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Published In: Species Plantarum 2: 988. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

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

 

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86. Parthenium L.

(Rollins, 1950)

Plants annual or perennial herbs (shrubs elsewhere), often somewhat aromatic when bruised or crushed, sometimes with rhizomes or a tuberous rootstock. Stems erect or ascending, unbranched below the inflorescence to many-branched, with fine longitudinal ridges, glabrous or sparsely to densely hairy and sometimes also with minute, sessile, spherical, yellow glands. Leaves basal and alternate, the basal and lower leaves short- to more commonly long-petiolate, the median and upper leaves mostly sessile. Leaf blades variously shaped, unlobed or 1 or 2 times deeply pinnately lobed, the margins otherwise entire or more commonly toothed or scalloped, also usually minutely hairy, the surfaces variously hairy, sometimes roughened to the touch, also usually with minute, sessile, spherical, yellow glands. Inflorescences usually small, more or less flat-topped terminal panicles, sometimes reduced to a small, loose terminal cluster, subtended by small, leaflike bracts at the branch points, the heads mostly with relatively short, densely hairy stalks. Heads radiate but sometimes appearing discoid. Involucre cup-shaped, the bracts in 2–4 subequal, overlapping series, those of the outer series usually somewhat narrower than the others. Involucral bracts ascending, straw-colored, sometimes greenish-tinged toward the tip, mostly hardened and leathery, the outer surface densely hairy, those of the inner series about as long as the outermost chaffy bracts. Receptacle convex to short-conical, not elongating as the fruits mature, with chaffy bracts subtending the ray and disc florets, these densely hairy on the outer surface toward the tip, those of the disc florets concave and wrapped around the florets. Ray florets (4–)5(–7), pistillate (with a 2-branched style exserted from the short tube at flowering), the corolla with a short (1–2 mm) or rarely absent ligule (then reduced to a minute tube), when present the ligule relatively broad, white or off-white, the short, tubular base (and the outer surface of the ligule) densely short-hairy, usually persistent at fruiting. Disc florets about 15–65, staminate (with a small, stalklike ovary and an undivided style), all but the outermost florets usually shed as an intact unit at fruiting, the corolla 1.2–2.0 mm long (only slightly surpassing the chaffy bract), off-white to pale cream-colored, minutely hairy on the outer surface of the lobes, not expanded at the base. Style branches with the sterile tip broad and bluntly pointed to rounded. Pappus of the disc florets usually absent, that of the ray florets of 2 scales or 2 or 3 slender awns. Fruits obovate to narrowly obovate in outline, flattened, the surface minutely hairy, dark gray to black, fused basally to the subtending chaffy bract as well as to the adjacent 2(3) disc florets and their chaffy bracts, the whole group shed intact as a unit at fruiting. About 16 species, North America to South America, Caribbean Islands.

Parthenium argentatum A. Gray is a shrubby member of the genus native to the Chihuahuan Desert region that is known as guayule. It has long been known to contain polyisoprenoid natural rubber in its tissues. During World War II, when the supply of commercial rubber (mostly produced from the latex of the rubber tree, Hevea brasiliensis (A. Juss.) Müll. Arg. [Euphorbiaceae]) from plantations in the Philippines and other Malesian islands was cut off, interest in guayule and its relatives was rekindled. In fact, the monograph of the genus by Rollins (1950) was an indirect result of his involvement with the search for alternative sources of rubber during the early 1940s. However, after the war ended, interest in research and cultivation of guayule lagged because extraction of the rubber requires the harvest of entire plants (as opposed to sustainable harvest from Hevea rubber by tapping latex from the tree’s trunk), which makes the process relatively costly and inefficient. On the other hand, the rubber refined from Parthenium supposedly causes fewer allergic reactions than does the rubber from Hevea, so there may be a market for rubber from Parthenium for the manufacture of items such as surgical gloves and condoms (Cornish and Siler, 1996). Currently, research on a stable domestic supply of natural rubber involving Parthenium and other genera continues in the United States, but at a level somewhat below that in the early 1940s.

Plants of Parthenium have a bitter flavor and usually are avoided by livestock and other grazing mammals.

 

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1 1. Leaves 1 or 2 times deeply pinnately lobed; plants annual, with taproots ... 1. P. HYSTEROPHORUS

Parthenium hysterophorus
2 1. Leaves unlobed, the margins coarsely toothed; plants perennial, with short and somewhat tuberous or long-rhizomatous rootstocks ... 2. P. INTEGRIFOLIUM Parthenium integrifolium
 
 
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