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a genus of annual plants of the family Elatinaceae. The stems are weak and usually root at the nodes. The entire leaves are opposite or verticillate, and the small and axillary flowers have a double perianth. The fruit is a pod. There are 25 species, distributed in the temperate and tropical regions of both hemispheres. The USSR has eight species, found in bodies of water and on coastal sands, silty shoals, and the shores of rivers and lakes. In some places the plants grow in masses, forming a dense green cover. All species have both aquatic and terrestrial forms (depending on their habitat).