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.
Erect or climbing herbs, destitute of tendrils, or more rarely shrubs, frequently clothed with hispid often stinging hairs. Leaves opposite or alternate, entire, lobed, pinnatifid or pinnate; stipules none. Flowers regular, hermaphrodite, solitary, racemose, or cymose or capitate; peduncles often opposite the leaves; pedicels bibracteolate. Calyx-tube ad-nate to the ovary, often ribbed and twisted; limb of 4 or 5 imbricated or contorted persistent lobes. Petals 4 or 5, inserted upon the throat of the calyx, sessile or clawed, flat or hooded. Stamens usually numerous, often in bundles opposite the petals, occasionally with intervening filiform or petaloid staminodes. Capsule commonly 1-celled, with straight or twisted ribs; seeds 1 or more, usually minute. Ten genera, containing about one hundred species, belong to this order, and, with the exception of one monotypic genus from tropical Africa, all are American.