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.