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.
Shrubs or herbs with opposite or verticillate exstipulate leaves. Flowers corymbose, spicate or capitate, rarely solitary. Calyx tubular, persistent, inferior. Corolla deciduous, irregular, often 2-lipped. Stamens 4 and didynamous, or only 2. Style terminating the ovary. Fruit 2- or 4-celled, nucamentaceous or drupaceous; cells 1-seeded. There are between 40 and 50 genera and about 600 species, common in the tropics, but rare in temperate regions.