This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", 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..
[Riddellia Nutt. Trans. Am. Phil. Soc. (II) 7: 271. 1841.] Branched annual or perennial woolly herbs, often nearly glabrous when old, with alternate leaves, and middle-sized heads of both tubular and radiate yellow or orange flowers, corymbose, or clustered at the ends of the branches. Involucre cylindraceous, its bracts 4-10 in 1 series, narrow, equal, densely white-woolly, separate, but erect and connivent, commonly with 1-4 scarious ones within, and occasionally a narrow outer one. Rays broad, becoming papery and whitish, persistent, 5-7-nerved, 2-3-toothed, pistillate. Receptacle small, naked. Disk-flowers perfect, fertile, their corollas with a short proper tube and elongated cylkndra-ceous limb, 5-toothed, the teeth glandular-bearded. Anthers obtuse and entire at the base. Style-branches of the disk-flowers capitellate. Achenes linear, striate. Pappus of 4-6 nerveless acute scales, glabrous or villous. [Greek, referring to the naked receptacle.]
About 7 species, natives of the southwestern United States and northern Mexico. Type species: Psilostrophe gnaphalodes DC.
Perennial, branched, 6'-2° high, loosely white-woolly. Basal and lower leaves spatulate, entire, dentate or rarely pinnatifid, mostly obtuse, 2'-4' long; upper leaves sessile, or nearly so, smaller, linear to spatulate, usually entire; heads several together in the clusters, 4"-6" broad, short-peduncled; rays few, lemon-yellow, commonly as wide as long, with 2 or 3 broad teeth or lobes at the summit; achenes glabrous, or sparingly pubescent; pappus scales linear-lanceolate to oblong-lanceolate, glabrous, shorter than the disk-corollas.
In dry sandy soil, Kansas to Texas and Arizona. Included, in our first edition, in P. Tagetinae (Nutt.) Kuntze, which has much larger yellow rays. June-Sept.