1 Shrubs.

  2 Leaflets 15-43 per pinna.

    3 Pinnae 4-7 pairs per leaf; fruits breaking into 10-15 segments; [ s. TX southwards]

    3 Pinnae 5-16 (-22) pairs per leaf; fruits breaking into 14-21 segments; [naturalized, in our region known from FL]

  2 Leaflets (3-) 4-12 per pinna.

      4 Pinnae 1-8 pairs per leaf; leaflets 5-12 per pinna; fruits slightly to somewhat constricted between segments, but not jointed into segments

1 Herbs, sometimes suffrutescent, trailing, sprawling, or climbing (by means of recurved, catclaw prickles).

        5 Plant unarmed (rarely with widely scattered prickles), the stem with strigose hairs with bulbous bases, 0.5-3 mm long; capitula 1.5-3× as long as the diameter, 9-25 mm long

        5 Plant well-armed with recurved, catclaw prickles; capitula spherical or nearly so, 1-1.2× as long as wide, 3.5-14 mm long.

          6 Pinnae (1) 2-5 pairs per leaf; leaflets either 10-25 per pinna or 4-6 per pinna; fruits not quadrangular, the valves breaking into segments.

             7 Pinnae (1-) 2 pairs per leaf; leaflets 4-6 per pinna; [native, s. & se. TX southwestwards]

             7 Pinnae 3-5 pairs per leaf; leaflets 10-25 per pinna; non-native, waif]

          6 Pinnae 3-8 (-11) pairs per leaf; leaflets 5-25 per pinna; fruits normally quadrangular (sometimes compressed in M. roemeriana) because of widening of the replum, the valves remaining intact and not separating.

               8 Leaflets 5-9 (-10) pairs per pinna

               8 Leaflets (9-) 10-17 (-19) pairs per pinna.

                 9 Venation of the lower leaf surface readily visible, pale; The midrib giving rise on both sides to 1 or more secondary veins.

                   10 Leaflets obtuse or with an obscure mucro; heads (not including the filaments) < 7 mm in diameter; [s. GA south to s. FL]

                   10 Leaflets sharply apiculate; heads (not including the filaments) 7-11 mm in diameter; [west of the Mississippi River, very rarely adventive eastwards].

                     11 Pod 20-45 mm long, densely armed on all ribs with spines that are basally confluent; valves of the pod as wide as or wider than the replum; [Coastal Plain, w. LA west to se. TX]

                     11 Pod 55-120 mm long, the spines separate from one another; valves of the pod narrower than the replum; [widespread west of the Mississippi River, Coastal Plain and inland (northwards)]

                 9 Venation of the lower leaf surface not visible (sometimes the upper surface with darkened and impressed veins).

                       12 Valves of the pod 3-6 mm wide, distinctly wider than the replum; [on limestone, caliche, and other calcareous substrates; sc. OK south through nc. TX to the Edwards Plateau, extending slightly into se. TX Coastal Plain]

                       12 Valves of the pod as wide as or narrower than the replum; [collectively widespread, usually not on calcareous substrates, at least not in TX]

                          13 Pinnae 4-8 pairs per leaf; seeds 3.2-5.2 mm long; [east of the Mississippi River, VA and KY south to s. FL and e. LA]

                          13 Pinnae 1-3 pairs per leaf; seeds (5-) 6-9 mm long; [west of the Mississippi River, sw. LA and TX southwestwards to COA]

1 Leaves variously modified from strict 2-even-pinnateness, either with 1) a mixture (on a tree) of 1-even-pinnate and 2-even-pinnate leaves, and/or 2) the pinnae or leaflets often subopposite or fully alternate, and/or 3) with an odd number of pinnae per leaf (the tip of the rachis with a pair of lateral pinnae and a terminal pinna, but the leaflets of the pinnae still in opposite pairs), and/or 4) the basal pair of pinnae evolutionarily replaced by a single pair of leaflets larger than the other leaflets.

  2 Flowers small, inconspicuous, in catkin like-racemes or racemes of racemes; perianth greenish-white or greenish-yellow, with a cylindrical hypanthium to 10 mm long and shorter perianth lobes; flowers mostly unisexual; large trees; fruits woody and indehiscent at maturity, 1-5 cm wide[ collectively widespread in our region, but absent from s. FL and s. TX].

