| 1 | Corollas red |
| 1' | Corollas white, or slightly tinged with pink (tube white with lobes lavender blue in N. cordatus) |
| 2 | Flower 1 per axil |
| 2' | Flowers 2 to several per axil (rarely 1 in N. porphyrotrichus) |
| 3 | Calyx lobes rounded at apex |
| 3' | Calyx lobes acute |
| 4 | Inflorescences pedunculate; calyx lobes sparsely villous or glabrous outside (cultivated) |
| 4' | Inflorescences epedunculate; calyx lobes hirsute or densely villous outside |
| 5 | Leaf blades subequal in a pair; corolla oblique in calyx |
| 5' | Leaf blades unequal in a pair; corolla erect in calyx |
| 6 | Leaf blades decurrent into petiole |
| 6' | Leaf blades not decurrent |
| 7 | Leaf blades purplish underneath |
| 7' | Leaf blades green underneath |
| 8 | Calyx lobes entire |
| 8' | Calyx lobes repand-serrate |
| 9 | Leaf blades acuminate at apex, cuneate at base |
| 9' | Leaf blades obtuse or acute at apex, rounded or subcordate at base |
| 10 | Calyx lobes with a few teeth; corolla tube 0.8-1.1 cm long |
| 10' | Calyx lobes serrate; corolla tube 2.8-5 cm long |
| 11 | Leaf blades equal in a pair, ovate, below minutely pilose; corolla tube white, 2.8 cm long, lobes white |
| 11' | Leaf blades mostly unequal in a pair, oblong-elliptic, below hirsute; corolla tube white, 3.3-5 cm long, lobes lavender |
