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

Figs 138, 139, 141

Spirolobium A.D. Orb., Voy. Amér . Mér . 8 (Atlas, Bot): t. 13. 1839, nom. rej. vs. Spirolobium Baill., Bull. Mens. Soc. Linn. Paris 1: 773. 1889 ( Apocynaceae). Type: Spirolobium australe A.D. Orb. [= Strombocarpa strombulifera (Lam.) A. Gray]

Sopropis Britton & Rose, N. Amer. Fl. 23: 182. 1928. Type: Sopropis palmeri (S. Watson) Britton & Rose [≡ Prosopis palmeri S. Watson (≡ Strombocarpa palmeri (S. Watson) C.E. Hughes & G.P. Lewis)]

Type.

Strombocarpa strombulifera (Lam.) A. Gray [≡ Mimosa strombulifera Lam.]

Description.

Low spiny, sometimes creeping, shrubs or small trees (Fig. 138B, C), 0.15-3 (18) m tall, multi-stemmed from base or sometimes with a short trunk, 10-30 (45) cm in 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, white or pale-grey, stout, glabrous, 0.5-2 cm long spiny stipules (Fig. 138H); brachyblasts congested, blackish, prominent or obscure, or sometimes absent. Stipules spinescent. Leaves bipinnate, always unijugate; obscure gland between pinnae; leaflets 3-30 pairs per pinna, well separated, alternate to opposite, veins lacking or weakly 1-3-veined. Inflorescences axillary, solitary, globose to ovoid-elliptic capitula (Fig. 138H) or shortly cylindrical spikes (Fig. 139B, C). Flowers bright lemon yellow (Figs 138H, 139B, C), young filaments sometimes reddish; sepals 5, valvate; petals 5, valvate, partially united; stamens 10, free, anthers with a minute, caducous, incurved claviform gland on the connective; pollen in tricolporate monads, pores with costae, exine smooth, perforated, columellae present; ovary sessile or shortly stipitate, stigma porate. Fruits indehiscent, lemon-yellow, straw-yellow or reddish-brown when ripe, slender, elongate, sometimes almost straight or falcate, but usually more or less tightly spirally coiled (Fig. 139I, J) with (1) 8-19 (24) regular coils; 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, testa hard, pleurogram present, not closed.

Chromosome number.

2 n = 28 (Bukhari 1997).

Included species and geographic distribution.

Ten species. Restricted to the New World and there occupying a markedly bicentric amphitropical distribution in arid and semi-arid regions of North America [southern USA, especially in the Sonoran Desert, Baja California, and northern Mexico (Coahuila)] and South America (south-central Peru to Argentina, Bolivia, and Chile) (Fig. 141).

Ecology.

Often abundant in cactus-rich, semi-desert Monte vegetation, deserts and arid mesetas, dry river-beds and washes (Fig. 138B, C) and in the hyper-arid Pampa del Tamarugal in northern Chile [ S. tamarugo (Phil.) C.E. Hughes & G.P. Lewis] (Fig. 138B), where it is the only tree present and dependent on moisture absorbed from fog. The indehiscent fruits are consumed by herbivores facilitating endozoochorous seed dispersal.

Etymology.

Strombo - (Italian = conch) and - carpa (Greek = fruit), in reference to the resemblance of the fruits to the spiral shells of some tropical marine molluscs (Fig. 139J).

Human uses.

Fruits browsed by cattle and sheep and much valued in arid deserts for that purpose (Bell and Castetter 1937). Wood valued for fuel, and occasionally cultivated ( S. tamarugo) (Pasiecznik et al. 2001).

Notes.

Strombocarpa is one of three genera segregated from Prosopis s.l. (Hughes et al. 2022a) and corresponds to Burkart’s section Strombocarpa Strombocarpa of Prosopis s.l., characterised by armature in the form of spinescent stipules (Fig. 138H) which it shares with its sister genus Xerocladia (Fig. 132; Ringelberg et al. 2022), and which are not found in either Prosopis s.s. nor the genus Neltuma . A subset of species, the so-called ‘Screw-Beans’ (in the USA), like the ecologically important velvet mesquite, S. pubescens (Benth.) A. Gray in North America, have highly distinctive spirally coiled fruits. Across the genus as a whole fruits range from weakly falcate, to strongly curved, annular and tightly coiled (see Hughes et al. 2022b, Fig. 5).

Taxonomic references.

Benson (1941); Burkart (1976); Hughes et al. (2022b); Palacios (2006).