Parthenium argentatum
Parthenium ligulatum
guayule
Colorado feverfew
blades lanceolate to oblanceolate, 15–25(–40+) × 6–15(–25+) mm, margins mostly entire, some with 1–2(–5) sharp teeth, faces densely strigillose (gray to white) and obscurely or not at all gland-dotted.
blades oblanceolate to spatulate, 5–18+ × 1–2(–4+) mm, margins entire, faces strigilloso-sericeous (gray) and obscurely gland-dotted.
1–2(–6+) mm.
0–4 mm.
5;
corolla laminae ovate, 1.2–1.5 mm.
5;
corolla laminae oval to elliptic, 1–2 mm, retuse.
20–30+.
15–25+.
outer 5 oval-elliptic, 2.5–3+ mm, inner 5 ± orbiculate, 3.5–4 mm.
outer 5 ± ovate, 2.5–3 mm, inner 5 ± orbiculate, 3–4+ mm.
radiate, in glomerules of 3–5+ at ends of ± ebracteate stalks 8–15(–20+) cm.
radiate, borne singly.
± obovoid, 2.5–3 mm; pappus-like enations 2(–4), erect to spreading, ± subulate, 0.3–0.8 mm.
oblanceoloid to pyriform, 3.5–4 mm; pappus-like enations 2, erect to spreading, ± subulate, 0.5–1 mm (a third, ± subulate element sometimes at apex of adaxial face).
= 36.
= 36, 72.
Parthenium argentatum
Parthenium ligulatum
Guayule has sometimes been used for commercial production of natural rubber. Hybrids between Parthenium argentatum and P. incanum have been noted.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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