Pithecellobium Martius [in Martius & Schrank, Hort. Reg. Monac. 188. 1829, nomen], Flora 20(Beibl. 2): 114. 1837, nom. et orth. conserv. (Camp & al, Brit- tonia 6: 65. 1947). — Sp. lectotypica (conserv.): P. unguis-cati (Linnaeus) Bentham = Mimosa unguis- cati Linnaeus. — Pithecolobium sect. Unguis-cati Bentham, London J. Bot. 3: 196, 197. 1844, nom. superfluum. — Etymology and orthography: Gr. pithecos, monkey + ellobion, earring; in allusion to the pendulous, contorted fruit, the spelling and meaning often modified to Pithecolobium, as though from pithecos + lobos, fruit, and later, by compromise, to the corrupt Pithecollobium. Throughout this text the variant spellings are quoted literatim, except that in all accepted contexts the superfluously conserved spelling Pithecellobium is adopted.

Spiroloba Rafinesque, Sylva Tellur. 119. 1838, ex parte, quoad S. [‘T"] unguiscati (Linnaeus) Rafinesque, caeteris exclusis.

Pithecolobium sect. Unguis-cati sensu Bentham, 1875: 570, max. ex parte (spp. 1, 2, et 12 exclusis). 1876: 432.

Pithecellobium sensu Britton & Rose, 1928: 17, sequ., magna ex parte; Britton & Killip, 1936: 124; Kostermans, 1954: 8; Mohlenbrock, 1963: 438, minori ex parte; Nielsen, 1981: 184.

Pithecellobium sect. Pithecellobium sensu Cardenas, 1974: 122, ex parte.

Xerophilous drought-deciduous and mesophilous evergreen, mostly macrophyllidious shrubs and trees seldom less than 1 m or more than 10 (but P. dulce potentially 15-20) m tall, the majority armed at nodes of long-shoots with lignescent stipules but these randomly lacking from some flowering branches, and in P. keyense all minute or obsolete. Indumentum of either erect or incumbent, pallid or seldom brownish hairs mostly <0.5 (exceptionally to 1.4) mm, often confined to inflorescence or to a dorsal patch on lfts, sometimes 0; branching apparently monopodial; fls always sessile, either capitate or spicate, the units of inflorescence borne either in primary lf-axils or on efoliate brachyblasts (these either extended or compressed), or paniculately pseudoracemose. Lf-formula mostly i—ii/1 —2(—3), the lfts 4-16(-20) per lf, but seldom ii—xi/7—23, the lf-blades amply plurifoliolate; lf-axes either subterete or openly sulcate ventrally, in P. furcatum broadly winged; lf-nectaries cupular, either sessile or shortly stipitate, between each pinna-pair and also (except P. hystrix) at tip of all pinna-rachises; lft-venation pinnate and usually also reticulate. Fls homomorphic (when spicate sometimes a little modified upward along receptacle but a distinct terminal fl never developed), pentamerous (random irregularities); calyx hemispherical, campanulate, or subcylindric (0.8—) 1—7.5 mm, faintly ribbed, shorttoothed; corolla tubular or trumpet-shaped, rarely turbinate (2.5—)3—13 mm, striately nerved or not, the lobes erect or ascending; androecium 16—76-merous, the tube as long or ±½-4 times as long as corolla, sometimes internally callous at base, the callosities rarely developed into a lobed disc; ovary either oblong or ellipsoid (never truncate), either sessile or long-stipitate; ovules 8-14. Pod oblong or linear in profile, backwardly recurved or coiled and sometimes also twisted, the shallowly undulate or evenly curved sutures broad but not prominent, the leathery or woody, red or fuscous valves biconvex over seeds, the cavity continuous or incipiently septate; dehiscence either through both sutures or through the ventral suture only (hence follicular); seeds following dehiscence dangling on and invested by a red, pink, or whitish, spongy arilliform funicle, plumply biconvex, the testa typically hard, lustrous black, with pleurogram, in the rest papery, brown or black, lacking pleurogram; endosperm 0. — Spp. 18, of lowland tropical North and South America, three extending N into subtropical Mexico, Bahamas, and peninsular Florida, one of these (P. dulce) cultivated and naturalized circumtropically.

The genus Pithecellobium, as first exactly defined by Britton & Rose (1928), differs consistently from all other Ingeae in one character: the seed-funicle modified into a spongy aril that cups the lower one- third or one-half of the seed. At dehiscence the seed is suspended on this red, pink, or white arilliform funicle, which serves as an elaiosome, attractive to birds and edible to man. The genus is further characterized, with one exception, by obligate or at least potential armament of lignescent stipules. The exception is the geographically marginal P. keyense, in which the stipules are reduced to small scale-like structures or are completely lost. This loss of armament is almost certainly a reversal to the primitive soft or rudimentary stipule prevalent in Ingeae. In the Americas, spinescent stipules are seen outside Pithecellobium in the genera Havardia, Painteria, Ebenopsis, and Sphinga, and in one species each of Zapoteca and Calliandra, these distinguished collectively by absence of petiolar nectaries, Calliandra further by distichous phyllotaxy. Some species of Havardia and related genera are difficult to separate from Pithecellobium at anthesis, although they are notably different in fruit. The leaflet-venation of Pithecellobium is primarily pinnate and thence reticulate, and the leafstalk nectaries are interpinnal. In the Havardia group, either the leaflet-venation is palmate (in smallest leaflets reduced to the midrib) or the first leaf-stalk nectary is borne on the petiole, below the first (or only) pair of pinnae. The fruits of Havardia, Sphinga, and Painteria are planocompressed and straight, or decurved and strongly compressed, but dehisce inertly by both sutures and lack the dispersal strategy of Pithecellobium; the fruit of Ebenopsis is lignified, tardily inertly dehiscent, and compartmented into locules by complete partitions.

The few spiny Asiatic Ingeae lack the specialized fruit and aril of Pithecellobium and have been referred by Nielsen (1981: 186) to Havardia, although we here hypothesize that they form a phylogenetically distinct group derived independently from a primitive al- bizioid stock. Pithecellobium, in consequence, is an endemic neotropical genus.

The dispersal of the four major groups of Pithecellobium corresponding with divisions I. II. III, and IV of the conspectus that follows suggests that the typical group, nearly ubiquitous in tropical lowlands north of the equator, both on the continents and the Antilles, is the primitive (though in context of tribe Ingeae highly evolved) core-element of the genus. The third group, which is continental only and is dispersed from Venezuela to Mexico, has its center of diversity in southeastern Mexico and the northwestern states of Central America. It is at the middle of this region that the presumably climactic second and fourth groups have arisen.

Cassens and Miller (1981) noted that Pithecellobium, Havardia, Sphinga (cited as Havardia), and Albizia sects. Albizia and Parviflorae are the only members of Ingeae whose wood has septate fibers and nonconfluent parenchyma. The Albizia-type, though, may be distinguished on the basis of differences in ray width, pore diameter, and vessel element length. Pithecellobium is further distinguished from Havardia and Sphinga by having a large number of gelatinous fibers.