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..
Perennial acaulesecnt or nearly acaukscent herbs. Leaves basal, leathery, pinnatifid or pinnately-toothed, petioled. Flowers basal, more or less tufted. Calyx pubescent, its tube linear-funnelform, its segments narrow, 2-3 times shorter than the tube. Petals white or pink, spreading. Stamens 8; filaments filiform, the alternate ones longer; anthers linear. United styles filiform; stigma 4-cleft. Capsules basal, woody, pyramidal, their angles retuse or obtuse, transversely wrinkled. Seeds sessile, in 1 or 2 rows, deeply furrowed along the raphe. [Greek, referring to the tuberculate edges of the valves of the capsule.]
Five or six species, of western North America, the following typical.
Oenothera caespitosa Nutt. Fras. Cat. 1813. Oenothera scapigera Pursh, Fl. Am. Sept. 263. 1814. P. Nuttallii Spach, Hist. Veg. 4: 365. 1835. Pachylophus caespitosa Raimann in Engl. & Prantl, Nat. Pfl. Fam. 3: Abt. 7, 215. 1893.
Acaulescent or nearly so, perennial or biennial from a thick woody root. Leaves clustered at the base, narrowed into a slender petiole, lanceolate, oblanceolate or oval, acutish. at the apex, densely pubescent, sinuate-dentate, often densely ciliate with white hairs, repand or pinnatifid, 3'-8' long, usually less than 1' wide; flowers few, white or rose, 1 1/2'-3' broad; petals obcordate; lobes of the calyx pubescent, narrowly lanceolate, reflexed-spreading, its tube 2'-7' long, dilated at the mouth, many times longer than the ovary; capsule sessile, ovoid, strongly tuberculate on each side, the angles ribbed; seeds densely and minutely tuberculate.
Plains, South Dakota to Nebraska, Colorado and Utah. June-July.