1 Flowers white or cream-white, 1-3 mm long; inflorescence peduncles very reduced, the pedicels appearing to arise directly from the stolons; traps 0.3-0.8 mm long; plants floating unattached in water (sometimes deposited on land by dropping water, but then the principal branch systems stranded on the soil surface); capsules ca. 1 mm long, fusiform, indehiscent, with 1 seed; seeds essentially smooth, unornamented; leaves absent; [subgenus Utricularia, section Utricularia]

1 Flowers yellow, pink, or purple (sometimes fading whitish), (2-) 5-20 mm long; inflorescence peduncles well-developed, the inflorescence clearly a raceme; traps 0.2-5.0 mm long; plants either attached (with principal branch systems within the soil), or floating unattached in water (sometimes deposited on land by dropping water, but then the principal branch systems stranded on the soil surface); capsules 1-8 mm long, globose, subglobose, or ovoid, with many seeds; seeds reticulate, papillose, echinate, multi-angled, or winged (rarely more-or-less smooth); leaves present (sometimes absent in the terrestrial species).

  2 Plants attached (with principal branch systems within the soil); leaves aerial, from near the rooted base of the plant (rosulate or from nodes of stolons), simple, linear or spatulate (sometimes absent); traps 0.2-1.1 mm long, most or all on a plant usually < 1.0 mm long; seeds reticulate-alveolate (also angled in U. resupinata), 0.2-0.4 mm long.

    3 Leaves 2-6 per plant, spatulate, the blade (2-) 3-6 mm wide; flowers whitish to pale yellow; bract at base of the pedicel basifixed, ca. 1 mm long, basally connate to 2 bracteoles; [subgenus Utricularia, section Foliosa]

    3 Leaves numerous per plant, linear, < 1 mm wide, either terete or flattened; flowers yellow (sometimes fading whitish) or pink; bract at base of the pedicel either tubular (1.2-1.5 mm long, attached circumferentially around the stem and bracteoles lacking), peltate, or ovate, attached on one side of the stem (bracteoles absent [resupinata, subulata] or present [cornuta, juncea, simulans]).

      4 Flowers pink; inflorescence 1 (-2)-flowered; leaves terete, septate, acute at its apex; bract at base of the pedicel tubular, 1.2-1.5 mm long, attached circumferentially around the stem (bractlets absent); [subgenus Utricularia, section Lecticula]

      4 Flowers yellow (sometimes fading whitish); inflorescence (1-) 2-15-flowered; leaves flattened, non-septate, acute to obtuse at the apex; bracts attached at their base (one end free) or near their middle (both ends free).

        5 Bracts, bracteoles, and sepals fimbriate with 10-20 subulate teeth; [subgenus Bivalvaria, section Aranella]

        5 Bracts, bracteoles, and sepals entire or nearly so.

          6 Bracts subtending the pedicels ovate (attached at their bases), free only at their upper end; pair of bracteoles associated with each bract present, linear to lanceolate; spur of the corolla oriented downward or backward, at approximately a right angle to the lower lip; leaves with acute apex; [subgenus Bivalvaria, section Stomoisia].

             7 Corolla 1.5-2.0 cm long; spur 8-12 mm long; raceme usually short, the (1-) 2-6 flowers crowded together, all of them chasmogamous

             7 Corolla 0.25-1.5 cm long; spur 5-7 (-9) mm long; raceme usually elongate, the (1-) 2-15 flowers well-spaced, often the lower (sometimes all) cleistogamous and much smaller than the chasmogamous flowers

  2 Plants floating unattached in water (sometimes deposited on land by dropping water, but then the principal branch systems stranded on the soil surface); leaves present and dissected into linear segments, normally submersed; traps 0.7-5.0 mm long, most or all on a plant > 1.0 mm long; seeds papillose, reticulate, ridged, angled, or winged, 0.5-2.0 mm long.

