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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
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