Shortly stoloniferous, forming dense mats; usually submerged except for capitulum. Leaves 3- 10 cm, 3-5 mm wide at the base, linear-subulate, pellucid, with a reticulum of opaque veins. Scape 7-20(-150) cm, 1(-3) from each rosette, fragile, the lower part enclosed by a loosely sheathing leaf. Capitulum 3-8 x 6-11 mm, hemispherical. Bracts 2-3 mm, greyish. Perianth of 2 whorls, each of 2 segments; those of female flowers c. 2 mm, free, greyish-black; outer segments of male flowers 3 mm, free, the inner united below, yellowish-grey. Inner segments of all flowers with black apex and conspicuously white-ciliate. Ovary 2-lobed; stigmas 2; seed 1 mm, smooth.