A small order of flowering plants, division Magnoliophyta (Angiospermae), which gives its name to the subclass Alismatidae of the class Liliopsida (monocotyledons). It consists of three families (Alismataceae, Butomaceae, and Limnocharitaceae) and less than a hundred species. They are aquatic and semiaquatic herbs with a well-developed, biseriate perianth that is usually differentiated into three sepals and three petals, and with a gynoecium of several or many, more or less separate carpels. Each flower is usually subtended by a bract. Butomus umbellatus (flowering rush) and species of Sagittaria (arrowhead, family Alismataceae) of this order are sometimes cultivated as ornamentals.