Thistles are annual, biennial, or perennial spiny herbs with alternate, toothed to more often pinnately lobed leaves and solitary to numerous smallish to large flowerheads. The flowerheads have disk flowers only, the flowers being all trumpet-shaped and with both stamens and pistils, or the plants sometimes have male and female flowers on separate plants by abortion. The involucral bracts are arranged in several series, about equal in length or usually more or less partly overlapping, usually some or all of them being spine-tipped, and in many species with a thickened, sticky-covered outer ridge. The receptacle is flat to slightly conic, densely bristly. The corollas are purple or reddish to yellowish or white, with slender tube and long narrow lobes. The filaments are usually covered with very small bumps. The anthers are tail-tipped at the base. The style has a thickened, minutely hairy ring and an abrupt change of texture below the branches, which are joined nearly throughout. The stigma-bearing lines are marginal, extending nearly or quite to the barely differentiated free tips. The achenes are hairless, attached by the base or nearly so, 4-angled or flattened, 4- to many-nerved. The pappus consists of feathery bristles (or that of the outermost flowers merely finely barbed), dropped in a ring. The genus consists of about 200 species worldwide, native to the n. hemisphere. About 50 are native to N. America. The name comes from the Greek kirsos, a swollen vein, for which thistles, called kirsion, were a reputed remedy.
Guide to Identify Presented Species of Genus Cirsium
FLOWERHEADS IN STALKED CLUSTERS
C. arvense Canada Thistle Perennial, 30-150 cm tall. Stems leafy, branched, not winged. Disturbed areas.
Flowerheads pink to purple, 1-2 cm high, often with weak, short spine tips..
Leaves alternate, lance-shaped, spiny-toothed, deeply pinnately lobed.
C. vulgare Bull Thistle Biennial, 30-150 cm tall. Stems branched, spiny-winged. Disturbed areas.
Flowerheads pink to purple, 2.5-4 cm high, without glutinous dorsal ridges.
Leaves alternate, lance-shaped, hairy and spiny-toothed, deeply pinnately lobed.