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Keyed in multiple places:

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Key A2: pteridophytes and lycophytes growing as floating or rooted aquatics

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1 Plant with at least some leaves palmately or pinnately lobed or 1-4× pinnately divided (‘fern-like’) and > 2 cm long
1 Plant either a floating aquatic with leaves <5 cm long, or with clover-like or linear leaves.
  2 Plant a floating aquatic
  2 Plant a rooted aquatic.
    3 Plant clover-like, with 4 leaf segments borne terminally
    3 Leaves linear.
      4 Plants cormose or with short rhizomes; leaves numerous, undivided leaves
      4 Plants with creeping rhizomes; leaves few, reduced to a winged petiole

Key A3: lycophytes and pteridophytes with leaves not ‘fern-like’ (unlobed, variously awl-shaped, scale-like, or terete)

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1 Stem obviously jointed; leaves scale-like, borne in a whorl at each of the distant joints; spores borne in a terminal strobilus with peltate scales
1 Stem (or rhizome) not jointed; leaves scale-like or larger, but if scale-like not borne in whorls at distant joints; spores borne variously, but if in a terminal strobilus the scales not peltate.
  2 Leaves linear, grass-like, 1-60 cm long, 20× or more as long as wide.
    3 Leaves solitary (though often the internodes very short from a thin, creeping rhizome); sporangia borne in a spherical (ca. 3 mm in diameter) sporocarp on a separate branch from the rhizome
    3 Leaves numerous from a corm or short-creeping rhizome; sporangia either borne in the expanded leaf bases (Isoetes in ISOETACEAE) or in 2 rows at the tip of the linear fertile leaves (Schizaea in SCHIZAEACEAE), or in a sub-marginal groove on either side of the midrib (Vittaria in PTERIDACEAE).
      4 Leaves straight and stiff, arching, or flaccid, from a 2-3-lobed corm; sporangia borne in the expanded, hyaline leaf bases
      4 Leaves either straight and stiff or notably spiral-curly, from a short-creeping rhizome; sporangia borne in 2 rows either at the expanded pectinate tip of the fertile leaves or along much of the length of the linear leaves.
        5 Leaves 10-60 cm long, straight, stiff; plants epiphytic or rarely epipetric, the leaves pendent
        5 Leaves 1-12 cm long, spiral-curly; plants terrestrial in peaty substrate, the leaves erect
  2 Leaves various (scale-like, awl-like, moss-like, or flat), but not linear and grass-like, mostly 1-10 (-20)× as long as wide.
          6 Leaves inconspicuous, reduced to a few nerveless scales (< 1.5 mm long), the internodes much longer than the leaves; sporangia yellowish, 3-locular, 1-2 mm in diameter; stems upright, repeatedly branched dichotomously
          6 Leaves either larger or, if scale-like, with 1 or more nerves and longer than the internodes (the leaves thus overlapping); sporangia yellowish to brownish, 1-locular, < 1 mm in diameter; stems either subterranean or surficial rhizomes or erect or ascending (and sometimes dichotomously branched in whole or in part in Huperzia in HUPERZIACEAE, and Diphasiastrum and Dendrolycopodium in LYCOPODIACEAE).
             7 Plant with leaves very numerous and overlapping along the creeping, ascending, erect, or pendant stems, the leaves scale-like or awl-like, 0.5-2 (-3) mm wide, typically acute, acuminate, or hair-tipped; sporangia either in terminal strobili (axillary to specialized, smaller leaves) or axillary to normal leaves; [Lycophytes].
               8 Sporangia borne in flattened or quadrangular strobili sessile at the tips of leafy branches; spores and sporangia each of two sizes, the megasporangia larger and borne basally in the strobili
               8 Sporangia borne either in the axils of normal foliage leaves, or in strobili sessile at the tips of leafy branches or stalked on specialized branches with fewer and smaller leaves; spores and sporangia each of one size.
                 9 Leafy stems erect or pendant, simple or dichotomously branched, the ultimate branches vertically oriented (upwards or downwards); sporophylls like the sterile leaves or only slightly reduced, in annual bands along the stem; vegetative reproduction by leafy gemmae near the stem apex (Huperzia) or lacking gemmae (Phlegmariurus)
                 9 Leafy stems prostrate or erect, if erect then generally branched, the ultimate branches spreading (horizontal) or ascending; sporophylls differing from sterile leaves, either broader and shorter, or more spreading, aggregated into terminal cones; lacking vegetative reproduction by gemmae.
             7 Plant with leaves not as above (see below for details); [Pteridophytes].
                   10 Leaves palmately lobed.
                     11 Leaves divided into separate sterile and fertile segments, the sterile leaf blade 30-90 cm long, ovate to lanceolate, entire-margined, obtuse, the longer fertile portion with 2 rows of sporangia somewhat imbedded in it
                     11 Leaves with fertile portions in patches near the tips of the lobes
                   10 Leaves unlobed (or if lobed, pinnately so).
                       12 Plant with 1 (-several) leaves, divided into separate sterile and fertile segments, the sterile leaf blade 0.3-90 cm long, ovate to lanceolate, entire-margined, unlobed, obtuse, the longer fertile portion with 2 rows of sporangia somewhat imbedded in it
                       12 Plant with many leaves, generally 5 or more, not divided into separate sterile and fertile segments, the leaves either (a) small, 0.3-1.6 cm long, obovate, scattered along a very thin creeping rhizome, or (b) larger, (2-) 8-30 cm long, cordate at base, the tip long-attenuate (often proliferous, bearing a plantlet at the tip).
                          13 Leaf blades (2-) 8-30 cm long, cordate at the base, the tip long-attenuate, often proliferous (bearing a plantlet at the tip); sporangia grouped into indusiate sori on the undersurface; leaf texture moderately thick; rhizome erect or ascending, 1.0-1.5 mm in diameter, the leaves clustered from its tip
                          13 Leaf blades 0.3-15 cm long, cuneate at the base, acute to rounded or obtuse at the tip, not proliferous; sporangia solitary in a marginal pocket on the leaf; leaf texture very thin; rhizome creeping on the surface of rock or bark, either 0.1-0.3 or 0.5-1.5 mm in diameter, the leaves scattered along it
                            14 Leaf blades 0.3-1.6 cm long; sporangia solitary in a marginal pocket on the leaves; leaf texture very thin; rhizome 0.1-0.3 mm in diameter
                            14 Leaf blades 2-15 cm long; sporangia borne in round sori on the lower surface of the leaves; leaf texture herbaceous to subcoriaceous; rhizome 0.5-1.5 mm in diameter

