Materials and methods

Herbarium specimens were collected in 2001, 2003, 2004, and 2014 following preparation techniques detailed by ^{Lot & Chiang (1986)}. Photos were taken in the field with a Nikon D5200 using an AF-S MICRO 105 ED lens and a SIGMA EM 140 ring flash. Pertinent literature on Polystemma was consulted: ^{Stevens (2005)}, ^{McDonnell & Fishbein (2016)}, and ^{Hernández Barón (2021)}. The morphological description was prepared at the QMEX herbarium, and specimens were distributed to ARIZ, FCME, MEXU, MO, and QMEX, acronyms according to ^{Thiers (2021)}. The conservation status was evaluated using IUCN Redlist criteria (^{IUCN 2019}), and the extent of occurrence (EOO) and area of occupancy (AOO) were determined by the methods described in ^{Bachman et al. (2011)}

Results

Polystemma fishbeiniana V.W. Steinm. & W.D. Stevens, sp. nov. (Figure 1).

Figure 1 Polystemma fishbeiniana. A, H. Flowering stems; B, G. Close-ups of flowers; C. Habit; D, E, I. Inflorescences; F. Fruit. 

Type. Mexico, Michoacán, municipio de Arteaga, along MEX 37, ca. 75 km (by road) N of Arteaga and 1 km S of El Descansadero, 18.643487 °N, 101.968979 °W, 08 July 2004, V.W. Steinmann & L. Alvarado Cárdenas 4401 (Holotype: QMEX; Isotypes: ARIZ, FCME, MEXU, MO).

Diagnosis. Species similar to other members of Polystemma in having white glandular trichomes that become crystalline in age; filiform corona appendages; and fusiform, smooth, mottled follicles; it differs by its small leaves, suffrutescent habit, and flowers abaxially yellowish with a blackish to dark burgundy red margin and central line with short lateral extensions.

Description. Suffrutescent perennial, forming a dense tangle 30-50 cm tall, plants solitary or sometimes climbing on associates and reaching 1 m tall, lower woody stems with a grayish, fissured, corky periderm, upper herbaceous stems twining, latex white, herbage with a strong fetid odor. Internodes 1.2-6.3 cm long, indumentum in two levels, longer trichomes acicular, 0.2-0.6 mm long, white, straight, spreading or slightly reflexed, shorter trichomes capitate-glandular, 0.05-1 mm long, pale brown to whitish. Leaves opposite, stipules absent or represented by an inconspicuous, papillose colleter 0.1-0.3 mm long, petiole slender, 0.5-1.4 mm long, with indumentum like the stems, lamina ovate to lanceolate, 0.6-1.9 cm long, 0.4-1.4 cm wide, base deeply lobate, the lobes widely separated by a profound sinus 0.2-0.4 cm deep, apex acute to obtuse, midrib prominent, with 1-3 pairs of lateral veins, colleteres papillose, 2-4 at the base of the lamina, indumentum like that of the stems. Flowers 3-6 in congested, umbelliform, extra-axillary cymes, peduncle 1-4.5 mm long, with indumentum like that of the stems, bracts subulate to linear, 1-2.6 mm long, pedicels 5.5-6.7 mm long, with indumentum like that of the stems, sepals narrowly triangular, lanceolate or subulate, free, 1.7-2.7 mm long, 0.3-0.7 mm wide, abaxially with indumentum like the stems, bearing a colleter in each sinus, aestivation imbricate and dextrose, corolla narrowly pyramidal and 4.8-6.5 mm long in bud, lobes 4.6-7.1 mm long, 0.9-1.1 mm wide proximally, united at the base into a short tube 0.7-1.5 mm long, adaxially blackish to dark burgundy red, abaxially yellowish with a blackish to dark burgundy red margin and central line with short lateral extensions, pilose inside with unicellular trichomes, glabrous outside, tapering to an acute apex, margin undulate, gynostegium sessile; corona blackish to burgundy red, external corona connate at the base into a structure 0.6 mm tall, 5-lobed above, each lobe subsequently 3-lobed, central lobe erect, ovate-cucullate, ca. 0.6 mm long, 0.3 mm wide, the lateral lobes filiform, somewhat reflexed, ca. 0.7 m long, internal corona with lobes reduced to small mounds between the central lobes of the external corona and the anthers; terminal appendages of the anthers adpressed to the style apex, reniform, white; style apex 1.5 mm wide, slightly convex, smooth. Pollinaria more or less horizontal on the anther margins, corpusculum approx. 0.18 mm long, 0.09 mm wide, ellipsoid, light brown, translator arms 0.09 mm long, pollinia ca 0.4 mm long, 0.26 mm wide, obovate, slightly asymmetrical, apex truncate with an excavated portion. Follicles narrowly fusiform, 1 or rarely 2 per flower, 7.2-11.3 cm long, 0.7-1.2 cm in diameter, mottled green-white; seeds ca. 40 per fruit, pear- to teardrop-shaped, flattened, 4.7-5.3 mm long, 3.2-3.9 mm wide, apex truncate, base rounded with an erose margin, coma 2.4-3.6 cm long, white.

Etymology. Polystemma fishbeiniana is named in honor of Mark Fishbein, our friend and colleague, with whom the first author had the pleasure of accompanying on trips to the tropical vegetation of northwestern Mexico in the early 1990s. He is a noted specialist of Asclepiadoideae and has made significant contributions to our understanding of the Mexican flora, including describing several species, including Polystemma canisferum A. McDonnell & Fishbein.

