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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].
                     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
                       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.
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].
                     11 Leaves phyllodial, with obvious cross-partitions (septa); leaves alternate
                 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 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 with a midvein; flowers and fruits solitary or in diffuse inflorescences.
                                  17 Leaves lacking a distinct lacunar band along the midvein.
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