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.
Leaves opposite, usually narrow and entire, and furnished with pellucid dots. Flower-head small; involucre of a single row of bracts; receptacle naked. Pappus bristly or scaly. An American genus of about thirty species, of which one hardy annual has lately been introduced.
1. P. angustifolia. - A dwarf annual 4 to 6 inches high with yellow fragrant flowers. North-west America.