KEY TO THE SPECIES

1Leaves in whorls or with occasional opposite pair
1'Leaves exclusively opposite
2Corolla doubled
2'Corolla not doubled
3Leaves subsessile, blade base subcordate
3'Leaves distinctly petiolate, blade base usually not subcordate
4Inflorescence containing empty bracts; anthers sagittate at the base
4'Inflorescence without bracts or all bracts with flowers in the axils; anthers cordate at the base
5Corolla tube 4.5-5.2 mm long; inflorescences very delicate; secondary veins 6-9 pairs
5'Corolla tube > 7 mm long; inflorescences variable but mostly not very delicate; secondary veins 4-26 pairs
6Sepals not ciliate, 3-14 mm long, leafy and often cordate, or linear but always narrower at the base than in the middle
6'Sepals ciliate or not, 0.9-6 mm long, not leafy or cordate, widest at the base
7Stamens inserted in lower half of corolla tube
7'Stamens inserted around middle or in upper half of corolla tube
8Leaves papery; cultivated
8'Leaves coriaceous; native
9Corolla tube ≥ 24 mm long
9'Corolla tube < 24 mm long
10Sepals not ciliate. — Peninsular Malaysia
10'Sepals ciliate. — Throughout Malesia (except New Guinea)
11Corolla puberulent on the lobes outside
11'Corolla glabrous on the lobes outside
12Leaves thickly coriaceous; sepals 3-6 mm long
12'Leaves not coriaceous; sepals 0.9-4 mm long
13Corolla lobes pubescent inside, at least in lower half, often also pubescent outside and at least ciliate
13'Corolla lobes glabrous inside or pubescent only at extreme base, mostly not ciliate
14Inflorescence and flowers robust; cultivated
14'Inflorescence and flowers not robust; native
15Head of bud in mature corolla bud acuminate or acute; sepals usually not ciliate
15'Head of bud in mature corolla bud rounded or obtuse; sepals ciliate or not
16Corolla tube mostly glabrous inside, 3.7-11 times as long as sepals; fruit 2-40-seeded. — Throughout Malesia
16'Corolla tube pubescent inside, 8-12 times as long as sepals; fruit 1- or 2-seeded. — Peninsular Malaysia