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Centaurea

See list of 13 species in this genus

Centaurea is a difficult genus that is plagued by misapplication of names (e.g., C. maculosa, 
 C. pratensis). Report of Centaurea bovina Velen. in New England by Kartesz (1999) was probably based on a collection of C. diffusa. The report of Plectocephalus americanus (Nutt.) D. Don in Sweet by George (1992) was based on a specimen of C. nigra (Fernald 1922a). Reference: Keil and Ochsmann (2006).

  • 1a. Involucral bracts tipped by 1 or more spines [Fig. 387]; disk corollas yellow, white to pink, or purple
    • 2a. Capitula sessile, closely subtended and partially concealed by a series of non-spine-tipped, foliaceous bracts; pappus composed of 2 series of awns—the outer series smooth or minutely barbellate and 9–10 mm long, the inner series minutely pubescent with spreading hairs and 2–5 mm long
    • 2b. Capitula sessile or borne on peduncles, not concealed by foliaceous bracts; pappus of capillary bristles or absent
      • 3a. Apical spine of involucral bracts 2–3 (–5) mm long; involucre 4–5 mm wide; flowers lacking pappus; apical appendage of involucral bracts decurrent on body of bract, therefore the base of the appendage concave; disk corollas usually white to pink or pale purple; stems angled but not winged
      • 3b. Apical spine of involucral bracts 5–25 (–30) mm long; involucre 7–12 (–15) mm 
wide; at least the central flowers of a capitulum with pappus; apical appendage 
of involucral bracts not decurrent on body of bract, therefore the base of the appendage ± truncate; disk corollas yellow or varying shades of purple; stems with decurrent wings (except C. calcitrapa)
  • 1b. Involucral bracts without spine-tips; disk corollas white to pink, red, or purple, or blue
    • 6a. Apical appendage of involucral bracts decurrent on body of bract, therefore, the base of the appendage concave [Fig. 389]; principal stem leaf blades entire or conspicuously pinnatifid with linear to oblong lobes
    • 6b. Apical appendage of involucral bracts not decurrent on body of bract, therefore the base of the appendage ± truncate [Fig. 388]; principal stem leaf blades entire to toothed (infrequently with a few small lobes on larger leaf blades)
      • 10a. Apical appendage of involucral bracts often recurved, long-tapering to a filiform-fringed tip, the rachis of the appendage 0.2–0.4 mm wide on the middle bracts
      • 10b. Apical appendage of involucral bracts ascending to appressed (rarely some with a loosely spreading apex), obtuse to acute at tip, the rachis of the appendage 0.5–3.5 mm wide on the middle bracts [Figs. 386,388]
        • 11a. Apical appendage of involucral bracts light brown to brown, those of the middle and outer series irregularly lacerate, those of the inner series often bifid; pappus absent; outer flowers of capitulum usually enlarged and falsely appearing as ray flowers (but the corollas still actinomorphic)
        • 11b. Apical appendage of involucral bracts brown to black, those of the middle and outer series regularly pectinate-fringed [Fig. 388], usually none of them bifid; pappus absent or present and then consisting of unequal bristles mostly 0.5–1 mm long; outer flowers of capitulum enlarged or not
          • 12a. Apical appendage of involucral bracts 1–2 (–2.2) mm long, with 5–8 fringe segments on each margin, relatively narrow, the appendage not completely obscuring the distal portion of adjacent involucral bracts; involucre definitely taller than wide in life, ca. 1.5 times as tall as wide; outer flowers of capitulum sometimes enlarged and falsely appear as ray flowers
          • 12b. Apical appendage of involucral bracts longer than 2 mm, the larger ones (3–) 4–6 mm long, with 7–15 fringe segments on each margin [Fig. 388], relatively broad, obscuring the distal portion of the adjacent involucral bracts; involucre nearly as wide or wider than tall in life; outer flowers of capitulum not enlarged

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