in Decken, Reisen Ost-Afrika 3(3): 11. 1879.
Petioles scattered along creeping stems, 0.3–3.5 m, shallowly to deeply grooved adaxially, base not strongly distinct from stem. Blades broadly deltate, papery to leathery, sparsely to densely hairy abaxially, rarely glabrous. Pinnae often opposite to subopposite [alternate]; proximal pinnae often prolonged basiscopically, each proximal pinna nearly equal to distal part of leaf in size and dissection (except in var. caudata). Segments alternate, numerous.
Alta., B.C., Man., N.B., N.S., Nfld. and Labr. (Nfld.), Ont., P.E.I., Que., Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., D.C., Del., Fla., Ga., Idaho, Ill., Ind., Iowa, Ky., La., Maine, Mass., Md., Mich., Minn., Miss., Mo., Mont., N.C., N.Dak., N.H., N.J., N.Mex., N.Y., Nev., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Va., Vt., W.Va., Wash., Wis., Wyo., Almost worldwide.
In accord with the most recent revision (R. M. Tryon 1941) of the genus, Pteridium is treated here as a single widespread species composed of two subspecies with 12 varieties. So treated, it is probably the most widespread species of all vascular plants, with the exception of a few annual weeds (F. H. Perring and B. G. Gardner 1976). The plants are generally aggressive, invading disturbed areas as weeds in pastures, cultivated fields, and roadsides. In Europe, it was harvested and burned to produce potash. Although croziers are eaten in many temperate cultures, bracken has been shown to contain thiaminase (and other compounds with mutagenic and carcinogenic properties).
Disagreement exists among taxonomists regarding the rank that should be accorded to the taxa treated herein as varieties. In a survey of the genus, C. N. Page (1976) noted uniform chromosome numbers and flavonoid compositions of the varieties. D. B. Lellinger (1985) separated the genus into at least two species based on morphology, recognizing as species the subspecies of R. M. Tryon (1941). J. T. Mickel and J. M. Beitel (1988) reported sympatric occurrence in Mexico of three taxa that maintained consistent characteristics and only rarely produced plants with combined characteristics. They suggested that these three taxa should be considered as species that occasionally hybridize. P. J. Brownsey (1989) reported that two different brackens in Australia formed sterile hybrids and should be treated as species. Modern systematic studies are needed to evaluate the status and rank of the four North American varieties. As treated below, Pteridium aquilinum var. pubescens, var. latiusculum, and var. pseudocaudatum are in subsp. aquilinum, and var. caudatum is in subsp. caudatum (Linnaeus) Bonaparte.
Varieties 12 (4 in the flora).
|1||Fertile ultimate segments only decurrent or more decurrent than surcurrent, mostly 1-2 mm wide; hairs on abaxial surface of blades abundant, straight, stiff, subappressed to spreading.||Pteridium aquilinum var. caudatum|
|1||Fertile ultimate segments adnate or equally decurrent and surcurrent, mostly 3-6 mm wide; hairs on abaxial surface of blades abundant to sparse, twisted and flexible, if abundant then lax, spreading.||> 2|
|2||Pinnules at nearly 90º angle to costa; outer indusium pilose on margin and often on surface; hairs on abaxial surface of blades abundant, lax, and spreading.||Pteridium aquilinum var. pubescens|
|2||Pinnules at 45º -60º angle to costa; outer indusium glabrous; hairs on abaxial surface of blades sparse or blades nearly glabrous.||> 3|
|3||Terminal segments of pinnules 2-4 times longer than wide; segment margins and abaxial surface of blade midrib and costae shaggy.||Pteridium aquilinum var. latiusculum|
|3||Terminal segments of pinnules ca. 6-15 times longer than wide; segment margins and abaxial surface of blade midrib and costae sparsely pilose to glabrous.||Pteridium aquilinum var. pseudocaudatum|