MARQUESAS ISLANDS: Ua Huka: Hanahouua valley, back of valley below cliff walls, 457 m elevation, 8°54.47S, 139°30.89W, 26 June 2004, S. Perlman & K. R. Wood 19072 (holotype: PTBG-041184!; Isotypes: P!, PAP!, US!).

Perennial woody herbs or subshrubs 20–50 cm tall, stems prostrate or sprawling to erect, branching from near base, with sparse lateral branches, glabrous or new growth sparsely pilose, mature twigs 2–3 mm diam, bark smooth, reddish brown to dark brown, with tufts of pilose hairs at thickened, persistent leaf bases. Leaves simple, spirally arranged; blade dark green above, yellow-green below, firm and moderately coriaceous when fresh, chartaceous when dry, glabrous, (15–) 20–47 × (12–)18–37 mm ovate to broadly ovate, base obtuse to rounded or truncate, apex obtuse, tip usually emarginate, venation subpalmate with 1–2(–3) pairs of secondary veins from base and 2–3 pairs along midrib above, venation raised and visible to 3° above and to 4° beneath; margin thin, plane; petiole (20–) 25–45 × 0.6–0.8 mm, sparsely scattered pilose or glabrescent, flattened, adaxially sulcate, distally with slight pulvinus. Inflorescences axillary near ends of branches, cymose, 5–13–flowered, 5–8 cm long, peduncles 4–5 cm long, terminating in apical flower and two monochasial lateral branches 18–40 mm long each with 2–6 flowers, bracts linear-subulate, 1–2 × 0.3–0.4 mm, sparsely puberulent with acicular and scattered glandular-tipped trichomes. Flowers (long-styled morph seen) with 5 calyx lobes 5–7 × 1.7–2 mm, narrowly ovate-oblong, 6–8-veined, apex acute, both surfaces minutely glandular-puberulent with capitate trichomes; petals yellow, 10–12 × 5 mm, narrowly obovate to oblong-elliptic, 7–9-veined, apex obtuse to rounded; stamens 10, in two series, the longer 7–8 mm long, the shorter 5.5–6 mm long, filaments connate basally, anthers broadly ellipsoid, 0.4–0.5 mm long, reniform to subcircular; gynoecium 10-14 mm long, with ovary 6–7 mm long, narrowly ovoid-cylindrical, externally glabrous, beak 4–5 mm long, styles 5, 1–2.5 mm long, stigmas slightly thickened, papillose, not bifid. Fruits ovoid-cylindrical, 10–11 × 2–2.5 mm, at maturity apparently straight and not twisting, externally glabrous, carpels 5, villous within, seeds ca. 40. Seeds ellipsoid, compressed, 0.8–0.9 × 0.5 mm, surface shiny, brown, rugose.

Known only from Ua Huka, Marquesas Islands.

Known only from two localities on Ua Huka, this new species occurs in shrubby and herbaceous vegetation on vertical basalts cliff above a mesic to wet lowland forest zone with Freycinetia impavida (Gaudich. ex Hombr.) B.C. Stone, Hibiscus tiliaceus L., Pandanus tectorius Parkinsonр and Pisonia grandis R. Br. Plants grow scattered on cliffs rooting in rock crevices (Figures 2A, B). Oxalis gagneorum occurs sympatrically or nearly so with Oxalis simplicifolia at both Hanahouua (Perlman and Meyer 19748) and Hane/Hokatu (Wood & Meyer 10530, 10551).

The specific epithet refers to the simple, unifoliolate leaves.

Following the criteria and categories of IUCN (2001) Oxalis simplicifolia is assigned a preliminary status of Critically Endangered (CR): B2a, B2b (i–iii); D): B2: total area of occupancy less than 10 km2 (ca. 5 km2). B2a, two populations known; b (i–iii), habitat continuing decline inferred. D, population estimated to number fewer than 250 individuals. The suitable habitat for Oxalis simplicifolia on Ua Huka (ca. 83 km2) is indicated as an endangered environment, threatened by feral animals and invasive plants, reducing the extent of the forest. Estimated population size is ca 100+ plants at the Hanahouua locality (Perlman & Meyer 19748), and “scattered” individuals were noted by the collectors at the Hane/Hokatu locality (Wood & Meyer 10530, 10551).

