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 of erect or rarely climbing habit, rarely shrubs. Leaves opposite or alternate, simple or compound. Inflorescence various. Flowers regular, pentamerous. Calyx inferior, 5-lobed. Corolla slightly perigynous, campanulate, funnel-shaped or salver-shaped. Stamens 5, inserted upon the corolla-tube and alternating with its lobes. Fruit a 3-celled 3-valved few or many-seeded capsule, splitting loculicidally. Seeds angular, winged or oval, with spongy testa and fleshy or horny albumen. A small order confined to temperate and cold regions, especially numerous in North-western America. There are 17 genera and upwards of 100 species.