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.
Taken in its entirety this is a very large order, and many of the tropical genera have a distinct calyx and corolla; but the following diagnosis includes only those genera coming within our province. Herbs with a milky juice, or shrubs, or trees. Leaves alternate or opposite, simple. Flowers inconspicuous, unisexual, often involucrate. Perianth none, or 2- to 5-lobed. Stamens 1 or more, sometimes very numerous. Fruit capsular, 2- or 3-celled; cells 1- or 2-seeded; seeds pendulous, albuminous.