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Published In: Species Plantarum 2: 909. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

Project Name Data (Last Modified On 7/9/2009)
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Project Data     (Last Modified On 8/10/2009)

 

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29. Centaurea L. (star thistle, knapweed)

Plants annual, biennial, or perennial herbs, sometimes with rhizomes. Stems erect to loosely ascending, unbranched or branched, usually finely angled or longitudinally ridged, not winged, or with slender, nonspiny wings. Leaves alternate and often also basal, sessile to short-petiolate or the basal ones sometimes long-petiolate, sometimes decurrent into slender wings with the margins unlobed and not spiny, the blades entire to deeply pinnately lobed. Inflorescences terminal, of solitary (clustered elsewhere) heads or panicles, the heads long-stalked to nearly sessile. Heads discoid but often appearing radiate, the involucre variously shaped, the florets all appearing similar and perfect or more commonly the marginal ones sterile, with an enlarged, raylike corolla. Receptacle flat or slightly convex, with numerous bristles. Involucral bracts with the body appressed, glabrous or cobwebby-hairy, the margins variously entire to fringed, the tip loosely ascending to spreading or reflexed, often with a spiny or flattened appendage. Florets numerous (as few as 25 in smaller-headed species, as many as 400 in C. americana). Pappus occasionally absent or more commonly of several series of bristles and/or scales, the outermost series shorter than the inner ones, the bristles, when present, with fine, ascending barbs but not plumose, mostly persistent at fruiting. Corollas white, pink, purple, blue, or yellow, often of two types, those of most of the florets discoid, slender, with slender, erect to spreading lobes; those of the marginal florets enlarged, appearing zygomorphic, one of the sinuses between the lobes much deeper than the rest and splitting the upper half of the tube, the portion above the split usually more or less fan-shaped. Fruits more or less oblong in outline, often appearing somewhat narrowed toward the base, somewhat flattened or less commonly somewhat 4-angled in cross-section, the basal portion asymmetrical, often appearing twisted to the side, with a slightly to strongly oblique or lateral attachment scar (only slightly so in C. repens), the surface glabrous or finely hairy, somewhat shiny, with longitudinal lines or stripes. Four hundred and fifty to 650 species, nearly worldwide, most diverse in Europe and Asia.

Generic delimitation in the subtribe Centaureinae Dumort. continues to be controversial. Recent molecular work has been correlated with data from cytology, pollen ultrastructure, and morphology to suggest that the genus is unnatural (Susanna et al., 1995; Wagenitz and Hellwig, 1996; Garcia-Jacas et al., 2000, 2001). On the one hand, plants long recognized in other genera, including Carthamus L. and Cnicus L. (blessed thistle), have been suggested as representing specialized groups nested within the revised concept of Centaurea. On the other hand, a number of species groups traditionally treated within Centaurea might be treated more properly in several segregate genera. For Missouri, at a minimum this would result in the recognition of two such segregates: 1) The two native North American basket flowers, the relatively widespread C. americana and the southwestern C. rothrockii Greenm., would be part of an odd, mostly New World group known as Plectocephalus D. Don; 2) Centaurea repens would be segregated into the monotypic Acroptilon Cass. Unfortunately, the dismemberment of Centaurea is not without problems. The conclusions of Garcia-Jacas et al. (2001) based on their molecular analyses were weakened by poor resolution at the more basal nodes of their phylogeny and by the failure of the DNA of many species groups to amplify for all of the sequences under study. Nomenclaturally, one problem is that the type species, the African C. centaurium L., corresponds to one of the segregate genera, which would require conservation of the name Centaurea with a new type species in order to preserve the traditional usage of that name (Greuter et al., 2001). For the present, it seems prudent to maintain a broad view of Centaurea, while acknowledging that in the future the taxonomic splitting of the genus probably will gain better support.

 

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1 1. Involucral bracts with the margins entire and somewhat papery; fruits nearly symmetrical at the base, the attachment scar appearing only slightly oblique ... 8. C. REPENS

Centaurea repens
2 1. Involucral bracts with the margins spiny, fringed, or irregular, not papery (except in C. cyanus); fruits noticeably asymmetrical at the base, the attachment scar appearing lateral or strongly oblique

3 2. Most or all of the involucral bracts with a spiny tip, the terminal spine longer and/or stouter than any lateral spines that may be present

4 3. Leaves not or only slightly decurrent, the stems angled but not winged; median and outer involucral bracts with the terminal spine 15 mm long; corollas pinkish-purple to reddish purple, rarely white

5 4. Pappus absent or of minute, bristly scales to 0.5 mm long; florets appearing all discoid, the marginal florets similar to the discoid florets but functionally sterile, the corolla 913 mm long; basal and lower stem leaves 2 times pinnately lobed ... 3. C. DIFFUSA

Centaurea diffusa
6 4. Pappus of numerous unequal bristles 35 mm long; florets of 2 types, the discoid florets with the corolla 1822 mm long, the raylike florets with the corolla 2530 mm long; basal and lower stem leaves 1 time pinnately lobed ... 4. C. DILUTA

Centaurea diluta
7 3. Leaves long-decurrent, the stems noticeably slender-winged; median and outer involucral bracts with the terminal spine 530 mm long; corollas yellow

8 5. Corollas 1012 mm long; involucral bracts with the terminal spine 510 mm long ... 5. C. MELITENSIS

Centaurea melitensis
9 5. Corollas 1320 mm long; involucral bracts with the terminal spine 1130 mm long ... 9. C. SOLSTITIALIS

Centaurea solstitialis
10 2. Involucral bracts without a spiny tip, at most with a short, firm tip, more commonly the tip fringed or irregular

11 6. Heads relatively large, the involucre 2545 cm long, wider than long, broadly bell-shaped; pappus bristles 814 mm long ... 1. C. AMERICANA

Centaurea americana
12 6. Heads smaller, the involucre 1218 cm long, longer than to about as long as wide, narrowly cup-shaped or ovoid to nearly spherical; pappus absent or the bristles 0.55.0 mm long

13 7. Principal leaves all deeply lobed, the smaller leaves toward the branch tips and occasionally also a few of the basal leaves entire or toothed ... 10. C. STOEBE

Centaurea stoebe subsp. micranthos
14 7. Leaves with the margins entire or toothed, occasionally a few of the lowermost ones with the margins lobed

15 8. Plants with dense, woolly hairs when young (appearing pale or whitened), the pubescence often partially reduced to woolly or cobwebby tufts at maturity, the leaf undersurface remaining persistently woolly; principal leaves 29 mm wide, linear or narrowly lanceolate; involucral bracts not strongly differentiated into a main body with an apical appendage, the broad, white or brownish- to purplish-tinged margins papery and with coarse, ascending, triangular teeth ... 2. C. CYANUS

Centaurea cyanus
16 8. Plants roughened or hairy with cobwebby hairs when young (not appearing pale or whitened), sometimes glabrous or nearly so at maturity; leaves variable, but at least some of the largest ones 1240 mm wide; involucral bracts differentiated into a smooth-margined green body and a brown to dark brown or black apical appendage, the appendage margins comblike with a fringe of stiff, spreading or loosely upward-curved, parallel bristles

17 9. Involucre about as long as wide; apical appendages of the involucral bracts relatively large, broader than the main body, the intact involucre often appearing solid dark brown to black; florets all discoid ... 6. C. NIGRA

Centaurea nigra
18 9. Involucre longer than wide; apical appendages of the involucral bracts relatively small, as wide as or narrower than the main body, the intact involucre with at least some green coloration visible; marginal florets usually raylike ... 7. C. NIGRESCENS Centaurea nigrescens
 
 
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