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1 Plant minute, consisting of filaments or thalli (undifferentiated into leaves, stems, and roots), generally a single cell thick, usually with abundant single-celled gemmae (specialized bud-like groups of cells for asexual reproduction), a free-living fern gametophyte, superficially resembling bryophytes in lacking vascular tissue, reproducing only vegetatively (by gemmae); [usually growing on vertical or overhanging bedrock (epipetric)]; [Pteridophytes]
1 Plant more complex, with stems (or rhizomes), leaves, roots, the leaves generally > 1 cell thick (except in sporophytes of Didymoglossum, Crepidomanes, Vandenboschia, and Hymenophyllum), with vascular tissue, reproducing by seeds or spores (and often also with various vegetative means of reproduction); [growing in very diverse habitats, including epipetric on bedrock]; [Lycophytes, Pteridophytes, Gymnosperms, Monocots, Basal Angiosperms, and Eudicots].
  2 Plants floating aquatics, never rooted to the substrate (though sometimes stranded by dropping water levels); plants often thalloid in structure (lacking clear differentiation of stems and leaves)
  2 Plants terrestrial, wetland, or aquatic, normally rooted to the substrate (sometimes becoming detached and then floating in the water column, though usually not on the water surface, and lacking obvious adaptations for surface flotation); plants generally with clear differentiation of stems and leaves (with some exceptions).
    3 Plants woody, either trees, shrubs, lianas (woody vines), subshrubs, or rosette shrubs, with perennating structures (buds) borne on long-lived, above-ground, woody stems or caudices.
      4 Stems fleshy and flattened, green and photosynthetic (becoming gray on older stems), the nodes scattered on the flattened pads and bearing glochidia and also often spines; leaves absent
      4 Stems not both fleshy and flattened, usually brown, gray, or tan (sometimes green and photosynthetic), lacking glochidia (sometimes bearing spines, prickles, or thorns); leaves present, usually obvious, but sometimes scale-like.
        5 Plants rosette shrubs or subshrubs, the leaves strongly basally disposed and few to many, the above-ground stem stout (> 1 cm in diameter), usually < 1 dm tall; leaf arrangement alternate (but often with very short internodes).
          6 Leaves ‘fern-like’, 1-pinnate-pinnatifid or more divided, deciduous; plants lacking both flowers and seeds, reproducing by spores; [Pteridophytes]
          6 Leaves either simple, 1-pinnate, or palmately compound, evergreen; plants bearing seeds, with or without flowers; [Gymnosperms, Monocots, and Eudicots].
             7 Leaves 1-pinnate; plants bearing seeds in cone-like strobili; [Gymnosperms]
             7 Leaves simple or palmately compound; [Monocots and Eudicots]
        5 Plants trees, shrubs, or lianas (woody vines), the leaves usually many and cauline (borne along the stem), the above-ground stem usually > 2 dm long, if shorter, then not stout (< 0.5 cm in diameter); leaf arrangement alternate, opposite, or whorled.
               8 Plants tree-ferns, reproducing by spores; [s. FL]
               8 Plants gymnosperms or angiosperms, reproducing by seeds; [collectively widespread].
                 9 Leaf venation dichotomous (with even Y-forks, the veins alike, no vein dominant); leaf fan-shaped, deltoid, 3-8 cm wide; leaves alternate, borne in clusters or short, spur shoots; [Gymnosperms]
                 9 Leaf venation various, parallel, pinnate-reticulate, palmate-reticulate, with differentiation into primary, secondary, and finer levels of venation, most vein branches showing dominance by one of the two veins; leaf shape various, but not fan-shaped and ginkgo-like; leaves alternate, opposite, whorled, or fascicled; [Gymnosperms, Eudicots, Basal Angiosperms, Monocots].
                   10 Leaves stiff or scarious, needle or scale-like, in ×-section flat, nearly terete, or variously angled, with or without an obvious midvein and generally lacking noticeable secondary venation; leaf arrangement alternate, opposite, whorled, or grouped into fascicles of 2-5 with a scarious sheath at the base; seeds not enclosed by an ovary or a true fruit, either borne naked on the upper surface of ovuliferous scales aggregated into a cone (the cone sometimes modified and fleshy and “berrylike) or the seed solitary and mostly or completely enclosed in a fleshy or leathery aril or receptacle; [Gymnosperms]
                   10 Leaves generally not stiff (some exceptions), usually broader and with well-developed leaf blades (therefore flat in ×-section), usually with a midvein and well developed secondary and tertiary venation (some exceptions); leaf arrangement alternate, opposite, or whorled; seeds borne in fruits, which develop from ovaries; [Eudicots, Basal Angiosperms, and Monocots].
                     11 Leaves alternate; [Eudicots, Basal Angiosperms, and Monocots].
                     11 Leaves opposite or whorled; [Eudicots].
    3 Plants herbaceous, herbs, or herbaceous vines (though sometimes with a tough, semi-woody texture), annual, biennial, or perennial, if the latter, with perennating structures borne below-ground (or on the ground surface) as crowns, offsets, etc., or as buds on woody rhizomes.
                              15 Plants aquatics, all of the plant (except sometimes the reproductive structures) normally submerged or suspended in water, or floating on its surface; {some ambiguously aquatic taxa keyed both here and under 14b}
                              15 Plants terrestrial or amphibious, all or most of the plant, including most of its leaves and its reproductive structures normally borne in the air, emergent plants may have their bases permanently submerged, and other wetland plants may be occasionally submerged by high waters.
                                16 Plants completely lacking chlorophyll (white, pink, orange, tan, red), strictly parasitic or mycotrophic; [Eudicots and Monocots]
                                16 Plants with chlorophyll (usually all or partially green, the green pigment sometimes wholly or partly masked by non-green pigments), at least in part autotrophic (many are also partially mycotrophic or parasitic).
                                  17 Plant reproducing by seeds, developing in fruits derived from flowers; [Eudicots, Basal Angiosperms, and Monocots].
                                    18 Plants epiphytic, normally growing attached to plants and not rooting in soil; [note that epiphytic Pteridophytes are not keyed here, and should be sought in Keys A4 and A6]
                                    18 Plants terrestrial, rooted in soil (sometimes on logs or in tree knotholes, hollows, or tree-limb crotches where soil has accumulated, but not truly epiphytic).
                                       19 [Eudicots and Basal Angiosperms; see combination of features in Table 1]
                                         20 Leaves strictly basal, or strongly “basally disposed” (the basal leaves the largest, and usually persistent through most of the growing season)
                                         20 Leaves cauline (if plant with basal leaves, these not noticeably the largest, often senescing early) [note: many taxa keyed in both leads].
                                           21 Leaves opposite or whorled or appearing whorled (a few plants have leaves or leaf-like structures which appear whorled but anatomically are opposite or alternate with leaflets divided to the stem).
                                               23 Leaves whorled (some taxa with normally opposite leaves can have occasional developmental errors that result in an individual plant having 3-whorled leaves; these are not accommodated in the key as “whorled” [if a plant does not key readily as “whorled”, try it as “opposite”]) or appearing so