Asplenium montanum

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 horizontal rhizomes, 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, narrowly 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]

The stipe (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, narrowly lance-shaped 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] the rachis (leaf axis), like the stipe, is a dull green, with occasional hairs.[4] The blade is triangular or lance-shaped, 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 triangular to lance-shaped, 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]

Asplenium montanum sori beneath a fertile frond

On fertile fronds, from 1 to 15 elliptical or narrow sori can be found on the underside of each pinna.[2][4] They are 0.5 to 1.5 millimeters long, covered by translucent, pale tan indusia with somewhat jagged edges.[7] Each sporangium holds 64 spores. The species has a chromosome number of 2n = 72 in the sporophyte; it is a diploid.[4]

This fern was at first identified by André Michaux, in 1803, as black spleenwort (Asplenium adiantum-nigrum).[10] Carl Ludwig Willdenow recognized and described it as a separate species, which he named Asplenium montanum, in 1810.[11] In 1901, John A. Shafer attempted to transfer it to the genus Athyrium as Athyrium montanum,[12] but this name is illegitimate as a later homonym of Athyrium montanum (Lam.) Röhl. ex Spreng. The species was segregated from Asplenium as Chamaefilix montana by Oliver Atkins Farwell in 1931.[13] The change was not widely accepted and current authorities do not recognize this segregate genus.[4]

A global phylogeny of Asplenium 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] The Pleurosorus clade has a worldwide distribution; members are generally small and occur on hillsides, often sheltering among rocks in exposed habitats. The Onopteris subclade has Aspidium-type gametophytes.[16] The closest relatives of A. montanum within the subclade are A. onopteris and its allopolyploid descendant, A. adiantum-nigrum.[15]

Illustration from How to Know the Ferns by Frances Theodora Parsons (1899); illustration by Marion Satterlee

One of the "Appalachian spleenworts", A. montanum is found in the Appalachian Mountains from Vermont and Massachusetts southwestward to Tennessee, Alabama, and Georgia, and to a lesser extent in the Ohio Valley in Ohio, Indiana, and Kentucky.[27] Arkansas populations were discovered in Garland County and Stone 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 near Graham Cave and has never been relocated. The site is thought to have been destroyed by road construction.[4][28] A collection by Farwell from the Keweenaw Peninsula of Michigan was considered valid by M. L. Fernald, but is of questionable authenticity; the population has never been relocated.[29]

Asplenium montanum grows on acidic rocks such as sandstone, 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 related A. 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 of calcium.[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 globally secure, but threatened at the edges of its range. It is known only historically from Missouri, and NatureServe considers it critically imperiled in Indiana, Massachusetts, Rhode Island, and Vermont, imperiled in New Jersey, and imperiled to vulnerable in Connecticut and New York.[1] The principal threat to New York populations is rock climbing.[32]

Asplenium montanum may be cultivated outdoors or in a terrarium. In either case, the soil used should be amended with chips of acidic rock.[7]