Herbs, perennial; rhizomatous, emergent aquatics in fresh or brackish water. Stems erect, glabrous. Leaves cauline, whorled; petiole absent; blade not fleshy, not leathery (fleshy or leathery in H. tetraphylla), margins entire. Inflorescences axillary, flowers solitary; bracts absent. Pedicels present (proximal) or absent (distal); bracteoles absent. Flowers bisexual or unisexual; caly× a minute rim adhering to summit of inferior ovary; petals 0; stamen 1, adnate to ovary, filaments glabrous; staminode 0; ovary 1-locular, placentation apical; stigma linear along surfaces of style. Fruits drupes. Seeds 1, brownish, globular, wings absent. x = 8.
North America, South America, Eurasia, introduced in Australia.
Species 4 (4 in the flora).
Leaf characteristics of Hippuris used here are derived from whorls on the emergent portions of the stems; morphology of submerged leaves differs sharply from that of emergent shoots.
M. E. McCully and H. M. Dale (1961) proposed that the taxa treated below all could be expressions of phenotypic plasticity of Hippuris vulgaris developed in different regimes of salts and photoperiod; this was not accepted by E. Hultén (1973), nor is it accepted here. Number of leaves in a whorl varies among plants and even on the same stem. Nevertheless, there are clear limits and discontinuities in leaf number and shape among taxa, which are well-correlated with less variable characters as well as with ecology and geography.
Hippuris has been placed in Halagoraceae or in Hippuridaceae as a monogeneric family. Molecular phylogenetic studies now place it in Plantaginaceae (D. C. Albach et al. 2005).
|1||Flowers unisexual; leaves 2–10 mm, midveins often conspicuous, lateral veins absent; stems 15–100 mm; rhizomes 1 mm diam.||Hippuris montana|
|1||Flowers bisexual; leaves 3–35 mm, midveins inconspicuous, lateral veins present, sometimes obscure; stems 80–500 mm; rhizomes (2–)3–7 mm diam.||> 2|
|2||Leaves on mid portions of emergent shoots in whorls of (7 or)8 or 9(–12), tips often curled in dried plants; filaments longer than anthers.||Hippuris vulgaris|
|2||Leaves on mid portions of emergent shoots in whorls of 3–6(or 7), tips not curled in dried plants; filaments equal to or shorter than anthers.||> 3|
|3||Leaves on mid portions of emergent shoots in whorls of (5 or)6(or 7), linear to narrowly oblong or lanceolate, 0.5–1.5 mm wide, apices subacute.||Hippuris lanceolata|
|3||Leaves on mid portions of emergent shoots in whorls of 3–5(or 6), oblanceolate or oblong to broadly obovate, 2–8 mm wide, apices obtuse, rounded, or blunt.||Hippuris tetraphylla|