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

Hughes, Colin E., Ringelberg, Jens J., Lewis, Gwilym P. & Catalano, Santiago A., 2022, Disintegration of the genus Prosopis L. (Leguminosae, Caesalpinioideae, mimosoid clade), PhytoKeys 205, pp. 147-189 : 147

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.

Figures

Figure 1. A Phylogeny of the Caesalpinioideae showing the placement of the Prosopis grade (boxed in red) within the subfamily, based on analyses of DNA sequences of 997 nuclear genes (Ringelberg et al. 2022) B the part of the phylogeny that includes all elements of Prosopis s. l. Genera recognised in the new generic system presented here are in bold. Pie charts show the fraction of gene trees supporting that bipartition in blue, the fraction of gene trees supporting the most likely alternative configuration in green, the fraction of gene trees supporting additional conflicting configurations in red and the fraction of uninformative gene trees in grey. Numbers above pie charts are Extended Quadripartition Internode Certainty (Zhou et al. 2020) scores. Branch lengths are expressed in coalescent units and terminal branches were assigned an arbitrary uniform length for visual clarity, see Ringelberg et al. (2022); the root is not drawn to scale C, D the two most likely alternative tree topologies which would allow for a monophyletic Prosopis s. l., either without (C) or with (D) Xerocladia and Indopiptadenia. In C and D numbers above pie charts = number of gene trees supporting the species tree, numbers below pie charts = number of gene trees conflicting with the species tree C lack of gene tree support (just 69 gene trees) for the alternative species tree topology where sections Algarobia + Monilicarpa (≡ Neltuma) are sister to section Strombocarpa (≡ Strombocarpa) vs. 573 genes supporting a sister group relationship between Strombocarpa and Xerocladia (as shown in D) D lack of gene trees (zero gene trees) supporting a monophyletic Prosopis s. l.

Figure 2. Variation in armature of Prosopis, Strombocarpa, Neltuma and Xerocladia A Neltuma denudans (nodal spines on a zig-zag stem) B N. humilis (paired striate spine-tipped branches) C Prosopis cineraria (scattered internodal prickles) D Neltuma sericantha (spine-tipped stems) E Strombocarpa burkartii (stipular spines) F Neltuma argentina (single nodal axillary spine) G N. kuntzei (spinescent shoots) H Strombocarpa ferox (stipular spines) I S. strombulifera (stipular spines) J Neltuma elata (variation in paired nodal spines on one specimen) K N. alba (paired nodal spines) L N. velutina (paired nodal spines) M Prosopis farcta (scattered internodal prickles) N Neltuma ruscifolia (single nodal axillary spine) O Xerocladia viridiramis (recurved, deflexed stipular spines) (5 cm scale bar). All specimens at K A drawn from Seijo 1489 B Tweedie s. n. C Willcox 299 D MERL 8792 E Acosta & Rosas 748 F Guaglianone et al. 1762 G Nee & Coimbra 35556 H Atahuachi et al. MA 1147 I Hunziker 2036 J Legname & Cuezzo 10396 (large and small spines from same specimen) K Hughes & Forrest 2312 L Harding & Balsinhas 140 M Guest et al. 17463 N Wood & Mamani 14063 O Kolberg & Tholkes HK 2493. Drawn by Andrew Brown, July 2021.

Figure 3. Variation in armature across Prosopis s. l. and allies A stipular spines of Strombocarpa strombulifera B internodal prickles on shoots and branches of Indopiptadenia oudhensis which it shares with its sister group, Prosopis s. s. illustrated in C; C internodal prickles of Prosopis farcta D stipular spines of Xerocladia viridiramis which it shares with its sister group, the genus Strombocarpa illustrated in A; E axillary nodal spines of Neltuma juliflora F spinescent straight cylindrical shoots of the subaphyllous Neltuma kuntzei. Photos courtesy of Guillermo Debandi (A) (see https // www. inaturalist. org / taxa / 78750 - Prosopis-strombulifera / browse _ photos), Dr. Omesh Bajpai and Dr. Lal Babu Chaudhary (B), Zeynel Cebeci (C) (see https // commons. wikimedia. org / wiki / FileProsopis _ farcta _ - _ Syrian _ mesquite _ 01), N. Dreber (D) (see http // www. southernafricanplants. com /), Colin Hughes (E, F).

Figure 5. Fruits of Prosopis, Strombocarpa, Xerocladia and Indopiptadenia A Prosopis cineraria B Anonychium africanum C Strombocarpa palmeri D Prosopis farcta E Strombocarpa ferox F S. strombulifera G S. pubescens H S. abbreviata (2 examples) I S. tamarugo J S. torquata K S. burkartii L Xerocladia viridiramis M Indopiptadenia oudhensis A-G, M (5 cm scale bar) H-L (1 cm scale bar with asterisk). All specimens at K A drawn from Gazanfar SG 4332 B Dembele & Sanogo ML- 146 and longitudinal section of fruit from Barter 1193 C Hughes et al. 1552 D van der Maesen 1627 E Atahuachi et al. MA 1147 F Hunziker 2036 G Acocks 1788 H Tweedie s. n. (from 2 type specimens) I Aronson 7742 J Vuilleumier 1019 K Acosta & Rosas 748 L Kolberg & Tholkes HK 2493 M Bajpai & Babu 264498. Drawn by Andrew Brown, July 2021.

Figure 7. Variation in fruits across Prosopis s. l. and allies A indehiscent pods of Anonychium africanum with thick pulpy mesocarp collected as fodder for livestock B plano-compressed pods of Indopiptadenia oudhensis lacking a thickened mesocarp and dehiscent along both sutures C indehiscent fruits of Prosopis farcta with a thick pulpy mesocarp D tightly coiled indehiscent screwbean fruits of Strombocarpa strombulifera E indehiscent pods of Strombocarpa ferox with a thick pulpy mesocarp F indehiscent fruits of Strombocarpa palmeri G small reniform to flabellate, flattened, indehiscent, 1 (- 2) - seeded, winged fruits of Xerocladia viridiramis which are unique within mimosoid legumes H indehiscent fruits of Neltuma articulata with a thick mesocarp and a hard bony segmented endocarp which remains closed I. Unripe indehiscent pods of Neltuma kuntzei with a thick pulpy mesocarp, these turning dark blackish-brown when ripe, reminiscent in colour to fruits of Anonychium. Photos courtesy of Marco Schmidt (A) (see Dressler et al. 2014), Dr. Omesh Bajpai and Dr. Lal Babu Chaudhary (B), Zeynel Cebeci (C) (https // en. wikipedia. org / wiki / Prosopis _ farcta), Dick Culbert (D) (see https // eol. org / pages / 640506, Colin Hughes (E, F, H, I), and Herta Kolberg (G) (see Plants of Namibia https // herbaria. plants. ox. ac. uk / bol / namibia).

Figure 8. The distributions of Indopiptadenia, Prosopis s. s., Anonychium, Xerocladia, Neltuma and Strombocarpa, based on 6,469 quality-controlled species occurrences from GBIF (www. gbif. org), DryFlor (www. dryflor. info), SEINet (www. swbiodiversity. org / seinet) and several other data sources (Ringelberg et al., in prep.). Map created using R packages ggplot 2 (Wickham 2016), sf (Pebesma 2018) and rnaturalearth (South 2017). The eight occurrence records, mapped in Bahia Brazil, are of Neltuma ruscifolia which is considered potentially native to that region (Burkart 1976 Oliveira & Queiroz 2020), while records of N. juliflora from Bahia, which is introduced and naturalised in that region, have been eliminated.