| 1 | Stem bearing 1-5 (mostly 2 or 3) leaves at the apical part: |
| 1' | Leaf with 3 prominent nerves, reaching often to the apex, connected with almost parallel and transverse veins: |
| 1'' | Leaves with lateral nerves pinnately arranged more or less at regular intervals; venation on the lower surface prominently and closely reticulate: |
| 1''' | Leaves villous or densely tomentose beneath and hairs covering almost the whole surface: |
| 1'''' | Leaf base distinctly cordate, the sinus rather narrow and the auricles or basal lobes often overlapping: |
| 1''''' | Leaves distinctly papillate beneath; papillae forming rings or curves: |
| 1'''''' | Flowers with funnel-shaped perianth, up to c. 12.5 cm long, the largest in this genus: |
| 1''''''' | Flowers with folded perianth more or less round in outline in side view, c. 7 cm ø, base cordate: |
| 1'''''''' | Flowers with stamens arranged in 4 whorls: |
| 1''''''''' | Flowers with stamens arranged in 3 whorls: |
| 1'''''''''' | Flowers with 6 stamens in 1 whorl; style lobes covered with (often hooked) hairs at the upper part: |
| 1''''''''''' | Flowers with 6 stamens in 1 whorl; anthers with connectives distinctly protruding 0.5-1 mm beyond them: |
| 1'''''''''''' | Style lobes 2 or rarely 3, glabrous: |
| 1''''''''''''' | Seeds boat-shaped, rather smooth and only sparsely granulate on both surfaces: |
