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.

Distribution

mostly tropical to warm-temperate New World, also introduced in Old World.

Discussion

Species 6 (3 in the flora).

Selected References

None.

Lower Taxa

Acanthospermum australe, Acanthospermum hispidum, Acanthospermum humile

Key

1	Fruits 7–9+ mm, 5–7-ribbed, terminal spines 0	Acanthospermum australe
1	Fruits 2–6 mm, 3-ribbed or not notably ribbed, terminal spines 2	> 2
2	Leaf blades rhombic-ovate to obovate, (20–)40–120(–150+) mm; fruits not notably ribbed, prickles ± scattered	Acanthospermum hispidum
2	Leaf blades ovate to lyrate, 10–30(–45) mm; fruits usually 3-ribbed, prickles mostly along 2 ribs and around apices	Acanthospermum humile