Small, tufted, grass-like annual, or moss-like perennial herbs. Stems slender, densely packed in perennial spp. Leaves linear or setaceous, all basal, or cauline and imbricate; sheaths open, distinct, membranous. Flowers minute, unisexual, without perianth, in terminal complex "spike-like" inflorescences with 2 or more distichous glume-like bracts. Each bract usually enclosing 1-several pseudanthia; each pseudanthium consisting of 1, 2 or 0 male flowers and 1-many superposed or collateral, sessile, often ± connate female flowers; pseudanthia frequently with 1-3 hyaline bracteoles. Male flower a single stamen, filament thread-like, anther 1-celled, versatile, opening lengthwise by a slit. Female flower a single unilocular ovary; style 1; ovule 1. Fruit with membranous pericarp, dehiscing longitudinally. Four genera with 30-35 spp., of Malesia, S.E. Asia, Australia, N.Z. and 1 sp. in S. America.

Fls unisexual, in a terminal "spike". Infl. us. complex with 1 pseudanthium or several within each of 2 or more glume-like floral bracts. Each pseudanthium of 1, 2 or 0 ♂ and 1– ∞ or 0, often connate ♀. Upper bractlets subtending fls, hyaline or 0. Tepals 0. ♂ a single stamen, filament thread-like, anther 1-celled, versatile, opening lengthwise by a slit, rarely 2-celled and basifixed. ♀ a single unilocular ovary, fls free or often 2 or more ± connate, collateral or superposed in 1–2 series. Styles 1–(3–10) to each fl., occ. styles from all fls connate at the base. Ovule solitary and pend. Fr. with membr. pericarp dehiscing longitudinally, (ind. in Hydatella). Seeds with cop. endosperm. Small, tufted, grasslike and annual, or mosslike and perennial herbs. Stems slender, densely packed in perennial spp. Lvs linear or setaceous, in annual spp. all crowded at base of stem, in perennial spp. imbricate and crowded along the stems. A small family of c. 5 genera and c. 40 spp. mainly in Australia and N.Z., a few in Malaysia, Southeast Asia and South America.