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Perennial wetland herb, rhizomatous. Stem erect, glabrous, square in section with distinct wings, 20–80 cm high. Leaves opposite, sessile, elliptic, with many translucent glands. Pentamerous flowers in branched cymes, 10–15 mm across, petals pale yellow. Fruit is a capsule containing narrow cylindric seeds, with fine reticulate surface.