Gymnocarpium robertianum (Hoffmann) Newman
limestone oak fern
Family: Dryopteridaceae

basal pinna

lower, proximal pinnule


Gymnocarpium robetianum is a rare fern, similar in appearance to the common G. dryopteris. Fronds are usually less than 50 cm long. The blade ranges from 5-19 cm long and is shorter than the stipe. The blade is broadly triangular, 2-3- 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. robertianum is known in the U.S. only from Iowa, Minnesota, Wisconsin and Michigan, and in Canada from Ontario east, plus a few locations in Manitoba. In Wisconsin it is a "special concern" species and is known from about a dozen scattered locations in the southwestern part of the state, in counties near Lake Superior, and on the Door Peninsula. It is most often found growing on limestone.

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