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Sometimes included in Centrolepidaceae.
Habit and leaf form. Small aquatic herbs. Glabrous annual, or perennial (rarely). Leaves basal. Plants very short stemmed, with a basal concentration of leaves. Hydrophytic to helophytic; rooted. Leaves submerged and emergent; alternate; spiral; sessile; shortly sheathing to non-sheathing. Leaf sheaths if detectable, not tubular; with free margins. Leaves simple. Leaf blades entire; flat (and linear), or solid (and filiform); linear; linear (to filiform); one-veined, or parallel-veined. Leaves eligulate. Leaf blade margins entire. Leaves presumably with a persistent basal meristem, and basipetal development. Vegetative anatomy. Plants without silica bodies. Leaf anatomy. Hairs absent. Extra-floral nectaries absent. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers functionally male and functionally female. Unisexual flowers present. Plants monoecious (the heads mostly unisexual). The unisexual flowers segregated in different inflorescences. Female flowers without staminodes. Male flowers without pistillodes. Floral nectaries absent. Autogamous, or pollinated by water.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in heads; more or less in ‘spikelets’. Inflorescences usually more or less scapiflorous; terminal; individually resembling flowers, usually unisexual, the males 4–10 flowered, the females 8–20 flowered; with involucral bracts (these opposite, membranous, glabrous, 1-veined); pseudanthial. Flowers pedicellate; bracteate (with two or rarely four erect or sheathing involucral bracts); ebracteolate; minute, or small; 1 merous. Hypogynous disk absent. Perianth absent. Fertile stamens present, or absent (female flowers). Androecial members definite in number. Androecium 1 (i.e. the male flower consisting of a single stamen). Androecium exclusively of fertile stamens. Stamens 1. Anthers basifixed; non-versatile; dehiscing via longitudinal slits; bilocular; tetrasporangiate. Fertile gynoecium present, or absent (male flowers). Gynoecium ostensibly 1 carpelled. The pistil 1 celled. Gynoecium monomerous, or syncarpous (perhaps pseudomonomerous, depending on interpretation); of one carpel, or eu-syncarpous; superior. Carpel if considered monomerous, non-stylate; 1 ovuled. Placentation apical. Ovary if considered pseudomonomerous, unilocular; 1 locular; shortly stipitate. Gynoecium non-stylate (if a tuft of filamentous structures represents stigmas), or stylate (if it is stylar). Styles free; apical. Stigmas with 4–10 stigmatic hairs. Placentation apical. Ovules non-arillate; anatropous.
Fruit and seed features. Fruit non-fleshy. The fruiting carpel if considered monomerous, indehiscent; an achene. Fruit indehiscent (shed entire); achene-like; 1 celled; 1 seeded. Seeds ‘almost’ non-endospermic. Perisperm present (starchy). Embryo rudimentary at the time of seed release. Testa operculate. Seedling. Germination hypogeal (seed remnant visible at the base of the plant).
Geography, cytology, number of species. World distribution: Western Australia, New Zealand.
Etymology. From the Greek "water" and "to come forth, arise"; the plant lives under water.
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