| | 1. Leaves 2-pinnate, with blades and axes |
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| | 2. Shrub, armed; flowers in spikes or heads |
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| | 3. Spines (modified stipules), straight, paired, only at nodes; inflorescences heads, generally < leaves |
| A. farnesiana var. farnesiana |
| | 3' Prickles, curved, not paired, irregularly scattered on stem; inflorescences spikes, generally > leaves |
| A. greggii |
| | 2' Tree, unarmed; flowers in heads |
| |
| | 4. Leaf with 3–6 pairs of 1° leaflets |
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| | 5. Leaf silvery blue; 2° leaflets linear, 5–7 mm, touching |
| A. baileyana |
| | 5' Leaf dark green; 2° leaflets lanceolate, 2–8 cm, generally not touching |
| A. elata |
| | 4' Leaf with > 6 pairs of 1° leaflets |
| |
| | 6. Twig prominently winged, ± glabrous; 1° leaflets 5–15 pairs, 3–7 cm, not overlapped; 2° leaflets 15–35 pairs, 5–15 mm, ± overlapped |
| A. decurrens |
| | 6' Twig ± angled, without wing-like ridges, hairy; 1° leaflets 8–25 pairs, 3–5 cm, overlapped; 2° leaflets 20–70 pairs, 2–5 mm, overlapped |
| |
| | 7. Leaf main axis with glands at junction of each pair of 1° leaflets; twig, leaf generally silvery hairy |
| A. dealbata |
| | 7' Leaf main axis with glands at junction of and generally between each pair of 1° leaflets; twig, leaf ± green |
| A. mearnsii |
| | 1' Leaves simple, with no blades but petioles and midribs blade-like |
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| |
| | 9. Leaf blade-like, with 2–3 veins more prominent than others, abruptly narrowed to a short, sharp tip |
| A. longifolia |
| | 9' Leaf needle-like or slightly flat, with 1 vein (midrib) more prominent than others, tapered to a spine-like tip |
| A. verticillata |
| | 8' Flowers in heads, these solitary or in racemes or panicles |
| |
| | 10. Shrub with stipular spines; leaf < 2 cm; heads solitary |
| A. paradoxa |
| | 10' Tree, shrub without stipular spines; leaf > 2 cm; heads in racemes or panicles |
| |
| | 11. Leaf with 2 or more prominent, longitudinal veins |
| |
| | 12. Shrub, densely branched, spreading from base; leaf narrowly oblong to obovate, generally 6–12 mm wide, simple throughout |
| A. cyclops |
| | 12' Tree, generally with a single trunk; leaf lanceolate or oblanceolate, generally 1–3 cm wide, even-2-pinnate at tip or not |
| A. melanoxylon |
| | 11' Leaf with 1 prominent, longitudinal midvein |
| |
| | 13. Twig not ridged; leaf widely lancolate to oblanceolate, curved to sickle-shaped; seed stalk ± 1/2 length of seed |
| A. pycnantha |
| | 13' Twig ridged; leaf linear-lanceolate to narrow-oblanceolate, straight or slightly curved, not sickle-shaped; seed stalk encircling seed in double fold |
| A. retinodes
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