Slender twining vines from a woody base, sometimes seeming shrubby when grazed; stems densely and persistently sericeous, eventually glabrescent; stipules interpetiolar (borne on stem beside petioles), 0.1–0.8 mm long, triangular or subulate, glabrous or sericeous; leaves decussate; lamina eglandular on surface, sometimes eglandular on margin but usually bearing a pair of stalked glands or eglandular processes near base; petiole sericeous or glabrescent, eglandular. Inflorescence sericeous or glabrescent, the flowers borne in umbels of 2–4 terminating lateral shoots; bracts 0.5–1.8 mm long, narrowly triangular or subulate, eglandular, persistent; peduncle (1–) 2–13 mm long; bracteoles like bract but usually smaller, borne at or somewhat below apex of peduncle, eglandular or one sometimes bearing a small stalked abaxial gland, persistent. Calyx bilaterally or radially symmetrical; sepals leaving petals exposed during enlargement of bud, flat and appressed in anthesis, the lateral 4 or all 5 bearing paired abaxial glands, the glands 0.6–1.1 mm long, elliptical or obovate, detached and often reflexed distally, well separated on the sepal; corolla bilaterally symmetrical; petals lemon-yellow, glabrous or bearing a few straight appressed hairs abaxially on claw or midrib, with the limb flat, the lateral 4 widely spreading, the posterior more erect, with a longer claw and a differently shaped limb; receptacle glabrous on both sides of filaments; androecium bilaterally symmetrical; fertile stamens 2, opposite posterior-lateral sepals, glabrous, nearly or quite distinct, erect and pressed against style, alike; staminodes (rudimentary filaments without anthers) 0–3, when present opposite anterior and anterior-lateral sepals, very rarely one of the filaments opposite an anterior-lateral sepal bearing an anther; gynoecium bilaterally symmetrical, 3-carpellate, the carpels all fertile, 1 anterior and 2 posterior, connate their whole length in ovary; ovary 1–1.5 mm high, densely sericeous; style 1, borne on anterior carpel, straight and erect, the stigma terminal, truncate or capitate. Fruit dry, breaking apart at maturity into 3 samaras (or fewer by abortion) borne on a pyramidal torus 1.2–2 mm high, with each face of torus elliptical or obovate; samara 9–15 (–17) mm long, sericeous proximally; dorsal wing well developed, elongated, 7–12 (–15) mm long, 4–6 mm wide, thickened on the adaxial edge with the veins bending toward the thinner abaxial edge; nut 1.5–2.5 mm high, 3–4.5 mm long, without lateral wings but reticulate and often parallel-rugose on sides, flared and rounded at base and emarginate below to form a shallow groove to accommodate the short but functional carpophore 1–2 mm long, the nut usually forming a spur 0.2–0.5 mm long at end of carpophore; ventral areole 1–2 mm high, 0.8–1.3 mm wide, deeply concave; embryo with the cotyledons subequal, flattened, folded back in the distal third. Chromosome numbers: n = 10 (C. californica and C. linearis), n = 20 (C. gracilis) (W. R. Anderson, 1993a); photos.
Cottsia is in the large Stigmaphyllon clade, in a subclade that consists of a group of genera that have the androecium reduced to six stamens (one opposite the posterior petal and the other five opposite the sepals) or fewer (Aspicarpa sens. lat., Camarea, Cottsia, and Janusia) and the style usually one on the anterior carpel, with a terminal stigma. The three North American species of Cottsia have traditionally been placed in Janusia because they resemble the South American type of Janusia, J. guaranitica, in several characters: herbaceous twining habit, flowers in umbels terminating short lateral shoots, and a small samara with a well-developed dorsal wing and a short but functional carpophore (a cartilaginous structure extending from the receptacle to the outer edge of the underside of the nut). Nevertheless, the latest phylogeny published for Malpighiaceae (Davis & Anderson, 2010 [pdf]) showed that Cottsia is sister to all the other genera of the Aspicarpa subclade. In several important respects Cottsia differs from Janusia guaranitica: its petals are lemon-yellow, not carrot-yellow, the three anterior stamens lack anthers, its filaments are distinct even at the base, the carpels are connate their whole length in the ovary, the samara lacks a lateral winglet, and there are no cleistogamous flowers. Cottsia is also interesting for having a base chromosome number of n = 10, a number that is common in less derived genera of the Stigmaphyllon clade (e.g., Banisteriopsis) but lower than any known otherwise in the Aspicarpa subclade (W. R. Anderson, 1993a). Anderson & Davis (2007, p. 159) suggested that the low chromosome number and lack of cleistogamy in Cottsia may indicate that its ancestor arrived in North America before polyploidy and cleistogamy evolved in the South American clades, followed by further reduction of the androecium in Cottsia. See the extensive discussion in Anderson & Davis (2007, pp. 157–165).
References: W. R. Anderson (1993a, pp. 343, 346–347, under Janusia), chromosome numbers, distributions, discussion of ploidy and hybridization; W. R. Anderson & C. Davis (2007, pp. 157–165), revision; the description and discussion given above were modified from the latter reference.
Etymology: Dubard and Dop, when describing the genus, coined the name Cottsia (basing it on an anagram of "Scott") to honor George Francis Scott Elliot (1862–1934), a Scottish botanist who collected plants in Madagascar in the late 1880s. They did that because their type, a single specimen in P, bore a label saying it was collected in Madagascar by Scott Elliot. Their specimen was mislabeled and did not come from Madagascar; it was actually a sheet of Palmer 263 in 1887, from Sonora, Mexico, a representative of what is now C. californica. For additional discussion see Anderson & Davis (2007, p. 157).