Gymnocarpium robertianum

Gymnocarpium robertianum, the limestone fern or scented oakfern, is a fern of the family Cystopteridaceae.

Description
Gymnocarpium robertianum has small (10–50 cm), deltate, two- to three-pinnate fronds. Fronds arise from creeping rhizomes and have long, delicate rachis. The sori are borne in round clumps on the underside of the blade and lack an indusium. This species differs from the closely related G. dryopteris in having a densely glandular rachis as well as a more sparsely glandular underside to the blade.

Gymnocarpium robertianum is thought to hybridise with G. appalachianum giving rise to Gymnocarpium × heterosporum W. H. Wagner. This hybrid was only known from Pennsylvania where it has now been eradicated. The hybrid between G. robertianum and G. dryopteris is called Gymnocarpium × achriosporum Sarvela. T his taxon is known from Sweden and Quebec.

Distribution and habitat
Gymnocarpium robertianum is a circumboreal species with populations in Europe, North America and the Caucasus Mountains.

This species is protected in Illinois and Michigan and is protected under the Flora Protection Order in Ireland.

This species is a calcicole and as such is restricted to alkaline rich areas. In the British Isles its preferential habitat is grykes in limestone pavement. In Michigan the species is most frequent in Thuja occidentalis swamps.