5. Centaurea melitensis L. (Maltese star thistle)
Pl. 251 i; Map
1044
Plants annual,
with a taproot, pubescent with cobwebby to woolly hairs, often appearing
somewhat pale or whitened, at least when young. Stems 10–60 cm long, erect or
ascending, unbranched or with loosely ascending branches, noticeably winged,
the wings with irregular and sometimes broadly toothed margins. Leaves 1–15 cm
long, often dotted with minute, yellow to brown resin glands; basal and lower
stem leaves with the blades mostly 5–35 mm wide, oblong to oblanceolate,
usually rounded at the tip, tapered gradually to a sessile or short-petiolate
base, deeply pinnately lobed, usually withering by flowering time; median and
upper stem leaves gradually reduced, mostly sessile, the base strongly
decurrent, the blades linear to oblong-lanceolate, entire or toothed. Heads
solitary or in small clusters of 2–4 at the branch tips (rarely on short,
axillary branches and thus appearing lateral). Involucre 10–15 mm long
(excluding spines), longer than to about as long as wide, broadly ovoid to
nearly spherical. Lower and median involucral bracts with the body ovate, the
margins entire, the outer surface more or less cobwebby-hairy, not concealed by
the appendages; the apical appendage well differentiated, spreading, more or
less narrower than the main body, straw-colored and commonly purplish-tinged,
the involucre with at least some green coloration easily visible (sometimes
becoming entirely straw-colored with age), the margins with 1 or 2 pairs of
short, spreading, spinelike, lateral bristles and a central spine, this 5–10 mm
long. Upper involucral bracts lanceolate, the appendages papery, tapered,
short-spined or merely irregularly toothed at the tip. Florets all discoid and
similar or the marginal florets similar in appearance but functionally sterile.
Pappus of many unequal bristles, these 2–3 mm long, white, usually persistent
at fruiting. Corollas 10–12 mm long, bright yellow. Fruits 2.5–3.5 mm long,
somewhat flattened, the attachment scar appearing lateral, the surface
yellowish brown to brown with fine, lighter, longitudinal stripes, glabrous or
more commonly finely hairy, sometimes somewhat shiny. 2n=24. June–September.
Introduced,
known only from historical collections from Jackson County (native of Europe,
Africa; introduced widely in the western U.S. and adjacent Canada, more
sporadically farther east). Railroads, roadsides, and open, disturbed areas.