Key C4: rooted aquatics with basal and simple, linear leaves

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1 Leaves thread-like or quill-like, about as thick as wide.
  2 Plants bulbous at base, and with the leaf bases expanded and containing sporangia; plant tufted or with very short rhizomes; [Lycophytes]
  2 Plants either somewhat bulbous or not at the base, the leaf-bases not containing sporangia; plant rhizomatous; [Pteridophytes, Eudicots, Monocots].
    3 New leaves unfurling with circinate vernation (a fiddlehead); plants reproducing by spores, from sporocarps on short stalks from the rhizome; [Pteridophytes]
    3 New leaves lacking circinate vernation; plants reproducing by flowers and seeds.
      4 Perianth differentiated, with either 3 sepals and 3 petals or 5 sepals and 5 petals; stamens either 7-many or stamens 4.
        5 Sepals 3; petals 3; stamens 7-many; [Monocots]
        5 Sepals 5; petals 5; stamens 4; [Eudicots]
      4 Perianth undifferentiated, with 0, 3, or 6 tepals; stamens 1, 2, or 3; [Monocots].
          6 Gynoecium of 2 or more pistils, each pistil with 1 carpel and with 1 stigma
          6 Gynoecium of 1 pistil, each pistil with (2-) 3 carpels and (2-) 3 stigmas.
             7 Fruit an achene; perianth absent
             7 Fruit a capsule; perianth of 6 tepals
1 Leaves ribbon-like or strap-like, distinctly flattened (sometimes only near the tip of the leaf).
               8 Subterranean portions of plant bearing bladder-traps; flowers yellow or purple, bilaterally symmetrical
               8 Subterranean portions of plant lacking bladder traps; flowers white, green, gray, radially symmetrical (except bilaterally symmetrical in Glossostigma in PHRYMACEAE).
                 9 Leaves broadened towards the tip; [Monocots, Eudicots].
                   10 Flowers 3-merous; [Monocots]
                   10 Flowers 4- or 5-merous; [Eudicots].
                     11 Leaves phyllodial, with obvious cross-partitions (septa); leaves alternate
                     11 Leaves not phyllodial; leaves opposite
                 9 Leaves parallel-margined or tapering towards the apex over much of their length; [Monocots].
                       12 Leaves tapering towards the apex over much of their length; plant either tufted and not rhizomatous, or short rhizomatous;.
                          13 Plant tufted, not rhizomatous; leaves spreading radially; inflorescence a tightly button-like head of very numerous small flowers, white, gray, tan, yellowish, or blackish; roots thickened, septate (not requiring magnification), unbranched
                          13 Planted short-rhizomatous; leaves distichous, equitant; inflorescence either a subglobular, ovoid, or cylindrical head, of spirally imbricate scales, or a diffuse corymb; roots not thickened, not septate, branched.
                            14 Inflorescence a diffuse corymb; rhizomes and roots bright red
                            14 Inflorescence a subglobular, ovoid, or cylindrical head of spirally imbricate scales subtending individual flowers; rhizomes and roots not bright red
                       12 Leaves parallel-margined; plant usually rhizomatous.
                              15 Plants of marine habitats, growing submersed in salt water; [FL and the Gulf Coast of AL, MS, and LA]
                              15 Plants of freshwater or slightly to somewhat brackish habitats; [collectively widespread].
                                16 Leaves lacking any midvein; flowers and fruits in globose heads
                                16 Leaves with a midvein; flowers and fruits solitary or in diffuse inflorescences.
                                  17 Leaves with a distinct, broad lacunar band along the midvein
                                  17 Leaves lacking a distinct lacunar band along the midvein.
                                    18 Stamens 3
                                    18 Stamens >6
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