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 or shrubs. Leaves opposite and alternate, usually entire. Flowers often showy, axillary and solitary, or in terminal racemes or panicles. Calyx-tube adnate to the ovary; limb with usually 2 or 4 valvate large lobes.
Petals 2 or 4, rarely more, fugacious, twisted. Stamens 1 to 8. Fruit variable. About twenty genera and 300 species are grouped together under this head. Most of the species are found in temperate regions.