Metastelma is a genus of flowering plants in the family Apocynaceae, placed within the subfamily Asclepiadoideae (the former milkweed family Asclepiadaceae, now a subfamily of Apocynaceae), tribe Asclepiadeae, and subtribe Metastelmatinae. It was first described by the Scottish botanist Robert Brown, with the original publication dating to his 1809 memoir in the Memoirs of the Wernerian Natural History Society (vol. 1, p. 5), though the genus is conventionally dated to 1810.
The genus comprises small to medium-sized twining or scrambling vines, characteristic of the broader milkweed alliance. Species in the genus typically bear small flowers and are found primarily in the Caribbean basin, Mexico, Central America, and adjacent parts of South America and the southern United States. The genus name Metastelma is used in some classifications for a core group of species; many formerly recognized Metastelma species have been transferred to the closely related genus Cynanchum under revised circumscriptions, reflecting ongoing revision of tribal boundaries within Asclepiadoideae.
Representative species include Metastelma parviflorum, M. blodgettii, M. decipiens, and M. pringlei, illustrating the genus's range across Caribbean islands and the North American mainland. The subtribe Metastelmatinae to which it belongs is a diverse New World lineage within the milkweed tribe Asclepiadeae.
Etymology
The name Metastelma is a botanical Latin compound; the precise derivation is not stated in available sources. The genus was established by Robert Brown in his 1809 memoir (Mem. Wern. Soc. 1: 5), making it one of the earlier described genera within what is now subfamily Asclepiadoideae.
Distribution
Metastelma is distributed primarily through the Caribbean islands, Mexico, Central America, and the northern margins of South America, with some species extending into the southern United States (e.g., Florida, Arizona). The genus's range overlaps broadly with that of related genera in subtribe Metastelmatinae.
Taxonomy Notes
Metastelma belongs to subfamily Asclepiadoideae, tribe Asclepiadeae, subtribe Metastelmatinae within family Apocynaceae. Many species formerly treated under Metastelma have been transferred to Cynanchum under modern phylogenetic revisions, including M. anegadense, M. bahamense, M. ephedroides, and numerous others. The genus boundary between Metastelma and Cynanchum remains subject to ongoing taxonomic revision. The original authorship rests with R.Br. (Robert Brown, 1809/1810).
