Herbs, perennial, submersed, floating-leaved, emersed, glabrous; rhizomes often present; stolons absent; corms absent; tubers absent. Roots not septate. Leaves sessile or petiolate; petiole triangular; blade with translucent markings absent, linear to ovate, base attenuate to rounded, margins entire, apex obtuse to acute. Inflorescences panicles, of 2–10 whorls, erect, emersed, rarely submersed; bracts delicate, smooth, apex acuminate, surfaces smooth. Flowers bisexual; pedicels ascending; bracts subtending pedicels, lanceolate, shorter than pedicels, apex acuminate; receptacle flattened; sepals erect, not sculptured, herbaceous; petals pink or white, entire; stamens 6–9, filaments filiform, glabrous; pistils 15–20, in ring around margin of flattened receptacle, not radiating in starlike pattern, distinct; ovules 1; styles lateral. Fruits without longitudinal ribs, strongly laterally compressed, abaxial wings absent, lateral wings absent, abaxially 2–3-ribbed, abaxial keel absent, glands absent. x = 7.
Widely distributed, nearly worldwide.
Much controversy surrounds the treatment of Alisma in North America. At present three distinct native species in North America are generally recognized (P. Rubtzoff 1964) as well as the probable occurrence of two introduced species, one in California and the other in Alaska.
Species 9 (5 in the flora).
|1||Style longer than or (equal to) ovary; anthers ellipsoid.||> 2|
|1||Style shorter than or (equal to) ovary; anthers ovoid to rounded.||> 3|
|2||Petals with obtuse apex, purplish white to purplish pink; style ± straight; leaf blade ovate to broadly lanceolate; Alaska.||A. plantago-aquatica|
|2||Petals with acute apex, purplish pink; style curved proximally; leaf blade narrowly elliptic to lanceolate; California.||A. lanceolatum|
|3||Achenes with 2 abaxial grooves, 1 abaxial ridge; leaves submersed, ribbonlike, or if emersed, blades linear-lanceolate to narrowly elliptic.||A. gramineum|
|3||Achenes with1 abaxial groove; leaves emersed, rarely submersed or floating, blades ovate to elliptic.||> 4|
|4||Fruiting heads 2–4 mm diam.; achenes 1.5–2.2 mm; petals 1–3 mm.||A. subcordatum|
|4||Fruiting heads 4.1–7 mm diam.; achenes 2.1–3 mm; petals 3.5–6 mm.||A. triviale|