Polystemma stevensii G.M. Hern.

Hernández-Barón, Gerald Matus, Juárez, Diana Isabel Trujillo & Steinmann, Victor W., 2023, Polystemma stevensii (Apocynaceae, Asclepiadoideae), a new species from Michoacán, Mexico, Phytotaxa 613 (2), pp. 194-200 : 195-199

Polystemma stevensii G.M. Hern. -Barón, Trujillo-Juárez & V.W. Steinm., sp. nov.

TYPE:— MEXICO. Michoacán: Mpio. Múgica, 5.8 km al NNE de la salida a Nueva Italia , sobre un sendero que va desde la autopista Morelia-Lázaro-Cárdenas al Río Marqués ; 19°02’01”N, 102°02’54”W, 350 m, 7 September 2010, V.W. Steinmann & Y. Ramírez-Amezcua 7159 (holotype: IEB; isotype: MEXU) . Figs. 1 , 2 .

Similar to P. viridiflorum in that both exhibit filiform corona appendages, but the new taxon differs by having a gynostegial corona with five lobes fused at the base to form a discontinuous flat ring (vs. not forming a discontinuous flat ring), an internal corona with two deltate lobes (vs. with one ligulate lobe), and a globose style head (vs. flat style head).

Twining vines, herbaceous or woody towards the base, with thick yellow cork in the oldest parts, perennial, climbing to 3 m tall, with copious white latex. Stems cylindrical, 2–3 mm in diameter, sparsely hispidulous to strigulose with mixed indumentum of white eglandular trichomes, 0.3–0.6 mm long and white bulbous trichomes ca. 0.1 mm long; internodes 7–10 cm long. Leaves opposite, stipular colleters absent, petiole slender, 2.9–3.4 cm long with mixed indumentum; leaf blade ovate to lanceolate, 3.4–7.2 × 1.5–3.6 cm, apex acute to acuminate, base deeply cordate, lobes diverging or converging, sinus 0.6–1.1 cm deep, margins entire, palmately 5-veined from the base, the midvein with 3 pairs of prominent secondary veins, abaxially and adaxially hispidulous to strigulose with scattered bulbous trichomes, more densely pilose abaxially; colleters 6–8, conical, 0.3–0.5 mm long, at the junction of the petiole and the blade adaxially. Inflorescences extra-axillary, 1 per node, racemiform cymes with 7–11 flowers, mixed indumentum, total length 3.4–6.1 cm long; peduncles 1.4–2.6 cm long, bracts narrowly lanceolate to subulate, 2–4.5 × 0.5–1.3 mm, apex acuminate; pedicels slender, 1.3–1.6 cm long in flower, to 2.2 cm long and thickened in fruit. Calyx lobes 5, almost free to the base, narrowly lanceolate, 5.2–6.3 × 1.3–1.7 mm, apex attenuate, abaxially pilose, adaxially glabrous. Corolla campanulate, dextrosly imbricate in bud, deeply 5-lobed, variable in color, ranging from greenish to brownish nearly throughout to burgundy red in the center and fading to yellowish green in the lobes; tube 2.5–3.2 mm long, smooth with vilose indumentum; lobes narrowly lanceolate, 5.8–11.8 × 1.2–4.2 mm, wide at the base to almost linear to the apex, apex rounded, coiled, abaxially puberulent, adaxially sericeous towards the base, glabrous towards the apex. Gynostegium 1.6–2 × 1.3–1.8 mm, sessile. External corona lobes 5, opposite the anthers, fused at the base to form a discontinuous flat and crenate ring, 0.2–0.4 mm high, white or yellow, each lobe with two pair of ascending and recurved appendages, the lower pair acicular, 0.3–0.6 mm long, the upper pair with filiform lateral appendages 1.8–2.5 × 0.1–0.2 mm, purple, with central inconspicuous lobes or small papilla. Internal corona lobes 5, a bicornate ligule, with two incurved deltate lobes, 0.4–0.7 × 0.3–0.5 mm, white to yellow. Anthers horizontal, 0.6–0.7 × 0.5–0.6 mm, terminal appendages of anthers hyaline, widely ovate, 0.7–0.8 × 0.8–0.9 mm. Style head prominent globose, dark purple or black when dried, 0.5–1.0 × 0.4–0.8 mm with a globose apex ca. 0.2 mm wide. Pollinia falciform, ca. 0.6 × 0.5 mm; corpuscle brown, rhombic, ca. 0.2 × 0.1 mm. Follicles narrowly fusiform, slightly wider in the proximal half, 16–19 × 0.8–1.3 cm, smooth, glabrous, mottled bluish green; mature seeds not seen.

Phenology:— Flowering occurs from June to September, with immature fruits found in September.

