The new species is distinguished from S. erecta and S. pulneyensis by the decumbent unbranched stem, absence of a distinct peduncle, the cymose 1–2 flowered terminal inflorescence and by the anthers which are half the length of filament. (Sonerila erecta and S. pulneyensis have branched stem, a distinct peduncle, inflorescence consisting of more than 2 flowers in a cyme and anther having same the length of the filament.)

INDIA. Kerala: Palakkad district, Pothumala, Nelliampathy hills, 10°30'09.6"N; 76°42'16.5"E, 1160 m 18 Oct 2015, Soumya M. & Maya C. Nair 1185 (Holotype CALI!, isotypes MH!, ERRCH!, GVCH!)

Decumbent, unbranched, delicate, succulent herbs attaining 6–10 cm height; the lower portion more or less trailing and bear perennating buds, while the upper portion curves upward and grows erectly. Stem translucent, fleshy, subterete with scattered multicellular and glandular trichomes which form a dense hairy nodal ring. Leaves opposite, fleshy, petiole 0.5–1.5 cm, adaxially grooved, with glandular trichomes; lamina ovate, 1.3–2.5 × 1–1.5 cm, base obtuse, green with pink tinge below, upper surface densely hirsute (0.08–0.09 × 0.03–0.06 cm), lower surface with scattered glandular trichomes (0.02–0.03 cm × 0.03–0.04 cm), margins finely serrate, acute at apex, prominently 3-nerved a pair of nerve obscurely seen near the margin. Inflorescence terminal, unbranched, condensed, a 1–2- flowered cyme. Peduncle more or less absent. Flowers 3-merous, pedicel 0.5–0.7 cm with few glandular trichomes, shorter than hypanthium, light green. Hypanthium 0.8–0.9 cm long, campanulate with scattered glandular trichomes, light green. Calyx lobes 3, 0.15–0.2 cm long, triangular, non-caducous, with sporadic glandular trichomes and pink tinge. Petals 3, 0.8–0.7 × 0.5 cm–0.45 cm orbicular-obovate, acuminate at apex with 3–4 glandular trichomes on the midrib of the abaxial side. Stamens 3; filaments 0.4–0.42 cm long, glabrous, white; anthers yellow, 0.2–0.22 cm, cordate at base, glabrous. Style 0.8–0.9 cm long, curved, deep pink towards the tip, stigma capitate, glabrous. Capsule campanulate, 0.8–0.9 cm long and 0.45 cm wide with occasional glandular trichomes, green. Seeds many, 0.07× 0.02 cm, minutely tuberculed, brown.

October–December.

The specific epithet honours Dr. P.K.K. Nair (1930-), eminent scientist, renowned as father of Indian palynology and founder director of the Environmental Resources Research Centre (ERRC), Thiruvananthapuram.

Sonerila nairii grows at altitudes of 1140–1160 m in shady rock surfaces within moist loose soils and under the evergreen canopies along the Pothumala hill tract of Nelliampathy hills. In these habitats, S. nairii grows close association with crustose lichens. The new species seems to prefer more or less moist lithophytic habitats in contrast to S. erecta and S. pulneyensis which grow in evergreen and riparian forests respectively. The distribution of the three taxa has been summarized in Fig. 3.

Figure 1. 

Sonerila nairii A Habit B Single flower and flower bud C Calyx D Petal-Adaxial side E Petal-Abaxial side F Stamen G Gynoecium H Ovary TS I Flower LS J Seed (from Soumya. M & Maya. C. Nair 1185) Illustration by Soumya. M.

Figure 2. 

Sonerila nairii A Habitat B Habit C Single flower and flower bud D Calyx E Petal-Adaxial side F Stamen G Gynoecium H Ovary TS I Flower LS J Seed (from Soumya. M & Maya. C. Nair 1185) Photos by Soumya. M & Maya. C. Nair.

Figure 3. 

Map showing the distribution of Sonerila nairii sp. nov., Sonerila erecta Jack and Sonerila pulneyensis Gamble.

Two populations comprising only a few individuals (5–10) of the species were recorded growing within a distance (50 meters) of each other. Apart from the type locality, the species has yet to be found anywhere else. Because the number of mature individuals is less than 50 and the species has a very restricted area of occupancy, we assign the species, the status of Critically endangered using IUCN Strategies and criteria (IUCN 2014).

(Paratypes). INDIA. Kerala: Palakkad district, Pothumala, Nelliampathy hills, 1 Nov 2015 Soumya M. & Maya C. Nair 1187 (ERRCH!) (Environmental Resources Research Centre Herbarium), 5 Nov 2015 Maya C. Nair & Soumya M. 98 (GVCH!) 12 Nov 2015 Maya C. Nair & Soumya M. 99 (GVCH!) (Government Victoria College Herbarium)

Sonerila nairii differs from S. erecta by having an unbranched decumbent stem, leaf margins with pink tinge; by the absence of a peduncle, a condensed, cymose, 1–2-flowered, terminal inflorescence. The anthers are half the length of the filaments and the stigma is capitate. From S. pulneyensis Gamble it differs in having decumbent and sparse glandular trichomes on the stem and pedicel, dimorphic hairs on the leaves, by the absence of a distinct peduncle; by the terminal inflorescence of 1-2 flowers borne in a condensed cyme. The hypanthium is green-coloured and the anthers are half the length of the filaments. Further differences between S. nairii, S. erecta and S. pulneyensis are given in Table 1.