               8 Flowers purple; leaves divided into verticillate segments with terminal traps; [subgenus Utricularia, section Vesiculina]

               8 Flowers yellow; leaves divided into alternate segments with lateral traps; [subgenus Utricularia, section Utricularia].

                 9 Peduncle with whorl of inflated leaf-like organs (floats).

                   10 Branches of the floats longer (3-8 cm long), with dissections throughout the distal half; leaves with the 2 primary divisions unequal; bracts of the scape longer than broad, entire; flowers (6-) 9-14 (-17) per scape; apex of corolla spur bifid

                   10 Branches of the floats shorter (1-4 cm long), with dissections at just the tip; leaves with the 2 primary divisions equal; bracts of the scape broader than long, the apex slightly to strongly 3-lobed; flowers (1-) 3-4 (-7) per scape; apex of corolla spur usually entire (rarely bifid)

                 9 Peduncle without whorl of inflated leaf-like organs (floats).

                     11 Main axes distinctly flattened in cross-section, up to 10 mm wide

                     11 Main axes round in cross-section.

                       12 Lower lip of corolla 3-lobed; seeds disk-shaped, not angular or winged; inflorescences of 2 types, the chasmogamous on erect peduncles 5-25 cm long bearing 2-8 flowers, the cleistogamous without a peduncle, the solitary pedicels borne directly on the stolons, 0.5-2 cm long, deflexed

                       12 Lower lip of corolla entire or slightly irregular, not 3-lobed; seeds angular or winged; inflorescences of 1 type (erect, chasmogamous).

                          13 Upper corolla lip smaller than the lower, entire; capsule circumscissilely dehiscent; seeds 0.7-1.0 mm long, 4-6-angled; corolla without stipitate glands on its external surface.

                            14 Leaves of one kind only, divided into numerous capillary segments bearing lateral traps; bracts scarcely auriculate; plant distinctly aquatic, floating in water and only rarely stranded; [of the Coastal Plain]

                            14 Leaves of 2 or 3 kinds, some divided into capillary or narrowly linear segments and bearing few or no traps, others divided into fewer capillary segments and bearing more-or-less numerous traps; bracts distinctly auriculate; plants typically in boggy situations, in shallow water or frequently stranded; [either of the Mountains at high elevations or of various physiographic provinces northward].

                              15 Broadest leaf segments with 9-20 lateral setae (use 10× magnification); spur of corolla cylindrical, distinctly longer than wide, the apex distinctly curved forward

                              15 Broadest leaf segments lacking lateral setae; spur of corolla shortly saccate to broadly conical, wider than long, the apex not curved forward

                          13 Upper corolla lip larger than the lower, obscurely 3-lobed; capsule laterally 2-valved or indehiscent; seeds 0.8-2.5 mm long, lenticular, with an irregular, lobed, or continuous wing; corolla (or at least the spur) with a few to many short stipitate glands (sometimes patchily distributed).

                                16 Vegetative shoots uniform, all bearing rather sparsely divided leaf segments bearing traps, seeds 0.8-1.1 mm long, with a continuous, circumferential wing, slightly to irregularly lobed.

                                  17 Lower corolla lip 8-10 mm long, about equaling or slightly shorter than the conical, 5-9 mm long spur; leaves usually forked twice

                                  17 Lower corolla lip 5-6 mm long, exceeding the blunt, 3.5-4.5 mm long spur; leaves usually forked once

                                16 Vegetative shoots of 2 kinds, some bearing leafy segments and few or no traps, others bearing reduced segments and more-or-less numerous traps; seeds 1.0-2.5 mm long, with an irregularly deeply lobed or partial wing.

                                    18 Plant anchored in mud up to 100 cm below water surface; green leafy shoots up to 40 cm long and 5 cm wide; peduncle flexuous, to 100 cm long, only the uppermost ca. 10 cm emergent

                                    18 Plant in shallow water or stranded; green leafy shoots usually not > 10 cm long and 2 cm wide; peduncle erect, straight, to 30 cm long, the uppermost 10-25 cm emergent