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.
Herbs, perennial, terrestrial, epiphytic or aquatic, often with milky, viscid or acrid sap; stems lianescent, tuberous, rhizomatous or reduced. Leaves alternate, 1–many, petiolate, usually some reduced to scale leaves (cataphylls) before or among normal leaves or inflorescences; petiole typically with distinct basal sheath, often pulvinate at or near apex; lamina usually broad, membranous to coriaceous, very variable in size and shape, simple or variously lobed, sometimes perforated; main venation pinnate, palmate, pedate or rarely parallel, finer venation reticulate or striate. Inflorescence pedunculate, consisting of fleshy ± cylindric spadix (spike) subtended by bract-like spathe; spathe usually spreading above and convolute below, rarely with margins connate near base, often variously coloured; spadix either uniform with bisexual flowers or monoecious with pistillate flowers at base, staminate flowers above, sterile flowers of varying shape often present at base, middle or apex, apical portion sometimes forming a sterile appendix. Flowers numerous, minute, sessile, bractless, naked or perigoniate, bisexual or unisexual; perigon (perianth) cup-like or composed of 4–9 free or ± connate tepals. Stamens opposite tepals, free or connate into synandria; anthers sessile or with elongated filaments, opening by lateral or apical slits or pores; connective often very thick. Ovary normally superior, 1–many-locular; locules each with 1–many ovules; placentas parietal, axile, basal or apical; stigma sessile or borne on short, conical, rarely attenuate style. Fruit a 1–many-seeded berry, rarely connate to form a syncarp, often brightly coloured. Seeds minute to large, variable in shape, with or without endosperm.
Notes: The standard monograph is by Engler & Krause, E.P. 21, 37, 48, 55, 60, 64, 71, 73, 74 (IV. 23A–F) (1905–20). The generic and tribal classification has been updated more recently by Bogner in Aroideana 1: 63–73 (1979). Colocasia esculenta, Xanthosoma sagittifolium and their related forms and varieties are important local food crops, and numerous genera are grown for their ornamental leaves and inflorescences, see p. 4.The family is one of the most important groups of ornamental plants in tropical horticulture and many hundreds of species are found in cultivation. The following are either common or based on a reliable record from East Africa. Williams (U.O.P.Z.) records Aglaonema pictum and several unidentified species of Philodendron from Zanzibar, but these require detailed verification and are not included here, although the genera are keyed out above.
Herbs with watery, bitter or milky juice, with a tuberous or elongated rhizome, rarely woody and climbing. Leaves solitary or few, sometimes appearing after the flowers, mostly radical, when cauline then alternate and distichous or spirally arranged, entire or variously divided, often hastate or sagittate, with a membranous sheath at the base. Flowers small, arranged on a spadix enclosed in a spathe, bisexual or monoecious, the males in the upper part, females below, rarely dioecious. Perianth present in the bisexual flowers or absent from the unisexual flowers. Stamens hypogynous, 2-4-8, opposite the perianth-segments; anthers opening by pores or slits, free or united. Ovary superior or immersed, 1-many-locular; style various or absent. Ovules parietal, axile, basal or apical. Fruit a berry, or coriaceous and rupturing, 1-many-seeded. Seeds mostly with copious endosperm.
Notes: Usually easily recognised by the characteristic spathe enclosing the inflorescence (spadix).