Shrubs or woody vines, branches scandent or trailing. Leaves bearing small glands on distal 1/2 of petiole and/or on blade margin near base; stipules borne on petiole at or just distal to base, distinct. Inflorescences axillary or terminal, few-flowered umbels, corymbs, or short pseudoracemes. Pedicels raised on peduncles. Flowers all chasmogamous, 6+ mm diam., showy with visible petals, stamens, and styles; calyx glands 8 (anterior sepal eglandular, 4 lateral 2-glandular); corollas bilaterally symmetric, petals lemon yellow, densely white-sericeous or -tomentose [glabrous] abaxially; stamens 10, all fertile [posterior 3 occasionally sterile]; anterior 7 anthers notably larger than posterior 3; pistil 3-carpellate, carpels completely connate in ovary; styles 3, cylindric, stout; stigmas on internal angle, oblate, large. Fruits schizocarps, breaking into 3 samaras; samaras bearing 2 semicircular lateral wings and 1 well-developed dorsal wing, like lateral wings but notably [somewhat] smaller; nut wall thick, tough. x = 10.
Tex., Mexico, Central America (to Nicaragua), South America.
Species 11 (1 in the flora).
In his revision of Callaeum, D. M. Johnson (1986c) cited a specimen of C. macropterum (de Candolle) D. M. Johnson, Palmer s.n. in 1869 (NY), as coming from Arizona, "without definite locality." That species occurs in Sonora, Mexico, fairly close to the U.S. border, but no other collection is known from north of the border. R. McVaugh (1956b) said that in 1869 Palmer collected in Arizona but was in southernmost Arizona only a short time; from Tucson he took a coach south into Sonora, where he collected in several areas, including the Yaqui River. There are two sheets of C. macropterum at NY labeled as Palmer s.n. in 1869, one with a label giving the locality as "Arizona," the other with a label giving the locality as "Yaqui River Sonora." The specimens look as if they were parts of the same gathering. Given McVaugh’s cautionary notes on the inaccuracies found in many of the labels placed on Palmer's collections after his return to Washington, I believe that the specimen cited by Johnson as coming from Arizona probably actually originated on the Yaqui River in Sonora, and am excluding C. macropterum from this treatment.