Limestone fern

Description

Gymnocarpium robertianum has small (10-50cm), 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 Gymnocarpium - achriosporum Sarvela this taxon is known from Sweden and Quebec.