Awkwardly branched arborescent shrubs mostly 2-5 m but sometimes (in savanna) flowering as erect shrubs 0.3-1.5 m, the older trunks and branches gray, lenticellate and glabrous, the young branchlets and all axes of lvs and inflorescence sordidly pilosulous with short spreading-ascending hairs, the stiffly papery or in age coriaceous lfts usually glabrous or almost so but rarely microhirtellous on both faces, the capitula of long whitish fls borne, either singly or 2-3 together, on long peduncles in the axil of coeval lvs or more shortly pedunculate in small functionally terminal and subterminal pseudoracemes. Stipules erect firm, narrowly lanceolate, 1.5-4(-5.5) mm, caducous. Lf-formula i or i-ii/14-22(-29); lf-stks of lvs with 1 pair of pinnae 5—16(—17) mm, of those with a second pair 2-5 cm, the interpinnal segment then 1.5-3.5 cm; a depressed elliptic, grossly comeous- margined nectary 1.6-4 mm in long diameter inserted immediately below first (or only) pair of pinnae, and a smaller suborbicular one below second pair (when present), further, much smaller ones between distal or between all but a few proximal pairs of lfts; rachis of the one pair or of the longer, distal pair of pinnae 7-15.5 cm, the longer interfoliolar segments 3-7.5 mm; paraphyllidia either linear-subulate 0.3-1 mm, or reduced to minute conic points, or absent, the posterior one exceptionally represented by a small 1ft; 1ft- pulvinules transversely elliptic 0.5-1 x 0.8-1.7 mm; lfts subequilong except for a few smaller proximal pairs and the longer narrower distal pair, the blades lanceolate from asymmetrical base where dilated and obtuse on anterior side and cuneate on posterior side, the longest ones (15-) 18-31 x (3.8-)4-6(7) mm, (3.9-)4-5.5(-5.7) times as long as wide, all obtuse but mucronulate at apex, prominently 2-nerved from pulvinule, the nearly straight or outwardly curved (but not sigmoid) midrib either subcentric or forwardly displaced to divide blade ±1:2, pinnately branched and the secondary nerves brochidodrome well within the plane margin, the posterior basal nerve produced to or beyond midblade, the tertiary connecting venules few and weak, the whole venation prominulous either on both faces or only beneath. Peduncles 4-45(-55) cm, charged immediately below the first fls with 1-3 deltate bracts bearing on inner face a nectary like those of lf-stks; capitula 3—12-fld, the globose or shortly clavate receptacle 2-5 mm diam, the ascending fls either subsessile or elevated on stout pedicel up to ±1 mm; floral bracts caducous (few seen), narrowly elliptic or linear-oblanceolate 2.5-5.5 mmm; calyx either cylindric or tumid and cylindro-campanulate (6.5—)7.5—15 x (3—)3.5—5 mm, densely minutely sordid-puberulent externally, many- nerved but the nerves not very prominent, the ovate or lanceolate teeth quite variable even at times in one calyx, 2.5-6 mm; corolla narrowly trumpet-shaped, pubescent like the calyx but in early anthesis a little more densely so (thus appearing paler), (15)17-32 mm, the lanceolate, in age subinvolute lobes (3)4-8 mm; androecium 30-53 mm, the stamen-tube a little shorter or a little longer than the corolla, the exserted filaments white or white tinged with pink distally; style up to 1 cm longer than longest filaments, a trifle dilated at apex; ovules ±14-20, often many aborting. Pods solitary or up to 3-4 per capitulum, subsessile, in profile narrowly oblong or broad-linear, straight or gently recurved, extremely variable in absolute width and in width relative to length, (6.5)7.5-21 x 1.5-3.2 cm, (2.6)3-14 times as long as wide, piano-compressed, the coriaceous brown valves plane except where low-umbonate over maturing seeds, framed by thickened sutures, reticulate-venulose and minutely sordid-puberulent overall; dehiscence and ripe seeds unknown.

Macrosamanea discolor is closely related to M. pubiramea, of which it has nearly the individual flower, but is distinguished, at least in typical form, by reduction of pinnae to one or two pairs per leaf, and by depauperate capitula of at most a dozen flowers. In width proportionate to length, its leaflets resemble those of M. pubiramea var. lindsaeifolia, with which it is rather widely sympatric and from which it may be derived. With some hesitation we recognize two varieties of M. discolor, seemingly differentiated by habitat but not always cleanly separable in the herbarium. The var. arenicola appears to occupy an enclave within the range of var. discolor and to be found in wet savanna rather than on flooded river banks, but its ecology requires study in the field.

The description of Macrosamanea aquatica and Pithecellobium subaquaticum from parts of one collection was an accidental duplication. The types are unquestionably conspecific with the original Inga discolor, discovered by Humboldt on the Orinoco a little more than 100 km upstream from Sanariapo. Bentham had lost sight of the genuine M. discolor, which lay buried in his concept of Pithecellobium adiantifolium, and when rediscovered was unidentifiable in Bentham’s monograph.