Asplenium montanum, commonly known as the mountain spleenwort, is a small fern endemic to the eastern United States.

It is found primarily in the Appalachian Mountains from Vermont to Alabama, with a few isolated populations in the Ozarks and in the Ohio Valley.

It grows in small crevices in sandstone cliffs with highly acid soil, where it is usually the only vascular plant occupying that ecological niche.

Members of the complex descended from A. montanum are among the few other vascular plants that can tolerate its typical habitat.

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.

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 dark bluish-green color and the widely spaced, deeply cut and indented pinnae differentiate A. 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.

[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.

[15] The Pleurosorus clade has a worldwide distribution; members are generally small and occur on hillsides, often sheltering among rocks in exposed habitats.

[18] In 1951, Herb Wagner, while reviewing Irene Manton's Problems of Cytology and Evolution in the Pteridophyta, suggested in passing that A. pinnatifidum itself might represent a hybrid between A. montanum and the American walking fern, Camptosorus rhizophyllus (now A. rhizophyllum).

[19] In 1953, he reported preliminary cytological studies on the Aspleniums and suggested that A. montanum had crossed with ebony spleenwort (A. platyneuron) to yield Bradley's spleenwort (A. bradleyi), noting that D. C. Eaton and W. N. Clute had already made tentative suggestions along those lines.

As A. pinnatifidum proved to be a tetraploid while A. montanum was a diploid, a hybrid between them would be a triploid, and Wagner showed that this was in fact the case for A. × trudellii.

[20] His further experiments, published the following year, strongly suggested that both A. bradleyi and A. pinnatifidum were allotetraploids, the product of hybridization between A. montanum and another Asplenium to form a sterile diploid, followed by chromosome doubling that restored fertility.

These substances were present in the chromatograms of all tested hybrids believed to descend from A. montanum at one or more removes: A. bradleyi, A.

[22] Four of the compounds present in the chromatograms of A. montanum and its descendants, fluorescing gold-orange under ultraviolet light, were subsequently identified as the xanthonoids mangiferin, isomangiferin, and their O-glucosides.

[23] The other two were identified as kaempferol derivatives, but could not be more precisely determined due to lack of material; the last was a trace compound which could not be studied.

[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.