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Monoecious or dioecious, glabrous. Leaves linear, lanceolate, broad-ovate, sub-orbicular, cordate or reniform, apex rounded, acute or apiculate, base cuneate, truncate or cordate; nerves 3-11, parallel, straight or curved, connected by fine parallel cross-veins, forming often a very characteristic pattern, midrib sometimes prominent; petiole sheathing at the base, often gradually merging into the blade. Spathe peduncled, elliptic or ovate, with 6 more or less prominent ribs or 2-10 wings; female and bisexual ones 1-flowered, male ones many-flowered. Flowers unisexual or bisexual, female and bisexual ones sessile, male ones pedicelled. Sepals 3, linear, oblong or ovate, green with searious margin, persistent. Petals 3, oblong, broad-obovate to orbicular, 2-3 times as long as the sepals, white or coloured. Stamens 6-15; filaments filiform, flattened; anthers linear or oblong, latrorsely dehiscent. Ovary oblong, narrowed at the top, incompletely divided by the parietal placentas into 6 cells; styles 6-15, bifid. (In the ♂ flowers 3 stylodia are present.) Fruit oblong, apex attenuate, pericarp thickened. Seeds ∞, minute, oblong or fusiform, with rather thick testa.
Aquatic perennials of fresh water; dioecious or hermaphrodite. Leaves all basal; juvenile submersed, ribbon-like with short petioles; older leaves with broad-ovate to suborbicular floating lamina, nerves 3-11, connected by fine parallel cross-veinlets, long-petiolate. Inflorescence scapose; spathes elliptic or ovate, often ribbed or winged, = or < perianth-tube, in male plants subtending many flowers, in female and bisexual plants subtending only one flower. Flowers with 3 green persistent sepals, 3 longer white petals. Male flowers pedicellate, stamens 6-15, ovary rudimentary; female flowers sessile, staminodia present or 0, ovary oblong, stigmas (3)-6-(9), bifid, ovules many; bisexual flowers with 6 or more stamens, 6 or more stigmas. Fruit a capsule ripening inside spathe. Seeds many. Spp. c. 40, mostly tropical, of Africa, S.E. Asia, Korea and Japan, Malaysia, Australia, New Caledonia and 1 sp. in S. America. Adventive sp.1.
Plants annual or perennial, of fresh waters. Rhizomes and stolons absent [present]. Erect stems rooted in substrate, unbranched, short. Leaves basal, submersed [floating], petiolate; blade lanceolate to reniform [linear], base cuneate to cordate, apex rounded to acute; midvein without rows of lacunae along sides, blade uniform in color throughout; abaxial surface without prickles or aerenchyma, smooth; intravaginal squamules entire. Inflorescences scapose, long-pedunculate; spathes winged or ribbed. Flowers bisexual [unisexual, staminate and pistillate on different plants], sessile [staminate pedicellate], floating or occasionally opening under water; petals white to pink or light violet, often yellow at base; filaments distinct; pollen in monads; ovary 1-locular; styles 3--9, not 2-fid. Fruits oblong, ridged, dehiscing irregularly. Seeds fusiform, covered with hairs.
Herbs, freshwater. Stems usually cormlike. Leaves all basal, petiolate, usually sheathing at base; blades submerged or sometimes floating, linear to broadly ovate, 3-11-veined. Spathes elliptic or ovate, usually 2-6-winged, apex bifid or trifid, with 1 to many flowers. Flowers bisexual or unisexual and plants dioecious. Male flowers with longer pedicel. Bisexual and female flowers shortly pedicellate or without pedicel. Sepals 3, greenish, linear, oblong, or ovate. Petals 3, colored, orbicular to broadly obovate, much longer and wider than sepals. Stamens 3-15; filaments linear, flattened; anthers laterally dehiscent; staminodes (absent or 1-)3, often present in female flowers. Ovary of 3, 6, or 9 (or more) carpels; ovules numerous; stigmas 2 per style, linear. Fruit cylindric, fusiform, or conic. Seeds numerous, small, often hairy.
Freshwater, monoecious or dioecious or bisexual [see also note under O. alismoides], perennial or annual; stolons absent. Leaves submerged, floating or emergent, all basal, usually differentiated into petiole and blade except when young; petiole with sheathing base; blade of mature leaves expanded, with midrib usually obvious. Spathes unisexual or bisexual; female and bisexual spathes 1-flowered; male spathes many-flowered. Flowers bisexual or unisexual, chasmogamous or cleistogamous. Perianth of 2 whorls of 3; inner segments larger and petaloid, white, maroon or yellow at base. Stamens 3–15, sometimes present as staminodes in female flowers. Ovary of 3–20 carpels; styles 3–20, bifid. Fruit an indehiscent utricle. Seeds numerous.
