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.