Habit and leaf form. Herbs. Annual, or perennial; plants with neither basal nor terminal concentrations of leaves. Stem internodes hollow. To 0.5 m high. Hydrophytic, or helophytic; when hydrophytic, rooted. Leaves of aquatics emergent; medium-sized; opposite, or whorled; ‘herbaceous’, or leathery, or membranous; petiolate to sessile; gland-dotted; aromatic, or without marked odour; simple; epulvinate. Leaf blades dissected, or entire; if dissected much-divided; pinnately veined. Mature leaf blades adaxially glabrous, or pubescent; abaxially glabrous, or pubescent. Leaves without stipules. Leaf blade margins serrate. Leaves without a persistent basal meristem. Leaf anatomy. Hydathodes present (occasionally), or absent. Hairs present, or absent; glandular hairs present, or absent. Stem anatomy. Secondary thickening developing from a conventional cambial ring.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Entomophilous, or pollinated by unusual means (sometimes cleistogamous under water).
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; axillary; in racemes. The terminal inflorescence unit cymose, or racemose. Inflorescences terminal. Flowers pedicellate, or sessile; bracteate; bracteolate, or ebracteolate; small; very irregular; zygomorphic; 5 merous; tetracyclic. Free hypanthium absent. Hypogynous disk present. Perianth with distinct calyx and corolla; 10; 2 -whorled; isomerous. Calyx present; 5; 1 -whorled; gamosepalous; lobed. Calyx lobes markedly shorter than the tube, or about the same length as the tube, or markedly longer than the tube. Calyx erect; imbricate, or valvate; tubular; unequal but not bilabiate (lobes subequal); persistent; accrescent; with the median member posterior. Calyx lobes linear. Corolla present; 5; 1 -whorled; gamopetalous; lobed. Corolla lobes markedly shorter than the tube, or about the same length as the tube. Corolla imbricate, or valvate; funnel-shaped; bilabiate; hairy abaxially, or glabrous abaxially; hairy adaxially, or glabrous adaxially; with contrasting markings; white, or cream, or pink, or purple, or blue. Androecium 4. Androecial members adnate (to the corolla); markedly unequal; coherent (by anther cells); 1 -whorled. Stamens 4; remaining included; didynamous; reduced in number relative to the adjacent perianth; oppositisepalous. Anthers cohering (in pairs); dehiscing via longitudinal slits; introrse; bilocular (cells equal, separate, stipitate); tetrasporangiate. Gynoecium 2 carpelled. The pistil 2 celled. Gynoecium non-petaloid; syncarpous; eu-syncarpous; superior. Ovary plurilocular; 2 locular. Gynoecium median; stylate. Styles 1; simple (deflexed at the apex with 2 lateral processes below the bend); attenuate from the ovary, or from a depression at the top of the ovary; apical. Stigmas 1; 2 - lobed. Placentation axile, or apical. Ovules 50 per locule (to ‘many’); pendulous to ascending; non-arillate; anatropous, or campylotropous, or hemianatropous.
Fruit and seed features. Fruit non-fleshy; dehiscent; a capsule. Capsules septicidal and loculicidal (secondarily loculicidal, initially marginicidal then loculicidal, resulting in a 4-valved capsule and an undivided placental column), or valvular (‘septifrugal’). Fruit 50 seeded (to ‘many’). Seeds endospermic. Endosperm oily. Seeds minute. Cotyledons 2. Embryo straight to curved.
Special features. Corolla tube exceeding the calyx; straight. The upper lip of the corolla incorporating 2 members, the lower 3; (posterior, adaxial) lip of the corolla bilobed. Lower (abaxial) lip of the corolla 3 lobed.
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, Northern Territory, and Queensland. Northern Botanical Province.
Etymology. From the Greek for "a marshy lake" and "to love"; refers to the habitat.