Perennial herbs or undershrubs. Stem jointed, often with deposition of calcium carbonate. Leaves simple, opposite, exstipulate. Flowers axillary, solitary or in dichasial cymes, hermaphrodite, regular. Calyx tubular, 4-6 lobed, persistent. Petals 4-7, clawed, free. Stamens 3-24. Carpels usually 3, syncarpous; ovary unilocular; placentation parietal with indefinite ovules. Capsule loculicidal, enclosed within the persistent calyx; seeds many, small.