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Keyed in multiple places:

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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
                                  17 Leaves monomorphic.
                                    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 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 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 (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
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