31. Metastelma R. Brown, Asclepiadeae. 41. 1810.
Swallow-wort, milkweed vine [Greek meta, change or instead, and stemma, girdle or crown, alluding to corona of separate scales in place of a crown]
Mark Fishbein
Basistelma Bartlett; Cynanchum Linnaeus subg. Metastelma (R. Brown) Woodson; Epicion Small
Vines, herbaceous or somewhat woody or corky at base, perennial; latex white. Stems twining, unarmed, glabrous or puberulent with eglandular trichomes. Leaves persistent or tardily deciduous, opposite, petiolate; stipular colleters usually present, interpetiolar; laminar colleters usually present. Inflorescences extra-axillary cymes, umbelliform or racemiform, sessile or pedunculate. Flowers: calycine colleters present or absent; corolla cream, yellowish cream, or green, campanulate, aestivation valvate; corolline corona absent; androecium and gynoecium united into gynostegium adnate to corolla tube; gynostegial corona of 1 whorl of 5 free, laminar segments; anthers adnate to style, locules 2; pollen in each theca massed into rigid, vertically oriented pollinium, pollinia lacrimiform, joined from adjacent anthers by translators to common corpusculum and together forming pollinarium. Fruits follicles, typically solitary, variously oriented, brown to dark brown, fusiform terete or subterete, glabrous or puberulent. Seeds ovate, flattened, winged, not beaked, comose, not arillate.
Species ca. 80 (7 in the flora): s 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 Metastelma. The microphyllous habit and minute flowers of Metastelma and Orthosia readily separate these genera from other Apocynaceae in the flora area, including Cynanchum. The phylogenetic separation of Cynanchum, Metastelma, and Orthosia has been demonstrated by S. Liede et al. (2005) among others. Orthosia scoparia has been included in Metastelma by some; see the discussion of Orthosia for features distinguishing these genera.
All but one species of Metastelma in the flora area has a mixed indumentum of flattened, opaque hairs toward the adaxial apex of the corolla lobes and stiff, acicular, translucent hairs toward the center and base of the lobes pointing downward into the tube. This distinctive indumentum provides an immediately diagnostic recognition feature for Metastelma; however, M. bahamense lacks the stiff, acicular hairs. In some of the species, the apical, flattened hairs are long and form a brush; in others, these hairs are quite short and may appear granular.
SELECTED REFERENCES Liede, S. and U. Meve. 2004. Revision of Metastelma (Apocynaceae–Asclepiadoideae) in southwestern North America and Central America. Ann. Missouri Bot. Gard. 91: 31–86. Liede, S. et al. 2014. Phylogenetics and biogeography of the genus Metastelma (Apocynaceae–Asclepiadoideae–Asclepiadeae: Metastelmatinae). Syst. Bot. 39: 594–612.
