6: 264. 1827
Illustrator: John Myers
Phenology: Sporulating summer–fall.
Habitat: Mostly on shaded rock and cliff faces but also occasionally on forest floors
Elevation: 0–2800 m
N.B., N.S., Ont., Que., Ark., Ariz., Conn., Del., Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Mo., Nebr., Nev., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., Tenn., Utah, Vt., Va., W.Va., Wis.
Long recognized as Cystopteris fragilis var. mackayi, C. tenuis was returned to species status by R. C. Moran (1983b). It is probably an allotetraploid originating from C. protrusa and an extinct diploid related to C. fragilis (C. H. Haufler 1985; C. H. Haufler and M. D. Windham 1991).
Cystopteris tenuis is common in eastern North America and less frequent at the northern and western perimeter of its range. In the center of its distribution (Minnesota, Iowa, Illinois, Wisconsin, Indiana, Ohio, Pennsylvania), the narrow, elliptic pinnae angled toward the blade apex and the rounded teeth make C. tenuis relatively distinct from C. fragilis and C. protrusa (although the early season, sterile leaves of C. protrusa often resemble those of C. tenuis). In the west and especially in the northeast, C. tenuis and C. fragilis are difficult to distinguish. For the most part, C. fragilis is confined to higher latitudes and elevations than C. tenuis, but the two species can be sympatric and occasionally form sterile tetraploid hybrids. Cystopteris protrusa and C. tenuis are infrequently sympatric, but where they are, sterile triploid hybrids can occur. Hybrids between C. tenuis and C. tennesseensis are recognized as C. × wagneri (R. C. Moran 1983). Hybridization between C. tenuis and C. bulbifera has also been reported (R. C. Moran 1982b). This hybrid, C. × illinoensis R. C. Moran, is known only from the type and needs to be studied further.