Pithecolobium sect. Samanea ser. Corymbosa [sic] Bentham in Hooker, London J. Bot. 3: 221. 1844. — Sp. typica (unica): P. corymbosum Bentham.
Pithecolobium sect. Samanea ser. Subarticulatae [sic] (Americanae) sensu Bentham, 1875: 586, magna ex parte, exclus. spp. no. 60, 62.
Unarmed trees and arborescent shrubs commonly 5-27 m, some precociously fertile when smaller, the young stems and foliage gray- or brown-pilosulous, the lfts nearly always conspicuously bicolored, the inflorescence of umbelliform units of dimorphic flowers mostly axillary to coeval or distally to hyster-anthous lvs, the meristem continuous beyond them, in one species (of uncertain affinity) appearing terminally paniculate; phyllotaxy spiral. Stipules linear or subulate, caducous, perhaps sometimes 0. Lf-formula i—vii/3—35; petiolar nectary between or close below insertion of first or only pinna-pair, either sessile or stipitate (rarely suppressed), the head variably plane, convex or shallow-cupular; lft-pulvinules ±0.5-2.5 mm; venation of lfts pinnate. Peduncles 2-11 (-14) cm; fls compactly racemose-capitulate, the axis including pedestal 1.5-6 mm, the peripheral fls slenderly pedicellate, mostly 5-merous, the 1-4 distal fls sessile or shortly stoutly pedicellate, 5-8-merous; calyx campanulate or turbinate-campanulate (1-)1.4-7mm, corolla (2.6—)3—11 (— 12) mm; androecium of peripheral fls 10-24(-36)-merous, 11-43 mm; ovary truncate at apex, 8-12-ovulate. Pods (that of H. acreana unknown) often several per capitulum, stiffly erect-ascending, sessile or shortly pseudostipitate, in profile linear or broad-linear, straight or almost so, compressed but low-biconvex over each seed and depressed between them, the valves leathery or sublignescent, the mature fruit lomentiform (4.5-)5-12.5 x 0.75-1.9 cm, readily breaking horizontally between seeds and less readily so through the shallowly undulate sutures into square or transversely oblong, 1- seeded articles closed at each end by half of the interseminal septum; seeds hard, brown, areolate, the pleurogram complete. — Spp. 4, widespread in riparian habitats in the Orinoco and Amazon basins and in the Guianas, in Brazil extending E to Maranhão and S to the Pantanal in Mato Grosso do Sul.
The relationships and diagnostic features of Hydrochorea are discussed under the genus Balizia, where Hydrochorea is shown to differ principally from Balizia and Abarema in its lomentiform fruit adapted to dispersal by water and brown (as opposed to partly translucent) seed-coat. The commoner species of Hydrochorea have been associated by students of Ingeae with those American albizias that have comparable jointed fruits (but not true loments). Consequently, they have been transferred successively to the Old World genus Cathormion Hasskarl, the neotropical genus Arthrosamanea, and latterly to a loosely defined and polymorphic Albizia. They form, nevertheless, a closely coherent group of species different from all the above in duration of meristem, features of inflorescence-architecture, leaf- venation, or shape of ovary, or in some combination of those. As the species of Hydrochorea differ mostly by autapomorphic characters, their relationships to each other remain unresolved (Fig. 4A).