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| 1 Plant a shrub or small tree, 3-10 m tall; stems and leaf blades glabrous and glaucous; [section Paniculatae] |
| 1 Plant an herb, 0.5-3 m tall; stems and leaf blades densely viscid-puberulent (or sparsely so to merely tuberculate in N. longiflora). |
| ..2 Corolla tube 1.2-1.7 cm long, greenish yellow, with limb 3-6 mm wide; leaves distinctly petiolate; [section Rusticae] |
| ..2 Corolla tube 2.5-12.0 cm long, cream, white, yellow, or pink, with limb 10-25 mm wide; leaves auriculate clasping. |
| ....3 Larger leaves on a plant 3.5-8 dm long; corolla tube 3.0-5.5 cm long, 4-7× as long as the average diameter, the limb 10-15 mm wide, pink or reddish (rarely white); [section Nicotiana] |
| ....3 Larger leaves on a plant 1-3 (-5) dm long; corolla tube 2.5-12.0 cm long, 10-50× as long as the average diameter, the limb 15-25 mm wide, white or lavender. |
| ......4 Rosette of a few leaves, not persisting; cauline leaves clasping and decurrent on the stem; [section Alatae] |
| ......4 Rosette persisting; cauline leaves clasping or auriculate, but not decurrent on the stem. |
| ........5 Corolla tube 2.5-3.5 cm long; [section Alatae] |
| ........5 Corolla tube 4-12 cm long. |
| ..........6 Corolla throat constricted; filaments unequal, stamens didynamous (2 different lengths); cauline leaf blades lanceolate or linear, leaves sessile, auriculate; [section Alatae] |
| ..........6 Corolla throat gaping; filaments equal, fused to corolla along their entire length, anthers appearing sessile; cauline leaf blades pandurate and clasping; [section Repandae] |