Habit and leaf form. Erect aquatic herbs; laticiferous. Perennial; plants with a basal concentration of leaves. Stem internodes hollow (in A. lanceolatum). Rhizomatous (rhizomes tuberous). Hydrophytic; rooted. Leaves emergent. Not heterophyllous. Leaves medium-sized to very large; alternate; petiolate (winged basally); sheathing; simple. Leaf blades linear, or ovate to elliptic; parallel-veined; cross-venulate, or without cross-venules. Axillary scales present. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Floral nectaries present. Nectar secretion from the gynoecium, or from the androecium.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. The terminal inflorescence unit cymose, or racemose. Inflorescences scapiflorous; paniculate, with flowers arranged in several successive pseudowhorls which are separated by a long axis; each pseudowhorl of 3–8(-12) flowers; with involucral bracts, or without involucral bracts. Flowers bracteate; regular; 3 merous; cyclic; tetracyclic. Floral receptacle developing a gynophore. Perigone tube absent. Perianth with distinct calyx and corolla; 6; 2 -whorled; isomerous; without spots, or spotted; different in the two whorls; white to pink (inner), or green (outer). Calyx 3; 1 -whorled; polysepalous; green. Corolla 3; polypetalous; white, or pink. Androecium 6. Androecial members branched (having three stamen pairs); free of the perianth; free of one another, or coherent (? in pairs next to the petals); 1 -whorled. Androecium exclusively of fertile stamens. Stamens 6; diplostemonous; alterniperianthial. Anthers dehiscing via longitudinal slits; extrorse. Gynoecium 10–20 carpelled (in a single whorl); apocarpous; eu-apocarpous; superior. Carpel with a gynobasic style; 1 ovuled. Placentation basal. Stigmas dry type; papillate; Group II type. Ovules ascending; anatropous, or amphitropous.
Fruit and seed features. Fruit non-fleshy; an aggregate. The fruiting carpels not coalescing. The fruiting carpel indehiscent; an achene (achenes obovate to elliptic, laterally compressed, when mature forming a circular or rounded-triangular whorl). Seeds non-endospermic; with starch. Cotyledons 1. Embryo strongly curved (horseshoe-shaped). Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; more or less circular in t.s. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Physiology, biochemistry. Photosynthetic pathway: C3.
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, New South Wales, and Victoria.
Additional characters Fruit shortly rostrate, or erostrate.