Perennial herbs with rhizomes or corms and fibrous roots; leaves basal or crowded near the stem base, distichous, with equitant, sheathing base and linear-ensiform or terete, unifacial blade. Flowering stem with few upwards gradually reduced leaves; inflorescence a terminal simple or compound spike; flowers sessile in the axils of dispersed, spathaceous bracts, bisexual, (nearly) hypogynous, zygomorphic; perianth petaloid, pale yellow or white to pink; segments 4, free or basally connate, the outer 2 median larger than the 2 lateral inner, the median upper frequently 3- or 2-cuspidate, composed of the 2 lateral outer tepals and the reduced median inner tepal; stamen solitary, anterior, its filament broad, inserted at the base of the lower tepal; anther bithecate and tetrasporangiate, basifixed to dorsifixed, straight and introrse, or curved (Philydrella), or helically twisted (Philydrum), opening by 2 longitudinal slits; staminodia usually absent; gynoecium of 3 united carpels; ovary superior, often slightly united at the base with the perianth and filament, ± zygomorphic (the median carpel often smaller), ± trilocular with axile placentas, or for the greatest part unilocular with deeply intruding parietal placentas; each placenta bearing numerous anatropous, heterotropous ovules; style simple, terminal, with a capitate, slightly trilobed stigma; septal nectaries or other nectariferous glands absent. Fruit a loculicidal, 3-valved capsule, or berrylike; seeds numerous, small; testa longitudinally or spirally striate, operculate, with enlarged chalazal cap or prolongation at micropylar and chalazal end; embryo small, straight, cylindrical, embedded in copious non-mealy endosperm containing starch, protein and oil.
The family is confined to Australia and tropical E Asia. Philydrum is the only genus with a wider distribution, ranging from Victoria along the E and N coastal parts of Australia to S New Guinea and from Malaysia and Thailand to S China, Taiwan and S Japan. Helmholtzia is restricted to tropical NE Australia, New Guinea and the Moluccas and Orthothylax to a limited area of (sub-)tropical E Australia. Philydrella is isolated from the rest of the family in SW Australia.
No observations on seed dispersal in natural habitats are available, but the tiny seeds of Philydrum and Philydrella may be dispersed by water; they are known to float for long periods. The same may be true for the elongated (scobiform) seeds of Orthothylax which also would be suitable for wind dispersal. It is unknown whether the berrylike fruits of Helmholtzia are endozoochorous or become dry and ultimately dehisce.
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