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Plants annual or perennial, propagated from seeds, winter buds, or rhizomes, submersed in fresh or rarely brackish water. Stems variable in length according to water depth, branched or unbranched, terete or compressed, rooting at the nodes. Leaves all submersed and/or floating, alternate or subopposite; submersed leaves pellucid, sessile or petiolate, linear to orbicular, apically subulate to ob-tuse, basally cuneate to rounded or cordate, the margins entire, the nerves 1-35; floating leaves coriaceous, mostly petiolate, rarely subsessile, elliptic to ovate, acute to obtuse at apex, cuneate to rounded or cordate at base, the margins en-tire, the nerves 3-51; stipules tubular, sheathing the stem and young inflores-cences, connate or convolute, either free or adnate to the base of submersed leaves, free from the base of floating leaves. Winter buds present or absent, with extremely shortened internodes, divided into inner and outer leaves; inner leaves few to numerous, unmodified or shortened and oriented at 90? with respect to outer leaves, rolled into a fusiform structure; outer leaves 1-5 per side, mostly similar to vegetative leaves, rarely corrugated near the base. Inflorescence a capitate or cylindric spike with 1-20 whorls of flowers, compact or moniliform, with 2-4 flowers in each whorl, mostly buoyed above surface of water at an-thesis. Flowers bisporangiate; perianth of 4 distinct, rounded, short-clawed, greenish segments; stamens 4, the filaments adnate to the perianth claw, the anthers bithecate, extrorse, pollen spherical, monoaperturate; gynoecium of 4 distinct, unilocular, uniovulate carpels, the placentation parietal, the ovule campylotropus. Fruit drupe-like, dorsally rounded or keeled; embryo coiled.
Herbs: rhizomes present or absent; tubers absent; turions present or absent. Stems terete or compressed, nodes occasionally with oil glands; turions with extremely shortened internodes, divided into outer and inner leaves; outer leaves 1--5 per side, similar to vegetative leaves or occasionally corrugate near base; inner leaves 1--10, rolled into fusiform structure, unmodified, or shortened and oriented at 90° angles to outer leaves. Leaves submersed or both submersed and floating, alternate to nearly opposite; stipules connate or not, if not, then convolute, tubular, sheathing stem and young inflorescences. Submersed leaves sessile or petiolate; stipules either free from or adnate to base of leaf blade for less than ½ length of stipule, if adnate, then extending past adnation as free ligule; blade translucent, linear to orbiculate, not channeled, flattened, base acute to perfoliate, margins entire or serrate, rarely crispate, apex subulate to obtuse; veins 1--35. Floating leaves petiolate, rarely nearly sessile; stipules free from base of leaf blade; blade elliptic to ovate, leathery, base cuneate to rounded or cordate, margins entire, apex acute to obtuse; veins 1--51. Inflorescences spikes or panicles of spikes, submersed or emersed, capitate or cylindric; peduncles stiff, if long enough then projecting inflorescence above surface of water. Flowers: pistils 1 or 4. Fruits abaxially rounded or keeled, flattened to turgid, beaked; embryo coiled 1 or more times. x = 13 or 14.
Rhizome present or absent, if present well differentiated from the stem, filiform, slender or robust, terete or ± compressed; stems terete or compressed, ± densely branched; turions present or absent. Leaves alternate, but mostly subopposite toward the inflorescence apex (involucral leaves), sessile or petiolate, 1-to many-nerved, all submerged and with filiform, grass-like or ± expanded blade, or dimorphic, the submerged ones thin and ± translucent, sometimes reduced to phyllodes, the floating ones ± coriaceous, opaque, petiolate,and usually broader; stipulesalways present,although sometimes fugacious, forming a sheath, connate or convolute around the stem; blade or petiole sometimes attached near the top of the sheath which projects beyond the joint as a ventralligule, sometimes attached farther down the sheath or arising from the node. Spikes cylindrical or subglobose, 3-to many-flowered, dense or lax, sometimes interrupted, mostly borne above the water and wind-pollinated, but sometimes submerged and water-pollinated. Tepals rounded at the apex. Fruitlets with soft mesocarp and multi-layered sclerified endocarp, with or without a distinct beak. Embryo unciform or spiral; cotyledon curved in the seed.
Herbs, perennial or annual, in fresh or brackish water, totally submerged or with floating leaves. Stems terete to compressed, rarely strongly compressed. Leaves alternate, occasionally opposite, mono-or dimorphic; stipules membranous, free or partially adnate to leaf base (sheaths); submerged leaves sessile or petiolate, linear or with thin blades lanceolate; margins entire, denticulate, or serrulate; floating leaves petiolate, with leathery blades lanceolate to ovate or broadly elliptic-oblong. Inflorescence a pedunculate spike, terminal or axillary, surrounded by sheath in bud, emergent, floating or submerged at anthesis. Perianth 4-merous, free, bractlike and shortly clawed, each inserted opposite a stamen. Stamens 4, united with perianth at base; anthers sessile, 2-celled, extrorse, dehiscence longitudinal. Carpels (1-)4(or 5), free; stigmas sessile or on short styles, expanded, capitate or peltate; ovule solitary, attached to adaxial side of carpel. Fruit drupaceous with fleshy exocarp and bony endocarp. Embryo curved or spiral, rarely erect; endosperm absent.
Characters of the family. 100, cosmop. The plants normally produce lvs (or phyllodia) on the submersed part of the stem. If the stem reaches the surface, some spp. also produce floating lvs of different shape and texture. In some spp. the midrib is paralleled by one or more rows of colorless, translucent lacunar cells, forming lacunar strips or bands that are usually easily observed by transmitted light. Lf-measurements are taken from the principal lvs of the main axis, not from those of short, lateral branches. In some spp. the sides of the achenes are depressed at the center, and the achenes are said to be pitted.
Aquatic perennial rhizomatous herbs. Stems erect, floating or creeping, often producing turions. Leaves alternate, each with an axillary membranous stipular sheath, perfoliate, sessile or petiolate, with 1–numerous veins, submerged or both submerged and floating and dimorphic. Stipules free. Inflorescence a capitate or cylindric spike, with 4–numerous whorls of 4 flowers per whorl. Perianth segments 4; anthers 4, 2-celled; carpels 4, superior, sessile, free. Peduncle often becomes bent when fruiting, submerging the fruit. Fruit often ridged or tuberculate, beaked. Embryo coiled inside fruit.
Characters of the family. All or most lvs alt. and all stipulate. Cosmopolitan. Nearly 100 spp.; only 1 of the 4 N.Z. spp. likely to be endemic.
Characters of the family. Native spp. 4, adventive 1.
