Leaves and stems:

At the top of the stem is a single compound leaf (frond) though appears like a whorl of 3 leaves. The leaf is generally triangular in outline, up to 6 inches long and nearly as wide, with 6 to 10 pair of branches (pinnae) oppositely arranged. The lowest pair of branches are typically twice compound, triangular in outline, to 3½ inches long, somewhat smaller than the rest of the leaf, stalked and connected to the main stem at a swollen node. Upper branches are once compound or merely lobed, stalkless, more oblong in outline but tapering to a pointed tip and generally curve upward toward the leaf tip. The leaves are held horizontally, nearly parallel to the ground, and typically bright yellow-green.

Each branch is divided into smaller segments (pinnules) that are oblong in outline but tending to curve upward toward the branch tip, divided into several rounded lobes or merely scalloped around the edges. The lowest pinnules are about as long as the second lowest. Veins are freely branched or sometimes merely forked. The upper surface is hairless, the lower surface of both leaves and stalks are sparsely to moderately glandular hairy, more densely so towards the leaf tip.

The main stem (stipe) is slender, up to about 12 inches long, dark purplish-brown at the base, greenish-brown to straw-colored above, glandular near the leaf but smooth at the base except for scattered tan scales.

Spores:

The sori (group of spores) are found on the underside of the leaf. They are circular and arranged around the edges of a pinnule, at the tip of a vein, and often merge at maturity. There is no extra tissue (indusium) that surrounds or covers the spores. Spores ripen to dark brown, though not all leaves have spores.

Notes:

Nahanni Oak Fern, formerly known as Gymnocarpium jessoense, is an uncommon fern found primarily in the arrowhead region of Minnesota. There are 3 Oak fern species in the state, all relatively small, under 16 inches tall, and have what appears to be 3 leaves whorled at the top of the stem which are held more or less horizontally, parallel to the ground. Gymnocarpium continentale is distinguished by its glandular leaf undersides and stems, and is almost always found on rocks—usually cool cliffs or talus slopes.

The other two are forest species: G. dryopteris is also hairless, lacking any glands; G. robertianum is glandular on both leaf surfaces and its pinnae and pinnules tend to be straight, not much curving, especially on the lower part of the leaf. G. continentale hybridizes with G. dryopteris and the hybrid (X intermedium) may be sparsely glandular but has sterile, blackish, malformed spores and sometimes also round, brown spores that may be fertile.