KEY 2 TO THE GENERA (based primarily on flower, fruit, and seed characters)

1Calyx lobes imbricate in bud
1'Calyx lobes valvate in bud
2Stamens 10 or less, free
2'Stamens more than 15
3Sterile or non-functional male flowers at base of inflorescence, the sterile flowers with long staminodes
3'Sterile flowers without staminodes
4Short-shoots terminated by spines or composed of many persistent fused stipules and (sometimes) bracts
4'Short-shoots without terminal spines
5Stipules subulate. Fruits subsessile, linear, with oblique or longitudinal seeds
5'Stipules ovate to lanceolate, striately veined. Fruits stipitate, narrowly to broadly oblong, seeds more or less transverse
6Shrubs or small trees armed with stipular spines or scattered spines, or with spine-tipped branches. Extrafloral nectaries generally present
6'Plants unarmed or armed with ± recurved prickles (then extrafloral nectaries absent)
7Flowers in glomerules, with persistent spathulate or peltate bracts. Fruits papery to semicoriaceous, dehiscing down both sutures or opening along the margins only. Seeds compressed but not flat
7'Flowers in spikes or, if in glomerules, then fruits otherwise
8Anther glands absent. Fruit valves separating, unwinged.
8'Anther glands present. Fruit valves ± winged, splitting at edges but not separating over the seed chambers
9Leaflets alternate
9'Leaflets opposite
10Plants unarmed
10'Plants armed with prickles on the internodes
11Scandent shrubs or lianas. The distal pair of pinnae transformed into tendrils. Flowers in spikes
11'Trees or erect shrubs. Flowers in glomerules
12Valves of fruit forming one-seeded segments at maturity leaving the sutures as an empty frame
12'Valves of fruit not forming one-seeded segments (India-Thailand, Indochina)
13Fruit ± separating into individual segments to leave a persistent replum or if central part of pod separating as one piece the peduncle not prickly.
13'Valves of fruit separating as one unit from a ± persistent replum. Peduncle prickly
14Filaments free or shortly united at the base only
14'Filaments united into a tube in the proximal part
15Leaves unipinnate
15'Leaves bipinnate
16Leaflets opposite or pinnae unifoliolate, proximal pair of leaflets rarely alternate, or leaflet unpaired
16'Leaflets alternate except perhaps the distal two; pinnae never unifoliolate
17Stipules spinescent, or branches armed with spines
17'Stipules not spinescent, in one case recurved and persisting; plants unarmed.
18Plants with ramous spines. Fruits falling apart in one-seeded segments. Seeds without aril
18'Plants with stipular spines. Fruits dehiscing along the sutures. Seeds with fleshy, funicular aril
19Seeds with pleurogram
19'Seeds without pleurogram
20Valves of fruit dehiscing elastically from the apex, recurving, margins thickened
20'Valves of fruit not dehiscing elastically from the apex, margins not or only slightly thickened
21Flowers in glomerules or corymbs
21'Flowers in racemes or spikes
22Flowers of the same inflorescence unit uniform
22'Flowers of the same inflorescence unit heteromorphic
23Fruits woody, curved or curled, indehiscent. Seeds biserially arranged
23'Fruits membranous-chartaceous, straight, dehiscent. Seeds uniserially arranged
24Fruit internally segmented. Seeds with an U-shaped areole.
24'Fruit usually not segmented inside; if segmented, then the seeds with a horseshoe-shaped areole
25Endocarp forming a closed envelope around each seed; seeds strongly flattened, circular
25'Endocarp not forming a closed envelope around each seed; seeds ± flattened, ellipsoid
26Fruits usually spirally contorted, reddish orange especially on the inner surface with contrasting bluish black seeds; if straight, then turgid with disc-like or bitruncate seeds
26'Fruits, where known, straight and flattened. Seeds strongly flattened, coin-like, with a narrow wing
27Leaflets petiolulate. Pods curved into a circle or contorted, reddish inside, dehiscent
27'Leaflets sessile. Pods straight, not reddish inside, usually indehiscent or tardily dehiscent