Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Petiole slender, with 2 bundles, scaly only at the base, much shorter than the blade in our spp.; blade mostly 2–4 times pinnate or ternate-pinnate; veins forking, free, reaching the margin, slightly thickened at the tip; sori rotund, each borne on a veinlet that continues to the margin; indusium hyaline, basiscopic (i.e., attached under the proximal margin of the sorus on the entering veinlet), ± hood-like, its free tip arched over the sorus (towards the distal margin of the pinnule) and commonly thrown back as the sorus expands, sometimes tiny and caducous; delicate woodland ferns with deciduous lvs, the petiole-bases not persistent. 10, cosmop.
Our spp. of Cystopteris enter into a polyploid complex that compromises the essential differences among the species. C. bulbifera and C. protrusa are diploid, 2n=84. C. fragilis is tetraploid (2n=168) in our area, but is considered to include some hexaploids and octoploids in the Old World. Two other established polyploids combine the features of C. bulbifera with those of C. protrusa (in one case) or C. fragilis (in the other). C. tennesseensis Shaver (tetraploid) may be an alloploid derived from C. bulbifera and C. protrusa, and C. laurentiana (Weath.) Blasdell may be an alloploid derived from C. bulbifera and C. fragilis var. fragilis. Both of these apparent alloploids usually have some poorly developed or abortive bulbils along the lf- rachis, and they tend to have some minute glands on the indusia. In C. tennesseensis the blade is widest at the base, as in C. bulbifera, whereas in C. laurentiana it is widest above the base, as in C. fragilis. C. laurentiana is northern, occurring from Nf. and Que. to Wis. and n. Minn., with a disjunct station in Centre Co., Pa. C. tennesseensis is more southern, occurring from Va. and Ky. to Kans. and Okla. There are also some ± sterile polyploids in this complex. C. ×illinoensis R. C. Moran (triploid, 2n=126) may reflect hybridization of C. bulbifera with C. fragilis var. mackayi, and C. ×wagneri R. C. Moran (tetraploid, 2n=168) may reflect hybridization of C. tennesseensis with C. fragilis var. mackayi.