Key for Aristolochia Linnaeus

1Leaves 3-lobed
1'Leaves not lobed
2Pseudostipules lacking; lateral lobes of leaves perpendicular to middle lobe; superior lobe of perianth triangular and emarginate at apex
2'Pseudostipules present; lateral lobes of leaves falciform, pointing partly forward; superior lobe of perianth ovate, with a long apical filament at least 10 cm long
3Leaves peltate
3'Leaves not peltate, or very slightly so
4Plant cauliflorous and leaves wider than long
4'Plant not cauliflorous or, if cauliflorous, then leaves longer than wide
5Limb of perianth not spreading, but at first reflexed backwards to the utricle and apically coming forward, but finally recurved outwards and forming a horse-shoe-shaped cavity around the mouth of the tube; fruit more than 6 cm long, external wall thick, woody
5'Limb of perianth spreading; fruit similar or thin-walled
6Upper part of the limb wider than long, apex rounded; fruit more than 6 cm long, external wall thick, woody
6'Upper part of the limb longer than wide, apex acute; fruit up to 6 cm long, external wall thin, chartaceous
7Perianth with only 1 superior lobe (with small inferior lobe in A. cremersii).
7'Perianth with 2 lobes (2 lateral lobes, or 1 superior and 1 inferior lobe).
8Perianth lobe hangmg, like a scallopped tape
8'Perianth lobe erect
9Inflorescences cauliflorous or axillary racemes
9'Flowers solitary or arranged in axillary inflorescences
10Flowers solitary; inside wall of dehiscing fruit not comb-like
10'Flowers in axillary inflorescences; inside wall of dehiscing fruit comb-like.
11Perianth large, utricle 3 x 1.5 cm, larger than the lobe
11'Perianth with the utricle smaller than the lobe
12Perianth lobe triangular, without fimbriae, but with dark violet spots
12'Perianth lobe with fleshy, dark purple fimbriae
13Perianth lobe triangular, acute, marginally reflexed
13'Perianth lobe distally ovoid, with acuminate apex
14Perianth lobe with terminal filiform appendage; seed wings apically truncate, seed base truncate
14'Perianth lobe triangular with acute apex; seed wings apically rounded, seed base cordate
15Perianth with 2 lateral lobes
15'Perianth with superior and inferior lobe
16Utricle less than 1.5 cm long, superior lobe linear, twisted
16'Utricle more than 2 cm long
17Superior lobe deeply 2-fid, up to 5 cm long
17'Superior lobe paddle-shaped, 1.5-3 cm long, wider in its distal part