Gymnocarpium jessoense (Koidz.) Koidz.
subsp. parvulum Sarvela
northern oak fern

Family: Dryopteridaceae

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Gymnocarpium jessoense is a rare fern, similar in appearance to the common G. dryopteris. Fronds are usually less than 40 cm long. The blade ranges from 3-14 cm long and is shorter than the stipe. The blade is broadly triangular, 2- pinnate-pinnatifid and ternate (divided into three roughly equal branches at the base). Sori are round, located on the underside of the blade and there is no indusium. The horizontal stems are elongate so leaves are not densely aggregated.

There are three similar species in this genus in Wisconsin, distinguished as follows. The abaxial (under) surface of rachis and blade of G. jessoense are clearly glandular and the adaxial (upper) surface is glabrous. G. dryopteris is glabrous on both surfaces and G. robertiana is glandular on both surfaces. Basal pinnae of G. jessoense are often curved toward the tip of the frond, and those of G. robertianum are usually straight and not pointing toward the tip of the frond. The reader is directed to the Flora of North America, upon which this description is based.

G. jessoense is found primarily from Wisconsin, Minnesota and Ontario, west through Canada to Alaska. Disjunct locations are also known from eastern Canada and the northeastern U.S. It is listed as a "special concern" species in Wisconsin where it has been recorded only from Bayfield County, often on or near acidic rocks.

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