Bladderworts are a particularly interesting and diverse genus of aquatic plants, well represented in Michigan by north-temperate standards (half the American species grow in the state). Over 200 species occur in the world, some epiphytic (with large simple leaves) and a considerable number terrestrial. Distinctions between roots, stems, and leaves are difficult; in fact, none of the species produce what can be called, anatomically, a true root. Yet, absence of a root does not mean the aquatic species are always drifting free; some species, as in the rootless Ceratophyllum, are well anchored in the substrate by modified structures of different origins but which function to "root" the plant in the substrate.

Some of the aquatic species produce turions (or “winter buds”) at the apices of branches. These consist of very short internodes with tightly compacted overlapping dissected leaves and a mucilaginous matrix. Turions are often conspicuous toward the end of the summer and in U. vulgaris, at least, have been shown to be induced by environmental change to short day lengths.

The short-stalked “bladders” which give these plants their name are actually little traps. Hairs at the opening of the bladder serve as triggers and when contacted by a little aquatic organism, mechanically cause the trap to spring open, drawing in a rush of water with whatever comes along: insects, crustaceans, tiny fish fry, even green plants. Enzymes inside the traps or resident bacteria are agents of digestion.

Several species may grow close to each other or even with vegetative parts intertangled. Utricularia cornuta and U. resupinata are often “terrestrial” together at the edges of softwater lakes.

1. Leaves (often not seen or collected) simple, filiform or very narrowly linear, entirely or mostly embedded in the substrate (“terrestrial”).

2. Calyx less than 2 mm long; bracts at base of pedicels and on scape peltate (attached at the middle); corolla yellow, ca. 5–6.5 mm long (smaller on cleistogamous flowers); very rare, in southwestern Lower Peninsula.

U. subulata

2. Calyx at least 2 mm long; bracts attached at their base; corolla purple or yellow (if yellow, then over 9 mm long, excluding the spur); widespread.

3. Corolla bright yellow, 9–17 mm long (excluding the spur), the spur (5–) 6–10 (–11) mm long and pointed downward; flowers (1–) 2–6 on scape; calyx (upper lip) (3–) 3.5–5.5 mm long.

U. cornuta

3. Corolla purple, ca. 6.5–9 mm long, the spur 1.5–3.5 mm long and upcurved; flowers solitary on scape; bracts tubular around scape, slightly flared at summit; calyx 2–3 mm long.

U. resupinata

1. Leaves at least once-forked, in most species more dissected, all or mostly on stems in water.

4. Leaves (branches) whorled; bladders lacking external hairs, borne at the ends of the ultimate leaf segments; corolla pink-purple with yellow marks at base of lower lip.

U. purpurea

4. Leaves (except for floating whorl in U. radiata) alternate (though divided at their base, so that 2–3 main segments may arise on one side of the stem, not truly whorled); bladders with whisker-like hairs at the mouth, borne on sides or base of leaf segments (or on separate branches); corolla yellow (pale and sordid in U. minor and often drying ± purplish in all species).

5. Plant bearing a whorl of leaves with strongly inflated petioles on the peduncle near water surface, very rare, in southwestern Lower Peninsula.

U. radiata

5. Plant without whorled or inflated leaves, ± common throughout Michigan.

6. Bladders borne on leafless branches (normally whitish and buried) separate from the leaves; leaf segments strongly flattened throughout, with marginal (as well as apical) spicules.

U. intermedia

6. Bladders borne on normal green leaves; leaf segments (at least ultimate ones) terete or thread-like, except in U. minor (which lacks marginal spicules) and occasionally U. vulgaris.

7. Principal leaves mostly forked 1–4 (–6) times (including a division into 2 or 3 main segments at the base), up to 0.9 (–1.4) cm long (plants consequently very slender in aspect), the leaf segments without marginal spicules.

8. Leaf segments hair-like; corolla bright yellow; lower lip (like the upper) turned upward, broadly rounded, the corolla saddle-shaped in aspect, with definite spur; bracts not auriculate (though clasping); leaves mostly forked 1 (–2) times; flowers 1 (–2) on scape.

U. gibba

8. Leaf segments flat; corolla pale or dingy yellow, with purplish tinge or stripes toward base of lower lip and only an obscure saccate spur; lower lip held ± horizontal, elongate-oblong (as result of downcurling of margins); bracts of scape and inflorescence auriculate; leaves mostly forked (2–) 3–4 (–6) times; flowers (2–) 3–8 on scape.

U. minor

7. Principal leaves mostly forked 5–17 (–22) times, (1–) 1.3–5 (–6.5) cm long (plants consequently more bushy in aspect), the leaf segments with minute marginal spicules.

9. Flowers (chasmogamous) ca. 5–8.5 mm long, on pedicels subtended by narrowly elliptic-oblong bracts unlobed at the base; cleistogamous flowers often present late in summer, on pedicels 2.5–7.5 mm long on submersed stems; bladders 0.8–1.7 (–2) mm long; leaves (1–) 1.3–2 cm long, extremely delicate and limp, forked 5–9 (–11) times; softwater lakes and acid pools in peatlands.

U. geminiscapa

9. Flower ca. 9–16 (–18) mm long, on pedicels subtended by ovate ± auriculate or cordate bracts; cleistogamous flowers none; bladders 0.8–5 (–6) mm long; leaves (1–) 1.5–5 (–6.5) cm long, coarse, usually forked 6–17 times; diverse ponds, lakes, and streams (in all but swiftly flowing waters).

U. vulgaris