Flora of North America species comparison

Cystopteris is a taxonomically difficult genus at the species level. Especially troublesome is the worldwide and polymorphic species C. fragilis sensu lato. To maintain it as a single species with several varieties would be easiest (and least controversial). This approach, however, may not accurately reflect true evolutionary history.

Although Cystopteris species are found in temperate climates worldwide at tetraploid to octaploid ploidy levels, extant diploid species are concentrated in North America. The diploid species are relatively distinct from one another and are the progenitors of numerous allopolyploid derivatives (see reticulogram). In addition, an extinct (or undiscovered) diploid may have been involved in the origin of some polyploids (shown as "C. hemifragilis" on the reticulogram).

Considerable overlap exists among the leaf morphologies in the species of Cystopteris, even among the diploid taxa. Consequently, the key requires observation of subtle and sometimes overlapping characteristics.

Several general recommendations can be made for identifying Cystopteris. (1) Field workers should be aware that whenever Cystopteris species occur together, hybridization is likely; hybrids usually have shriveled and malformed spores. (2) Species of Cystopteris frequently occur as highly reduced plants, especially in stressful habitats such as high elevations, high latitudes, and cold and/or dry climates. Such stunted plants can be fertile, but leaf and stem characters required to distinguish species can be obscured. (3) Because of the importance of examining stem and spore features in distinguishing species, collectors should always attempt to obtain complete, fertile specimens.

Species ca. 20 (9 in the flora).

(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.)