| 1 | Inflorescence developing in a shallow depression, under a bubble-like calyptra of one piece formed from the stem periderm and falling or rupturing irregularly as the inflorescence develops beneath it |
| 1' | Inflorescence externally visible from an early stage, not developing under a calyptra |
| 2 | Inflorescence a head with an involucre of enlarged imbricate or valvate bracts completely or partially enclosing the flowers |
| 2' | Inflorescence not a head, or if so, with floral bracts not enlarged and imbricate or valvate, nor forming an involucre around the entire inflorescence |
| 3 | Involucral bracts 2, connate at the margins |
| 3' | Involucral bracts 4 or more, free, usually imbricate |
| 4 | Flowers in the inflorescence 6 in 2 opposite triads, sessile |
| 4' | Flowers in the inflorescence 8-12 in 2 opposite rows, pedicellate and with a bract at the apex of each pedicel |
| 5 | Flowers in the inflorescence (at least the outer ones) grouped in triads |
| 5' | Flowers in the inflorescence not grouped in triads |
| 6 | Petals free |
| 6' | Petals fused to the middle or higher |
| 7 | Inflorescence in reality a subumbellate raceme of triads tightly crowded at the apex of the axis, with narrow involucral segments developed from the bracts of the outer flowers and fused to the pedicels and rays |
| 7' | Inflorescence distinctly capitate, the flowers inserted on a flat receptacle, with broad involucral segments developed from nonfertile bracts |
| 8 | Inflorescence triads in the axils of enlarged bracts, with each flower surrounded by 3 smaller bracts |
| 8' | Inflorescence triads not in the axils of enlarged bracts (except the outer triads), with each flower subtended by a single small bract |
| 9 | Involucral bracts subtending the individual flowers |
| 9' | Involucral bracts tightly enclosing the whole inflorescence |
| 10 | Flowers in simple dichasia (triads or rarely tetrads), these in most species aggregated into larger racemose or umbellate inflorescences |
| 10' | b. Flowers single in the inflorescences (racemes, spikes, umbels, heads or solitary flowers) |
| 11 | Petals fused to the middle or higher (sometimes with the corolla tube deeply split on one side) |
| 11' | Petals free completely or almost to the base (sometimes coherent for some time after anthesis) |
| 12 | Anthers dorsifixed |
| 12' | Anthers basifixed |
| 13 | Inflorescence a subumbellate raceme of triads tightly crowded at the apex of the axis, with narrow involucral segments developed from the bracts of the outer flowers and fused to the pedicels and rays |
| 13' | Inflorescence umbellate or racemose but lacking an involucre of enlarged bracts |
| 14 | Inflorescence umbellate |
| 14' | Inflorescence racemose |
| 15 | Inflorescence umbellate, sometimes contracted to a head |
| 15' | Inflorescence racemose |
| 16 | Inflorescence a raceme with whorls of triads |
| 16' | Inflorescence a raceme with decussate triads |
| 17 | Petals fused to the middle or higher (sometimes with the corolla tube deeply slit on one side) |
| 17' | Petals free completely or almost to the base (sometimes coherent for some time after anthesis) |
| 18 | Corolla 6-merous |
| 18' | Corolla 5- or 4-merous |
| 19 | Bracts 3 under each flower, sometimes partly united |
| 19' | Bracts single under each flower |
| 20 | Inflorescence a solitary flower on a short pedicel which is sometimes articulate. |
| 20' | Inflorescence a spike or raceme |
| 21 | Inflorescence axis decussately flattened, with the flowers borne in hollows |
| 21' | Inflorescence axis terete or quadrangular, with the flowers not borne in hollows |
| 22 | Flowers strongly reflexed upwards from a vertical axis; corolla thick, more than 60 mm long |
| 22' | Flowers not reflexed on the axis; corolla thin, less than 60 mm long |
| 23 | Fruit obovoid, club-like, distinctly stipitate |
| 23' | Fruit ovoid or ellipsoid, not stipitate |
| 24 | Inflorescence racemose (sometimes few-flowered and subumbellate); corolla 5- or rarely 4-merous, regular or slightly zygomorphic; leaves mostly alternate |
| 24' | Inflorescence a few-flowered umbel; corolla (in Malesia) 4-merous, zygomorphic; leaves opposite |
| 25 | Anthers dorsifixed, versatile |
| 25' | Anthers basifixed, immobile |
| 26 | . Inflorescence a simple umbel or solitary flower |
| 26' | Inflorescence a raceme, spike or contracted to a head |
| 27 | Inflorescence sessile, capitate, a very condensed spike without involucre |
| 27' | Inflorescence a raceme or spike |
| 28 | Flowers hermaphrodite; anthers linear |
| 28' | Flowers mostly unisexual; anthers globose or subglobose. |
