Tithonia
Gen. Pl., 189. 1789.
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.
Distribution
sw United States, Mexico, Central America, introduced in se United States, West Indies, South America, and Old World.
Discussion
Species 11 (3 in the flora).
Selected References
Lower Taxa
Key
| 1 | Perennials, subshrubs, or shrubs; phyllaries 16–28; ray laminae 48–69 mm; disc florets 80–120+; cypselae 4–6 mm | Tithonia diversifolia |
| 1 | Annuals; phyllaries 12–16; ray laminae 9–15 or 20–33 mm; disc florets 40–60 or 60–90; cypselae 5–9 mm | > 2 |
| 2 | Leaf blades ± deltate to pentagonal, often 3- or 5-lobed, abaxial faces soft-pubescent; rays usually orange, rarely yellow, laminae 20–33 × 6–17 mm | Tithonia rotundifolia |
| 2 | Leaf blades mostly deltate, rarely, if ever, lobed, abaxial faces sparsely hirsute (hairs larger on veins); rays yellow, laminae 9–15 × 4–6 mm | Tithonia thurberi |