Gymnocarpium robertianum (Hoffm.) Newman

Limestone oak fern, Robert's oak fern, Northern oak fern

Etymology Resembling Geranium robertianum.
Description Rhizome: long-creeping, dark brown or black, forking.
Frond: 40 cm high by 15 cm wide, deciduous, monomorphic, blade/stipe ratio: 3:2 to 2:1.
Stipe: grooved, dull green, scaly at the base, soon falling, vascular bundles: 2, oblong.
Blade: 2-pinnate-pinnatifid, triangular, stiff, dull surface, very glandular, especially on the lower surface and the rachis.
Pinnae: 9 to 12 pair, lowest pair similar in form (not size) to the next pair; pinnules lower basal pinnule of the lowest pinnae cut like adjacent pinnules (unlike D. dryopteris); costae grooved above, continuous from rachis to costae; margins entire to slightly crenate; veins free, simple or forked.
Sori: round, closer to the margin, on both edges of the segments, indusium: absent, sporangia: brownish, maturity: midsummer.
Dimensionality: vertical stipe bends towards the horizontal in the rachis, but not as much as in the other species.
Culture Habitat: calcareous woodlands or swamps, uncommon. Distribution: cicumboreal. Hardy to -40°C, USDA Zone 2, requires a cool summer.
Distinctive Characteristics The similarity of the lowest pinnae pair to the second pair, the glandularity (hand lens), consequent rank smell, and a generally more robust plant distinguish it from the other species.
Polypodium robertianum Hoffm.
Aspidium calcareum Baumg.
Lastrea robertiana Newm.
Phegopteris calcarea Fée
Phegopteris robertiana (Hoffm.) Ascherson
Thelypteris robertiana Slosson
Gymnocarpium robertianum
Gymnocarpium robertianum. bottom left: rhizome scales, above: pinnule, sori, and glandular hairs on lower surface, center: sorus, again with glands visible at 100% on a good monitor.  Illustration from Scandinavian Ferns by Benjamin Øllgaard and Kirsten Tind, Rhodos, 1993.
Gymnocarpium robertianum
Gymnocarpium robertianum.  Illustration by V. Fulford from Ferns and Fern Allies of Canada, William J. Cody and Donald M. Britton, 1989, © Agriculture Canada, used with permission.
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