Leaves and stems:

Leaves (fronds) are mostly erect, 4 to 12 inches long, up to 3 inches wide, narrowly elliptic to diamond-shaped in outline, broadest near the middle, compound with 6 to 15 pairs of opposite leaflets (pinnae). Pinnae are mostly narrowly egg-shaped to triangular in outline. The largest leaflets have 5 to 14 pairs of lobes, oblong with a rounded or blunt tip; edges are smooth to minutely toothed and the larger lobes often further shallowly lobed.

Veins are unbranched, branched or forked, mostly obscure except for an enlarged pore (hydathode) near the vein tip which is most easily seen on the upper surface but is often also obscure. Both surfaces are sparsely to moderately covered in a mix of short glandular hairs and longer non-glandular, multi-cellular hairs; the longer hairs are mostly concentrated along the midvein.

The lower stem (stipe) is reddish-brown at least at the base, covered in scattered brown scales, some with a darker stripe or spot in the center. The upper stem (rachis) is grooved, green to straw-colored at least towards the tip, has scattered hair-like scales. The entire stem is sparsely to moderately covered in a mix of short glandular and longer, non-glandular, multi-cellular hairs. Plants form a loose to tight clump, the old stem bases persisting to the next year, broken off at varying points so are of varying lengths.

Spores:

The sori (group of spores) develop on the underside of fertile fronds starting in early to midsummer. They are circular and arranged in a row between the midrib and the edge of a pinna lobe. Spores mature to dark brown or black. Surrounding the sori is tissue (indusium) split into several short, oblong lobes that are frayed at the tip, but are mostly covered up by the sori. There is no visual difference between fertile and sterile fronds.

Notes:

Rocky Mountain Woodsia is one of 6 Woodsia species in Minnesota, found on cool, shaded, north or east-facing cliffs and talus slopes, mostly in or near the Boundary Waters Canoe Area Wilderness in northern Cook County. According to the DNR, about 19 sites were located from 1929 through the 1940s, 9 of which were relocated in the 1980s. Botanical surveys since then have located 7 more sites but there is little hope of finding many more. Due to its rarity and specific habitat requirements, Woodsia scopulina was listed as a Threatened species in 1984.

Rocky Mountain Woodsia is distinguished from the other Woodsia ferns by its stems that are chestnut brown at least at the base, green to straw-colored at least at the frond tip and lack a joint near the base, persistent old stem bases of varying lengths, scales only near the stem base and none along the rachis, indusia with several oblong lobes frayed at the tip but mostly covered up by the sori, and sparsely to moderately covered all over with a mix of short glandular hairs and longer, non-glandular, multi-cellular hairs. Most similar are Blunt-lobed Woodsia (Woodsia obtusa), which has indusia with broader lobes, and Oregon Woodsia (Woodsia oregana), which has short, hair-like indusia; both of these species lack the longer, multi-cellular hairs on pinnae and stems. Other Woodsia species may have jointed stems with old stem bases of equal lengths, scales on the rachis, have non-glandular hairs or glandular hairs but not both.

W. scopulina is one Woodsia resembling Cystopteris ferns, which also mostly grow on rocks, but Cystopteris ferns lack the enlarged pore (hydathode) at vein tips (a distinctive trait of Woodsia), lack persistent stem bases, and most lack glandular hairs. There are 3 recognized subspecies of W. scopulina: subsp. appalachiana (a.k.a. Woodsia appalachiana) has a limited range in Arkansas and Appalachia, scales mostly with a dark central stripe, longest hairs with 5 to 8 cells, and indusia lobes are not frayed at the tip; subsp. scopulina is a western species with smaller spores (measure in micrometers) and scales usually a solid color; subsp. laurentiana has a divided range between the northern Great Lakes region (including Minnesota) and the mountainous regions of western North America, and has larger spores than subsp. scopulina and at least some scales with a dark central spot or stripe.