32. Orthosia Decaisne in A. P. de Candolle and A. L. L. P. de Candolle, Prodr. 8: 526. 1844.
Swallow-wort, milkweed vine [Greek Orthosie, goddess of prosperity, and one of the Horae, daughters of Zeus and Themis]
Mark Fishbein
Amphistelma Grisebach
Vines herbaceous, woody only at base [scandent]; latex white. Stems twining, unarmed, glabrous. Leaves caducous [persistent], opposite, petiolate; stipular colleters interpetiolar; laminar colleters present or absent. Inflorescences axillary (extra-axillary) cymes, umbelliform to shortly racemiform [paniculiform], sessile [pedunculate]. Flowers: calycine colleters apparently absent; corolla yellow to light green (cream), campanulate [rotate or tubular], aestivation valvate; corolline corona absent; androecium and gynoecium united into a gynostegium adnate to corolla tube; gynostegial corona of 1 whorl of 5 segments connate at base; anthers adnate to style, locules 2; pollen in each theca massed into a rigid, vertically oriented pollinium, pollinia lacrimiform, joined from adjacent anthers by translators to common corpusculum and together forming a pollinarium. Fruits follicles, typically paired, variously oriented, brown, linear, smooth, glabrous. Seeds naviculate, not winged, linear, comose, not arillate. x = 10.
Species ca. 40 (1 in the flora): se United States, Mexico, West Indies, Central America, South America.
R. E. Woodson Jr. (1941) adopted a very broad (and evidently polyphyletic) circumscription of Cynanchum, including Orthosia. The isolated placement of Orthosia relative to Cynanchum and presumed relatives such as Metastelma was demonstrated by S. Liede et al. (2005). Superficially, the species are very similar to those of Metastelma in the small, linear leaves and tiny cream to yellow flowers. However, the often caducous leaves; deltate, glabrous corolla lobes; and commonly paired inflorescences and follicles provide a ready means in our region to distinguish Orthosia from Metastelma.
