Lachemilla mexiquense differs from Lachemilla aphanoides (Mutis ex L. f.) Rothm. by its caespitose and stoloniferous habit, creeping stems, basal leaves and basal stipule persistent, campanulate-elongate hypanthium and the presence of a single carpel.

MEXICO. Estado de México, Municipio Ocuilan, 4 km NE of Santa Martha on road Santa Martha–Huitzilac, 19.07567°N, 99.36215°W, alt. 3,050 m, 30 June, 2015, Morales-Briones D. F. & Tenorio-Lezama P. 683. (holotype: ID!; isotype: MEXU!, QCA!).

Caespitose herbs, stoloniferous; stems creeping, mat-forming, branches sometimes rooting, pilose. Basal leaves 3-parted, 6–20 × 5–15 mm, chartaceous, lateral segments bifid, segments obovate to cuneate, margin incised-dentate, lower surface pilose, upper surface sparsely pilose to glabrescent; petioles 12–35 mm long; stipules 5–15 mm long, adnate to the petiole at base, free, entire and acute at apex, membranaceous, greenish-white. Stem leaves 3-parted, 7–12 × 4–7 mm, chartaceous, lateral lobes entire or bifid, segments obovate to cuneate, margin deeply cleft, lower surface pilose, upper surface sparsely pilose to glabrescent; petioles 3–5 mm long; stipules 3–8 mm long, adnate to the petiole at base, free at apex, 6-lobed at apex, membranaceous and greenish-white at base, chartaceous and green at apex. Inflorescences axillary or terminal, glomerulate, 6–10 flowered cymes; floral bracts lobed, spreading; pedicels 1–1.5 mm long, pilose at apex. Flowers 1.2–1.5 mm long; hypanthium campanulate-elongate 1–1.2 × 0.6–0.8 mm, glabrous outside, glabrous inside, green when young, reddish at maturity; episepals 4, ovate, 0.6–0.7 × 0.5–0.7 mm, glabrous, apex acute; sepals 4, lanceolate, 0.5–0.6 × 0.2–0.3 mm, glabrous, apex acute; stamens 2, filaments 0.2–0.3 mm long; carpels 1, stigma clavate. Achenes ovoid-globose, 0.9–1.1 × 0.6–0.8 mm, glabrous, one-seeded. Seeds ovate, 0.7–0.8 × 0.4–0.6 mm, pink, glabrous.

Lachemilla mexiquense is only known from the State of Mexico, municipality of Ocuilan, at ca. 3050 m altitude (Figures 2B, 3). Lachemilla mexiquense grows at the border of dense forest of various species of Pinus. This species lives in sympatry with Lachemilla procumbens (Rose) Rydb., L. vulcanica (Schltdl. & Cham.) Rydb., and L. aphanoides (Mutis ex L. f.) Rothm. It was collected in flower and fruit in late June.

Figure 1. 

Lachemilla mexiquense. A Habit B Basal leaf and stipule C Flowering branch D Flower. Illustration by P. Lu-Irving.

Figure 2. 

Lachemilla mexiquense. A Habit B Type locality.

Figure 3. 

Geographic distribution of Lachemilla mexiquense.

The specific epithet refers to the demonym for State of Mexico where the type specimen was collected.

Lachemilla mexiquense has a very limited geographic distribution, and is only known from the type locality (Figures 2B, 3). It occurs right outside the limits of the Cumbres del Ajusco National Park and Lagunas de Zempoala National Park. The type locality has been severely impacted by human activities, including conversion to agriculture (sheep and cow grazing). Following the IUCN (2014) guidelines, based on the reduced geographic distribution and altered land use at the type locality, this species should be categorized as endangered (EN), at least until other populations are discovered.

Lachemilla mexiquense resembles L. aphanoides by having tripartite leaves with bifid lateral segments and glomerulate inflorescence with entirely glabrous flowers. Nevertheless, L. mexiquense differs from L. aphanoides by its caespitose habit, creeping stems, and stolons that form dense mats. Also, the basal leaves and basal stipules are persistent, and flowers are smaller (1.2–1.5 mm long) with a campanulate-elongate hypanthium and single carpel. Lachemilla rupestris (Kunth) Rothm., a species from Andean South America with similar habit, differs from L. mexiquense by having entire lateral segments of the leaves, yellow-brown membranaceuos basal stipules, and larger flowers (2.5–3 mm long) with a turbinate-campanulate hypanthium, sericeous-hirsute pubescence, and 2–4 carpels.

In addition, phylogenetic analyses of chloroplast and nuclear DNA (Morales-Briones et al. unpubl. data) clearly separate L. mexiquense from L. aphanoides and L. rupestris. The chloroplast phylogeny place it as sister species of the ‘Orbiculate group,’ which encompasses species with stoloniferous habit, palmately lobed or cleft leaves, and flowers disposed in profuse terminal cymes, like Lachemilla pectinata (Kunth) Rothm. The nuclear phylogeny fails to confidently resolve the phylogenetic position of L. mexiquense, suggesting that it may be of hybrid origin, a common pattern seen throughout Lachemilla.