Trees, shrubs, or subshrubs; stipules large to very large (1.5–11 cm long), intra- and interpetiolar, the 4 at each node pressed or fused together to form a sheath enclosing the shoot apex, deciduous, leaving large scars; leaves eglandular. Inflorescence an axillary shoot with 1 internode and 1 node below a pseudoraceme of single or clustered non-decussate flowers; pedicels sessile, the floriferous bract and bracteoles clustered at the base of the pedicel, eglandular, persistent or deciduous. Sepals leaving outermost petal exposed in enlarging bud or concealing it almost up to anthesis, all biglandular, the glands borne mostly below free part of sepals; corolla bilaterally symmetrical, the posterior petal more or less different from the lateral 4; petals mostly white, occasionally described as pinkish or pale yellow, glabrous or abaxially hairy; lateral petals entire, erose, or denticulate; androecium radially symmetrical; stamens 10, all fertile, glabrous except for a few hairs at adaxial base of filaments; filaments distinct, alike or longer opposite petals than opposite sepals; anthers alike, the outer locules bearing longitudinal wings widening from base to apex, the connective bearing an apical appendage; pollen radially symmetrical, 3-colporate; receptacle bearing straight basifixed hairs on both sides of the androecium; gynoecium radially symmetrical; ovary with the 3 carpels distinct, all fertile; styles 3, attached ventrally or subapically, slender and subulate with minute apical or subapical stigmas. Fruit dry, comprising 3 cocci borne on a flat or pyramidal torus; cocci subspheroidal, unwinged, smooth, indehiscent, with a brittle papery wall. Chromosome number unknown.
Seven species, all Brazilian, most from near rivers in forests that are periodically flooded, in the Amazon drainage; one species is known from a white-sand campina in southern Amazonas, and one from the cerrados of Piauí. [map]
Acmanthera resembles its sister genus, Pterandra, in the winged anthers and the distinct carpels growing into smooth cocci. They differ in the inflorescence, which is much less reduced in Acmanthera than in Pterandra, and in the very long, narrow, deciduous stipules of Acmanthera. The other Amazonian genus with winged anthers is Lophanthera; that has glands on the leaves and some bracteoles, and its carpels are connate along a central axis in the ovary. The cocci of Acmanthera often have inflated aerenchymatous tissue at the base, presumably an adaptation for dispersal by water.
Photos: A. latifolia, A. parviflora