Perennial or annual marsh or aquatic herbs, erect, or rarely with floating leaves; leaves basal, with elongated petioles sheathing but open at the base and linear-lanceolate to ovate-rounded often sagittate blades, the principal nerves parallel with the margins and converging at the apex of the blade, the transverse nerves often close and parallel. Flowers often whorled, racemose or paniculate, bisexual or rarely polygamous, actinomorphic. Torus flat to globose. Perianth 2-seriate, the outer 3 imbricate, persistent, green and sepal-like, the inner 3 petaloid, imbricate and deciduous or rarely absent. Stamens hypogynous, 6 or more, rarely 3, free; anthers 2-locular, extrorse. Carpels free or rarely united at the base, sometimes in a single whorl; style persistent; ovules solitary or several, basal or on the inner angle. Fruit a bunch or whorl of achenes, rarely dehiscing at the base. Seeds curved, with horseshoe-shaped embryo; endosperm none.
Perennial, rarely annual, aquatic, swamp or marsh herbs, lactiferous. Rhizome very short; roots short, fibrous. Leaves erect, rarely floating or submerged, basal; petiole with an expanded, sheathing base; leaf-blade entire, linear-lanceolate to ovate, with a decurrent to sagittate base, acute to rounded apex. Inflorescence compound or simple, of whorls of branches or flowers, rarely pseudo-umbellate or with solitary flowers; bracts 2 or 3 at the base of each whorl, and sometimes several bracteoles. Flowers regular, bisexual or unisexual. Sepals 3, persistent, herbaceous. Petals 3, deciduous, rarely 0. Stamens 3, 6, 9 or more; filaments filiform or flattened; anthers 2-celled, dehiscing longitudinally and laterally. Carpels superior, free or joined at the base, 3–∞, in a whorl or spiral, unilocular; style terminal or ventral; ovules 1, 2 or many, basal and erect, or situated on the ventral suture. Fruit indehiscent. Seeds oblong, indented laterally to follow the form of the horseshoe-shaped embryo, smooth, wrinkled or ridged, without endosperm.
Notes: In common with most water-plants members of this family show great morphological variability, especially in the size and shape of the leaves. This has led to much confusion in that extreme forms of a species have often been described as distinct species or varieties. Such extremes are prevalent under tropical conditions, notably the more robust forms. However, among specimens examined from the tropical African areas, there was always a good proportion of the “ typical ” forms which, in most cases, have been described from European or Asian material.
Monoecious, polygamous or dioecious, annual or perennial herbs with latex, glabrous to stellate pubescent with submersed, floating or emergent leaves; in fresh or brackish water. Roots fibrous, septate or aseptate, few to many; from a stout rhizome or stolon at base of stems or lower nodes. Stems ± short (of variable length), fleshy or spare, branched or unbranched erect, or corm-like or stolons (Luronium), rhizome occasionally terminated by tubers (Sagittaria); turions (winter buds) not formed. Leaves basal, alternate or spirally inserted; sessile or petiolate; petioles terete to triangular, mostly considerably longer than the blade, sheathing base, without auricles, intravaginal scales present; blade linear or orbicular to lanceolate, ovate to rhomboid, with or without pellucid dots or lines, margins entire or undulate, apex acute or acuminate, obtuse to round-acute, base without basal lobes and attenuate or otherwise truncate, cordate sagittate or hastate, with parallel primary veins from base of blade to apex and reticulate or transverse secondary veins. Inflorescence scapose, mostly erect, but some floating, forming racemes of panicles by verticillate branching, involucrate umbels in some, without subtending spathe, bracteate, bracts whorled linear, entire, obtuse to acute or membranous acuminate. Flowers hypogynous, perfect or imperfect, subsessile to (long)-pedicellate, perianth actinomorphic of 6 separate elements in 2 whorls, the outer 3 sepal-like, green, or 6 tepals in two whorls, the outer 3 not green, inner structures delicate, caducous, outer elements erect, enclosing flower, then fruit. Stamens free, 6 (in pairs, alternating with petals), 9 (in two whorls: outer whorl of 6, inner whorl of 3); alternatively, many (∞) of these stamens will be densely and irregularly spaced. Anthers 2-locular, basifixed or versatile, tetrasporangiate, dehiscing by longitudinal slits. Fruit types variable, styles/styluli central terminal, ventrally biased terminal (almost dorsal in Luronium natans) or lateral. Achenes 6 (Caldesia), 6+ in untidy spiral (Baldellia), 20 in one ‘whorl’ (Alisma) or ∞ (-100) spirally on orbicular to conical receptacle (Echinodorus, Sagittaria). Schizocarp: 6-10 fused elements (Damasonium). Follicles (Limnocharis) 6-18 in a ring, ± basally adnate. All fruits contain campylotropous (horse shoe-shaped) apotropous ovules, epitrous in Luronium, bitegmic with a macropodous embryo, lacking endosperm at maturity; some seeds glandular (mucilage) (e.g. Butomus). Achenes: one basal campylotropous ovule, few with funicular aril, rarely with operculum. Schizocarp: according to the 6 species 1-2-(6) ovules on marginal placentae ± funicular aril. Follicle: 1-locular, ∞ ovules restricted to veins / vein endings, on lateral sides of follicle, funicular aril noticeable, but weak in Limnocharis and absent from Butomus. Strong dorsal side of follicle free of seeds.