    3 Leaves characteristically a mixture (on a tree) of 1-even-pinnate (mainly on spurs) and 2-even-pinnate (mainly on new growth), the pinnae and the leaflets strictly opposite or subopposite; leaflets 1.5-4 cm long, acute to rounded at the apex; trunks with simple, trident, or multiply branched thorns to 20 cm long (or unarmed)

    3 Leaves all 2-even-pinnate, the basalmost "pinna pair" usually replaced by a pair of leaflets larger than the others, the pinnae and the leaflets often 'straying' to subopposite or fully alternate arrangement (some pinnae appearing odd-pinnate); leaflets 3-6 cm long, acuminate at the apex; trunks unarmed

  2 Flowers either with conspicuous petals or aggregated into spikes or spherical heads with showy stamens; petals either (Caesalpinoids) yellow, orange, red, or (Mimosoids) the tubular perianth whorls less conspicuous than the stamens, these yellow to orange; flowers bisexual; small shrubs, shrubs, small or medium trees, or large trees (if large trees, then of tropical parts of our region); fruits papery or chartaceous (rarely woody), often < 2 cm wide; [collectively widespread, including of s. FL and s. TX].

      4 Leaves with pinnae and leaflets mostly subopposite or fully alternate; [exotic, s. FL]; [Mimosoid]

      4 Leaves with an odd number of pinnae per leaf (the tip of the rachis with a pair of lateral pinnae and a terminal pinna), but the leaflets in opposite pairs; [native or exotic, TX and OK]; [Caesalpinoid].

        5 Shrub or small tree, 5-50 dm tall; sepals ovate; leaflets not glandular, or with glands only along the leaflet margin

        5 Sprawling shrub, less than 6 dm tall; sepals linear; leaflets glandular punctate across the lower surface (these orange in life, drying black)

1 Leaves strictly 2-even-pinnate (with pinna pairs borne opposite one another and no pinna terminal on the rachis, and with leaflets also born in opposite pairs).

          6 Pinna pairs 1-6 (-7) per leaf; leaflets 4-ca. 250 per leaf.

             7 Leaflets 1.5-14 mm long.

               8 Glands absent on petioles and rachises.

                 9 Dwarf shrub, 1-6 dm tall; branches unarmed.

                 9 Shrub, small tree, or scrambling liana; branches armed with straight nodal spines or internodal catclaw prickles.

                   10 Branches armed with paired, straight, nodal spines, these simple or 3-branched; branches and larger stems green, photosynthetic; flowers caesalpinoid, 2-2.5 cm in diameter.

                   10 Branches armed with catclaw prickles scattered along internodes; branches and stems brown; flowers caesalpinoid or mimosoid.

                     11 Scrambling liana; flowers caesalpinoid, 1.5-2 cm across

               8 Glands (stalked, columnar, or domed) 1 or more on petioles or rachises, on the petiole or at rachis nodes.

                       12 Branches unarmed.

                          13 Stamens connate basally into a tube; spherical inflorescences whitish or cream, 15-23 mm in diameter

                          13 Stamens free; spherical inflorescences yellow, 6-8 mm in diameter

                       12 Branches armed with straight nodal spines or internodal catclaw prickles.

                            14 Branches armed with catclaw prickles scattered along internodes.

                            14 Branches armed with paired, straight, nodal spines.

                              15 Leaflet pairs 2-6 per pinna; inflorescences cylindrical.

                                16 Petiolar/rachis glands at nodes of the rachis, short-columnar; stamens connate basally into a tube

                                16 Petiolar/rachis gland on the petiole; stamens free

                              15 Leaflet pairs 6-25 per pinna; inflorescences spherical (or cylindrical in Prosopis laevigata).

                                  17 Inflorescences cylindrical; leaflet pairs 15-27 per pinna

                                  17 Inflorescences spherical; leaflet pairs 6-20 per pinna.

                                    18 Dwarf shrub; pinna pair 1 per leaf; leaflet pairs 6-11 per pinna; fruit spirally twisted into a cylinder

                                    18 Shrub or small tree; pinna pairs 2-5 (-6) per leaf; leaflet pairs 9-20 per pinna; fruit flat, straight.

                                       19 Spherical inflorescences 15-23 mm in diameter; stamens connate basally into a tube; fruits flat, about 1 cm wide, 2-4 mm thick

                                       19 Spherical inflorescences 6-13 mm in diameter; stamens free; fruits turgid, cylindrical, about 1 cm in diameter (wide and thick)

             7 Leaflets 10-60 mm long (at least the larger leaflets on a plant > 14 mm long).

                                         20 Glands absent on petioles and rachises.

                                           21 Pinna pairs 1 per leaf; branches unarmed; flowers red-pink, mimosoid, aggregated into spherical inflorescences 5-7 cm in diameter

                                           21 Pinna pairs 2-5 per leaf; branches armed with catclaw prickles scattered along internodes; flowers yellow to orange, caesalpinoid, each flower 0.7-2 cm across.

