This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
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.