This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", 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..
Annual or perennial herbs, with opposite petioled thick entire leaves, one of each pair somewhat larger than the other. Stems ascending, erect or prostrate, branching, mostly glandular-pubescent, with clustered or solitary numerous-flowered involucres on long axillary peduncles. Flowers sessile, usually conspicuous. Perianth-tube elongated, tubular or funnelform, the limb spreading, 5-lobed, the lobes obcordate or emarginate. Stamens 3-5, unequal, inserted on the tube of the perianth; anthers linear-oblong, included. Style filiform. Fruit dry, 1-5-winged or ridged, the wings broad or narrow, reticulate-veined. Seed cylindric, smooth, shining; one of the cotyledons is abortive, the seedling appearing monocotyledonous. [Name from the Greek, graceful.]
Perennial, viscid-pubescent, stem erect or ascending, usually much branched, 1°-2° high. Leaves oval, ovate or oblong-elliptic, petioled, obtuse or acutish at the apex, cuneate, truncate or rounded at the base, l'-2 1/2' long; bracts of the involucre 5 or 6, ovate or elliptic, white, 5"-8" long; flowers greenish-white, very numerous in the involucres, 8"-12" long, fragrant, opening at night; fruit 4"-6" high, coriaceous with 5 or sometimes fewer, undulate coarsely reticulated ridges about 1" wide, which do not close over its summit.
In dry soil, South Dakota to Nebraska, Colorado, Texas and Mexico. Reported from Iowa. June-Aug.