                                             22 Liana climbing and sprawling over other vegetation; flowers 0.7-1 cm across

                                             22 Shrub or small tree; flowers 1.8-2 cm across

                                         20 Glands (stalked, columnar, or domed) 1 or more on petioles or rachises, on the petiole or at rachis nodes.

                                               23 Leaflet pair 1 per pinna

                                               23 Leaflet pairs 3-14 per pinna

                                                 24 Branches usually armed with paired, straight, nodal spines; inflorescences cylindrical; stamens 10

                                                 24 Branches unarmed; inflorescences spherical; stamens many.

                                                    25 Stamens free

                                                    25 Stamens connate basally into a tube.

                                                      26 Petiolar gland near base of petiole; spherical inflorescences either 0.9-1.7 cm in diameter, or 5-6 cm in diameter

                                                      26 Petiolar gland closer to first pinna pair than to petiole base; spherical inflorescences 2-2.6 cm in diameter

          6 Pinna pairs 4-25 per leaf (at least the larger leaves on a plant with > 6 pinna pairs); leaflets 150-3000 per leaf.

                                                        27 Leaflets 10-40 mm long, at least the larger on a leaf > 15 mm long.

                                                          28 Leaf rachis eglandular; flowers caesalpinoid, each yellow-orange and 3-4 cm across

                                                          28 Leaf rachis with cupular glands at nodes; flowers mimosoid, pink, aggregated into a spherical head 5-6 cm in diameter

                                                        27 Leaflets 1-15 mm long.

                                                            29 Glands absent on petioles and rachises.

                                                              30 Branches armed with catclaw prickles scattered along internodes; shrub; flowers mimosoid, white, cream, or pink, aggregated into spherical heads 0.9-2 cm in diameter.

                                                              30 Branches unarmed; tree; flowers caesalpinoid, each yellow and 2-3.5 cm across

                                                            29 Glands (stalked, columnar, or domed) or nonglandular spheroid projections 1 or more on petioles or rachises, on the petiole or at rachis nodes.

                                                                 31 Branches unarmed.

                                                                   32 Fruits 30-60 cm long, woody, persistent and indehiscent on the tree; flowers caesalpinoid, scarlet and yellow, 8-10 cm across; rachis with spheroidal projections at the nodes; petiole lacking glands

                                                                   32 Fruits < 20 cm long, papery, coriaceous, or woody; flowers mimosoid, white, cream, pink, or pale yellow, aggregated into spherical heads 1-6 cm in diameter; rachis lacking spheroidal projections; petiole bearing domed or saucer-shaped glands along the petiole or at the 1st pair of pinnae.

                                                                     33 Spherical heads pink, 2.5-6 cm in diameter

                                                                     33 Spherical heads white, cream, or pale yellow, 0.5-2.2 cm in diameter.

                                                                       34 Fruits strongly coiled; stamens many; tree

                                                                       34 Fruits strait or slightly curved; stamens 10; tree or shrub.

                                                                         35 Stipules persistent and evident, 3-10 mm long; fruits 5.5-8.5 cm long, 2-5 mm wide

                                                                         35 Stipules caducous; fruits 8-20 cm long, 10-20 mm wide

                                                                 31 Branches variously armed, with nodal spines, thorn spurs, or catclaw prickles scattered along internodes.

                                                                           36 Branches armed with catclaw prickles scattered along the internodes

                                                                           36 Branches armed either with woody thorn spurs or paired nodal spines.

                                                                              37 Branches armed with stout thorn spurs (these simple or branched); inflorescences cylindrical, bicolored, the base of pink or whitish staminodes, the tip of yellow stamens; fruits flattened, but also curved and contorted

                                                                              37 Branches armed with paired nodal spines, these either narrow and needle-like or massive, hollow, and with an entrance hole; fruits turgid, straight or slightly curved