Strombocarpa (Benth.) Engelm. & A. Gray, Boston J. Nat. Hist. 5: 243. 1845.

Spirolobium A.D. Orb., Voy. Amér . Mér . 8 (Atlas, Bot): t. 13. 1839, nom. rej., non Spirolobium Baill. 1889. ( Apocynaceae).

Prosopis sect. Strombocarpa Benth., J. Bot. (Hooker) 4: 351. 1841.

Sopropis Britton & Rose in N.L. Britton & al. (eds.), N. Amer. Fl. 23: 182. 1928.

Type.

Prosopis strombulifera (Lam.) Benth. [= Strombocarpa strombulifera (Lam.) A. Gray].

Description.

Low spiny, sometimes creeping, shrubs or small trees, 0.15-3 (-18) m high, multi-stemmed from the base or sometimes with a short trunk to 10-30 (-45) cm diameter, usually densely and intricately much-branched, some species forming long underground, spreading, horizontal runners (gemmiferous roots or rhizomes), armed with strongly decurrent, straight, cinereous spiny stipules (Figs 2E, H, I and 3A), 0.1-3.5 (-5.5) cm long, brachyblasts congested, blackish. Leaves always unijugate, the petiole (0.5-) 2-15 mm, the pinnular rachises 1-4 cm long, with 3-30 pairs of well separated, alternate to opposite leaflets, these oblong or elliptic-oblong, obtuse to subacute, veins lacking or weakly 1-3-veined, 2-12 × 0.6-4 mm, glaucous, puberulous or glabrescent. Inflorescences axillary, solitary, globose, ovoid-elliptic heads to 1.5 cm diameter at anthesis or shortly cylindrical-spicate, 3-8 cm long. Flowers small, bright or lemon yellow, young filaments red; calyx, 1.5-2.3 mm long; corolla 3-4 (-6) mm long, the petals linear, partially united, villous within; stamens and style exserted, anthers with a minute, caducous, incurved claviform gland arising from the connective. Fruits densely clustered with 1-21 per flower head, indehiscent, lemon-yellow, straw-yellow or reddish-brown when ripe, slender, elongate, straight or falcate (in S. palmeri and S. ferox; Figs 5C, E and 7E-F), but usually more or less tightly spirally coiled (like corkscrews) with (1-) 8-19 (-24) regular coils, forming a cylindrical body 1.8-5.5 × 0.6-1.5 cm (Figs 5F, G and 7D) or irregularly and more openly coiled; exocarp crustaceous, mesocarp thin or more usually thick and pulpy, tannic, reddish, endocarp delicately segmented in longitudinal or transverse seed chambers which are easy to open or hard and closed. Seeds ovate or reniform ovoid, grey-green, 3-6 (-7) × 3-4 mm.

Geographic distribution.

Ten species. Restricted to the New World and there occupying a markedly bicentric amphitropical distribution in arid and semi-arid regions of N. America (southern U.S.A., especially in the Sonoran Desert, Baja California and northern Mexico (Coahuila)) and S. America (south-central Peru to Argentina and Chile) (Fig. 8).

Habitat and uses.

In cactus-rich semi-desert Monte vegetation, deserts and arid mesetas, dry river beds and washes and in the hyper-arid Pampa del Tamarugal in northern Chile ( S. tamarugo), where it is the only tree present and dependent on moisture absorbed from fog. Fruits browsed by cattle and sheep and much valued in arid deserts for that purpose. Wood valued for fuel, and occasionally cultivated ( S. tamarugo).

Etymology.

Strombo - (Italian. = conch) and - carpa (Gk. = fruit), referring to the resemblance of the fruits to the spiral shells of tropical marine molluscs (see Figs 5F, G and 7D).

Affinities.

Strombocarpa is robustly supported in recent molecular phylogenies as sister to the African monospecific genus Xerocladia (Fig. 1; Ringelberg et al. 2022). These two genera share the diagnostic synapomorphy of stipular spines which are not found elsewhere in Prosopis s.l.