257. Acanthospermum Schrank, Pl. Rar. Hort. Monac. 2: plate 53. 1820.
[Greek acantha, prickle, and sperma, seed, alluding to prickly "fruits"]
John L. Strother
Annuals (sometimes persisting), 10–60(–120) cm. Stems erect to ± prostrate (repeatedly "forked"). Leaves cauline; opposite; petiolate or ± sessile; blades mostly elliptic to deltate, rhombic, or ovate, sometimes lyrate, ultimate margins entire or toothed, faces usually pilosulous to sericeous or scabrellous, sometimes glabrate or glabrescent, usually gland-dotted. Heads radiate, 1(–3) in "forks" of branches (terminal, appearing axillary by sympodial growth). Involucres ± hemispheric, 3–5 mm diam. (becoming ± rotate in fruit). Phyllaries persistent (outer) or falling, 10–13 in 2 series (outer 4–6 herbaceous, inner 5–8 each investing a ray ovary, enlarging in fruit to form a perigynium, shed with enclosed cypsela). Receptacles convex, paleate (paleae cuneate to spatulate, ± conduplicate or flattish, membranous). Ray florets 5–8, pistillate, fertile; corollas yellowish (tubes shorter than to equaling laminae, laminae ovate to elliptic or linear). Disc florets 3–8(–12+), functionally staminate; corollas yellowish, tubes shorter than funnelform or campanulate throats, lobes 5, deltate. Cypselae each enclosed within and shed with an often hardened, ± prickly perigynium (the ultimate "fruits" plumply ellipsoid to fusiform, or ± compressed); pappi 0 or rudimentary. x = 11.
Species 6 (3 in the flora): introduced; mostly tropical to warm-temperate New World; also introduced in Old World.
Blake, S. F. 1921. Revision of the genus Acanthospermum. Contr. U.S. Natl. Herb. 20: 383–392.