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.
A large order of herbs, shrubs, or rarely trees. Leaves opposite, or whorled below, but often alternate in the upper part. Calyx usually persistent, inferior, 5 - cleft. Corolla regular or irregular, 4- or 5-lobed, imbricate or valvate in bud. Stamens 4, didynamous, with or without a rudimentary fifth, or 2 (rarely 5) perfect ones. Fruit a 2-celled capsule opening by valves or pores, rarely baccate. Seeds small, albuminous, few or many. There are about 180 genera and 1,800 species included in this family. They occur in all parts of the world. The ornamental herbaceous species are very numerous.