Including Blyxaceae Nak., Elodeaceae Dum., Enhalaceae Nak., Halophilaceae J.G. Agardh, Stratiot(ac)eae Link, Thalassiaceae (Aschers and Gurke) Nak., Vallisneriaceae Dum.
Habit and leaf form. Vegetatively diverse aquatic herbs. Mostly perennial; with a basal aggregation of leaves, or with neither basal nor terminal aggregations 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. Lamina 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 stipulate, or exstipulate. Axillary scales (and sometimes serial axillary buds) present. Lamina margins often with thick-walled prickle-hairs. Leaves with a persistent basal meristem, and basipetal development.
General anatomy. Accumulated starch other than exclusively `pteridophyte type'.
Leaf anatomy. Stomata present, or absent; when present, paracytic. Hairs absent.
Lamina without secretory cavities. The mesophyll without calcium oxalate crystals. Minor leaf veins without phloem transfer cells (5 genera). Vessels absent.
Stem anatomy. Secondary thickening absent. Xylem without vessels. Sieve-tube plastids P-type; type II.
Root anatomy. Root xylem with vessels, or without vessels. Vessel end-walls scalariform.
Reproductive type, pollination. 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, floral, fruit and seed morphology. 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; free, or joined; 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.
Androecium 2-3 (rarely), or 4-100 (i.e. to `many'). Androecial members unbranched, or branched (the members opposite the calyx sometimes paired); 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; al\ter\ni\peri\anthial, or op\po\siti\peri\anthial. Anthers dehiscing via short slits; generally extrorse; bisporangiate, or tetrasporangiate. Endothecium developing fibrous thickenings, or not developing fibrous thickenings. The thickenings spiral. Anther epidermis persistent. Microsporogenesis successive. The initial microspore tetrads isobilateral, or decussate, or linear. Anther wall initially with one middle layer, or initially with more than one middle layer; of the `monocot' type. Tapetum amoeboid. Pollen shed in aggregates (as threadlike chains, in the marine forms), or shed as single grains. Pollen grains nonaperturate; 2-celled, or 3-celled.
Gynoecium (2-)3-6(-20); syncarpous; synovarious to synstylovarious; inferior. Ovary 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; bitegmic; crassinucellate. Outer integument contributing to the micropyle (rarely), or not contributing to the micropyle. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating; persistent (into early embryogeny). Synergids pear-shaped (sometimes with filiform apparatus), or hooked. Endosperm formation helobial. Embryogeny caryophyllad.
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). Seeds with starch. Cotyledons 1. Embryo achlorophyllous (1/1); straight.
Physiology, biochemistry. Cyanogenic (seldom), or not cyanogenic. Alkaloids absent (one species). Iridoids absent. Proanthocyanidins present (Hydrocharis), or absent (3 genera); in Hydrocharis, cyanidin. Flavonols absent. Ellagic acid absent. Saponins/sapogenins absent. Anatomy non-C4 type (Elodea, Hydrilla, Vallisneria).
Geography, cytology. Temperate to tropical. Cosmopolitan, except frigid zones. X = 7-12.
Taxonomy. Subclass Monocotyledonae. Superorder Alismatiflorae; Hydrocharitales. Species 80. Genera 16; Apalanthe, Appertiella, Blyxa, Egeria, Elodea, Enhalus, Halophila, Hydrilla, Hydrocharis, Lagarosiphon, Limnobium, Maidenia, Nechamandra, Ottelia, Stratiotes, Thalassia, Vallisneria.
Economic uses, etc. Including some important aquarium and watergarden ornamentals.
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