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Centaurea
See list of 13 species in this genusCentaurea 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).
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1a. Involucral bracts tipped by 1 or more spines [Fig. 387]; disk corollas yellow, white to pink, or purple
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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
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2b. Capitula sessile or borne on peduncles, not concealed by foliaceous bracts; pappus of capillary bristles or absent
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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
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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)
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4a. Disk corollas pale purple, pink-purple, or purple; stem leaves not decurrent
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4b. Disk corollas yellow; stem leaves long-decurrent
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5a. Apical spine of involucral bracts 10–15 (–30) mm long; marginal flowers of capitulum lacking pappus; disk corollas eglandular; pappus of inner flowers mostly 3–5 mm long, ca. twice as long as cypsela body
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5b. Apical spine of involucral bracts 5–8 mm long; marginal flowers of capitulum with pappus; disk corollas glandular; pappus of inner flowers mostly 1.5–3 mm long, ca. as long as cypsela body
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1b. Involucral bracts without spine-tips; disk corollas white to pink, red, or purple, or blue
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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
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7a. Plants annual; stem leaf blades linear
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7b. Plants perennial; stem leaf blades oblong or lanceolate to ovate
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8a. Appendage of involucral bracts decurrent nearly to the base of the bract; leaf blades entire (rarely the lower ones obscurely dentate); involucre 10–15 mm wide
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8b. Appendage of involucral bracts decurrent only in the apical half of the bract [Fig. 389]; leaf blades pinnatifid; involucre narrower than 10 mm or wider than 15 mm
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9a. Involocural bracts lacking prominent, longitudinal veins; pappus (3–) 4–5 (–6) mm long; involucre 18–25 mm wide; cypsela bodies 4–5.5 mm long; corollas of fertile flowers 20–25 mm long
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9b. Involucral bracts with several, prominent longitudinal veins [Fig. 389]; pappus 1–2.5 (–3) mm long (rarely absent); involucre 6–8 mm wide; cypsela bodies 2–3.5 mm long; corollas of fertile flowers 12–15 mm long
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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)
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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
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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]
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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)
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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
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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
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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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Each photo represents one species in this genus.