Protrusa means protruding. See the Distinctive Characteristics.
Rhizome: short-creeping, covered with old stipe bases, tan to golden hairs, and at the apex scales tan to light brown, lanceolate.
Frond: 40 cm high by 10 cm wide, deciduous, monomorphic, but early fronds smaller, sterile, less divided, blade/stipe ratio: 1:1.
Stipe: stipes rising 1-several cm behind the rhizome tip, grooved, dark at base, straw-colored to green above, scales lanceolate, at the base, and sometimes up the rachis, vascular bundles: 2, round or oblong.
Blade: 2-pinnate-pinnatifid, ovate-lanceolate, herbaceous, glabrous.
Pinnae: 6 to 12 pair, lanceolate, widest just below the middle, perpendicular to the rachis, ± opposite; costae grooves above continuous from rachis to costae; veins free, simple or forked, ending usually at teeth.
Sori: round, in 1 row between midrib and margin, indusium: ovate, forming a hood over the sorus, but shriveling with maturity, beneath sorus on midrib side, sporangia: brown to black.
Dimensionality: lowest pinnae pair bending forward, down.
Habitat: on soil (rather than limestone rocks of C. fragilis) in woods.
Distribution: central to eastern North America.
Hardy to -30°C, USDA Zone 4, but given the distribution, the source is likely important.
Similar to C. fragilis in almost all respects, but the rhizome protruding well beyond the cluster of fronds. Additionally, this is broader, less than 2.5 times longer than wide; C. fragilis is narrower, more than 2.5 times longer than wide.
Cystopteris fragilis (Linnaeus) Bernhardi var. protrusa Weatherby