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Key A6: medium to large ‘fern-like’ pteridophytes, epipetric on rock or walls, or epiphytic on tree trunks

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1 Leaf vine-like, 0.3-10 m long, the branching dichotomous, 1 branch of each dichotomy terminating in a pair of pinnae, the pinnae often widely spaced (> 10 cm apart)
1 Leaf not vine-like, 0.3-3 m long, the branching not as described above, the pinnae regularly and more-or-less closely spaced (mostly < 10 cm apart).
..2 Leaves (at least the sterile leaves if the leaves are dimorphic) 1-pinnate-pinnatifid or less divided (the pinnae entire, toothed, lobed or pinnatifid).
....3 Leaves dimorphic, the sterile leaves 1-pinnate, the fertile leaves bipinnate; sori covering the entire undersurface of the fertile leaves
....3 Leaves monomorphic; sori not covering the undersurface of the leaves.
......4 Sori marginal, continuous, covered by a reflexed false indusium along the leaf margin; pinnae usually opposite, linear, not toothed or lobed
......4 Sori neither marginal nor continuous, slightly to entirely covered by an elongate or roundish indusium (sometimes ciliate, toothed, or divided into narrow segments); pinnae usually at least in part alternate, mostly lanceolate, toothed, lobed, or pinnatifid.
........5 Sori elongate, the indusium flap-like, attached along the side; leaf blades < 7 cm wide when > 30 cm long
........5 Sori circular or globular, the indusium peltate, reniform, or cuplike; leaf blades > 5 cm wide when > 30 cm long.
............ 7 Leaves 1-pinnate, the pinnae toothed and each with a slight to prominent lobe near the base on the side toward the leaf tip; indusia peltate, reniform, or crescent-shaped.
............ ..8 Leaves pale green, thin in texture; pinnae articulate to rachis, deciduous with age; rhizome bearing elongate, thin, wiry stolons; indusia reniform or crescent-shaped
............ ..8 Leaves dark-green, subcoriaceous to coriaceous; pinnae not articulate and deciduous with age; rhizome not producing stolons; indusia peltate
............ 7 Leaves 1-pinnate-pinnatifid or 2-pinnatifid, the pinnae pinnatifid, generally lacking a prominent basal lobe; indusia either reniform or cuplike.
............ ........11 Indusium cuplike, attached beneath the sorus and consisting of 3-6 lanceolate to ovate segments
..2 Leaves (at least the sterile leaves if the leaves are dimorphic) 2-pinnate or more divided (the pinnae divided to their midribs).
............ ............ 13 Sori marginal and borne on the underside of the false indusium (modified, marginal flaps of the leaf margin); petioles and rachis shiny black or reddish-black; pinnules fan-shaped or obliquely elongate
............ ............ 13 Sori not marginal, borne on the undersurface of the leaf blade (if marginal, as in Pteridium and Dennstaedtia, borne on the undersurface of the leaf); petioles darkened only basally (if at all), rachis green, tan, or reddish; pinnules not notably fan-shaped or obliquely elongate.
............ ............ ..14 Outline of leaf blade narrowed to base, the widest point > 7 pinna pairs above the base, the lowermost pinnae < 1/4 as long as the longest pinnae; rhizomes long-creeping, the leaves scattered or in small tufts, forming clonal patches
............ ............ ..14 Outline of the leaf blade slightly if at all narrowed to the base, the widest point < 5 pinna pairs from the base, the lowermost pinnae > 1/2 as long as the longest pinnae; rhizomes short-creeping, the leaves clustered, not forming clonal patches (or with rhizomes long-creeping, leaves scattered, forming clonal patches in Dennstaedtia in DENNSTAEDTIACEAE).
............ ............ ....15 Rhizomes long-creeping, leaves scattered, forming clonal patches; vascular bundles in the petiole 1, U-shaped (even in the lower petiole); sori very small, marginal in sinuses, the indusium cup-like, 2-parted, the outer part a modified tooth of the leaf blade; leaf blades conspicuously puberulent with septate glandular hairs
............ ............ ....15 Rhizomes short-creeping, the leaves clustered, not forming clonal patches; vascular bundles in the petiole 2-7 (sometimes uniting to 1 in the upper petiole); sori mostly larger, mostly not marginal, the indusium not as above (though cup-like in Woodsia obtusa); leaf blades either glabrous, glabrescent, with flattened scales, or puberulent with glandular trichomes.
............ ............ ......16 Vascular bundles 2 in the petiole (or uniting near the leaf blade into 1).
............ ............ ........17 Leaves 25-65 cm wide, with whitish, straight, acicular hairs; [species adventive and weedy]
............ ............ ........17 Leaves 5-25 (-30) cm wide, with scales and minute glands (sometimes also with septate hairs); [native species].
............ ............ ..........18 Leaves 1-pinnate-pinnatifid; indusium cup-like, attached beneath the sorus and consisting of 3-6 lanceolate to ovate segments
............ ............ ..........18 Leaves 2-pinnate-pinnatifid; indusium flap-like, pocket-like, or hood-like, attached at one side of the sorus and arching over it.
............ ............ ............ 19 Leaves 10-30 cm wide, the tip acute to acuminate; indusium flap-like
............ ............ ............ 19 Leaves 4-9 cm wide, the tip long-attenuate; indusium pocket-like or hood-like
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