| | 1. Subshrub or shrub; stems persisting 2+ seasons |
| |
| | 2. Spines of fruit heads straight, flat or round in X-section |
| |
| | 3. Cauline leaves ± compound; fruit heads (sub)glabrous |
| A. dumosa |
| | 3' Cauline leaves few-toothed to shallowly lobes; fruit heads long-soft-silky |
| A. eriocentra |
| | 2' Spines of fruit heads hooked, round in X-section |
| |
| | 4. Cauline leaves sessile, clasping |
| A. ilicifolia |
| | 4' Cauline leaves petioled |
| |
| | 5. Leaf blades long-triangular, 15+ cm; fruit heads long, body ± glabrous or gland-dotted |
| A. ambrosioides |
| | 5' Leaf blades round, < 10 cm; fruit heads round, body white-woolly |
| A. chenopodiifolia |
| | 1' Annual or perennial herb |
| |
| | 6. Annual from slender taproot |
| |
| | 7. Cauline leaves mostly opposite, palmately lobed |
| A. trifida |
| | 7' Cauline leaves mostly alternate, pinnately lobed and divided |
| |
| | 8. Spines of fruit heads many, scattered, sharply pointed; distal staminate phyllary tips ± black-lined |
| A. acanthicarpa |
| | 8' Spines of fruit heads few, below beak, blunt; staminate phyllary tips uniformly green |
| A. artemisiifolia |
| | 6' Perennial from rhizome-like roots or caudex |
| |
| | 9. Fruit head spines many, scattered, or in several rows |
| |
| | 10. Fruit heads 5–10 mm; spines straight |
| A. chamissonis |
| | 10' Fruit heads 2–5 mm; spines hooked |
| A. confertiflora |
| | 9' Fruit heads spines 0–few, vestigial, mostly on upper half |
| |
| | 11. Leaves simple or sparsely 1-pinnate, lobes toothed; plants generally 4–20 dm |
| A. psilostachya |
| | 11' Leaves 3–4-pinnately divided into many tiny segments; plants generally 1–3.5 dm |
| A. pumila
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