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..
Glabrous annual or perennial herbs, with opposite linear and undivided, or finely dissected leaves, and long-peduncled heads of both tubular and radiate flowers, or the rays wanting. Involucre hemispheric or campanulate, of 2 distinct series of bracts, the outer short, narrow and somewhat spreading, the inner united nearly to or beyond the middle into a cup, their tips scarious-margined. Receptacle flat, chaffy, the 2-nerved broad white scarious chaff subtending the disk-flowers and achenes. Ray-flowers, when present, neutral, the rays yellow, entire or toothed. Disk-flowers perfect, fertile, their corolla with a slender tube and 5-toothed limb. Anthers obtuse and entire at the base. Style-tips acute. Achenes, some or all of them, papillose on the back, oblong or linear, slightly compressed or terete, wingless. Pappus of 2 retrorsely hispid awns or scales, or sometimes none. [Greek, nipple-seed.]
About 10 species, natives of the south central United States, Mexico and southern South Amer ica. Type species: Thelesperma scabiousoides Less.
Rays large; pappus-awns shorter than the width of the achene.
Leaves not rigid, their segments filiform-linear; annual or biennial.
1. T. trifidum.
Leaves rigid, their segments linear; perennial.
2. T. intermedium.
Rays inconspicuous, or none; awns longer than the width of the achene; perennial.
3. T. gracile.
Sci. 9: 182. 1890.
Annual or biennial; stem branched, 1°-3° high. Leaves numerous, not rigid, 1 1/2' - 2' long, bipinnately divided into filiform or linear-filiform segments; heads several or numerous, 12"- 15" broad; outer bracts of the involucre about 8, subulate-linear, equalling or more than half as long as the inner, which are united not higher than the middle; rays 6-10, somewhat spatulate, 3-lobed; disk purple or brown; achenes linear-oblong, straight, or slightly curved, the outer ones strongly papillose; awns of the pappus not longer than the width of the summit of the achene.
In dry soil, South Dakota, Missouri and Nebraska to Colorado, Texas, New Mexico and northern Mexico. June-Aug.
Perennial from a deep woody root and slender rootstocks; stem rigid, usually much branched, 1°- 1 1/2° high. Leaves usually numerous, 1 1/2 '- 2' long, bipinnately divided into entire, rigid, linear segments, but less compound than those of the preceding species; outer bracts of the involucre lanceolate-subulate, usually much shorter than the inner ones, which are united to about the middle: rays and achenes similar to those of the preceding.
In dry soil, on plains, Nebraska and Wyoming to Colorado and New Mexico. In our first edition included in T. ambiguum A. Gray, of the Southwest. June-Aug.
Bidens gracilis Torr. Ann. Lyc. N. Y. 2: 215. 1827. T. gracile A. Gray, Kew. Journ. Bot. 1: 252. 1849.
Perennial from a deep root; stem rigid, branched, 1°-3° high, the branches nearly erect. Leaves rigid, erect or ascending, 2-3' long, pinnately or bipinnately divided into linear segments, or the upper linear and entire; heads 6"- 10" broad; rays usually none, sometimes present and 2"-3" long; outer bracts of the involucre 4-6, oblong or ovate, mostly obtuse, very much shorter than the inner ones, which are united to the middle or beyond; disk yellow or brownish; outer achenes slightly papillose; pappus awns longer than the width of the summit of the achene.
On dry plains, Nebraska and Wyoming to Texas, northern Mexico and Arizona. May-Aug.