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a genus of plants of the family Isoëtaceae. The plants are perennials that live underwater (aquatic forms) or in extremely wet areas (amphibious and terrestrial forms). There are approximately 70 species, distributed throughout the world. They are only rarely encountered in the tropics. The USSR has four species. The plants, which are 5–20 cm high, usually grow in groups or form small thickets. Two species, I. lacustris and I. setacea (formerly I. echinospora), grow on the sandy bottoms of lakes, creeks, and oxbows in the central and northern zones of the European USSR and, occasionally, in Siberia. Two other species grow in the Far East.