Distribution and habitat. This new species is known only from two locations in the Zicuirán-Infiernillo Biosphere Reserve, in the Balsas Depression of central Michoacán, Mexico. One location is on the west side of the Cerro El Barril, and the other is about nine kilometers (airline) to the south-southwest, along the tollfree portion of Highway 37 between Las Canas and El Descansadero; at 275-400 m elevation. The vegetation is xeric scrub or open thorn forest with a dominance of columnar cacti and a carpet of Bouteloua spp. Associated shrubs and trees include Pachycereus tepamo S. Gama & S. Arias and Stenocereus quevedonis (González Ortega) Bravo (Cactaceae), Pseudosmodingium perniciosum (H.B.K.) Engl. (Anacardiaceae), Crossopetalum managuatillo (Loes.) Lundell (Celastraceae), Cordia seleriana Fern. (Cordiaceae), Croton flavescens Greenm., Euphorbia arteagae Buck & Huft, and Jatropha jaimejimenezii V.W. Steinm. (Euphorbiaceae), Erythrostemon macvaughii (J.L. Contr. & G.P. Lewis) Gagnon & G.P. Lewis, Lonchocarpus balsensis M. Sousa & J.C. Soto, Senna wislizeni (A. Gray) Irwin & Barneby, Cenostigma eriostachys (Benth.) Gagnon & G.P. Lewis (Fabaceae), Krameria sonorae Britton (Krameriaceae), Ruprechtia fusca Fern. (Polygonaceae), Randia laevigata Standl. and R. thurberi S. Watson (Rubiaceae), Bonellia macrocarpa (Cav.) B. Ståhl & Källersjö subsp. pungens (A. Gray) B. Ståhl & Källersjö (Primulaceae) and Karwinskia johnstonii R. Fernández (Rhamnaceae).

Conservation status. Polystemma fishbeiniana is a rare species and only about 50 plants have been observed. However, a careful census of mature individuals has not been conducted, and the actual number is surely greater because it occurs in a little-explored area with abundant suitable habitat, including sites between the two known locations. Also lacking is sufficient information to determine if there has been a reduction in population size and to what extent. The known EOO is 6 km² and the AOO is 16 km², but both of these values are likely underestimates. Although endemic to a biosphere reserve, the locations are close to small towns and subject to extensive cattle and goat grazing. Furthermore, there is evidence that drought also negatively affects the populations because in August of 2020, after a very delayed start of the rainy season, many plants that had been alive for more than a decade were deceased. Thus, a reduction in habitat quality has been observed and will likely continue. Given these conditions and following the IUCN Red List criteria (^{IUCN 2019}), the conservation status of P. fishbeiniana is assessed as Endangered, EN B1ab(iii)+2ab(iii). This takes into account that AOO and EOO are predicted to be larger but not enough to change the status of Endangered, which would require an EOO of 5,000 km² and an AOO of 500 km², as well as the discovery of more locations.

Phenology. 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.

Additional specimens examined. Mexico. Michoacán: municipio de Arteaga, along MEX 37, 7 km (by road) N of Las Canas, 18.626546 °N, 101.971740 °W, 24 June 2001 (fls), V.W. Steinmann 1694 (QMEX); municipio de Arteaga, along MEX 37, ca. 75 km (by road) N of Arteaga and 1 km S of El Descansadero, 18.643487 °N, 101.968979 °W, 7 October 2001 (fts), V.W. Steinmann 2060 (QMEX); municipio de La Huacana, ca. 0.5 km NE of Los Ranchos, at western base of Cerro El Barril, 18.70555556 °N, 102.0083333 °W, 30 August 2003 (fls, young fts), V.W. Steinmann & M. Lara-Camacho 3471 (QMEX); municipio de La Huacana, Cerro El Barril, ca. 1 km (en línea recta) NNE de Los Ranchos, 18.7122222 °N, 102.009722 °W, 14 March 2014 (fts), V.W. Steinmann & Y. Ramírez-Amezcua 7805 (QMEX).

Discussion

The genus Polystemma was first proposed by ^{Joseph Decaisne (1844)} in his treatment of the tribe Asclepiadeae for Alphonse de Candolle’s Prodromus Systematis Naturalis Regni Vegetabilis. Nearly a century later, ^{Woodson (1941)} synonymized the genus with Matelea Aublet, but ^{Stevens (2001}, ²⁰⁰⁵⁾ resurrected it to accommodate plants having a combination of white glandular trichomes that become crystalline in age; filiform corona appendages; and long, fusiform, smooth, and mottled follicles. All of these features are possessed by Polystemma fishbeiniana. The characterization and delimitation of Polystemma is still incomplete. Although only seven species have been formally placed in the genus, ^{Stevens (2005)} and ^{McDonnell & Fishbein (2016)} mention that it contains approximately 20 Mexican species. ^{Hernández Barón (2021)} provided a treatment of 12 Mexican species, but many of these species do not yet have combinations in Polystemma or are undescribed. Polystemma fishbeiniana was treated as Polystemma sp. 2, and information about its morphological distinctions and distribution were provided.

Polystemma fishbeiniana is a distinctive species readily distinguished from its congeners by having a suffrutescent habit, leaves less than 2 cm long, and a corolla abaxially yellowish with a blackish to dark burgundy red margin and central line with short lateral extensions. All of these traits are otherwise unknown in the genus, and the other species have leaves that are larger than 2 cm long, a non-suffrutescent habit, and flowers that lack the characteristic pattern of yellow with black or dark burgundy. It is one of four species of Polystemma occurring in Michoacán, the others being P. guatemalense (Schltr.) W.D. Stevens, P. viridiflorum Decne., and an undescribed species that was treated by ^{Hernández Barón (2021)} as Polystemma sp. 3. A key to distinguish these taxa, as well as the other members of Polystemma, is provided in ^{Hernández Barón (2021)}.