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Published In: Genera Plantarum 2: 541. 1791. (Gen. Pl.) Name publication detail

Project Name Data (Last Modified On 8/11/2017)
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Project Data     (Last Modified On 8/21/2009)


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104. Vernonia Schreb. (ironweed)

(Jones and Faust, 1978)

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.

The name 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, 1998).

Readers should 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 than others.


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1 1. Involucral bracts (except the outermost ones) linear to narrowly lanceolate, long-tapered and somewhat curled to a threadlike, sharply pointed tip ... 1. V. ARKANSANA

Vernonia arkansana
2 1. Involucral bracts ovate to oblong-obovate or oblong-lanceolate, rounded, angled or short-tapered to an appressed to spreading (but not appearing curly), rounded to sharply pointed tip

3 2. Leaf blades with the undersurface glabrous, appearing dotted with minute, impressed resin glands (these sometimes difficult to observe in fresh material, but darkening and becoming more noticeable in dried leaves) ... 3. V. FASCICULATA

Vernonia fasciculata
4 2. Leaf blades with the undersurface moderately to densely but sometimes minutely hairy, the hairs obscuring any glandular dots

5 3. Involucral bracts angled or more commonly short-tapered to a sharply pointed tip, this loosely ascending or recurved-spreading, usually with abundant minute, impressed resin glands on both sides of the noticeably keeled midvein ... 2. V. BALDWINII

Vernonia baldwinii
6 3. Involucral bracts rounded or broadly angled to a bluntly pointed tip, sometimes abruptly short-tapered to a minute, sharp point, glandless or with sparse, minute, impressed resin glands, the midvein not or only inconspicuously keeled

7 4. Disc florets 13–30 per head; stems minutely hairy, sometimes becoming nearly glabrous toward the base; leaf blades minutely hairy on the undersurface, especially along the veins (occasionally sparse, longer hairs also present along the veins) ... 4. V. GIGANTEA

Vernonia gigantea
8 4. Disc florets 32–60 per head; stems with longer, spreading or often bent to tangled hairs toward the tip, sometimes minutely hairy toward the base; leaf blades with longer, often bent or tangled hairs, more or less evenly distributed on the undersurface ... 5. V. MISSURICA Vernonia missurica
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