Habit and leaf form. Aquatic herbs. Plants prickly to spiny (leaves and petioles), or unarmed. Mostly perennial, or annual. Leaves basal. Plants with neither basal nor terminal concentrations of leaves; without a stolon but sometimes with a creeping rhizome, usually with an erect corm-like stem. Hydrophytic; non-marine; rooted. Leaves submerged, or floating. Heterophyllous. Leaves alternate; spiral to distichous; ‘herbaceous’; petiolate (at maturity); sheathing (usually), or non-sheathing; simple; epulvinate. Leaf blades entire; elliptic to orbicular (adult leaves); one-veined, or pinnately veined, or palmately veined, or parallel-veined; cross-venulate, or without cross-venules. Leaves without stipules. Axillary scales present (2–10 per leaf, not fringed). Leaves with a persistent basal meristem, and basipetal development. Leaf anatomy. Hairs absent. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male, or functionally female, or functionally male and functionally female, or hermaphrodite, functionally male, and functionally female. Unisexual flowers present. Plants monoecious, or dioecious, or polygamomonoecious. Female flowers and bisexual flowers solitary, or aggregated in ‘inflorescences’ (then spathes few-flowered, long-pedunculate, usually reaching the water surface; bisexual flowers sometimes cleistogamous); with staminodes (often), or without staminodes. Male flowers aggregated in ‘inflorescences’ (many per spathe); with pistillodes (usually 3-lobed). Floral nectaries present (usually 3), or absent. Nectar secretion (when manifest) from the androecium (from staminodial nectaries). Pollinated by water, or entomophilous.
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; in cymes. The terminal inflorescence unit (when flowers clustered) cymose. Inflorescences scapiflorous, or not scapiflorous; axillary; spatheate (the spathe formed of (1-)2 connate bracts). Flowers long- pedicellate (in male flowers, the pedicel soon disintegrating), or subsessile to sessile (in female flowers); small; regular; 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present, or absent. Perianth with distinct calyx and corolla; 6; 2 -whorled; isomerous; different in the two whorls; white, or yellow, or orange, or blue, or purple, or pink, or red (basally). Calyx 3; 1 -whorled; polysepalous; regular; persistent. Corolla 3; 1 -whorled; polypetalous; white, or yellow, or orange, or blue, or purple, or pink, or red (basally). Petals clawed (usually), or sessile. Fertile stamens present, or absent (female flowers). Androecium 3–15. Androecial members free of the perianth; free of one another; 1–5 -whorled (usually in whorls of 3). Androecium including staminodes (often, the innermost or outermost members often constituting staminodal nectaries). Stamens 3–15 (? stamens and staminodia varying in number and distribution from species to species and sometimes from flower to flower). Anthers dehiscing via short slits; generally extrorse; bisporangiate, or tetrasporangiate. Pollen shed in aggregates, or shed as single grains. Fertile gynoecium present, or absent (male flowers). Gynoecium 3–20 carpelled (or more). The pistil 1 celled (with 3–20 or more placentae). Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary unilocular; 1 locular (with or without intruding partial partitions). Styles 3–20; free, or partially joined; forked; apical. Stigmas dry type; non-papillate; Group II type. Placentation laminar-dispersed, or parietal. Ovules in the single cavity 12–100 (i.e. ‘many’); pendulous to ascending; non-arillate; anatropous.
Fruit and seed features. Fruit fleshy; dehiscent, or indehiscent (then disintegrating at maturity); a capsule, or capsular-indehiscent (usually pulled underwater as they mature by the spiralling peduncles). Capsules when dehiscent splitting irregularly (underwater). Dispersal by water. Fruit many-seeded. Seeds ellipsoid or fusiform; scantily endospermic; conspicuously hairy, or not conspicuously hairy; with starch. Cotyledons 1. Embryo straight. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; dorsiventrally flattened. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Geography, cytology, number of species. Native of Australia. Not endemic to Australia. Australian states and territories: Western Australia, South Australia, Northern Territory, Queensland, New South Wales, Victoria, and Australian Capital Territory. Northern Botanical Province, Eremaean Botanical Province, and South-West Botanical Province.
Wheeler, Judy; Marchant, Neville; Lewington, Margaret; Graham, Lorraine (2002). Flora of the south west, Bunbury, Augusta, Denmark. Volume 1, introduction, keys, ferns to monocotyledons. Australian Biological Resources Study. Canberra.
Jacobs, S. W. L. (1994). Ottelia ovalifolia subsp. chrysobasis (Hydrocharitaceae), a new Australian subspecies.
Wheeler, J. R.; Rye, B. L.; Koch, B. L.; Wilson, A. J. G.; Western Australian Herbarium (1992). Flora of the Kimberley region. Western Australian Herbarium. Como, W.A.
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