Leaves and stems:

Leaves (fronds) are erect to drooping, 1 to 6 inches long (typically less than 4), less than 1 inch wide (typically less than ½ inch), narrowly lance-oblong in outline, broadest near the middle, once compound with 7 or more pairs of leaflets (pinnae), usually opposite though may be more alternate near the stem tip. Pinnae are mostly lance to triangular in outline, the lowest more fan-shaped tending to be wider than long and the upper longer than wide. The largest leaflets have 1 to 3 pairs of lobes, rounded or blunt at the tip; edges are toothless to somewhat scalloped. Both surfaces are hairless and lack any scales on the midveins.

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.

Stems are green to straw-colored throughout with a small, swollen joint about halfway between the base and the lowest pinnae; a few brown scales are below the joint but there are no scales or hairs above it. 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 mid to late summer. 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. Pinnae lobes often fold down around the sori later in the season. There is no obvious difference between sterile and fertile fronds.

Notes:

Of the 6 Woodsia species in Minnesota, this is one of the least common, found primarily along Lake Superior's rocky north shore and river gorges and cliffs near the Canadian border. According to the DNR, it was first discovered in 1929 near Grand Portage, in a few more locations in the years that followed, plus over a dozen more in more recent biological surveys, but all populations are small, usually fewer than 20 plants. Development and recreational pressures put the risk of extinction of any one of these quite high. It was listed as a Threatened species in 1984.

Smooth Woodsia is distinguished from the other Woodsia ferns by its small stature, the jointed stem that is green or straw-colored throughout, total lack of any hairs or scales above the joint, and the persistent old stem bases that are all about the same length. Most similar are Alpine Woodsia (Woodsia alpina) and Rusty Woodsia (Woodsia ilvensis), both of which also have jointed stems with old stem bases about the same length, but both of which have darkened stem bases and at least some hairs and scales above the joint on the rachis and/or pinnae.

Other Woodsia species lack jointed stems, have old stem bases of varying lengths, or have glandular hairs. 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.