Habit and leaf form. Herbs. Plants rootless; ‘carnivorous’. Trapping mechanism active. The traps consisting of the curiously modified, steel-trap-like leaf blades, which spring shut when adaxial receptors are touched (Dioonaea-like). Leaves cauline (ass.). Plants with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves; rhizomatous, or tuberous. Hydrophytic; free floating. Leaves submerged; small to medium-sized; whorled; 5–9 per whorl; with blades, or bladeless (absent from flowering whorls); petiolate (connate at base, dorsally flattened, turgid, broadened upwards, apices rounded-truncate with leaf lamina articulate at its midpoint and with several outward projecting bristles lateral and dorsal to lamina attachment); simple. Leaf blades entire (ass.); orbicular (2 more or less semi-circular halves). Leaves with stipules, or without stipules. Leaf blade margins dentate (denticulate, inflexed). Leaves without a persistent basal meristem. Vernation conduplicate. Leaf anatomy. Hairs present; glandular hairs present. Extra-floral nectaries absent. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Entomophilous.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; inflorescence pedunculate and emergent from water. The terminal inflorescence unit cymose, or racemose. Flowers pedicellate; ebracteate (ass.); ebracteolate (ass.); regular; 5 merous; cyclic; tetracyclic, or pentacyclic. Free hypanthium present, or absent. Hypogynous disk absent. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx present; 5; 1 -whorled; polysepalous; imbricate; regular; persistent (marcescent). Sepals elliptic to ovate. Corolla present; 5; 1 -whorled; polypetalous; contorted (convolute); regular; white, or pink (tinged). Petals obovate (narrowly); shortly clawed. Androecial members definite in number. Androecium 5. Androecial members free of the perianth; all equal (ass.); free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 5; all more or less similar in shape (ass.); isomerous with the perianth; oppositisepalous. Anthers dorsifixed, or basifixed; dehiscing via longitudinal slits; extrorse; tetrasporangiate. Pollen shed in aggregates; in tetrads. Gynoecium 3(–5) carpelled. The pistil 1 celled. Gynoecium syncarpous; synovarious, or synstylovarious; superior. Ovary unilocular; 1 locular. The ‘odd’ carpel when G3 posterior. Ovary sessile (ass.). Gynoecium stylate. Styles 5; free, or partially joined; apical. Stigmas 5 (ass.). Placentation parietal, or basal. Ovules in the single cavity 20–50 (many); ascending; non-arillate; anatropous.
Fruit and seed features. Fruit non-fleshy; indehiscent; capsular-indehiscent (the pericarp decaying to release the seeds); 5 celled; 20–100 seeded (numerous or several). Seeds copiously endospermic. Endosperm oily. Embryo well differentiated (small). Cotyledons 2. Embryo straight.
Etymology. After Ulysse Aldrovandi (1522–1605), Italian botanist who founded the natural history museum at Bologna.