Woody vines, sometimes described as shrubs; leaves usually opposite, sometimes subopposite or alternate; stipules very small, triangular, borne on stem between petioles; petiole eglandular or biglandular near apex; lamina eglandular or bearing 2–many small glands impressed in abaxial surface between midrib and margin, on very margin at base in A. concinna but usually somewhat set in from margin in other species. Inflorescence with the flowers borne in elongated terminal or axillary pseudoracemes or panicles; bracts large (2–8 mm long) and spreading, often lanceolate and short-petiolate, often bearing 1 (2) submarginal or marginal glands on each side near base; pedicels pedunculate; bracteoles like bracts but narrower, shorter, and less often bearing glands. Sepals leaving petals exposed during enlargement of bud, the lateral 4 bearing large paired abaxial glands (occasionally the 2 glands adjacent to anterior sepal absent), the anterior eglandular (rarely bearing 1 or 2 small glands); corolla bilaterally symmetrical, the posterior petal erect, its claw usually longer and thicker and its limb usually smaller than in the lateral 4 petals; petals yellow or yellow turning red-orange in age (especially the claws), abaxially densely velutinous, tomentose, or sericeous, at least on proximal half (only thinly sericeous in A. concinna), adaxially glabrous or densely to sparsely hairy on distal half; limb of petals entire or slightly erose; androecium radially symmetrical or nearly so; stamens 10, all fertile; filaments connate at base, straight, subequal or longer opposite sepals than opposite petals; anthers alike; gynoecium bilaterally symmetrical; carpels 3, connate their whole length in ovary, all fertile; styles 3, erect and straight to recurved in age, with large internal stigmas, the apex dorsally rounded or truncate or acute to short-hooked. Fruit dry, breaking apart into samaras separating from a short or moderately high pyramidal torus; samara butterfly-shaped to depressed-elliptical with lateral wings dominant, chartaceous with many fine parallel veins, cleft to nut at apex, continuous at base or cleft part-way or completely to nut, the margin entire or undulate to coarsely dentate; dorsal wing small, distinct at apex and base or confluent with lateral wings at base; nut almost always smooth between lateral and dorsal wings; ventral areole broadly ovate or rotund to very narrowly elliptical. Chromosome number unknown.
Ten species, all South American, found in diverse habitats from northern Colombia to Rio Grande do Sul, Brazil. [map]
Among the South American genera with mascagnioid fruits, Amorimia is distinguished by its leaf glands on the abaxial surface, its abaxially hairy petals, its large spreading gland-bearing bracts, and its straight erect styles. W. Anderson (2006b) suggested on the basis of morphology that this is the one mascagnioid genus most likely to be closely related to Mascagnia, and that suggestion is supported by the molecular data of C. C. Davis and W. R. Anderson (2010 [pdf]; family phylogeny), which places Amorimia as sister to the large and complex Malpighia clade, in which Mascagnia is basal. Amorimia is easily distinguished from Mascagnia, which has glabrous petals, small mostly eglandular bracts, a membranous samara with a prominent reticulum of arching anastomoses, and a fleshy disk subtending the samaras. For discussion of the reasons for breaking up the old polyphletic Mascagnia sens. lat., see W. Anderson (2006b).
The latest study of the phylogeny of Malpighiaceae (Davis & Anderson, 2010 [pdf]) includes six species of Amorimia (A. amazonica, A. exotropica, A. kariniana, A. pubiflora, A. rigida, and A. velutina). The monophyly of that group received 99% bootstrap support. Unfortunately, the one species that stands out as odd in the genus, and about whose generic position there is room for doubt, is A. concinna, known from only a few collections in northern Colombia, and we have not yet succeeded in obtaining sequences for that species
Reference: W. R. Anderson (2006b) gave descriptions of the genus and of new species, brief notes on the species that were not new, and a key to all the species (Amorimia only: pdf; entire paper: pdf); unpublished notes on selected species of Amorimia.
Uses: Several species of Amorimia have been shown to poison ruminants (cattle, goats, and sheep) when eaten (Schons et al., 2011, Lee et al. 2012 [pdf], and references cited therein). Schons et al. called the plant they studied Mascagnia sepium (Adr. Juss.) Griseb. and "Amorimia sepium" [no such combination has ever been published, nor is such publication intended here]. Their plant was very probably Amorimia amazonica (Nied.) W. R. Anderson; that identification was made in 2012 by W. R. Anderson on the basis of the photograph in the publication by Schons et al. and the fact that the specimen was collected in Rondônia, Brazil. Mascagnia sepium is a species of Mascagnia sens. str., known only from southeastern Brazil. There is no evidence that any species of Mascagnia sens. str. is poisonous.