Distribution and habitat:— Endemic to the Balsas Depression of central Michoacán, Mexico where it is known from four localities at elevations of 350 to 750 m ( Fig. 3 ). Three of the localities are within the Zicuirán-Infiernillo Biosphere Reserve. The vegetation at these localities is tropical deciduous forest (following Rzedowski 1978), and for more details of the region see Steinmann (2021).

Etymology:— The specific epithet honors Dr. Warren Douglas Stevens. His contributions have been essential for advancing our knowledge of American Asclepiadoideae and the genus Polystemma .

IUCN conservation assessment:— Little is known about the biology and population demographics of Polystemma stevensii . The locality north of Nueva Italia was severely impacted by the construction of the Francisco J. Múgica Dam in 2013, and some plants were lost. Two other localities are adjacent to the highway from Arteaga to Nueva Italia. The remaining site is on a pristine ridgetop approximately two kilometers from the nearest roads and town. The known extent of occurrence (EOO) is 285 km 2 and the area of occupancy (AOO) is 16 km 2, but both values are likely underestimates because the region is still relatively unexplored. Although P. stevensii occurs in forest margins along roadsides, it does not appear to be favored by disturbance. Three localities are within the Zicuirán-Infiernillo Biosphere Reserve, but the area is subject to anthropogenic pressures such as agriculture and livestock grazing. It is uncertain how these factors will affect the survival of the species, but we anticipate some future loss of habitat area, extent, and quality as well as a decrease in AOO and EOO.Although the EOO and AOO are almost certainly underestimates, more than 15-fold increases in both would be required for the endangered category to be changed to vulnerable. Therefore, following the IUCN Red List criteria ( IUCN, 2022), we assign a preliminary category of endangered, EN B1ab(i,ii,iii) +2ab(i,ii,iii).

Notes:— Polystemma stevensii is similar to P. viridiflorum in that both have filiform corona appendages. However, P. viridiflorum has a corolla with ovate lobes and a glabrous, purple corolla tube lacking obvious venation. In contrast, P. stevensii has a corolla with lanceolate lobes and a sericeous, green to yellowish corolla tube with reticulated venation. Furthermore, the apex of the style head in P. stevensii is conspicuously globose, whereas in P. viridiflorum it is flat. Polystemma stevensii is only known from the Balsas Depression ( Fig. 3 ), whereas P. viridiflorum occurs from Michoacán to Nicaragua. Polystemma now includes eight species, of which five are endemic to Mexico and two of these are endemic to Michoacán.

Additional specimens examined (paratypes): —Michoacán: mpio. Múgica, 5.8 km al NNE de la salida a Nueva Italia, sobre un sendero que va de la autopista Morelia-Lázaro Cárdenas al Río Marqués, 19°02’01”N, 102°02’54”W, 350 m, 23 June 2009 (fls), Y. Ramírez-Amezcua & V.W. Steinmann 1716 (IEB); ibid., 07 September 2010 (fls and immature fts), V.W. Steinmann & Y. Ramírez-Amezcua 7159 (IEB); mpio. Múgica, 5.8 km al NNE de la salida a Nueva Italia, sobre un sendero que va de la autopista Morelia-Lázaro Cárdenas al Río Marqués, 19°02’03”N, 102°03’02”W, 400 m, 11 July 2009 (fls), Y. Ramírez-Amezcua & V.W. Steinmann 1727 (IEB); mpio. La Huacana, ca. 2 km (by air) ENE of Los Ranchos, along the ridge top of Cerro El Barril, 18°42’35”N, 102°00’00”W, 750 m, 30 August 2003 (fls), V.W. Steinmann & M. Lara-Camacho 3456 (IEB); mpio. Arteaga, sobre la carretera de Nueva Italia a Arteaga, 19.6 km al SO de la desviación al Infiernillo, 18°26’57.57”N, 102° 4’25.83”W, 595 m, 06 September 2020, V.W. Steinmann & Y. Ramírez-Amezcua 8387 (IEB); mpio. Arteaga, 9.2 km al SO de la desviación al Infiernillo y 0.5 km al N de El Guayabito; 18°29’2.64”N, 102° 1’36.00”W, 360 m, 06 September 2020, V.W. Steinmann & Y. Ramírez-Amezcua 8399 (IEB).

Figures

FIGURE 1. Polystemma stevensii. A. Flowering branch. B. Frontal view of a flower. C. Detail of gynostegial corona showing appendages and five lobes fused basally to form a flat discontinuous ring. D. Pollinarium. E. Branch with a follicle. Illustration by Ericka B. Cortez.

FIGURE 2. Polystemma stevensii. A. Stems with yellow cork B. Branch with inflorescences. C. Flowers with some floral visitors. D. Frontal view of a flower. E. Frontal view of a flower, showing the gynostegium. F. Lateral view of a flower, showing the gynostegium. Photos by Victor W. Steinmann.

FIGURE 3. Maps showing the occurrence of Polystemma stevensii.