Flora of North America species comparison

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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Cystopteris bulbifera usually occurs on moist calcareous cliffs, but it also grows on rock in dense woods and occasionally occurs terrestrially in northern swamps. Blades on most individuals are narrowly deltate and distinctively long-attenuate. The rachises, costae, and indusia are densely beset with gland-tipped, unicellular hairs. Mature specimens often have deciduous bulblets. These characteristics readily distinguish C. bulbifera from other diploid species.

Hybridization and allopolyploidy involving Cystopteris bulbifera and other North American Cystopteris species have generated several species. In the eastern portion of its range, C. bulbifera and C. protrusa are the diploid progenitors of the tetraploid C. tennesseensis (C. H. Haufler et al. 1990). In northeastern North America, C. bulbifera has hybridized with tetraploid C. fragilis, ultimately resulting in the hexaploid C. laurentiana (R. F. Blasdell 1963). In the southwest, the diploid C. reevesiana and disjunct representatives of C. bulbifera are the progenitors of the tetraploid C. utahensis (C. H. Haufler and M. D. Windham 1991). In addition to these fertile allopolyploids, sterile hybrids are also possible when C. bulbifera is sympatric with its polyploid derivatives. Sterile hybrids between C. bulbifera and C. tennesseensis have been identified from several localities. Cystopteris bulbifera may hybridize with C. tenuis to form C. × illinoensis (C. H. Haufler et al. 1990; R. C. Moran 1982b). Diploid sexual C. bulbifera may be distinguished from these allopolyploid species and sterile hybrids because the hybrid-derived taxa (1) will normally have less prominent glandular hairs, (2) will have misshapen bulblets, (3) will more likely have blades that are widest above the base, and (4) will have large spores (in sexual allopolyploids) or malformed spores (in sterile hybrids).

(Discussion copyrighted by Flora of North America; reprinted with permission.)