Background:

Polystemma (Apocynaceae subfamily Asclepiadoideae) is a small genus currently with seven species characterized by having white glandular trichomes becoming crystalline in age; filiform corona appendages; and fusiform, smooth, mottled follicles. Exploration in the Balsas Depression of Michoacán, Mexico resulted in the discovery of an unknown species of Polystemma.

Questions:

Are the unknown Polystemma plants occurring in the Balsas Depression of Michoacán new to science? If so, how are they distinguished from other species of the genus, what are their ecological preferences, what is their distribution, and are they endangered?

Studied species:

Species of Polystemma.

Study site and dates:

Balsas Depression, Michoacán, Mexico; 2001, 2003, 2004, 2014.

Methods:

Fieldwork to collect herbarium specimens and photograph plants; review of literature on Polystemma to determine taxonomic status; preparation of a morphological description; evaluation of conservation status using IUCN Redlist criteria.

Results:

Polystemma fishbeiniana is proposed as a new species and illustrated with photographs. It occurs at two locations in the xeric scrub and open thorn forest of the Zicuirán-Infiernillo Biosphere Reserve, at elevations from 275 to 400 m. Flowering occurs from June to September, with mature fruits present from October to March; dehisced fruits can persist on the plants for almost a year. Its conservation status is assessed as Endangered, EN B1ab(iii)+2ab(iii).

Conclusions:

Polystemma fishbeiniana is distinguished from congeners by its suffrutescent habit, small leaves, and flowers abaxially yellowish with a blackish to dark burgundy red margin and central line with short lateral extensions. These traits are otherwise unknown in the genus.

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