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, sometimes woody at the base, often enlarged at the joints. Leaves opposite, entire, with or without stipules. Flowers regular, hermaphrodite, more rarely unisexual by abortion, often in dichotomously branched cymes. Sepals 4 or 5, persistent, free or cohering in a tubular calyx, imbricate. Petals of the same number as the sepals, either hypogynous or perigynous, entire or bifid or fringed. Stamens 8 to 10 or fewer, inserted with the petals. Torus usually small. Styles 2 to 5. Capsule 1-celled, or rarely 2- to 5-celled at the base, membranaceous or crustaceous, dehiscing in valves or at the top, rarely transversely. Seeds numerous, amphi-tropal, albuminous, often reniform, embryo curved. Species very numerous; especially abundant in the temperate zone of the northern, but extending to the utmost plant limit in the frigid zones of both hemispheres.