30. Cirsium Mill. (thistle)

Plants biennial or perennial. Stems erect, branched or less commonly unbranched, in some species noticeably spiny-winged, in unwinged species angled or ridged, variously glabrous or hairy. Leaves basal and alternate, spiny along the margins, usually densely pubescent with white, woolly hairs on both surfaces or only the undersurface, those of the basal rosette with the margins scalloped or toothed but often unlobed, the stem leaves often pinnately lobed, in wing-stemmed species the leaf bases decurrent along the stem, the wings wavy or scalloped to evenly lobed, spiny along the margins. Inflorescences terminal on the branches, the heads long-stalked to nearly sessile, solitary or in small clusters at the branch tips. Heads discoid, the involucre variously shaped, the florets all appearing similar and perfect (or the plants incompletely dioecious in C. arvense). Receptacle flat or short-conical, with numerous bristles. Involucral bracts (except sometimes the innermost ones) tapered to a spiny tip (spineless or nearly so in C. muticum). Pappus of numerous often unequal, long bristles (the marginal florets sometimes with somewhat fewer, less plumose bristles), these fused at the base, plumose (featherlike with numerous long, capillary side branches), shed more or less as a unit before fruiting. Corollas cream-colored or pink to purple, rarely white. Fruits appearing basally or more commonly somewhat obliquely attached, oblong or slightly narrower at the usually somewhat asymmetrical base, often slightly curved or arched in profile, somewhat flattened and sometimes slightly 4-angled in cross-section, the tip usually with an angular rim or raised crown surrounding a small, knoblike or conical projection, the surface somewhat shiny, straw-colored or light brown, grayish brown or brown. Two hundred to 350 species, widespread in the Northern Hemisphere.

In addition to the species treated below, C. hillii (Canby) Fernald (C. pumilum ssp. hillii (Canby) R.J. Moore & Frankton) should be searched for in eastern Missouri. Steyermark (1963) misdetermined two historical specimens of this taxon from the St. Louis metropolitan region as C. pumilum (Nutt.) Spreng. but correctly noted that they were actually collected in St. Clair County, Illinois. On that basis, he excluded these specimens from the Missouri flora. In 1979, Marlin L. Bowles of the Morton Arboretum (Lisle, Illinois) photographed a plant of probable C. hillii in a small remnant upland prairie strip between a road and a railroad near the town of Ely in Ralls County. However, the photographs were insufficient in detail to confirm with certainty the identity of the species. Perhaps because of construction in the vicinity, repeated attempts by several botanists to relocate Hill’s thistle at this site have failed. The main distribution of C. hillii is to the north and east of Missouri (Moore and Frankton, 1966), but western Illinois populations are known from sites immediately adjacent to northeastern Missouri. The species will not key well below. Cirsium hillii is a perennial with long, thickened, hollow roots giving rise to stout stems. The stem leaves are generally narrowly oblong-elliptic, pinnately lobed with relatively short, irregularly triangular lobes, and with the undersurface finely hairy or cobwebby, but the green color not persistently hidden by the pubescence. The heads are relatively large, with the involucre 3.5–5.0 cm long and the narrow involucral bracts having a slender, sticky dorsal ridge (the outer and median ones are tapered to a short, slender, ascending, spiny tip).

In most of our thistles, the heads are subtended by one to several small leaves and thus appear sessile or short-stalked. Unlike the situation in some western species, these leaves are relatively small and few-lobed in the taxa occurring in Missouri. In C. carolinianum, the accessory leaves are highly reduced and widely spaced along the long stalk, and the heads thus appear long-stalked.