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| 1 Acaulescent herb; calyx tube straight, radially symmetrical; stamens 12; [subfamily Asaroideae]. |
| ..2 Leaves deciduous, pubescent, paired |
..2 Leaves evergreen, glabrous, not paired |
| 1 Caulescent herb, herbaceous vine, or liana; calyx tube bent, bilaterally symmetrical; stamens 6; [subfamily Aristolochioideae, tribe Aristolochieae]. |
| ....3 Woody, twining vine; leaves 3-35 cm wide; [subtribe Isotrematinae] |
| ......4 Gymnostemium lobes 6; calyx limb 1- or 2-lobed, if 2, the lobes dissimilar in shape; [subtribe Aristolochiinae] |
| ......4 Gymnostemium lobes 3; calyx limb 3-lobed, the lobes similar in size and shape and arranged radially around the tube orifice at 120 degree angles; [subtribe Isotrematinae] |
| ....3 Erect to ascending herb or twining, herbaceous vine; leaves 0.7-6.5 cm wide. |
| ........5 Leaf bases cuneate; gynoecium and androecium 5-merous |
| ........5 Leaf bases cordate; gynoecium and androecium 3-, 5-, or 6-merous. |
| ..........6 Twining herbaceous vine; gynoecium and androecium 5-merous |
| ..........6 Erect to ascending herb; gynoecium and androecium 3- or 6-merous. |
| ............ 7 Leaf blade as wide as long, or wider than long; leaf venation palmate; [subtribe Aristolochiinae] |
| ............ 7 Leaf blade narrower than long; leaf venation pinnate (3-veined from the base, pinnate above); [subtribe Isotrematinae] |