Herbs annual, aquatic, rooted or floating. Stem submerged, slender, unbranched, internodes elongate; adventitious roots developing from leaf scars, highly dissected, leaflike, photosynthetic. Leaves dimorphic; stipules deeply cleft; submerged leaves opposite, sessile, simple, linear, caducous; floating leaves crowded terminally into a rosette, petiolate; petiole inflated about the middle; leaf blade rhombic to deltoid, distal half of margin coarsely dentate. Flowers solitary in upper leaf axils, flowering at water surface, bisexual, 4-merous, actinomorphic. Floral tube (hypanthium) developed, partly epigynous. Sepals 4, valvate, persistent as hardened horns of fruit. Petals 4, white or lilac, deciduous. Stamens 4, antesepalous; anthers introrse, versatile. Ovary surrounded by a coronary disk, partly inferior, becoming inferior in fruit, 2-loculed; ovules anatropous, pendulous, 1 per locule, 1 ovule undeveloped after anthesis; placentation axile. Stigma capitate, deciduous. Fruit indehiscent, (0-)2-4-horned, turbinate, cup-shaped, or elongate rhombic, exocarp succulent, ephemeral, endocarp stony, with a thin to prominent crest between and along horns, fruit topped by a dome-shaped or tetragonal to rounded crown, crown apex a pointed beak or tuft of hairs. Seed 1; cotyledons unequal, 1 large, starchy, retained in fruit, 1 small, scalelike, germinating from fruit apex, through pore of stylar canal; endosperm absent.

Trapa is allied morphologically to the Lythraceae by the partly inferior position of the ovary together with a host of other features, including basically opposite, simple leaves, development of the floral tube which persists in fruit, valvate sepals, 4-merous flowers, introrse and versatile anthers, axile placentation, and seeds without endosperm. Trapa is sufficiently similar to the Lythraceae and Onagraceae to have been considered for membership within either family or, as has been done here, as a closely related family. Molecular evidence suggests the closest relative is Sonneratia (Lythraceae).

Dimensions for the fruit body are described as height × width × thickness.

Wan Wenhao. 2000. Trapaceae. In: Chen Chiajui, ed., Fl. Reipubl. Popularis Sin. 53(2): 1-26.

One genus and two species: subtropical and temperate regions of Africa, Asia, and Europe; introduced in Australia and North America; two species in China.

(Authors: Chen Jiarui (陈家瑞 Chen Chia-jui), Ding Bingyang (丁炳扬); A. Michele Funston)