Marquesas Islands: Ua Huka: Hanahouua valley, back of valley on ridge between Hanahouua and Hanalei, 8°54.47S, 139°30.87W, 488 m, 28 July 2005, S. P. Perlman, J.-Y. Meyer 19748 (PTBG); Hane/Hokatu cliff, zone, 520 m, 11 Dec 2003, K. R. Wood, J.-Y. Meyer 10512 (PAP, PTBG, US).

Although the majority of Oxalis species have palmately compound leaves with three (rarely to nine) leaflets, several taxa with unifoliolate leaves occur in South America in subgenus Thamnoxys (Lourteig 1994). At least three South African taxa are also unifoliolate: Oxalis monophylla L., Oxalis salteri L.Bolus, and Oxalis flava L. var. unifoliolata Dreyer & Oberl. (Dreyer et al. 2010). These are small, bulbous, acaulescent plants with white or yellow flowers apparently unrelated to Oxalis simplicifolia. Certain species, e.g. Oxalis renifolia Kunth and a few other South American taxa can have one and three leaflets on the same branch (E. Emshwiller, pers. comm. 2009). Following Lourteig’s (1994, 2000) monograph Oxalis simplicifolia keys to subgenus Monoxalis (leaves simple, 1-foliolate, stigmas linguiform) which consists of two herbaceous species from the southwestern United States and Mexico (Oxalis dichondrifolia A. Gray and Oxalis robusta Kunth), neither of which bears any morphological similarity to Oxalis simplicifolia.

This new species may be related to the Marquesas endemic Oxalis gagneorum, from which it differs by its glabrous simple, unifoliolate leaves with ovate to broadly ovate blades having subpalmate venation of one to three basal vein pairs, minutely glandular puberulent calyx lobes, shorter corolla lobes 8–12 mm long, shorter stamens 5–8 mm long, capsules not twisting at maturity (they often twist in Oxalis gagneorum), and smaller seeds 0.8–0.9 × 0.5 mm (Table 1). Fosberg and Sachet (1981: 3–5, Fig. 1) stated that the relationships of Oxalis gagneorum were obscure and suggested that it may be related to Oxalis novaecalidoniae Kunth & Schlechter, a species belonging to section Caledonicae (= section Neocalidonicae), but that mature seeds were needed for more accurate placement. Lourteig (2000) placed Oxalis gagneorum in section Rhombifoliae along with several neotropical species characterized by “lianoid” transversely striate stems [a character not apparent in material of either Marquesan species studied by us] and trifoliolate leaves with oblong to rhomboidal leaflets and lacking stipules, but expressed doubt as to its relationships due to the poor material available for study (i.e., lacking stigmas and seeds). Unfortunately, it has not been possible to obtain DNA sequences from samples of either Marquesan species thus far (E. Emshwiller, pers. comm. 2009), and consequently their phylogenetic relationships remain unclear.

Oxalis species often have different floral morphs, frequently tristylous or sometimes distylous (Weller et al. 2007). Examination of material of Oxalis gagneorum revealed most flowers appear to be homostylous with styles about equaling the stamens. Due to a paucity of flowering collections of Oxalis simplicifolia, only the long-styled floral morph with both whorls of stamens shorter than the style is known (illustrated in Fig. 1). There may be several possible explanations for this: either Oxalis simplicifolia might be a clonally reproducing species with a single morph (as are several Oxalis species in Mexico), or it could be an autogamous species that reproduces sexually but has only a single morph (S. Weller, pers. comm. 2009). Further collections and field studied are clearly necessary to resolve this question.