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Centaurea

(Genus)

Overview

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Annuals, biennials, or perennials, 20-300 cm, glabrous or tomentose. Stems erect, ascending, or spreading, simple or branched. Leaves basal and cauline; petiolate or sessile; proximal blade margins often ± deeply lobed, (spiny in C. benedicta ), distal ± smaller, often entire, faces glabrous or ± tomentose, sometimes also villous, strigose, or puberulent, often glandular-punctate. Heads discoid, disciform, or radiant, borne singly or in corymbiform arrays. Involucres cylindric or ovoid to hemispheric . Phyllaries many in 6-many series, unequal, proximal part appressed, body margins entire. distal parts expanded into erect to spreading, usually ± dentate or fringed, linear to ovate appendages, spine. tipped or spineless. Receptacles flat, epaleate, bristly. Florets 10-many; outer usually sterile, corollas slender and inconspicuous to much expanded, ± bilateral; inner fertile, corollas white to blue, pink, purple, or yellow, bilateral or radial, often bent at junction of tubes and throats, lobes linear-oblong, acute; anther bases tailed, apical appendages oblong; style branches: fused portions with minutely hairy nodes, distinct portio ns minute. Cypselae ± barrel-shaped, ± compressed, smooth or ribbed, apices entire (denticulate in C. benedicta ), glabrous or with fine, 1-celled hairs, attachment scar. lateral (with or without elaiosomes) ; pappi 0 or ± persistent, of 1-3 series of smooth or minutely barbe d, stiff bristles or narrow scales . x = 8, 9, 10, 11, 12, 13, 15.

Species ca. 500: introduced; Eurasia, n Africa, widely introduced worldwide.

Taxonomic limits of Centaurea have been controversial. The genus has great morphologic diversity, and studies have revealed much cytologic (e.g., N. Garcia-Jacas et al. 1996) and palynologic (e.g., G. Wagenitz 1955) variation as well. During the nineteenth and twentieth centuries, various taxonomists attempted, with limited success, to divide Centaurea into smaller genera or workable infrageneric taxa. The relations of several satellite genera have been controversial as well.

Recent molecular phylogenetic studies (A. Susanna et al. 1995; N. Garcia-Jacas et al. 2000, 2001) have begun to clarify relationships within Centaurea and between Centaurea and other genera. These studies make it clear that Centaurea as traditionally defined is polyphyletic, and that generic boundaries should be realigned if monophyletic taxa are to be recognized. Some taxa traditionally included within Centaurea (e.g., the two native North American species, Centaurea americana and C. ro throckii) fall outside the redefined generic boundaries and are here treated in Plectocephalus. Others usually placed into segregate genera (e.g., Cnicus benedictus) are firmly nested within Centaurea. Because the type species of Centaurea (C. centaurium Linnaeus, an African species) falls outside the main lineage of the genus, a proposal has been made to conserve Centaurea with a different type species (W. Greuter et al. 2001), thereby maintaining the nomenclatural stability of most of the numerous species that do fall within the principal Centaurea clade.

Although several Centaurea species are widely established as members of the North American flora, and some of these are widely distributed invasive weeds, some of the taxa listed by J. T. Kartesz and C. A. Meacham (1999) are apparently waifs and not permanent members of the flora. These taxa are discussed informally immediately below.

Although Cnicus has usually been recognized as a distinctive monotypic genus, it has been merged into Centaurea by various authors (e.g., K. Bremer 1994; G. Wagenitz and F. H. Hellwig 1996) . Recent molecular systematic studies (N. Garcia-Jacas et al. 2000) provide additional evidence that it is nested within Centaurea.[1]

Photos

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Taxonomy

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The Genus Centaurea is further organized into finer groupings including:

Bibliography

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Footnotes

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  1. David J. Keil, Jörg Ochsmann "Centaurea". in Flora of North America Vol. 19, 20 and 21 Page 52, 57, 58, 67, 83, 84, 96, 171, 172, 176, 177, EFloras.org.

Sources

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Last Revised: September 22, 2009
2009/09/22 16:56:43