Common name. Tape-grass Family.
Habit and leaf form. Vegetatively diverse aquatic herbs. Mostly perennial; plants with a basal concentration of leaves, or with neither basal nor terminal concentrations of leaves. Hydrophytic, or helophytic; marine (Thalassioideae, Halophiloideae), or non-marine (Hydrocharitoideae); free floating, or rooted. Leaves submerged, or emergent, or floating, or submerged and emergent, or submerged and floating. Heterophyllous (usually), or not heterophyllous. Leaves alternate, or opposite, or whorled; spiral, or distichous; membranous, or ‘herbaceous’; petiolate, or sessile, or petiolate and sessile; sheathing to non-sheathing. Leaf sheaths with free margins. Leaves simple; epulvinate. Leaf blades entire; linear, or oblong to orbicular (usually with ribbonlike submerged leaves); one-veined, or pinnately veined, or palmately veined, or parallel-veined; cross-venulate, or without cross-venules. Leaves with stipules, or without stipules. Axillary scales (and sometimes serial axillary buds) present. Leaf blade margins often with thick-walled prickle-hairs. Leaves with a persistent basal meristem, and basipetal development. Stem anatomy. Secondary thickening absent.
Reproductive type, pollination. Fertile flowers hermaphrodite (rarely), or functionally male, or functionally female, or functionally male and functionally female. Unisexual flowers present. Plants monoecious, or dioecious, or polygamomonoecious (rarely). Floral nectaries present, or absent. Nectar secretion (when manifest) from the androecium (from staminodial nectaries). Entomophilous (some Hydrocharitoideae), or pollinated by water (mostly, sometimes from free-floating male flowers).
Inflorescence and flower features. Flowers solitary, or aggregated in ‘inflorescences’; in cymes. The terminal inflorescence unit (when flowers clustered) cymose. Inflorescences scapiflorous, or not scapiflorous; axillary; few flowered cymes; spatheate (the spathe formed of (1-)2 connate bracts). Flowers small; regular, or somewhat irregular (Vallisneria). The floral asymmetry involving the perianth. Flowers 3 merous; partially acyclic. The gynoecium acyclic. Perigone tube present, or absent. Perianth with distinct calyx and corolla (usually), or of ‘tepals’; 6, or (2–)3; 2 -whorled; isomerous; if not resolvable into calyx and corolla, sepaloid, or petaloid; similar in the two whorls (then semipetaloid), or different in the two whorls; white, or yellow, or red, or purple, or blue. Calyx (2–)3; 1 -whorled; polysepalous (sometimes from a hypanthium); regular. Corolla when present, 3; 1 -whorled; polypetalous (attached to the gynoecium or to the perigone tube). Petals clawed, or sessile. Fertile stamens present, or absent (female flowers). Androecium 2–3 (rarely), or 4–100 (i.e. to ‘many’). Androecial members unbranched, or branched (the members opposite the calyx sometimes paired). Androecial sequence determinable, or not determinable. Androecial members usually maturing centripetally; free of the perianth; free of one another; 1–10 -whorled (the whorls trimerous, but sometimes with pairs opposite the calyx). Androecium exclusively of fertile stamens, or including staminodes (the innermost or outermost members often constituting staminodal nectaries). Stamens 2–25; reduced in number relative to the adjacent perianth to polystemonous; alterniperianthial, or oppositiperianthial. Anthers dehiscing via short slits; generally extrorse; bisporangiate, or tetrasporangiate. Pollen shed in aggregates (as threadlike chains, in the marine forms), or shed as single grains. Fertile gynoecium present, or absent (male flowers). Gynoecium (2–)3–6(–20) carpelled. The pistil 1–20 celled. Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary unilocular; 1 locular (but often with deeply intruding partial partitions). Styles (2–)3–6(–20) (but often individually bifurcated, and then seeming to be twice as many as the carpels); partially joined; apical. Stigmas dry type; non-papillate; Group II type. Placentation laminar-dispersed, or basal (e.g. Elodea). Ovules in the single cavity 12–100 (i.e. ‘many’); pendulous to ascending; non-arillate; orthotropous (rarely), or hemianatropous to anatropous.
Fruit and seed features. Fruit fleshy, or non-fleshy; dehiscent, or indehiscent (often opening by decay); a capsule, or capsular-indehiscent. Capsules splitting irregularly (underwater). Dispersal by water. Seeds scantily endospermic (Otelia), or non-endospermic (usually); with starch. Cotyledons 1. Embryo achlorophyllous (1/1); straight. Seedling. Hypocotyl internode present. Mesocotyl absent. Seedling collar not conspicuous. Cotyledon hyperphyll elongated; assimilatory; dorsiventrally flattened. Coleoptile absent. Seedling macropodous. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.
Geography, cytology, number of species. World distribution: cosmopolitan, except frigid zones. X = 7–12. 80 species.
Economic uses, etc. Including some important aquarium and watergarden ornamentals.