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.
Herbs with opposite exstipulate leaves and capitate involucrate flowers. Calyx superior, enclosed by a bracteolate invo-lucel; limb persistent, cup-shaped, lobed, or with five or more rigid bristles. Corolla-tube funnel-shaped, often curved. Stamens 4; filaments filiform, free, ex-serted. Fruit indehiscent, covered by the hardened involucel, containing one pendulous albuminous seed. Nearly 150 species are known, included in six genera, mostly Asiatic. The Wild Teasel, Dipsacus sylvestris, is a familiar example of this small order. In this the floral bracts are spinescent and exceed the florets.