Trees, shrubs, or subshrubs; leaves eglandular. Inflorescence lateral; flowers single (i.e., all cincinni 1-flowered); pedicels sessile or both peduncles and pedicels absent; bracts and bracteoles eglandular. Sepals all biglandular or all eglandular; petals mostly white or pink, green or yellowish in a few species; lateral petals entire, erose, or denticulate; receptacle bearing straight basifixed hairs on both sides of the androecium; androecium radially symmetrical; stamens 10, all fertile; filaments distinct, alike or longer opposite petals than opposite sepals, mostly glabrous except for hairs at adaxial base (adaxially hirsute in 1 species); anthers alike, glabrous; pollen radially symmetrical, 3 (4)-colporate; 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; cocci subspheroidal, unwinged, smooth, indehiscent, with a brittle papery or corneous but not bony wall.
Distribution: South America and Panama.
The Acmanthera clade is supported strongly by both molecular and morphological evidence, as is evident in the phylogenetic tree shown above. That tree is from Davis & Anderson (2010 [pdf]); the numbers above the branches are bootstrap values. In all three genera the carpels are distinct even in the ovary, with each developing into a dry, unwinged, brittle-walled, subspheroidal coccus; such fruits are found in no other clade of Malpighiaceae. In the Galphimia clade, which has similar-looking mericarps, the carpels are connate along a central axis in the ovary and the fruit is schizocarpic.
The petals in the Acmanthera clade are mostly white or pink, greenish in a few species, sometimes turning yellowish in age—the truly yellow petals found in many basal Malpighiaceae, e.g., Galphimia, Byrsonima spp., are completely unknown here. The clade is also interesting for the fact that the inflorescences are strictly lateral and all the flowers are borne singly (one-flowered cincinni); in the other clades at the base of the family terminal inflorescences are common and at least some genera have retained in some species the presumably ancestral several-flowered cincinni.
In other characters the Acmanthera clade fits well into the larger grouping that has been called subfamily Byrsonimoideae: erect (non-vining) habit, well-developed intrapetiolar stipules, sepals all biglandular or all eglandular, small, radially symmetrical, tricolporate pollen, and slender subulate styles with tiny apical or slightly subapical stigmas. The Acmanthera clade and its sister, the Byrsonima clade, are alike in having their leaves, bracts, and bracteoles completely eglandular.
The Acmanthera clade has the formal tribal name Acmanthereae W. R. Anderson, Leandra 7: 11. 1978 ["1977"], with its type the genus Acmanthera (Adr. Juss.) Griseb.