This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial biennial or annual herbs, with opposite or rarely verticillate leaves, and perfect gamopetalous flowers in dense involucrate heads. Stipules none. Flowers borne on an elongated or globose receptacle, bracted and involucellate. Calyx-tube adnate to the ovary, its limb cup-shaped, disk-shaped, or divided into spreading bristles. Corolla epigynous, the tube usually enlarged at the throat, the limb 2-5-lobed. Stamens 2-4, inserted on the tube of the corolla and alternate with its lobes; filaments distinct; anthers versatile, longitudinally dehiscent. Ovary inferior, i-celled, style filiform; stigma undivided, terminal, or oblique and lateral; ovule 1, anatropous. Fruit an achene, its apex crowned with the persistent calyx-lobes. Seed-coat membranous; endosperm fleshy; embryo straight.
About 7 genera and 140 species, natives of the Old World. Scales of the elongated receptacle prickly pointed.
Scales of the receptacle not prickly, herbaceous, capillary, or none. No receptacular scales.
Scales of the receptacle about as long as the flowers.