7. Cystopteris Bernhardi, Neues J. Bot. 1(2): 26. 1805.
Bladder fern, brittle fern [Greek kystos, bladder, and pteris, fern, alluding to the indusium, which is inflated when young]
Christopher H. Haufler
Robbin C. Moran
Michael D. Windham
Plants terrestrial or on rock. Stems short- to long-creeping, stolons absent. Leaves monomorphic, dying back in winter. Petiole 1/3--3 times length of blades, base often swollen and persisting as trophopod over winter; vascular bundles 2, lateral, round or oblong in cross section. Blade ovate-lanceolate to deltate, 1--3-pinnate-pinnatifid, gradually reduced distally to a pinnatifid apex, membranaceous to herbaceous. Pinnae not articulate to rachis, segment margins crenulate, dentate, or serrate; proximal pinnae not reduced or 1 pair slightly reduced, sessile or petiolulate, equilateral or ± inequilateral, if inequilateral basiscopic side more narrowly cuneate; costae adaxially grooved, grooves continuous from rachis to costae; indument absent or of uniseriate, multicellular hairs in pinnae axils or of unicellular, gland-tipped hairs abaxially, absent adaxially. Veins free, simple or forked. Sori in 1 row between midrib and margin on ultimate segments, round; indusia ovate to lanceolate, hoodlike and arching over sorus toward margin, attached to receptacle base on costal side, persistent to ephemeral or often obscure at maturity. Spores brownish, echinate, or verrucate. x = 42.
Species ca. 20 (9 in the flora): worldwide.
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.
Blasdell, R. F. 1963. A monographic study of the fern genus Cystopteris. Mem. Torrey Bot. Club 21: 1--102. Haufler, C. H., M. D. Windham, D. M. Britton, and S. J. Robinson. 1985. Triploidy and its evolutionary significance in Cystopteris protrusa. Canad. J. Bot. 63: 1855--1863. Haufler, C. H. and M. D. Windham. 1991. New species of North American Cystopteris and Polypodium, with comments on their reticulate relationships. Amer. Fern J. 81: 7--23. Haufler, C. H., M. D. Windham, and T. A. Ranker. 1990. Biosystematic analysis of the Cystopteris tennesseensis (Dryopteridaceae) complex. Ann. Missouri Bot. Gard. 77: 314--329.