VI. SAMANEA MERRILL
Samanea Merrill, J. Wash. Acad. Sci. 6: 46. 1916. — Sp. typica: Samanea saman (Jacquin) Merrill = Mimosa saman Jacquin = Pithecolobium saman (Jacquin) Bentham = Albizzia saman (Jacquin) F. v. Mueller. Calliandra sect. Samanea Grisebach, Fl. Brit. W. I. 225. 1864.
Pithecolobium sect. Samanea ser. Carnosae [sic] Bentham, Trans. Linn. Soc. London 30: 587. 1875, ex parte, omnibus praeter lectotypum, P. saman, exclusis.
Samanea sensu Britton & Rose, 1928: 34. quoad sp. typ., caeteris expulsis; Mohlenbrock, 1963: 435, parva ex parte, seriebus Coriaceis et Parvifloris omissis.
Albizia sensu Nielsen, 1981: 180, fig. 1(2), minori ex parte.
Macrophyllidious trees, some attaining great age and size, the branching sympodial, new growth arising yearly from buds below the prior season’s determinate inflorescence, the lfts folding forward slowly at night or after shock. Stipules herbaceous, lanceolate, early deciduous. Lf-formula iii—vi(—vii)/3—8(—9); lft-venation pinnate. Capitula axillary to coeval or quickly hysteranthous lvs, umbelliform, the fls dimorphic, the peripheral ones at least shortly pedicellate, the terminal one stouter, sessile; calyx of peripheral fls vase-shaped 4.5-11 mm, the corolla 7-14.5 mm; androecium (16-)20-36-merous, the tube included, the tassel pink or reddish; ovary (sub)sessile, narrowly ellipsoid. Pods subsessile, in profile broad-linear, straight or nearly so, either compressed but plump or biconvex, the valves composed of thin, glabrous or densely puberulent exocarp, thick pulpy, when ripe pitchlike mesocarp, and crustaceous-lignescent endocarp either adherent or narrowly septiferous between seeds; dehiscence 0, the seeds released by weathering, or by predators, or following excretion by cattle; seeds transverse on slender compressed funicle, oblong-ellipsoid, the testa hard opaque, closely investing the embryo, areolate. Germination in S. tubulosa (M. Nee 39105, NY) is epigeal, the first two eophylls subopposite, the proximal one simply pinnate, 4—6-foliolate, the distal one bipinnate, each pinna 4-foliolate. — Spp. 3, endemic to tropical continental North and South America, native from El Salvador southeastward to E Brazil, Paraguay, and NE Bolivia, but one species (S. saman) planted and subspontaneous northward into Mexico and long established in the West Indies and in parks, gardens, and agroforestry plantations of the Old World.
The genus Samanea, internally uniform in characters of substance, belongs unequivocally among the albizioid Ingeae, of which it has the dimorphic terminal flower and the hard areolate seeds. Indehiscence of the fruit, on which emphasis has been placed in former classifications, is a mechanical consequence of an incrassate pericarp. It has evolved independently in various neotropical Ingeae (e.g., Enterolobium, Balizia, and Albizia ex parte, as well as in Inga itself) and also in some Madagascan Albizia (Capuron, 1970: 22 passim). Indehiscence becomes taxonomically significant only within particular geographic and coincident morphological contexts. In Madagascar differentiation of Albizia must have proceeded pari passu with speciation, for the endemic species, while extraordinarily diverse in form, texture, and dehiscence of the fruits, have preserved evidence of a common lineage in the peculiar displacement of the areole to the base of the seed, a feature encountered nowhere else in the genus. Samanea possesses no known feature unique in tribe Ingeae. However, though the fruits of Samanea are technically indistinguishable from those of Enterolobium, they are never confused. Evolution of its fruit, by acquisition of sweet nutritive pulp in the mesocarp, loss of dehiscence, and hard seeds that pass undigested through the gut of herbivores, appears to be a single (if plurifaceted) biological specialization that arose prior to the differentiation of the three modern species. Samanea is certainly monophyletic, and on the basis of cladistic analysis sectional status within a more comprehensive Albizia is indefensible.
Samanea is most closely related to Pseudosamanea, and at anthesis S. saman and Ps. guachapele are often mistaken for one another. Samanea differs from Enterolobium, which has a similar dispersal mechanism, in organization of the inflorescence, and in the straight fruit. If Samanea were subordinated to Albizia nothing but tradition would prevent subordination of Enterolobium to equivalent rank.
When Merrill proposed a genus Samanea to accommodate the Rain Tree (and perhaps other species, obliquely alluded to), he traced in detail its checkered taxonomic history, which involved successive transfers from genus to genus; it is superfluous to repeat the story, implicit in the synonymy of S. saman set out below. Britton & Rose (1928: 1, in clave, 34) expanded the definition of Samanea so as to include along with the type species Balizia leucocalyx, Abarema macradenia, and Cojoba filipes. Mohlenbrock (1963: 435) expanded Samanea yet further to embrace all of Bentham’s series Carnosa, Coriacea, and Parviflora of Pithecolobium. He seems to have been unaware that the fruits of all Coriacea (= our genus Macrosamanea) as well as those of some Parviflora are inertly dehiscent, as is that typical of Albizia. Capuron (1970) recognized Samanea in Merrill’s sense, whereas Cárdenas (1974: 125) reverted to an orthodox Pithecellobium ser. Carnosa Bentham. Finally Nielsen (1981: 180) returned Samanea to a comprehensive circumtropical Albizia.
Cassens and Miller (1981) differentiated the wood of Samanea from that of all other New World Ingeae on the basis of the following syndrome of character states: septate fibers and confluent parenchyma both lacking, intervascular pits 7-10 [im, and mostly biseriate rays.