| 1 | Corolla with almost straight tube and narrow throat, glabrous inside. Nutlets usually 1 or 2. Sect. Stenostoma. |
| 1' | Corolla with broad, distinctly incurved tube and inflated throat. Nutlets usually 1-4. |
| 2 | Calyx teeth triangular or lanceolate, much shorter than the tube. |
| 2' | Calyx teeth linear-subulate or narrow lanceolate, as long as or longer than the tube. Flowers in lax, axillary, peduncled, often branched cymes. |
| 3 | Flowers few in a verticillaster and forming an axillary spurious raceme. Calyx (in flower) 10-12 mm long |
| 3' | Flowers many in densely congested axillary verticillasters. Calyx 6-8 mm long. |
| 4 | Calyx teeth narrow lanceolate, nearly as long as the tube. Plant generally covered with golden yellow or dark brownish tomentum |
| 4' | Calyx teeth linear-subulate, considerably longer than the tube. Plant generally covered with ash-grey tomentum |
| 5 | Corolla tube hirsute inside, and included in the calyx; lips small. Nutlets usually 1-3. Sect. Podosiphon |
| 5' | Corolla tube glabrous inside, and exceeding the calyx; lips large and prominent. Nutlets usually 4. Sect. Gomphostemma. |
| 6 | Calyx tube not ribbed. Ovary glabrous or punctate. Flowers in lax, peduncled, branched cymes. Stem ascending |
| 6' | Calyx tube often conspicuously ribbed. Ovary villous or hispid. Flowers more or less congested in axillary verticillasters. Stem erect. |
| 7 | Calyx teeth generally shorter than the tube. Flowers c. 10-15 in a verticillaster. Bracts linear-lanceolate. Calyx (in flower) 10-15(-20) mm long |
| 7' | Calyx teeth often considerably longer than the tube. Flowers c. 20 in a verticillaster. Bracts subulate. Calyx (in flower) 18-25 mm long |
