2. Xanthium strumarium L. (common cocklebur)

X. strumarium var. canadense (Mill.) Torr. & A. Gray

X. italicum Moretti

X. pensylvanicum Wallr.

X. speciosum Kearney

X. varians Greene

X. wootonii Cockerell

X. strumarium var. glabratum (DC.) Cronquist

X. chinense Mill.

X. inflexum Mack. & Bush

Pl. 292 g, h; Map 1242

Stems 10–150(–200) cm long, sparsely to moderately roughened-pubescent with short, stout, broad-based, ascending hairs and often also with minute glandular hairs or inconspicuous, sessile glands, spineless. Leaves all or mostly long-petiolate. Leaf blades 2–18 cm long, broadly ovate or ovate-triangular to more or less kidney-shaped or nearly circular, mostly shallowly to deeply cordate at the base (often broadly short-tapered to the petiole within the notch), variously rounded to bluntly or sharply pointed at the tip, unlobed or with 3 or 5 usually shallow, broad, palmate main lobes, the margins otherwise coarsely and irregularly toothed, the surfaces sparsely to moderately roughened with short, stout, loosely appressed, broad-based hairs, sometimes only along the veins, usually also glandular, the undersurface not appearing whitened. Pistillate heads mostly 2–4 per spike, the bur 15–30(–35) mm long, the beaks 2, usually relatively prominent and incurved, the surface variously (see below) nearly glabrous or with sessile glands, stalked glands, and/or short, spreading to loosely appressed hairs, also with numerous slender to slightly broad-based spines, these hooked or curled at the tip. 2n=36. July–November.

Scattered to common throughout the state (nearly worldwide, but probably introduced in the Old World). Banks of streams, rivers, and spring branches, swamps, sloughs, bottomland prairies, bottomland forests, and margins of ponds, lakes, sinkhole ponds, marshes, fens, and seeps; also fallow fields, crop fields, pastures, barnyards, ditches, railroads, roadsides, and open, disturbed areas.

Patterns of morphological variation in X. strumarium are very subtle. Characters of the burs used to differentiate variants within the complex have included size, color, relative length and shape of the beaks, spine density and pubescence, and pubescence of the bur surface. None of these features can be observed if mature burs are absent from a given sample. Löve and Dansereau (1959) suggested that recent expansion of the ranges of formerly more isolated entities followed by hybridization in mixed or adjacent populations accounts in part for the difficulties in distinguishing taxa within the group. Wiegand (1926) was the first botanist to combine the various North American taxa into a single species (under the name X. orientale L.) but listed four informal subgroups occurring in the northeastern United States. Cronquist (1945) formalized Wiegand’s treatment into three varieties of X. strumarium, combining two different hairy-burred entities listed by Wiegand into a single variety and noting the presence of common intermediates between his taxa. Löve and Dansereau (1959) treated X. strumarium in a provisional (informal) sense hierarchically to include two major subunits (which they suggested might be subspecies) divided into nine provisional varieties and with three additional putative intervarietal hybrids. Infraspecific combinations for most of their taxa have yet to be validly published. In Missouri, for those specimens with mature burs, there appear to be more specimens intermediate for any given differentiating character than there are morphological extremes. Thus no attempt has been made in the present treatment to segregate varieties. For those who enjoy tormenting themselves with attempts to assign infraspecific names to cocklebur specimens, the following key has been adapted from that of Cronquist (1945). Of the Missouri specimens that can be keyed successfully, there are no discernable differences in abundance or distribution between the two supposedly distinct varieties. In Cronquist’s treatment, var. strumarium, which is most common in Europe but occurs sporadically in the United States (not reported from Missouri), differs from the two widespread varieties in its yellowish green burs mostly 15–20 mm long with straight beaks and minute pubescence. For convenience, in the list of synonyms of X. strumarium above, the specific epithets applied to Missouri plants by Steyermark (1963) are in two groups under the corresponding varietal names in Cronquist’s classification.