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.