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.