104. Vernonia Schreb.
Plants with a stout,
often short-rhizomatous rootstock. Stems 1 to several, several-branched toward
the tip, erect or ascending, usually with fine longitudinal lines or ridges.
Leaves alternate (basal leaves usually absent at flowering), sessile or
short-petiolate, the margins toothed (occasionally entire in V. arkansana),
the undersurface sometimes dotted with minute, impressed resin glands.
Inflorescences irregularly branched terminal panicles, sometimes appearing
somewhat flat-topped, the heads not grouped into secondary headlike clusters at
the branch tips. Heads sessile to long-stalked, with 9 to more commonly
numerous florets. Involucre urn-shaped to short-cylindrical, hemispherical or
somewhat bell-shaped, the bracts in several overlapping series, the inner series
progressively longer, variously shaped, flattened or with a slightly raised
midvein dorsally, green or more commonly purplish-tinged to uniformly dark
purple. Pappus of an inner series of numerous capillary bristles and an outer
series of minute scales or bristles, persistent at fruiting, with minute,
ascending barbs. Corollas reddish purple to purple or rarely white, relatively
deeply lobed, the sinuses between the lobes all similar. Fruits narrowly oblong
to narrowly oblong-triangular in outline, not flattened, with 8–10
relatively narrow ribs, usually hairy, at least along the ribs, often with
minute resin dots between the ribs, grayish brown to brown. About 500 species,
North America to South America, West Indies, Asia, Africa.
The taxonomy of Vernonia
has not yet stabilized. Robinson (1999) studied the American species and
confirmed the conclusions of earlier workers that the traditional, broad
concept of Vernonia was unnatural. He suggested that Vernonia in
the strict sense should be restricted to about 20 species of North America and
the Bahamas plus 2 temperate South American species, but he noted that further
studies were still necessary to assess the numerous Old World taxa.
ironweed may be attributable to the grayish cast of plants in some of the
species, which is caused by the relatively dense pubescence on the leaves and
sometimes also the stems. In the Midwest, ironweeds are a familiar sight in
overgrazed pastures, presumably because the plants are unpalatable to cattle.
Most species of Vernonia produce toxic sesquiterpene lactones, although
the midwestern species have not yet been implicated directly in livestock or
human poisoning. Lactones (vernoniosides, especially vernolepin) extracted from
the African V. hymenolepis A. Rich. have been shown to inhibit tumor
formation (Burrows and Tyrl, 2001). Some of the North American species were
used medicinally by Native Americans, principally as pain relievers (Moerman,
note that the North American ironweeds have a notorious reputation for
promiscuity, and that the numerous hybrids likely to be encountered in nature
are fertile. In Missouri, all possible parental combinations have been recorded
among the five species growing in the state, although some hybrids are less frequent