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See list of 12 species in this genus

Trap measurements were performed on dried specimens. References: Crow and Hellquist (1985), Taylor (1989).

  • 1a. Peduncle subtended by a whorl of conspicuously inflated branches that serve as floats to hold the peduncle above the surface of the water [Fig. 710]; leaf-like branches repeatedly dichotomous, forking 6 or more times into two equal divisions and lacking a main axis
    • 2a. Raceme with 1–5 (–7) flowers [Fig. 710]; peduncle above the inflated branches 3–10 cm long; inflated branches 10–40 mm long, with nearly parallel margins for much of the proximal half, abruptly tapering to the peducle, bearing dissected branches only near the tip [Fig. 710]; bracts 2–3 mm long, sometimes ± 3-lobed at the apex; sepals 2.5–3.5 mm long; spur entire at tip (very rarely bifid); leaf-like branches divided at the base into 
2 ± equal branches
    • 2b. Raceme with (4–) 9–14 (–17) flowers; peduncle above the inflated branches 10–30 cm long; inflated branches 30–80 mm long, gradually tapering to the peduncle from a point distal to the middle, bearing dissected branches in the distal half; bracts 3–5 (–7) mm long, entire at the apex; sepals 3–5 mm long; spur bifid at apex; leaf-like branches divided at 
the base into 2 unequal branches
  • 1b. Peduncle not subtended by a whorl of inflated branches; leaf-like branches simple, branched fewer than 6 times, or repeatedly branched but then with a flexuous main axis with branches emerging from the sides
    • 3a. Leaf-like branches whorled; traps located at the tips of branches; corolla red-purple with a yellow spot on the lower lip (rarely white)
    • 3b. Leaf-like branches alternate (note: the branches are divided at the very base in some species and appear opposite) [Fig. 711]; traps scattered along the branches [Figs. 709,711]; chasmagamous (i.e., petaliferous) corolla yellow or purple with cream to yellow spot on the lower lip in U. resupinata
      • 4a. Leaf-like branches mostly simple; traps 0.2–1.1 mm long; plants with stems anchored in the substrate
        • 5a. Corolla purple with a cream to yellow spot on the lower lip; peduncle with a single flower subtended by a tubular bract; leaf-like branches with 1 or more transverse cross-septa
        • 5b. Corolla yellow (white to red-purple in cleistogamous flowers of U. subulata); peduncle with 1–10 flowers, each flower subtended by non-tubular bracts; leaf-like branches without cross-septa
          • 6a. Inflorescence with yellow, chasmogamous flowers, these with a downward directed spur 7–14 mm long and a lower corolla lip 9–16 mm long; pedicels 1–2 mm long; each flower subtended by a basifixed bract and a pair of bractlets; capsules ovoid, 3.8–5 mm long; intact leaf-like branches with a mass of brown cells at the apex, often forming a callus tip
          • 6b. Inflorescence either with white to red-purple cleistogamous flowers or with yellow chasmogamous flowers, the latter with a spur 2.4–5 mm long that is appressed to the lower corolla lip 4–7 mm long; pedicels 4–15 mm long; each flower subtended by a peltate bract; capsules globose, 1–1.5 mm long; intact leaf-like branches without an apical mass of brown cells
      • 4b. Leaf-like branches divided 1–8 or more times [Figs. 709,711]; traps (0.5–) 1–5 mm long ( U. gibba frequently with many traps less than 1 mm long); plants free-floating or partially anchored in substrate (sometimes stranded during low water periods)
        • 7a. Ultimate segments of leaf-like branches flat, scarcely narrower than the primary branches; branches dimorphic—chlorophyllous ones above the substrate with few or no traps and achlorophyllous ones buried in the substrate with abundant traps [Fig. 709]
          • 8a. Corolla 10–16 mm long, the lower lip wider than long and with a well-developed palate; ultimate segments of chlorophyllous leaf-like branches with an apical seta and numerous lateral setae; chlorophyllous branches without traps [Fig. 709]; traps 1.5–4 mm long
          • 8b. Corolla 6–8 mm long, the lower lip longer than wide and with a poorly developed palate; ultimate segments of chlorophyllous leaf-like branches with only an apical seta, if any at all; chlorophyllous branches with traps; traps (0.6–) 0.9–1.8 (–2) mm long
        • 7b. Ultimate segments of leaf-like branches capillary, much finer than the primary branches; branches monomorphic (i.e., all chlorophyllous and trap-bearing) or dimorphic in U. striata
          • 9a. Flowers dimorphic—petaliferous, chasmogamous flowers in an erect, emersed, 2- to 8-flowered raceme and nearly apetalous, cleistogamous flowers that are borne singly at the tips of solitary or paired, submersed peduncles 5–20 mm long; lower lip of chasmogamous corolla with 3 short, but evident, rounded lobes; peduncle without scales below the lowest flowers; seeds circular in outline, unwinged; turions 2–5 (–6) mm thick
          • 9b. Flowers monomorphic, all petaliferous and borne in erect, emersed racemes with (1–) 2–14 flowers; lower lip of corolla ± entire or retuse, but without 3 lobes; peduncles with 1–3 (–4) scales below the lowest flowers (usually lacking scales in U. gibba);
            • 10a. Leaf-like branches 5–15 mm long, few-branched, divided 1–4 times; traps 0.5–2.5 mm long; corolla with scattered, minute stipitate-glands (at least on the spur); raceme with (1–) 2–6 flowers; fruit usually produced, a capsule dehiscing by 2 lateral valves; seeds irregularly circular or elliptic in outline, surrounded by a conspicuous wing; turions wanting, less than 1 mm thick
              • 11a. Leaf-like branches monomorphic, all chlorophyllous and trap-bearing; corolla 6–12 mm long (up to 25 mm long in some coastal plain populations from MA south); upper surface of seeds ± smooth or with some bluntly conical processes that are confined to the central area of the seed
              • 11b. Leaf-like branches dimorphic—chlorophyllous ones above the substrate divided 3 or 4 times with few traps and achlorophyllous buried in the substrate divided 1–4 times with abundant traps; corolla 15–25 mm long; upper surface of seeds covered with numerous, bluntly conical processes that extend even onto the wings of the seed
            • 10a. Leaf-like branches 15–60 mm long, divided 6 or more times [Fig. 711]; traps 1.5–5 mm long; corolla without stipitate glands; raceme with (3–) 6–14 flowers; fruits infrequently produced, a pyxis; seeds prismatically 4- to 6-angled, surrounded by a very narrow wing; turions coarse, 5.5–13.5 mm thick [Fig. 711], the component branches ciliate with stout, gray hairs

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 Show photos of:   Each photo represents one species in this genus.