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 7½ inches long and nearly as wide, with 9 to 12 pair of branches (pinnae) oppositely arranged. The lowest pair of branches are typically twice compound, triangular in outline, to 5 inches long, somewhat smaller than the rest of the leaf, stalked and connected to the main stem at a swollen node. The second lowest branch is also usually stalked and much like the lowest but smaller. Upper branches are once compound or merely lobed, stalkless, more oblong in outline but tapering to a pointed tip and mostly straight, perpendicular to the stem. The leaves are held horizontally, nearly parallel to the ground, and medium green to bright yellow-green.

Each branch is divided into smaller segments (pinnules) that are oblong in outline, mostly straight and parallel to the midrib, divided into several rounded lobes or merely scalloped around the edges. The lowest pinnules are as long as or shorter than the second lowest; when the second lowest branch is stalkless, the lowest pinnules are typically shorter. Veins are freely branched or sometimes merely forked. The lower surface of both leaves and stalks are moderately to densely glandular hairy, more densely so towards the leaf tip; the upper surfaces are also glandular but more sparsely so.

The main stem (stipe) is slender, up to about 12 inches long, mostly dull green, glandular near the leaf becoming smoother with scattered tan scales near the base.

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:

Northern Oak Fern is a rare species in Minnesota. According to the DNR, it has only been documented 14 times in Minnesota and about a third of those populations have not been seen since at least the 1970s. Populations in the southeast counties are found mainly on talus slopes while those farther north are in a type of conifer swamp known as a forested rich peatland. It was listed as a Special Concern species in 2013, is also Special Concern in Wisconsin, and is considered rare in all of its US range.

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 robertianum is distinguished by leaves and stems glandular on all surfaces, and forested habitat. Of the other two species: G. dryopteris lacks any glands; G. continentale (formerly G. jessoense) is glandular only on the lower surfaces of leaves and stems and its pinnules and pinnae tend to be curved upward, towards the branch or leaf tip.