Pontederiaceae
A small natural order of erect or floating, aquatic herbs, mostly American, rare in tropical Asia and Africa, absent in Europe. Flowers hermaphrodite, scarcely irregular, or sometimes regular, fasciculate or scattered at the sides of a simple or branched rachis, racemose, spicate, or sub-paniculate, terminal, in a sessile or pedunculate sheath; perianth inferior, free of the ovary, the tube evolute or rarely absent; lobes (rarely segments) six, more or less distinctly biseriate; stamens six or three; filaments free; ovary three-celled. Perfect leaves on a rhizome or floating stem, long-stalked; blade floating or emersed; submersed leaves sometimes reduced to linear petioles (without a blade). Pontederia vaginalis is used in various forms as medicine in Japan. The order comprises four genera--Eichhornia, Heteranthera, Monochoria, and Pontederia--and scarcely thirty-five species.