| | 1. Leaf simple, linear or oblanceolate, rarely with 3 leaflets |
| |
| | 2. Leaves persistent; inflorescence axis not a thorn |
| P. schottii |
| | 2' Leaves deciduous by early summer; inflorescence axis extended beyond flowers as a thorn |
| P. spinosus |
| | 1' Leaves all odd-1-pinnate or some simple |
| |
| | 3. Fruit 2–4.5 mm, included; inflorescence a spike or raceme, dense, spheric or short-cylindric; pedicel lacking bractlets |
| |
| | 4. Terminal leaflets of major leaves (or some of them) > lateral leaflets; glands of twigs << 0.5 mm wide |
| P. emoryi |
| | 4' Terminal leaflets of all leaves ± = lateral leaflets; glands of twigs ± 0.5 mm wide |
| P. polydenius |
| | 3' Fruit 7–10 mm, exserted; inflorescence a ± open, sometimes spike-like raceme; pedicel with bractlets |
| |
| | 5. Fruit glabrous, with many small glands forming longitudinal lines; Desert (eastern half San Bernardino Co.) |
| P. fremontii |
| | 6. Leaflet linear, < ± 1 mm wide; Sonoran Desert (Whipple Mtns, extreme se San Bernardino Co.) |
| var. attenuatus |
| | 6' Leaflet narrowly elliptic to ovate or obovate, ± 1.5–3 mm wide; Desert Mountains (east-central San Bernardino Co.) |
| var. fremontii |
| | 5' Fruit glabrous or finely hairy, with few or several large, scattered glands; East of Sierra Nevada to San Bernardino Mountains, Mojave Desert (except eastern half of San Bernardino Co.) |
| P. arborescens |
| | 7. Leaflets generally continuous with axis |
| var. simplicifolius |
| | 7' Leaflets generally (or at least some) jointed to axis |
| |
| | 8. Calyx 7–9 mm, generally conspicuously hairy |
| var. arborescens |
| | 8' Calyx 5–7 mm, glabrous or sparsely hairy |
| var. minutifolius
|