Leaves and stems:

Leaves (fronds) are compound, 3 to 14 inches long, up to 3 inches wide, with 8 to 15 pairs of branches (pinnae) that are more or less opposite and perpendicular to the stem. The frond is widest at or a little below the middle, the lowest pinnae pair slightly to distinctly shorter than the second lowest, but there is often not much difference in size from the widest point down to the base. Pinnae are narrowly triangular in outline, tapering to a pointed tip, stalkless or nearly so at the base, the largest pinnae with 5 or more pairs of lobes or leaflets (pinnules).

Pinnules are irregularly toothed and smooth on both surfaces, lacking any hairs, glands or scales. Pinnule veins are branched, sometimes forked, and usually terminate at the tip of a tooth or lobe, though there may be a minute indentation where the vein terminates. The pinnules closest to the stem on the lowest pinnae are stalkless or on a very short stalked up to .5 mm long.

Stems are grooved, the upper stem (rachis) straw-colored to greenish on mature plants, becoming darker reddish-brown with 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 some shriveled stems may persist to the next season.

Spores:

The sori (group of spores) mature in mid-summer 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. A lance to egg-shaped, translucent tissue (indusium) partly covers the spores and is attached on inner curve but usually withers away before spores reach maturity. Spores ripen to dark brown or black. Nearly all leaves have spores.

Notes:

Fragile Fern is common in Minnesota, found in the cracks and crevices of cliffs, rock outcrops, rocky slopes, banks and ravines, usually in shade. It is a rather variable species, but if you see a fern clump growing on rock (or thin soil over rock) with fronds (including the stem) not more than 14 inches long, lacks any hairs, scales or glands on the pinnae or upper stem, the lowest pinnae pair is at least slightly smaller than the second lowest, and sori are on most fronds in a row about midway between the pinnule midvein and edge (without indusia when mature), chances are it is Cystopteris fragilis. It may be mistaken for a Woodsia species, which also grow on rocks but most of which have scales, hairs and/or glands on the pinnules, indusia and/or stalks, and an enlarged pore near the vein tip, most easily seen on the upperside of the pinnule.

Of the other Cystopteris species in the state, the frond is most similar to C. protrusa or C. tenuis, both of which also lack any glandular hairs and have fronds broadest above the base, but both of which grow in soil. C. protrusa is never on rock, its stem is usually green to straw-colored throughout and has a rhizome covered in yellowish hairs that extends well past the clump of frond stems; C. tenuis may be on rock or in soil, its pinnae are usually more angled up than perpendicular to the stem (but not always obviously so) and veins terminate both at the tip of a lobe/tooth and in the notches. The pinnules closest to the stem on the lowest pinnae of these three 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 on the upper stem and back of pinnules and also have bulblets, though bulblets on C. laurentiana, the child of C. fragilis and C. bulbifera, are few and misshapen.