| | 1. Ovary superior; fruit 2–3 × sepals |
| N. longiflorus |
| | 2. Corolla 3–3.5 mm, tube 2–2.5 mm; filament tube ± 2–3 mm |
| var. breviflorus |
| | 2' Corolla 5–8 mm, tube 2.5–5 mm; filament tube 3.5–7.5 mm |
| var. longiflorus |
| | 1' Ovary partly to completely inferior; fruit < or slightly > sepals |
| |
| | 3. Stems < 1 cm, cushion-like; inflorescence generally head-like, dense |
| N. twisselmannii |
| | 3' Stems generally > 1 cm, spreading, decumbent, or erect; inflorescence an open raceme |
| |
| | 4. Stem below branches silver-gray, shiny, sometimes dark; corolla yellow with brown marks |
| N. rubescens |
| | 4' Stem below branches reddish, brown, or dark purple; corolla white, pink, lavender, or purplish |
| |
| | 5. Stems spreading to decumbent; pedicels ± thread-like, 0.2–0.3 mm diam |
| N. rigidus |
| | 5' Stems generally erect to stiffly ascending (spreading in N. gracilis, N. secundiflorus, N. sigmoideus); pedicels thread-like, < 0.2 mm diam |
| |
| | 6. Inflorescence axis straight or weakly zigzag |
| |
| | 7. Bracts 2–9 mm, flat, not enfolding pedicel base; corolla white; fruit bell-shaped |
| N. ramosissimus |
| | 7' Bracts 2–4 mm, enfolding pedicel base; corolla pink or lavender; fruit hemispheric |
| |
| | 8. Corolla 1.5–2 mm, white with lavender lines; fruit ± 1.5 mm; seeds elliptic |
| N. gracilis |
| | 8' Corolla 3–5 mm, lavender or pink, yellow-blotched at base; fruit ± 2–2.5 mm; seeds subspheric |
| N. secundiflorus |
| | 6' Inflorescence axis strongly zigzag |
| |
| | 9. Corolla divided 1/2 or less to base |
| N. pinnatifidus |
| | 9' Corolla divided nearly to base |
| |
| | 10. Pedicels clearly S-curved; bracts ovate; flowers spreading |
| N. sigmoideus |
| | 10' Pedicels straight, slightly curved, or reflexed; bracts linear, elliptic, or lanceolate (sometimes ovate in N. glanduliferus); flowers spreading or reflexed |
| |
| | 11. Fruit ± hemispheric (base and tip rounded); sepals spreading or reflexed |
| N. glanduliferus |
| | 12. Pedicels spreading, tip curved; sepals 1.5–2.5 mm |
| var. glanduliferus |
| | 12' Pedicels stiffly ascending, tip straight; sepals ± 1–1.5 mm |
| var. orientalis |
| | 11' Fruit fusiform or obconic (base acute, tip acute, ± pointed, or rounded); sepals erect |
| |
| | 13. Pedicels straight, base to tip; seed surface narrowly ridged |
| N. capillaris |
| | 13' Pedicels straight below, tip curved; seed surface with clearly pitted rows or deeply impressed lines |
| |
| | 14. Seed surface with clearly pitted rows; corolla 2.5–5 mm; leaf blades irregularly serrate |
| N. interior |
| | 14' Seed surface with deeply impressed, vertical lines; corolla 1.5–2 mm; leaf blades entire or obscurely toothed |
| N. montanus
|