297. Tithonia Desfontaines ex Jussieu, Gen. Pl. 189. 1789.
Sunflowerweed [From Greek mythology, Tithonus, son of Laomedon and consort of Aurora, symbolic of old age; perhaps alluding to gray to white induments of some plants]
John C. La Duke
Annuals, perennials, subshrubs, or shrubs [trees], 70–500[–700] cm. Stems erect, branched. Leaves all or mostly cauline; opposite (proximal) or mostly alternate; petiolate or sessile; blades often (1-), 3-, or 5-nerved, mostly deltate or pentagonal [lanceolate, linear], sometimes 3- or 5-lobed, bases ± truncate or auriculate [attenuate] (sometimes decurrent onto petioles), ultimate margins serrate to crenate, faces glabrate, ± hirsute, pilose, soft-pubescent, or villous, often gland-dotted. Heads borne singly (peduncles usually distally dilated, fistulose). Involucres campanulate to hemispheric, 10–20+ mm diam. Phyllaries persistent, 12–28+ in 2–5 series (linear to broadly rounded, unequal to subequal, apices acute to rounded). Receptacles hemispheric to convex, paleate (paleae persistent, embracing cypselae, striate, ± 3-toothed, middle teeth larger, stiff, acute or acuminate to aristate). Ray florets 8–30, neuter; corollas yellow or orange. Disc florets 40–120[–200+], bisexual, fertile; corollas yellow, tubes shorter than throats (bases of throats bulbous and hairy), lobes 5, ± triangular (anthers black, brown, or tan, bases cordate-sagittate, appendages ovate; style branches relatively slender, appendages penicillate or lanceolate to attenuate). Cypselae (black or brown) ± compressed or flattened, often 3- or 4-angled or biconvex, ± cuneiform in silhouette (sometimes with basal elaiosomes); pappi 0, or ± coroniform (of ± connate scales, 1–2 scales sometimes subulate to aristate). x = 17.
Species 11 (3 in the flora): sw United States, Mexico, Central America; introduced in se United States, West Indies, South America, and Old World.
SELECTED REFERENCE
La Duke, J. C. 1982. Revision of Tithonia. Rhodora 84: 453–522.
