Family Trapaceae

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. 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); (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, distally dentate; (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 calcium oxalate crystals. The mesophyll crystals druses. Minor leaf veins without phloem transfer cells.

Stem anatomy. Primary vascular tissue comprising a ring of bundles; bicollateral. Secondary thickening absent. Xylem with vessels. Vessel end-walls simple.

Reproductive type, pollination. Plants hermaphrodite; homostylous.

Inflorescence, floral, fruit and seed morphology. Flowers solitary (on short, pubescent pedicels); 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); 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 (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, biochemistry. Not cyanogenic. Iridoids not detected. Proanthocyanidins absent. Saponins/sapogenins 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 3 core angiosperms; core eudicot; Superorder Rosanae; malvid; Order Myrtales (as a synonym of Lythraceae).

Species 30. Genera 1; only genus, Trapa.

Illustrations.
• Technical details: Trapa.