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a genus of perennial aquatic plants of the family Primulaceae. Roots are absent. The leaves are whorled and pectinately pinnatisect. The leafless peduncle, which rises above the water, bears a racemose inflorescence at its tip. The flowers are not identical because of heterogony. There are two species: H. palustris and H. inflata. The former grows in Asia Minor and in stagnant and slow-running waters in the European USSR. H. inflata, which has inflated internodes, occurs in North America.