Plants perennial, of fresh or brackish waters. Rhizomes and stolons present. Erect stems rooted in substrate, unbranched, short. Leaves basal, submersed, sessile; blade linear, base grading into sheath, apex obtuse to apiculate; midvein with 4--5 rows of lacunae on each side, balde appearing 3-zoned with light-colored middle zone bordered on each side by darker zone; abaxial surfacely without prickles or aerenchyma; intravaginal squamules entire. Inflorescences cymose, long-pedunculate; spathe not winged. Flowers unisexual, staminate and pistillate on different plants, submersed or floating, sessile (staminate) or pedicellate (pistillate); petals transparent. Staminate flowers: filaments distinct, released from spathe and floating to surface; anthers spheric; pollen in monads. Pistillate flowers floating; ovary 1-locular; styles 1, not 2-fid. Fruits cylindric to ellipsoid, ridged, dehiscing irregularly. Seeds ellipsoid, glabrous. Vallisneria is generally considered to have 1-flowered pistillate inflorescences. A few populations in southern United States and Central America, however, have cymes with up to 30 flowers. The United States populations include those in Florida, Alabama, and Mississippi.
Dioecious; staminate fls minute, numerous in a head subtended by a short-pedunculate, ovoid, bivalved spathe arising from the base, with 3 sep (one smaller), one pet-vestige, and 2 stamens, the fls at anthesis separating individually and floating to the surface; pistillate fls mostly solitary and sessile in a tubular, apically bifid spathe on a long, slender scape (reaching the surface), with 3 sep, 3 pet-vestiges, 3 staminodia, and 3 broad, bifid stigmas; pistillate scape coiling after anthesis and retracting the fr; ovules scattered over the ovary-wall; fr elongate, cylindric, indehiscent, many-seeded; vigorously stoloniferous, aquatic perennials with long, ribbon-like basal submersed lvs from a very short, erect crown. 2, cosmop.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.