Gigantic to minute herbs, perennial; terrestrial, epiphytic, hemiepiphytic or epilithic or aquatic (free floating or rooted). Often with milky, viscid or acrid sap. Roots often aerial; hair-like or absent in the Lemnoideae. Stems lianescent, tuberous, rhizomatous, or not differentiated into stem or leaf (Lemnoideae), in which case the plant is reduced to a minute, fleshy or flattened plant body bearing hair-like roots on under surface, or roots absent. Cataphylls often variously ribbed and persistent, may remain intact or weather into fibres. Leaves alternate, basal or cauline, sometimes distichous, 1 to many; normally differentiated into petiole and expanded blade; blade and petiole often variegated or mottled with various shades of green, yellow and grey. Petioles often elongate, sheathed at least basally, often pulvinate apically, basally or rarely centrally, usually smooth, sometimes hairy, warty or prickly. Blades simple to compound, very variable in size and shape, from elliptic or ovate to sagittate or hastate, less commonly trifid or trisect, dracontioid (i.e. trisect with each primary division further divided), pinnatifid or pinnatisect to quadripinnatifid (Gonatopus); sometimes perforated. Venation: midrib almost always differentiated; primary veins usually pinnate, palmate, rarely parallel, finer venation reticulate, parallel-pinnate or arising from primary veins at a wide angle and arching strongly toward leaf margin (e.g. Colocasia). Inflorescences terminal or axillary, solitary or clustered in axils, an unbranched spadix (spike) subtended by a single spathe (bract), in Lemnoideae the inflorescence is within a minute dorsal cavity of the plant body (Wolffia, Wolffiella) or in paired lateral budding pouches (Spirodela, Landoltia, Lemna); spathe herbaceous, free or adnate to spadix, spreading, reflexed or convolute, sometimes constricted below middle and differentiated into tube below and blade above; tube margins usually convolute, sometimes connate (e.g Stylochaeton); spathe entirely deciduous after anthesis, or tube persistent to fruiting and blade marcescent to deciduous after anthesis, or spathe entirely persistent until fruiting. Spadix usually cylindric, erect, often fleshy and thick, flowers usually dense, often divided into distinct floral zones with lower part female and a male zone above, sterile flowers of varying shape often present at base, middle or apex, apical portion sometimes forming a sterile appendix. Flowers usually numerous, sessile (except Pedicellarum), very small, protogynous, lacking floral bracts, 2-3-merous, bisexual or unisexual, naked or with a perigone (perianth); tepals 4-6(-8), usually free, sometimes united and thickened. Androecium of (1-)3-6(-9) stamens, free or united into synandria; anthers sessile or with elongated filaments, opening by lateral slits or pores; connective often very thick. Gynoecium syncarpous; ovary usually 1-3 locular, rarely more (e.g. Philodendron, most Spathicarpeae); ovules 1-many per locule; placentas parietal, axile, basal or apical; stylar region usually well-developed, variable, rarely connate with those of neighbouring gynoecia; stigma wet at anthesis, sometimes distinctly lobed. Fruit berry-like, 1 to many seeded, sometimes juicy, free or rarely fused into syncarps (Syngonium), often colourful; seeds minute to large, variable in shape, with or without endosperm.