Gymnocarpium disjunctum (Rupr.) Ching

Western oak fern, Pacific oak fern

Etymology Probably named for the disjunct distribution in northeastern Asia and northwestern North America.
Description Rhizome: long-creeping, dark brown, forking.
Frond: 40 cm high by 15 cm wide, deciduous, monomorphic, blade/stipe ratio: 2:1 to 3:1.
Stipe: grooved, straw-colored, darker, purplish at the base, scaly at base, vascular bundles: 2, oblong.
Blade: 3-pinnate pinnatifid, triangular to pentagonal, herbaceous, vivid green, only a few glands below.
Pinnae: 6 to 10 pair, the lowest pair each similar to the remainder of the blade, only slightly smaller; margins entire to crenate; veins free, simple or forked.
Sori: round, in 1 row between midrib and margin, indusium: absent, sporangia: brownish.
Dimensionality: blade displayed horizontally.
Culture Habitat: woodlands. Distribution: northeastern Asia (Sakhalin Island, Kamchatka) and northwestern North America (Alaska to Oregon and Idaho). Hardy to -30C, USDA Zone 4.
Distinctive Characteristics Larger than G. dryopteris and more divided.
Polypodium disjunctum Rupr.
Dryopteris disjuncta (Rupr. ex Schur) Morton
Gymnocarpium dryopteris (L.) Newman ssp. disjunctum (Rupr.) Sarvela
Gymnocarpium disjunctum
Gymnocarpium disjunctum.  in habitat, Washington:: Snohomish County Perry Creek, 2006.  Robbin Moran
Compare to Distinguishable from G. dryopteris by generally a larger plant and more divided; the spore of G. dryopteris is also larger. G. dryopteris is distributed throughout the Northern Hemisphere, whereas G. disjunctum is confined to the coastal northwest of North America and the Pacific coast of Russia. See the discussion by Ed Alverson.
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