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
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.