Annual or perennial aquatics and marsh plants, sometimes laticiferous. Leaves basal and erect, sometimes floating, rarely all submerged, sometimes some reduced to phyllodes, lanceolate to sagittate, rarely broad-elliptic to ovate, entire, with a hydathode on the apex, curvinerved, nerves more or less parallel and gradually joining the marginal nerve, connected by ascending cross-veins; petiole sheathing, mostly with air-channels, often septated. Inflorescences mostly erect, racemose or paniculate; peduncle sometimes hollow, mostly with air-channels. Bracts 3(-2) per whorl of flowers or branches. Flowers actinomorphous, bisexual or unisexual (and then rarely with rudiments of the other sex). Sepals 3, imbricate, green, parallel-nerved, convex, persistent. Petals 3, imbricate, white or faintly coloured, marcescent. Stamens 3-~, free, in a whorl; filaments filiform or dilated; anthers 2-celled, basifix, sometimes versatile, latrorsely lengthwise dehiscent. Carpels 2-~, free, in the Mal. spp. spirally arranged on the receptacle (in extra-Mal. Alisma in a whorl); style 1, ventrally or terminally inserted on each carpel, persistent. Ovule 1 (in extra-Mal. Damasonium 2 or more), basal, campylotropous, rarely anatropous (Damasonium), micropyle extrorse, rarely introrse ( Luronium). Achenes in a head (or whorl in Alisma), free, rarely connate at the base. Seeds oblong or horseshoeshaped; testa membranous; embryo horseshoe-shaped; albumen 0; radicula extrorse, rarely introrse (Luronium). Distribution. About 10 genera with c. 70 spp., all over the temperate and tropical zones except in the Pacific area (Micronesia, Melanesia, and Polynesia). The largest genera are Sagittaria and Echinodorus both centering in the New World.