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 branched stems, stipulate, odd-pinnate leaves, the leaflets pinnatifid, and greenish perfect bracted flowers in dense oblong spikes. Calyx-tube urn-shaped, constricted at the mouth, 4-winged, its 4 lobes with scarious margins. Stamens 2 or 4; filaments short. Pistil 1; style terminal; stigma brush-like; ovule 1, suspended. Achene enclosed in the dry calyx-tube. [Greek, diminutive of Poterium.]
Two species, one of northwestern America and the following typical one.
Sanguisorba annua Nutt.; T. & G. Fl. N. A. 1: 429. 1840.
Poteridium annuum Spach, Ann. Sci. Nat. III. 5: 43. 1846.
Glabrous, 4'-15' high. Leaflets 7-15, broadly obovate, 6" long or less, pectinate-pinnatifid, with linear-oblong segments; spikes 1 1/4' long or less, about 3 1/2" thick; bracts ovate, shorter than the flowers; calyx-lobes oval, apiculate, 1" long; stamens usually 4; fruiting calyx-tube strongly 4-winged.
Plains, Kansas to Arkansas and Texas. Summer.