Leaves and stems:

Leaves (fronds) are compound, 6 to 18 inches long, up to 5 inches wide, with 12 to 16 pairs of branches (pinnae) that are more or less opposite and perpendicular to the stem. The frond is widest 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 lance-oblong to 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; veins are branched, sometimes forked, and terminate at the tip of a tooth or lobe or in the notch between. Both surfaces lack any hairs, glands or scales, though there are usually sparse glandular hairs scattered along the central frond stalk and pinnae midribs or in the pinnae axils. Occasionally, a hairy, sterile bulblet forms at the base of a pinna.

Stems are grooved, the upper stem straw-colored to greenish on mature plants, the lower stem darker brown with a few tan scales near the base. The plant grows in an asymmetrical clump, the fronds erect to ascending to arching, or hanging down when growing in a rock wall. 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 cup-shaped, translucent tissue (indusium) partly covers the spores, is attached on inner curve and is sparsely covered in glandular hairs. Spores ripen to dark brown or black. Most leaves have spores.

Notes:

Laurentian Fragile Fern is uncommon in the cracks and crevices of cliffs and canyon walls, on moss-covered boulders, shaded banks and rock outcrops, mostly on or near the north shore of Lake Superior with a few populations in the southeastern counties. It is identified by fronds widest above the base (though the lowest pinnae is not always much smaller than the second lowest), veins terminate at both the tips of lobes/teeth and the notches between, sparse glandular hairs on indusia and along the frond central stalk (rachis) and pinnae midribs and/or in the axils, occasional hairy bulblets at the base of a pinna, and usually growing on rocks. Of note is that many of our specimens had what at first appeared to be numerous whitish glands dotting the underside of pinnules; these turned out to be stomates (pores) rather than glands, so keep that in mind.

Thought to originally be a hybrid between C. bulbifera and C. fragilis, C. laurentiana is more similar to C. bulbifera, which has fronds widest at the base and a longer taper to the frond tip, is more densely glandular hairy with glands on the pinnule surface as well as on stalks, has larger, smooth and more numerous bulblets that are fertile, drop off and form new plants, and fresh spring fronds emerge with bright red or maroon stems, where C. laurentiana emerges on green stems. C. fragilis has smaller fronds than these two, widest at or below the middle, and lacks any glands or bulblets.

The only other Cystopteris species in MN that has any glandular hairs is C. tennesseensis, the child of C. bulbifera and C. protrusa; it also has sparse glandular hairs on both the stalks and indusia, veins that terminate in both the lobe notches and tips, and may have misshapen bulblets, but its fronds are widest at or near the base; it also has smaller spores but a microscope is required to see this, and this species hasn't been seen in Minnesota in at least 50 years so C. laurentiana is much more likely to be encountered. The other two Cystopteris species in Minnesota (C. protrusa and C. tenuis) lack any glandular hairs or bulblets and have fronds widest at or just below the middle.