Mezcala C.E. Hughes & J.L. Contr., 2022

Hughes, Colin E., Ringelberg, Jens J., Luckow, Melissa & Jimenez, Jose Luis Contreras, 2022, Mezcala - a new segregate genus of mimosoid legume (Leguminosae, Caesalpinioideae, mimosoid clade) narrowly endemic to the Balsas Depression in Mexico, PhytoKeys 205, pp. 191-201 : 191

Mezcala C.E. Hughes & J.L. Contr. gen. nov.

Diagnosis.

Mezcala is distinguished from Desmanthus s.s. and Kanaloa by the presence of a claviform anther gland with an orbicular head on a filiform stalk on the apex of the anthers, this best seen in bud and often caducous after anthesis, versus absence of anther glands; by the aggregation of pollen into tetrahedral tetrads as opposed to pollen shed as eumonads; and by its sub-cyclindrical, lignified fruits that are held erect above the shoots and which are tardily dehiscent along both sutures from the apex as opposed to the dorsi-ventrally flattened pods with chartaceous or coriaceous valves and passive dehiscence found in species of Desmanthus s.s and Kanaloa .

Type.

Mezcala balsensis (J.L. Contr.) C.E. Hughes & J.L. Contr. = Desmanthus balsensis J.L. Contr.

Description.

(modified from Luckow 1993: 59-60). Small multi-stemmed erect treelet or large shrub 1-3 m tall. Young shoots angled, woody, glabrous or with amorphous red glandular protrusions, reddish-brown when very young, soon exfoliating a waxy white cuticle; older stems terete, reddish-brown to grey, wrinkled, glabrous with conspicuous lenticels, branches geniculate; trunks with checkered grey bark. Stipules persistent, 1.5-3 mm long, setiform with striate, membranous wings, glabrous, red or green, the fused bases clothing short shoots on the older branches from which new leaves or side shoots arise. Leaves 2.5-4.5 cm long, petiole 5-9 mm long, rachis 11-18 mm long, red granular tissue scattered along the axes and concentrated at the junctions of the leaflets with the pinna, and pinnae with the rachis; pinnae 2-4 (-5) pairs, 9-20 mm long, the lowest pair bearing a stipitate nectary 0.4-0.7 mm in diameter on a 0.5-1 mm-long stipe, the tip orbicular, crateriform and flared; leaflets 8-14 pairs per pinna, inserted several millimeters above the base of the pinna, shortly petiolate, 2.5-3.5 × 0.8-1.2 mm, oblong, oblique to square basally, the apex acute, glabrous, finely ciliate along the margins, venation obscure except the nearly central midvein. Capitula 1-2 per leaf axil, borne on peduncles 1-3 cm long. Bracts subtending each flower 1-2.5 × 0.25-0.5 mm, deltate setiform, pale reddish or purple when dry, membranous with a single opaque midvein, peltate and short pedicellate at the centre of the capitulum, sessile at the base, persistent. Flower buds obovate, apically rounded. Capitula 0.5-1 cm long, containing 30-50 sterile, functionally male and hermaphrodite flowers, sterile or male flowers rarely absent, proportions of each flower type variable. Sterile flowers 0-5; calyx 1-1.75 × 0.5-1 mm, obconic, minutely 5-lobed; petals 2-2.5 × 0.2-0.4 mm, lanceolate, white or pale green; staminodia 10, 2.5-5 mm long, the same widths as the filaments of functional stamens, white. Male flowers 12-30, borne above the sterile flowers but with a perianth and androecium like that of the hermaphrodite flowers. Hermaphrodite flowers 5-25; calyx 1.4-2.7 mm long, obconic, the tube 1.3-2 mm long, 0.8-1.2 mm in diameter, rimmed with 5 free acute lobes 0.3-0.5 mm long; petals 2-3.5 × 0.3-0.5 mm, oblanceolate, pale green with white margins, glabrous; stamens 10; 3.5-5.5 mm long, anther apically with a minute orbicular gland borne on a filiform stalk, caducous; ovary 1-1.5 mm long, linear, glabrous, style 3.5-6 mm long, always more than three times the length of the ovary, exserted beyond the stamens. Fruiting peduncles 1-3 cm long, bearing 1 (-4) pods held erect above shoots and tardily dehiscent from the apex along both sutures, also splitting irregularly and transversely along valves, 3.2-5.5 (-10) × 3.3-5 × 0.25-5 mm, linear-oblong, straight to slightly arcuate, apex acute, valves initially fleshy, glabrous, bright emerald-green when unripe, becoming woody or sub-woody and turning dark brown when ripe. Seeds 5-13 per pod, 4.4-6 mm × 2.5-3.5 mm, longitudinally inserted, square to rhomboidal, 4-angled, deep reddish-brown; pleurogram 0.5-1 mm wide, 0.7-1.5 mm deep, deeply U-shaped, often asymmetric with unequal arms.

Geographic distribution.

Mezcala is a narrowly restricted endemic genus, known from just a handful of localities in the central Balsas Depression in Guerrero, Mexico (Fig. 3 ). A large majority of the collections are from karst limestone ridges above the gorge of the Río Xochipala, a few km from the village of Xochipala in the Municipio Eduardo Neri, with two outlying localities to the east, close to Tlalcozotitlán, in Municipio Copalillo, and south-east of Olomatlán, in Municipio Tecomatlán, in the extreme south-east of the State of Puebla. Given that Mezcala is undoubtedly globally rare, with an extremely restricted range, and is only known from a handful of populations, it is clear that the conservation status of the genus, although not formally assigned an IUCN threat category here, is likely to be vulnerable or potentially endangered.

Habitat.

Locally common, or in places close to Xochipala even abundant, in typical succulent-rich, grass-poor, seasonally dry deciduous tropical forest (SDTF) and dry scrubland with Bursera Jacq. ex L. ( Burseraceae), Bourreria P. Browne ( Boraginaceae), Neobuxbaumia mezcalaensis (Bravo) Backeb. ( Cactaceae), and Bauhinia andrieuxii Hemsl., Conzattia multiflora (B.L. Rob.) Standl., Haematoxylum brasiletto H. Karst., Lysiloma tergeminum Benth. and species of Mimosa L. (all Leguminosae), on dry karst limestone with shallow freely drained soils.

Etymology.

Mezcala is named with reference to the indigenous Mezcala culture, which like the genus Mezcala itself, is little-known, elusive, distinctive and narrowly endemic to central Guerrero, and which blossomed in this area 700-200 BC. Vestiges of the Mezcala culture are found today along the Río Balsas and its tributaries ( Reyna Robles 2020), including an important archaeological site at Xochipala, the type locality of M. balsensis . This is the second mimosoid legume genus named after an indigenous Mexican cultural group following the earlier example of Hernández (1986) who coined the generic name Zapoteca H.M. Hern. Adding a second name of similar derivation recognizes the diversity and importance of, and threats to, both endemic legumes and indigenous cultures in Mexico.

Figures

Figure 3. Distribution of Mezcala balsensis in the central Balsas Depression in Guerrero, Mexico. Map based on 15 quality-controlled species occurrence records from GBIF (www. gbif. org), SEINet (www. swbiodiversity. org / seinet), and Contreras (1986), created using R packages ggplot 2 (Wickham 2016), sf (Pebesma 2018), and rnaturalearth (South 2017), with data layers depicting Rio Balsas and borders of Mexican states downloaded from the North American Environmental Atlas (www. cec. org / north-american-environmental-atlas).