Fiddlehead:

Fiddleheads emerge in spring on green stems sparsely covered in tan scales.

Leaves and stems:

Leaves (fronds) are compound, 3 to 12 inches long, up to 2½ inches wide, with 6 to 14 pairs of branches (pinnae) that are more or less opposite and usually ascending, sometimes more perpendicular to the stem. The frond is widest at or a little below the middle, the lowest pinnae pair usually noticeably shorter than the second lowest. Pinnae are lance-oblong to narrowly triangular in outline, tapering to a pointed tip, short-stalked to stalkless and often curve upward toward the frond tip. The largest pinnae have 5 or more pairs of lobes or leaflets (pinnules).

Pinnules are irregularly toothed and smooth on both surfaces, lacking any hairs, glands or scales, and teeth are mostly rounded. Pinnule veins are branched, sometimes forked, and terminate at the tip of a tooth or lobe or in the notch between them. The pinnules nearest the stem on the lowest pinnae are typically stalkless.

Stems are grooved, and commonly straw-colored to greenish on the upper stem and darker purplish-brown with a few scattered tan scales below the leaf. The plant grows in an asymmetrical clump, the fronds erect to ascending to arching. Old stem bases persist, and small colonies may form from short, creeping rhizome.

Spores:

The sori (group of spores) mature in summer to fall and are found on the underside of the leaf. They are circular and arranged in a row along one or both sides of the pinnule midvein about halfway to the pinnule edge. An egg to cup-shaped, translucent tissue (indusium) partly covers the spores and is attached on inner curve but usually withers away as spores mature. Spores ripen to dark brown or black. Nearly all leaves have spores.

Notes:

Mackay's Fragile Fern is common in Minnesota, found in a variety of shaded, rocky habitats from cliffs to talus slopes to river banks, less often in soil on the forest floor. It can be very difficult to distinguish from the related C. fragilis; the two were once treated as varieties of the same species. Both grow on rocks but C. tenuis may be distinguished by the arrangement of the pinnae—angled and/or curving upward—as well as pinnule teeth that are more rounded than the more sharply pointed teeth of C. fragilis. While these can be good indicators, they are not always obvious—the pinnae arrangement in particular seems more pronounced on fresh leaves, less so on older ones.

C. protrusa is also similar but grows only in soil, not on rocks, and has a distinctive rhizome that is covered in yellow hair and extends up to 2 inches beyond the clump of fronds, where there is no such extension on the other two. The pinnules closest to the stem on the lowest pinnae of these three species might be distinctive, with C. tenuis usually having stalkless pinnules, C. protrusa usually stalks .5 to 1 mm long, and C. fragilis stalkless or stalks up to .5 mm. This trait is variable so it alone should not be a determining factor.

The remaining Cystopteris species in MN, C. bulbifera, C. laurentiana and C. tennesseensis, all have at least sparse glandular hairs and bulblets on the upper stem and back side of pinnules, though bulblets on the latter two are few and misshapen.