mare's-tail, any plant of the genus Hippuris, perennial, submerged aquatic herbs of temperate and frigid regions. Mare's-tails have an erect stem bearing whorls of small leaves, so they resemble some of the horsetails (which are also sometimes called mare's-tails); however, unlike horsetails, they are flowering plants, with the inconspicuous flowers borne in the leaf axils. Mare's-tails are cultivated in bog gardens. They are classified in the division Magnoliophyta, class Magnoliopsida, order Callitrichales, family Hippuridaceae.
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