatives, but could not be more precisely determined due to lack of material; the last was a trace compound which could not be studied.[24] A chloroplast phylogeny has suggested thatA. montanum is the maternal ancestor ofA. bradleyi.[16]
The allotetraploid hybrid species derived fromA. montanum canbackcross withA. montanum to form triploid hybrids. The backcross hybrid betweenA. montanum andA. pinnatifidum isA. × trudellii, as suggested by Wherry. He also collected specimens of the backcross hybrid betweenA. montanum andA. bradleyi from a cliff nearBlairstown, New Jersey in 1935.[25] The hybrid received no further attention until 1961, when it was described and named in Wherry's honor as Wherry's spleenwort (A. × wherryi).[9] The sterile diploidA. montanum × platyneuron, precursor toA. bradleyi, was collected in 1972 atCrowder's Mountain,Georgia;[26]A. montanum × rhizophyllum, precursor toA. pinnatifidum, has never been found.
One of the "Appalachian spleenworts",A. montanum is found in theAppalachian Mountains from Vermont and Massachusetts southwestward to Tennessee, Alabama, and Georgia, and to a lesser extent in theOhio Valley in Ohio, Indiana, and Kentucky.[27] Arkansas populations were discovered inGarland County andStone County in 2002 and 2008, respectively.[28] One outlying population in Missouri, collected in 1960, is considered historical;[27] it is represented by one specimen collected nearGraham Cave and has never been relocated. The site is thought to have been destroyed by road construction.[4][28] A collection by Farwell from theKeweenaw Peninsula of Michigan was considered valid byM. L. Fernald, but is of questionable authenticity; the population has never been relocated.[29]
Asplenium montanum grows on acidic rocks such assandstone, in crevices[4][7] into which moisture seeps from within the rock strata. It has been found at altitudes up to 2,000 meters (7,000 ft).[4] Like the closely relatedA. bradleyi,A. montanum requires that the thin soil in its favored crevices be subacid (pH 4.5–5.0) to mediacid (pH 3.5–4.0), and it is intolerant ofcalcium.[30][31] This habitat is unfavorable to most other plants, but its allotetraploid descendants and their backcross hybrids may occur alongside it.[4]
Asplenium montanum is considered byNatureServe to be globally secure (G5), but threatened at the edges of its range. It is known only historically from Missouri, andNatureServe considers it critically imperiled (S1) in Indiana, Massachusetts, New Jersey, Rhode Island, and Vermont, and imperiled (S2) to vulnerable (S3) in Connecticut and New York.[1] The principal threat to New York populations isrock climbing.[32]
Asplenium montanum may be cultivated outdoors or in aterrarium. In either case, the soil used should be amended with chips of acidic rock.[7]
