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Key A4: small ‘fern-like’ pteridophytes, epipetric or epiphytic, growing on rock, tree bark, or walls

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1 Leaves pinnatifid or bipinnatifid, the pinnae not fully divided from one another (the rachis winged by leaf tissue most or all of its length).
..2 Leaves pinnatifid, the pinnae not lobed.
....3 Leaf blades with a long-attenuate apex, blade lobed for up to 2/3 its length; sori elongate
....3 Leaves without a long-attenuate apex, blade lobed for > 4/5 of its length; sori round
..2 Leaves bipinnatifid, at least the lowermost pinnae deeply lobed.
......4 Leaves of a very delicate texture, 1 cell thick; sori borne in cups on the leaf margins; [rock outcrops with high air humidity]
......4 Leaves of an herbaceous, subcoriaceous, or coriaceous texture, > 1 cell thick; sori otherwise; [various habitats, not strictly of moist sites].
........5 Leaf blades 1-7 cm long, hairy with some hairs > 1 mm long
........5 Leaf blades > 7 cm long, glabrescent or hairy with hairs < 1 mm long.
..........6 Lowermost (and other) pinnae with numerous, rather even lobes
..........6 Lowermost pinnae with a few, irregular lobes (the upper pinnae unlobed)
1 Leaves pinnate, pinnate-pinnatifid, 2-pinnate, or even more divided, to 5-pinnate (the rachis naked for most of its length, often winged in the apical portion).
............ 7 Leaves pinnate or pinnate-pinnatifid.
............ ..8 Leaves of a very delicate texture, 1 cell thick; sori borne in cups on the leaf margins; [rock outcrops with high air humidity]
............ ..8 Leaves of an herbaceous, subcoriaceous, or coriaceous texture, > 1 cell thick; sori otherwise or sporangia not grouped in sori; [various habitats, not strictly of moist sites].
............ ....9 Pinnae > 1 cm wide; leaves subcoriaceous to coriaceous
............ ......10 Sporangia borne on 2 specialized fertile pinnae (in rows on segments of the 2 basalmost fertile pinnae of the leaf); [native; TX westwards]
............ ......10 Sporangia borne on the lower surface of green leaves; [collectively widespread]
............ ........11 Veins free; [native; s. FL]
............ ........11 Veins anastamosing (rejoining one another) to form a netlike pattern of areoles.
............ ..........12 Costae with a groove on the upper side, sometimes with tan, filiform scales; margins of segments denticulate, crenate, or spinulose
............ ..........12 Costae rounded or flat on the upper side, with red, multicellular hairs; margins of segments entire or with rounded lobes
............ ....9 Pinnae < 1 cm wide; leaves herbaceous to subcoriaceous.
............ ............ 13 Sori on the undersurface of the leaf, located away from the margins
............ ............ 13 Sori on the undersurface of the leaf, marginal and more-or-less hidden beneath either the unmodified revolute leaf margin or under a modified, reflexed false indusium
............ 7 Leaves bipinnate or more divided.
............ ............ ..14 Leaf blade pentagonal, broadly triangular, or narrowly triangular in outline, ca. 1-2.5× as long as wide.
............ ............ ....15 Leaf blade narrowly triangular in outline, mostly 2-2.5× as long as wide.
............ ............ ......16 Sporangia borne either in many sori on fertile blades or on fertile pinnules borne at the base of the otherwise sterile blades
............ ............ ......16 Sporangia borne in a solitary sorus terminal on the truncate apex of wedge-shaped leaf segments
............ ............ ....15 Leaf blade either ovate, pentagonal, or broadly triangular in outline, mostly 0.8-2× as long as wide.
............ ............ ........17 Leaves dimorphic, fertile leaves with longer and narrower segments than the sterile leaves
............ ............ ..........18 Leaf segments strongly revolute; sporangia scattered along veins or submarginal, not grouped into indusiate sori; lower surface of leaf segments either densely covered with a whitish farina or distinctly bluish-green
............ ............ ..........18 Leaf segments planar; sporangia borne in round, indusiate sori; lower surfaces of leaf segments green.
............ ............ ............ 19 Leaf blade pentagonal in outline, the terminal pinna by far the largest; rhizome 5-8 mm in diameter; indusia present, thick, persistent, and reniform; [introduced species, rarely naturalized]
............ ............ ............ 19 Leaf blade broadly triangular in outline, the basal pinnae by far the largest; rhizome ca. 1 mm in diameter; indusia absent; [native species of mountain peaks from n. NC and VA northwards]
............ ............ ..14 Leaf blade elongate, mostly lanceolate, generally > 4× as long as wide (except in Adiantum, with leaf blade often only 1.5-3× as long as wide, but not notably triangular or pentagonal in outline).
............ ............ ............ ..20 Sori marginal, usually more-or-less hidden under the revolute margin of the pinnule
............ ............ ............ ..20 Sori not marginal, either exposed, or slightly to strongly hidden by indusia.
............ ............ ............ ....21 Leaf blades 3-12 cm long; sori elongate, covered by a flap-like, entire indusium
............ ............ ............ ....21 Leaf blades 4-30 (-50) cm long; sori globular, surrounded or covered by an entire, ciliate, or divided indusium.
............ ............ ............ ......22 Veins reaching the margin; indusium attached under one side of the sorus, hood-like or pocket-like, arching over the sorus; petioles glabrous or sparsely beset with scales, the petiole bases not persistent
............ ............ ............ ......22 Veins ending short of the margin; indusium attached under the sorus, either cup-like (divided into 3-6 lanceolate to ovate lobes which surround the sorus from below) or of minute numerous septate hairs, which extend out from under the sorus on all sides; petioles often densely beset with scales, the petiole bases persistent

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.
..........6 Leaves pinnatifid
..........6 Leaves 1-pinnate or, 1-pinnate-pinnatifid, or 2-pinnatifid.
............ 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
............ ....9 Segment margins
............ ....9 Segment margins serrate or spinulose
............ 7 Leaves 1-pinnate-pinnatifid or 2-pinnatifid, the pinnae pinnatifid, generally lacking a prominent basal lobe; indusia either reniform or cuplike.
............ ......10 Vascular bundles in the petiole 3-7
............ ......10 Vascular bundles in the petiole 2.
............ ........11 Indusium reniform, arching over the sorus
............ ........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).
............ ..........12 Pinnae dimorphic, sori limited to the basal 2 pinnae
............ ..........12 Pinnae not dimorphic.
............ ............ 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, 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 (3-) 5 (-7) in the petiole
............ ............ ......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