| 1 | Female and male flowers in the same inflorescence. |
| 1' | Female and male flowers in different inflorescences. |
| 2 | Leaves distichous. Male flowers sessile, bisymmetric or zygomorphic |
| 2' | Leaves spirally arranged, rarely subopposite. Male flowers pedicellate, actinomorphic. |
| 3 | Male specimens only. (Note: flowers at proximal and distal parts of inflorescence often not typically developed.) |
| 3' | Female specimens only. (Note: in some cases difficult to key out properly.) |
| 4 | Male flowers actinomorphic, 4- or 5-merous, rarely 3- or 6-merous. Tepals all lanceolate, acute. |
| 4' | Male flowers bisymmetric or zygomorphic, 4-merous, rarely 5- or 7-14-merous. Lateral tepals narrow, acute, median tepals wide, truncate. |
| 5 | Pedicels 14-18 mm, during anthesis much reflexed. Lateral tepals extremely narrow and acute, median tepals very wide, square, truncate. Synandrium completely compressed in anterior-posterior direction (Borneo, Malaya) |
| 5' | Pedicels absent or at most up to 6 mm. |
| 6 | Leaves always distichous |
| 6' | Leaves spirally arranged or opposite, decussate. |
| 7 | Leaves spirally arranged, gradually increasing in size upwards on the stem, the upper ones elliptic, concealing inflorescence during anthesis. Tuber elongated and regularly branched. |
| 7' | Leaves opposite, decussate. |
| 8 | Leaves 6-8 pairs, gradually increasing in size upwards on the stem, the uppermost ones almost orbicular in outline, cuculiate, completely concealing the inflorescence during anthesis. Tuber spherical, not branched (Borneo) |
| 8' | Leaves 2-4(-5) pairs, all of nearly the same size; in case of 4 leaves only, the two pairs often very close to each other, apparently whorled, patent during anthesis. Tubers branched (3-12 branches) with slightly elongated parts |
| 9 | Leaves whorled or opposite and decussate. |
| 9' | Leaves distichous or spirally arranged. |
| 10 | Leaves 4, distinct, apparently whorled at the upper part of the stem |
| 10' | Leaves opposite and decussate. |
| 11 | Leaves gradually increasing in size upwards on the stem; the uppermost leaves almost orbicular, cuculiate, concealing the flowering inflorescence |
| 11' | Leaves almost equal in size except for the lowermost 1-3 pairs |
| 12 | Leaves distichous |
| 12' | Leaves spirally arranged. |
| 13 | Spadicles without flowers in lower part. (Note: easily observed with handlens on cross-section of inflorescence.) (Borneo and Malaya only) |
| 13' | Spadicles with flowers in lower part. |
| 14 | Upper leaves not covering the flowering inflorescence. |
| 14' | Upper leaves totally or partially covering the flowering inflorescence. |
| 15 | Stem elongated, slender. Female inflorescence ellipsoid. Tubers elongated and repeatedly branched |
| 15' | Stem very short and stout. Tubers not elongated. |
| 16 | Female inflorescence subspherical-ellipsoid. Leaves coarsely longitudinally striate (Java only) |
| 16' | Female inflorescence subspherical, markedly depressed. Leaves smooth (Java only). |
