| | 1. Shrub or small tree; herbage glabrous, glaucous |
| N. glauca |
| | 1' Annual or perennial herb; herbage generally glandular-hairy |
| |
| | 2. Corolla 50–75 mm; cauline leaves generally elliptic; basal leaves tapered to winged petiole |
| N. sylvestris |
| | 2' Corolla 15–50 mm; cauline leaves generally lanceolate; basal leaves petioled |
| |
| | 3. Cauline leaves clasping; perennial herb, base often slightly woody; corolla open during day |
| N. obtusifolia |
| | 3' Cauline leaves not clasping; annual; corolla closed during day |
| |
| | 4. Cauline leaves generally ± sessile (except lowest) |
| |
| | 5. Corolla tube 15–20 mm, limb 8–10 mm wide; filaments attached ± at 1 level below middle of corolla tube; calyx lobes very unequal |
| N. clevelandii |
| | 5' Corolla tube 25–50 mm, limb 20–50 mm wide; filaments attached at various levels above middle of corolla tube; calyx lobes ± unequal |
| N. quadrivalvis |
| | 4' Cauline leaves petioled |
| |
| | 6. Calyx dark-striped, lobes narrowly lanceolate, unequal, generally > tube, not pock-marked; plant densely glandular |
| N. acuminata var. multiflora |
| | 6' Calyx not striped, lobes triangular, ± equal, < tube, pock-marked; plant glabrous to ± sparsely glandular |
| N. attenuata
|