| # The Families of Angiosperms |

L. Watson and M.J. Dallwitz

### Trapaceae Dum.

Alternatively Hydrocaryaceae Raimann; ~ Onagraceae, Lythraceae sensu lato.

Habit and leaf form. Herbs. Annual. Hydrophytic, or helophytic; rooted (initially), or free floating (often, ultimately). Leaves submerged and floating. Conspicuously heterophyllous (the leaves submerged-dissected and aerial-rosulate-rhombic, though the opposite to subopposite, elongate, filiform-dissected organs at the submerged nodes have been interpreted as photosynthetic roots or stipules). Leaves alternate (aerial), or opposite to whorled (submerged); when emergent/aerial, long petiolate (the petioles equipped midway with an inflated, aerenchymatous float); simple (emergent/aerial), or compound (submerged). Lamina dissected (submerged), or entire (emergent/aerial); of the aerial leaves, more or less rhombic and distally dentate; of the emergent members, palmatifid. Leaves (at least the aerial members) stipulate. Stipules small, cleft; caducous.

Leaf anatomy. Stomata present (on the emergent leaves); anomocytic. The mesophyll without sclerenchymatous idioblasts; containing crystals. The crystals druses. Minor leaf veins without phloem transfer cells.

Axial (stem, wood) anatomy. Primary vascular tissues comprising a ring of bundles; bicollateral. Internal phloem present. Secondary thickening absent.

The vessel end-walls simple.

Reproductive type, pollination. Unisexual flowers absent. Plants hermaphrodite; homostylous.

Inflorescence, floral, fruit and seed morphology. Flowers on short pubescent pedicels, solitary; axillary; regular; 4 merous; cyclic; tetracyclic. Free hypanthium present. Hypogynous disk present; intrastaminal; more or less annular (angular, often eight-lobulate).

Perianth with distinct calyx and corolla; 8; 2 whorled; isomerous. Calyx 4; 1 whorled; gamosepalous (with basal tube); regular; persistent; accrescent (two or all four members forming indurated, hornlike or spiny projections on the fruit), or non-accrescent; valvate. Corolla 4; 1 whorled; polypetalous; imbricate. Petals sessile.

Androecium 4. Androecial members free of the perianth; free of one another; 1 whorled. Androecium exclusively of fertile stamens. Stamens 4; isomerous with the perianth; oppositisepalous; alternating with the corolla members; shortly. Anthers dorsifixed; versatile; dehiscing via longitudinal slits; introrse; tetrasporangiate. Endothecium developing fibrous thickenings. Anther epidermis persistent. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or decussate. Anther wall initially with more than one middle layer (2 or 3). Tapetum glandular. Pollen grains aperturate; 3 aperturate; colpate (three prominent meridional crests joined at the poles, alternating with the three colpi); 2-celled.

Gynoecium 2 carpelled. Carpels reduced in number relative to the perianth. The pistil 2 celled. Gynoecium syncarpous; eu-syncarpous; partly inferior (becoming almost completely inferior in the fruit). Ovary 2 locular. Gynoecium transverse; stylate. Styles 1 (elongate); apical. Stigmas 1; capitate. Placentation axile to apical. Ovules 1 per locule; pendulous; with dorsal raphe; anatropous; bitegmic (but no true micropyle); crassinucellate. Embryo-sac development Polygonum-type. Antipodal cells not formed. Synergids pear-shaped. Hypostase present. Endosperm formation not occurring. Embryogeny solanad.

Fruit non-fleshy (the thin exocarp evanescent, the stony endocarp persistent); indehiscent; a nut (or ‘false drupe’, woody or bony, more or less top-shaped, variously sculptured or horned or spiny); 1 seeded (one locule aborting). Seeds non-endospermic; large. Seeds with starch. Embryo well differentiated. Cotyledons 2 (very unequal, the one large and thick, the other a scale inserted lower). Embryo straight.

Seedling. Germination phanerocotylar and cryptocotylar (the larger, starch-packed cotyledon remaining within the fruit on germination, the small one emerging with plumule and radicle through the terminal pore left by the fall of the style).

Physiology, phytochemistry. Not cyanogenic. Iridoids not detected. Saponins/sapogenins absent. Proanthocyanidins absent.

Geography, cytology. Paleotropical. Temperate to tropical. Warm Eurasia, Malaysia and Africa. X = about 18, or 24. Supposed basic chromosome number of family: 18, or 24.

Taxonomy. Subclass Dicotyledonae; Crassinucelli. Dahlgren’s Superorder Myrtiflorae; Myrtales. Cronquist’s Subclass Rosidae; Myrtales. APG III core angiosperms; core eudicot; Superorder Rosanae; malvid. APG IV Order Myrtales (as a synonym of Lythraceae).

Species 30. Genera 1; only genus, Trapa.

General remarks. Differing from Lythraceae (q.v.) in foliar, floral and fruit morphology (tetracyclic flowers with imbricate corolla, free androecium, part-inferior ovary with pendulous ovules, fruit a sculptured, horned or spiny nut); also in terms of the compiled data in stem anatomy (no secondary thickening, no internal phloem), embryology and perhaps cytology.

Illustrations. • Trapa natans: Bot. Reg. 3 (1817). • Le Maout and Decaisne: Trapa natans.

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications (Dahlgren; Dahlgren, Clifford, and Yeo; Cronquist; APG). See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 1992 onwards. The families of flowering plants: descriptions, illustrations, identification, and information retrieval. Version: 8th June 2026. delta-intkey.com’.

Contents