Habit and leaf form. Aquatic herbs. Annual, or perennial; plants with neither basal nor terminal concentrations of leaves; rhizomatous (rhizome not clearly distinct from erect branches, unbranched). Stem growth not conspicuously sympodial. Hydrophytic; rooted. Leaves submerged; small, or medium-sized; alternate, or opposite (subopposite when subtending an inflorescence); when alternate, distichous; sessile; sheathing (sheath open, broad at the base, tapering to a slender 2-lobed summit). Leaf sheaths tubular; with free margins. Leaves simple. Leaf blades entire; linear (filiform); one-veined; cross-venulate, or without cross-venules. Leaves eligulate; without stipules. Axillary scales present. Leaf blade margins entire. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite (usually protandrous). Pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’. Inflorescence few-flowered (usually 2-flowered). Flowers subumbellate in racemes, or in spikes. The terminal inflorescence unit racemose. Inflorescences spikes emergent, with long peduncles which twist spirally especially before and after flowering. Flowers ebracteate; small; regular; cyclic; tricyclic. Hypogynous disk absent. Perianth absent. Androecium 2 (with an expanded connective). Androecial members all equal; free of one another; 1 -whorled. Androecium exclusively of fertile stamens. Stamens 2; filantherous, or with sessile anthers. Anthers dehiscing via longitudinal slits; extrorse; unappendaged. Pollen reniform. Gynoecium (2–)4–12(–26) carpelled; apocarpous; eu-apocarpous; superior. Carpel non-stylate; apically stigmatic; 1 ovuled. Placentation marginal. Stigmas dry type; non-papillate; Group II type. Ovules pendulous; non-arillate; campylotropous.
Fruit and seed features. Fruit non-fleshy; an aggregate. The fruiting carpel indehiscent; drupaceous (the drupelets of each flower resembling an umbel due to the considerable elongation of the carpel stipes during ripening). Dispersal usually by floating of the head of fruits. Fruit 1 seeded. Seeds non-endospermic; with starch. Cotyledons 1. Embryo slightly curved. 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. 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, Australian Capital Territory, and Tasmania. Eremaean Botanical Province and South-West Botanical Province.
Additional characters Fruit rostrate, or erostrate.
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.; Brock, Margaret A. (1982). A revision of the genus Ruppia (Potamogetonaceae) in Australia.
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