Leaves and stems:

Leaves (fronds) are erect to drooping, 2 to 6 inches long (typically 3 to 5), less than 1 inch wide, narrowly lance-oblong in outline, broadest below the middle, once compound with 8 to 15 pairs of opposite leaflets (pinnae). Pinnae are mostly lance to triangular in outline with the smallest ones fan-shaped. The largest leaflets have 1 to 3 pairs of lobes, rounded or blunt at the tip; edges are toothless to somewhat scalloped. The upper surface is hairless, the lower occasionally has a few hairs or scales along the midvein but is essentially hairless.

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, often as a white spot.

Stems are initially green, covered in a mix of sparse, long, white hairs and lance-linear, tan scales. The lower stem (stipe) turns dark purplish to black with age and has a small, swollen joint about halfway between the base and the lowest pinnae; the upper stem (rachis) is grooved, green and smooth or has just a few scattered hairs and scales on the underside. Plants form a loose clump, the old stem bases persisting to the next year, broken off at the joint and all more or less the same length.

Spores:

The sori (group of spores) develop on the underside of fertile fronds in early to midsummer. They are circular and arranged around the edge of the pinnae lobes. Spores mature to dark brown. Surrounding the sori is hair-like tissue (indusium) that is initially white and tightly wound into a ball, but unravels and turns rusty brown as spores develop. The indusium often persists but may become obscure. There is no obvious difference between sterile and fertile fronds.

Notes:

Of the 6 Woodsia species in Minnesota, this is one of the less common, found primarily along Lake Superior's rocky north shore and the bedrock river gorges and cliffs in the arrowhead region. According to the DNR, while there are a fair number of records in the state, population sizes are all quite small and development and recreational pressures put the risk of extinction of any one of these quite high. It was listed as a Special Concern species in 1996 and elevated to Threatened in 2013.

Alpine Woodsia is distinguished from the other Woodsia ferns by its sparse scales and hairs on the rachis, sparse hairs on the pinnae, the jointed stem that is dark at the base and green above, and the persistent old stem bases that are all about the same length. It is said to have developed as a hybrid between Smooth Woodsia (Woodsia glabella) and Rusty Woodsia (Woodsia ilvensis), and does have some characteristics intermediate between the two, but all three of these share the traits of jointed stems with old stem bases about the same length, and lack of any glandular hairs. Smooth Woodsia is the smallest Woodsia (rarely over 4 inches), with a stem that is green to straw-colored throughout and lacks any hairs or scales above the joint. Rusty Woodsia is densely hairy and scaly throughout the stem and pinnae, and the largest pinnae have at least 4 pair of lobes.

Other Woodsia species lack jointed stems, have old stem bases of varying lengths, or have glandular hairs. W. alpina may hybridize with W. ilvensis, the hybrid (X gracilis) having some intermediate characteristics, such as scales on the pinnae, and malformed spores. Woodsia is sometimes confused with Cystopteris ferns, which also mostly grows on rocks, but Cystopteris ferns lack the enlarged pore (hydathode) at vein tips (a distinctive trait of Woodsia), lack persistent stem bases, do not have hair-like indusia, and sori are in one row between the midvein and edge, not along the edge.