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 simple petiolate leaves and capitate flowers surrounded with a finely divided involucre. Perianth funnel-shaped or salver-shaped. Stamens 5. A small genus of North-western American plants. The name is said to be from aBpos elegant, in allusion to the involucre.