Annual or perennial herbs, rarely shrubs, usually swollen at the joints. Leaves opposite or alternate, and frequently unequal. Flowers bisexual, capitate or solitary. Perianth inferior, coloured, tubular or funnel-shaped, constricted in the middle. Stamens definite, hypogynous. Fruit a 1-celled 1-seeded utricle enclosed in the enlarged often indurated tube of the perianth. Seeds adhering to the walls of the ovary. There are about 16 genera and 100 species, chiefly tropical.