Woody vines or shrubs, when shrubby the branches often twining; stipules usually present, interpetiolar, distinct, triangular, minute (0.2 mm long) or small (up to 1.5 mm long); leaves decussate; petiole eglandular or biglandular at or somewhat below apex; lamina with glands none or usually marginal, sessile, the distal glands (if present) minute, the 2 most proximal glands usually enlarged. Inflorescence axillary or terminal, paniculate or dichasial, the flowers borne in short, usually dense pseudoracemes of 6–45; bracts and bracteoles eglandular, persistent; pedicels mostly short-pedunculate, sometimes sessile. Sepals leaving petals exposed during enlargement of bud, the lateral 4 bearing large paired abaxial glands and the anterior eglandular, the glands attached for most of their length below free part of sepal; corolla bilaterally symmetrical; petals yellow, glabrous (very rarely sericeous on abaxial midrib), the posterior petal different in size, shape, and stance from the lateral 4, and its margin often more deeply divided with the divisions (especially proximally) ± glandular-thickened; receptacle glabrous on both sides of filaments; stamens 10, all fertile; anthers glabrous or pilose, the connective not or hardly exceeding locules at apex; gynoecium 3-carpellate, the carpels 1 anterior and 2 posterior, all fertile, mostly distinct in the ovary; styles 3, distinct, mostly ± alike; stigmas terminal, truncate or capitate. Fruit dry, breaking apart at maturity into 3 samaras (or fewer by abortion) separating from a low pyramidal torus; dorsal wing of samara well developed, elongated, thickened on the adaxial edge with the veins bending toward the thinner abaxial edge, a shallow triangular or rounded appendage usually present on adaxial edge at base; nut of samara spherical, smooth-sided or bearing a single ridge or winglet on each side parallel to areole; locule of nut glabrous within; carpophore absent or present but short (up to 1.5 mm long), broad (1–2 mm wide), and non-functional. Chromosome number: n = 10 (in B. acapulcensis var. llanensis [Gates, 1982]).
Ten species in the neotropics from South America to Mexico, mostly in wet forests and savannas in South America, often in drier vegetations in Central America and Mexico (moist forests, seasonally dry forests, tropical deciduous forests, dry thickets). [Map] — Regional key to genera: Central America.
Bronwenia is a member of the very large Stigmaphyllon clade; in fact, it is basal in that clade, sister to all the other genera in the clade (Davis & Anderson, 2010 [pdf]). Many of its morphological characteristics are shared with other clades of neotropical wing-fruited Malpighiaceae: twining habit; distinct triangular interpetiolar stipules; biglandular lateral sepals, the anterior sepal eglandular; bilaterally symmetrical corolla; yellow petals; ten fertile stamens with the connective not or hardly enlarged; a base chromosome number of n = 10. In two respects it shows synapomorphies that are predominant in the Stigmaphyllon clade, terminal stigmas and samaras with a dominant dorsal wing thickened on the adaxial edge. Curiously, it has the carpophore at the base of the samara absent or rudimentary and non-functional, while in most of the more derived genera of the clade the carpophore is well developed and functions in dispersal by allowing the samara to dangle until the wind is strong enough to break the connection to the receptacle; that carpophore is peculiar to the Stigmaphyllon clade. The absence of dramatic unique synapomorphies makes recognition of Bronwenia difficult, but several characters are helpful: 1) The leaf glands (if present) are usually marginal; 2) the flowers are borne in dense pseudoracemes of 6–45, not in umbels; 3) the pedicels are mostly short-pedunculate; 4) the calyx glands are attached below the free part of the sepal; 5) the petals are always yellow and glabrous or nearly so; and 6) the nut of the samara is spherical and smooth-sided or bears at most a single ridge or winglet on each side.
Gates (1982) treated the species of Bronwenia as Banisteriopsis subg. Banisteriopsis. She did that because she believed the earliest lectotype for Banisteriopsis was B. cornifolia (H. B. K.) C. B. Rob., designated by Cuatrecasas in 1958 [pdf], but a different lectotype, for which the correct name is now B. muricata (Cav.) Cuatrec., was designated by Morton in 1931 [pdf]. With B. muricata the lectotype of Banisteriopsis, the name Banisteriopsis goes with the large group that Gates called subg. Hemiramma (Griseb.) B. Gates and at the level of genus her subg. Banisteriopsis has to have a new name, Bronwenia (Anderson & Davis, 2007).
Bronwenia peckoltii W. R. Anderson & C. Davis is atypical in its leaf glands and connation of the carpels in the ovary. It is imperfectly known, from a single collection without mature fruits, and its assignment to Bronwenia is open to question; see the discussion in Anderson & Davis, 2007.
References: Gates (1982), revision of Banisteriopsis subg. Banisteriopsis; W. R. Anderson & C. Davis (2007, pp. 137–146), description of genus, key to species, new combinations, synonymies, and one new species; the description and discussion given above are based on the latter reference.
Etymology: The name Bronwenia honors Bronwen Elizabeth Gates (b. 1945), who published a revision of Banisteriopsis sens. lat. and Diplopterys sens. str. (Gates, 1982).