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Published In: Species Plantarum 2: 987. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

Project Name Data (Last Modified On 8/11/2017)
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Project Data     (Last Modified On 8/10/2009)

 

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95. Xanthium L. (cocklebur)

Plants annual, with taproots. Stems usually few- to many-branched, erect or ascending, sometimes from a spreading base, finely ridged or grooved, glabrous or sparsely to moderately roughened or short-hairy. Leaves mostly alternate, sometimes opposite at the basal few nodes, sessile to long-petiolate. Leaf blades variously shaped, unlobed or more commonly with 3–7 main lobes, the margins entire to somewhat wavy or coarsely and irregularly toothed, the surfaces glabrous to densely hairy. Inflorescences of separate staminate and pistillate heads in small, dense axillary spikes (the uppermost spikes sometimes entirely staminate, the lowermost spikes occasionally entirely pistillate), the staminate heads usually several toward the tip of the spike, the pistillate heads usually solitary or in a small cluster of 2–4 at the base of the spike, each subtended by a small bract. Heads discoid (but this not evident in pistillate heads), the staminate heads spreading in several directions. Involucre of the staminate heads 2–4 mm in diameter, saucer-shaped, symmetrical, the 6–18 involucral bracts in 1–3 series, free to the base, green. Involucre of the pistillate heads with the main body ellipsoid to oblong-ellipsoid, the involucral bracts several to numerous in several overlapping series, closely enclosing the florets and all but the outermost few fused into a bur, the outer surface with sharp spines that are hooked or curled at the tip, often more or less 1- or 2-beaked at the tip (where an opening allows exsertion of the stigma), green, turning brown to orangish brown or brownish yellow at fruiting. Receptacle of the staminate heads conical (impossible to observe in pistillate heads), somewhat elongating as the florets mature, with chaffy bracts subtending the florets, these narrowly linear to narrowly lanceolate, usually hairy and sometimes also glandular, not wrapped around the florets. Staminate heads with 20–60 disc florets, these with a minute, nonfunctional ovary and undivided style, the stamens with the filaments more or less fused into a tube and the anthers free but positioned closely adjacent to one another in a ring, the corolla 1–2 mm long, narrowly bell-shaped to nearly tubular, shallowly 5-lobed, white to pale yellow, sometimes purplish-tinged toward the tip, usually minutely hairy and often also glandular. Pistillate heads with 2 florets in separate chambers, the corolla absent. Pappus of the staminate and pistillate florets absent. Fruits 8–11 mm long, narrowly elliptic to narrowly oblanceolate in outline, somewhat flattened with rounded angles, grayish brown to dark gray or black, nearly smooth or more commonly with noticeable branched veins, glabrous, often somewhat shiny, completely enclosed in the persistent pistillate involucre and dispersed as an intact bur. Three species, nearly worldwide, but probably introduced in the Old World.

Based on a cladistic analysis of morphological data, Karis (1995) suggested that Xanthium might best be treated as a specialized subgroup within Ambrosia. However, this was in disagreement with the results of earlier, similar studies (Bolick, 1983), and molecular studies of chloroplast DNA variation (Miao et al., 1995b) also did not confirm the hypothesis.

The species-level taxonomy of Xanthium remains somewhat controversial. More than 50 species have been accepted by various authors in different parts of the world (over 20 in North America), differentiated mostly on subtle details of bur morphology. Beginning with Wiegand (1926) and Cronquist (1945), some North American authors began questioning the validity of recognizing most of the taxa present on the continent at other than infraspecific levels. The classification followed here is adapted from that proposed by Löve and Dansereau (1959), who accepted only two well-marked species originally described by Linnaeus in 1753. Steyermark (1963) discussed the confusing taxonomic situation within the genus and preferred to recognize eight species in Missouri, seven of which are here considered part of the complex pattern of ecological and morphological variation within X. strumarium. The third species in the genus is X. ambrosioides Hook. and Arn., an Argentinean taxon related to X. spinosum but differing in its more or less spreading stems, more finely divided leaves, and densely hairy burs. Löve and Dansereau (1959) suggested that this taxon might better be treated as a variety of X. spinosum but did not discuss their reasons for doing so.

The burs of Xanthium can be hazardous to livestock, causing mechanical injury and sores to mouth parts and other soft tissues, as well as blockages in the throat and intestines. Additionally, the embryonic tissue in the seed and the cotyledons of the seedling contain highly toxic diterpene glycosides such as carboxyatractyloside (gummiferin), which are poisonous to mammals and birds when ingested at even low concentrations (Burrows and Tyrl, 2001). Ecologically, Xanthium is a colonizer of disturbed open-soil habitats with an interesting behavior in that one of the two fruits within each bur generally germinates quickly and readily, with the other fruit waiting in reserve in case the first seedling fails to survive. Xanthium strumarium (in the broad sense) has been a model species for physiological research on seed germination and plant phenology. In particular, the physiological and biochemical mechanisms that result in the onset of flowering in response to shortening day length have been well documented (see Löve and Dansereau [1959] for a review of the older literature) in various populations at different latitudes.

 

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1 1. Stems with prominent, slender, 3-parted spines at the nodes; leaves lanceolate to more or less elliptic; burs 9–13 mm long, the beak absent or 1, short and straight ... 1. X. SPINOSUM

Xanthium spinosum
2 1. Stems lacking spines; leaves broadly ovate to more or less kidney-shaped or nearly circular; burs 15–30 mm long, the beaks 2, relatively prominent and somewhat incurved at the tips ... 2. X. STRUMARIUM Xanthium strumarium
 
 
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