Leaves and stems:

Leaves (fronds) are once compound, lance-oblong in outline, widest above the base, abruptly tapering at the tip end, 18 to 40 inches long, 3 to 10 inches wide, with 20 to 40 pairs of leaflets (pinnae). Pinnae of sterile fronds are mostly lance-oblong, pointed at the tip, rounded to straight across the base, hairless, toothless or shallowly scalloped and often wavy around the edges, and have veins forked near the base. Pinnae are sometimes held rotated, the blade parallel to the ground. Sterile fronds are often arching.

Pinnae of fertile fronds are narrower, more lance-linear, often have a single small lobe (auricle) at the base, and veins that are not forked, or less regularly forked than sterile fronds. Fertile fronds tend to be more erect than the sterile fronds.

The stem (stipe) is grooved, mostly green except reddish brown at the base, and smooth except for a few brown scales near the base. The plant grows in an asymmetrical clump, the fronds spreading in all directions. Colonies may form from short creeping rhizomes.

Spores:

The sori (group of spores) are found on the underside of the pinnae and mature in late summer. Sori are linear-oblong and straight or slightly curved and are attached along the pinnae veins, creating a herringbone pattern on the back of the pinnae. A linear tissue (indusium) partly covers the spores and is persistent. Spores ripen to black.

Notes:

A rare fern in Minnesota, we are on the northwest tip of Narrow-leaved Spleenwort's range. The name “spleenwort” is a bit of a misnomer, a holdover from the time this species was included in Asplenium. Older references list it as the synonym Athyrium pycnocarpon, and other references list Diplazium pycnocarpon. Such is the way of taxonomy. According to the DNR, Narrow-leaved Spleenwort is confined to wooded bluffs near streams or seeps in the “driftless area” of our southeast counties. It was listed as a Special Concern species in 1984 and elevated to Threatened in 1996. It is currently a Special Concern species in Wisconsin. Narrow-leaved Spleenwort somewhat resembles Christmas Fern (Polystichum acrostichoides), another rare fern that inhabits our southeast counties, which is distinguished by its bristly-toothed pinnae and round sori that mostly cover the underside of pinnae.