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Key to Hymenophyllaceae

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1 Gametophytes only present, not in association with or in close proximity to filmy-fern sporophytes.
..2 Gametophytes thalloid, ribbonlike, flattened
..2 Gametophytes filamentous in part or in whole, forming felt-like mats.
....3 Gametophytes strictly filamentous; [of inland physiographic provinces, on ceilings and back walls of rock grottos]
....3 Gametophytes filamentous and also bearing flattened blades tipped by gemmae; [of rotting wood in the Coastal Plain]
1 Sporophytes present.
......4 Leaves simple to slightly lobed, < 2 cm long; rhizomes filiform, <0.5 mm in diameter.
........5 Leaves glabrous or with simple hairs; rhizomes densely covered with dark-colored hairs
........5 Leaves stellate pubescent; rhizomes glabrous or with sparse light-colored hairs
......4 Leaves pinnatifid to pinnate-pinnatifid, > 2.5 cm long; rhizomes filiform or moderately stout.
..........6 Rhizomes erect, the leaves clustered at the tip; leaves 1-pinnatifid; [of rotting wood in the Coastal Plain]
..........6 Rhizomes long-creeping, the leaves scattered along the rhizome; leaves 1-pinnatifid to 1-2 pinnate-pinnatifid; [of inland physiographic provinces, of moist rock].
............ 7 Leaves with dark stellate hairs in the lobe sinuses; [of Coastal Plain of s. FL]
............ 7 Leaves lacking stellate hairs; [of inland provinces, rarely Coastal Plain of AL and MS].
............ ..8 Rhizomes filiform, < 0.5 mm in diameter, glabrous or with sparse light-colored hairs; indusium ("involucre") bivalvate (deeply divided into 2 flaps); receptacle not exserted from between the deeply bilobed indusium; margins of finest leaf divisions with sharp teeth pointing towards the division apex (at 20× magnification)
............ ..8 Rhizomes moderately stout, 0.8-1.5 mm in diameter, densely clad with dark-colored hairs; indusium ("involucre") tubular or funnelform, sometimes slightly 2-lobed; receptacle long and whiplike, exserted from the mouth of the tubular (slightly bilobed) indusium; margins of finest leaf divisions entire or with obscure crenations (at 20× magnification)

Key to Hymenophyllum

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1 Sporophytes present.
..2 Leaf blade with stellate hairs; [subgenus Sphaerocionium]
..2 Leaf blade glabrous; [subgenus Hymenophyllum]
1 Gametophytes only present.
....3 Gemmae present; margin crenate, composed predominantly of cells with concave outer walls; archegonia and antheridia rare; plant forming sprawling, ribbon-like forms; branches filamentous to broad; proliferations abundant, arising marginally and centrally; [subgenus Sphaerocionium]
....3 Gemmae absent; margin entire, composed predominantly of straight-sided cells; archegonia and antheridia common, often present on the same gametophyte; plant typically forming rosettes; branches always broad; proliferations few, always marginal; [subgenus Hymenophyllum]
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