35. Polystemma Decaisne in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 8: 602. 1844.
Milkweed vine [Greek polys, many, and stemma, garland or wreath, alluding to numerous filiform divisions of corona segments in P. viridiflorum, the type species]
Mark Fishbein
Angela McDonnell
Labidostelma Schlechter; Rothrockia A. Gray
Vines or scandent herbs, woody at base; latex white. Stems weakly to strongly twining, unarmed, mixed indumentum of long, eglandular trichomes and short, glandular trichomes that become filled with white-crystalline inclusions with age. Leaves drought-deciduous [persistent], opposite, petiolate; stipular colleters interpetiolar; laminar colleters present. Inflorescences extra-axillary, umbelliform or racemiform, pedunculate. Flowers: calycine colleters apparently absent; corolla cream [green, brown, purple, orange, yellow], tubular-campanulate [rotate], aestivation contort-dextrorse; corolline corona absent; androecium and gynoecium united into a gynostegium adnate to corolla tube; gynostegial corona of 1 whorl of 5 segments united at base; anthers adnate to style, locules 2; pollen in each theca massed into a rigid, horizontally oriented pollinium, pollinia lacrimiform, each with proximal, sterile, excavated region, joined from adjacent anthers by translators to common corpusculum and together forming a pollinarium. Fruits follicles, solitary, variously oriented, striate, lance-ovoid to fusiform, smooth, glabrous. Seeds winged, not beaked, ovate, flattened, comose, not arillate.
Species ca. 20 (1 in the flora): sw Arizona, Mexico, Central America.
The combination of glandular trichomes that develop crystalline inclusions with age and long, smooth, striate follicles is diagnostic for Polystemma and distinguishes this genus from Matelea (W. D. Stevens 2009). However, most species with these characteristics remain in Matelea, where they were placed by R. E. Woodson Jr. (1941). The genus as circumscribed here is supported by molecular phylogenetic evidence (A. McDonnell et al. 2018). A single species of Polystemma barely enters the flora region in southernmost Arizona.
A second species of Polystemma was discovered in the United States by S. Carnahan in 2019 in the southern foothills of the Santa Rita Mountains, Pima County, Arizona (Carnahan 3807 [OKLA]). The species, which is distributed south to central Sonora, Mexico, is in the process of being described. It is easily distinguished from P. cordifolium by its corollas, which are deep purple to reddish brown with spreading lobes, and by its flat, pentagonal style apex. The species will key to Polystemma in the key to genera, except for the style apex character that points instead to Matelea. However, like P. cordifolium, this species has the characteristic glandular trichomes with crystalline inclusions that are lacking in Matelea.
