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..
Perennial or annual herbs, with alternate entire dentate incised or pinnatifid leaves, and large or middle-sized heads, of tubular purple violet white or rarely yellow flowers. Involucre ovoid or globose, its bracts imbricated in many series, appressed, fimbrillate, or dentate. Receptacle flat, densely bristly. Marginal flowers usually neutral and larger than the central ones, which are perfect and fertile, or flowers all perfect and fertile in some species. Corolla-tube slender, the limb regular or oblique, 5-cleft or 5-lobed, the segments sometimes appearing like rays. Anthers sagittate at the base. Style-branches short, somewhat connate, obtuse. Achenes oblong or obovoid, compressed or obtusely 4-angled, usually smooth and shining, obliquely or laterally attached to the receptacle, surmounted by a disk with an elevated margin. Pappus of several series of bristles or scales, rarely none. [Greek, of the Centaurs, who were said to use it in healing.]
About 350 species, mostly natives of the Old World. Type species: Centaurea Centaurium L.
Bracts of the involucre lacerate or fimbriate, not spiny.
Heads 2' broad or less; achenes laterally attached; introduced species. Annual; leaves entire.
1. C. Cyanus.
Perennials or biennials, or C. maculosa annual. Bracts of the involucre laciniate or entire.
2. C. Jacea.
Bracts of the involucre, or their tips, pectinate-fringed.
Lower bracts of the involucre pectinate-fringed to below the middle. Leaves entire or merely dentate.
3. C. nigra.
4. C. Scabiosa.
Lower bracts of the involucre pectinate-fringed only at the tips. Leaves entire, toothed, or the lower lyrate.
5. C. vochinensis.
All but the upper leaves pinnatifid into linear segments.
6. C. maculosa.
Heads 2'-4' broad; achenes obliquely attached: native western species.
7. C. americana.
Bracts of the involucre tipped with stout spines. Flowers purple; stem wingless.
8. C. Calcitrapa.
Flowers yellow; stem winged by the decurrent leaf-bases. Spines slender, purplish, 5" long or less, branched below.
0. C. melitensis.
Spines stout, yellow, 6"-10" long, with smaller ones at the base.
10. C. solstittahs.
Cenlaurea Cyanus L. Sp. Pl. 911. 1753.
Annual, woolly, at least when young; stem leafy, slender, branched, 1°-2 1/2° high, the branches ascending. Leaves linear or linear-lanceolate, mucronate, 3'-6' long, the basal and lower ones mostly remotely dentate, the upper, or sometimes all of them, entire; heads l'-1 1/2' broad, on long naked peduncles; involucre campanulate, its bracts greenish-yellow, or the inner with darker tips and margins, appressed, fimbriate with scarious teeth; flowers blue, purplish, pink or white, the marginal ones neutral with large radiant corolla-limbs; achenes slightly compressed, or 4-angled; pappus bristles unequal, nearly as long as the achene.
In waste places, escaped from gardens, and in ballast, Quebec to Ontario, western New York, Nebraska and Virginia. Witches'-bells or -thimbles, corncentaury, corn-bottle or -binks. Brushes. Hurt-sickle. Blue-bonnets. Blaver. Blue poppy. Bachelor's-buttons. Blue caps. Barbeau. French pink. July-Sept.