This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
[Vincetoxicum Moench, Meth. 717. 1794] Perennial twining herbaceous or slightly woody vines (some species erect herbs), with opposite or rarely verticillate or alternate leaves, and small yellowish green or purplish flowers in axillary cymes. Calyx 5-parted, minutely glandular within. Corolla rotate, deeply 5-cleft, the segments spreading, somewhat twisted. Crown flat or cup-like, entire, 5-lobed or 5-parted, the lobes not appendaged. Stamens attached to the base of the corolla, their filaments connate into a tube; anthers appendaged by an inflexed membrane. Pollen-masses solitary in each sac, pendulous. Stigma flat or conic. Follicles acuminate, glabrous. Seeds comose. [Greek, dog-strangling.]
About 100 species, natives of both the Old World and the New. Besides the following, 2 native species occur in the southeastern United States. Type species: Cynanchum acutum L.
Asclepias nigra L. Sp. Pl. 216. 1753. Vincetoxicum nigrum Moench, Meth. 317. 1794. Cynanchum nigrum Pers. Syn. 1: 274. 1805.
Twining, or at first erect, puberulent, slender, 2°-5° high. Leaves ovate or ovate-lanceolate, entire, thin, acuminate at the apex, rounded at the base, pinnately veined, petioled, 2-5' long, 1/2'-2 1/2' wide; petioles 2"- 12" long; pedicels 1 1/2"- 3" long; flowers dark purple, about 2}" broad; corolla-segments pubescent within; crown fleshy, 5-lobed; follicles on nearly straight fruiting pedicels, about 2' long, glabrous.
In waste places, escaped from gardens, Massachusetts to Pennsylvania and Ohio and in British Columbia. Introduced from Europe. June-Sept.