Argostemma separatum Aranas, Hsu, & Alejandro, 2020

Hsu, Hao Wei C., Aranas, Denmarc R., Alejandro, Grecebio Jonathan D. & Schumann, Sigrid Liede-, 2020, A new species of Argostemma (Rubiaceae) with free stamens in star-shaped corolla from the Philippines, Phytotaxa 437 (2), pp. 113-117 : 114-116

Argostemma separatum Aranas, Hsu, & Alejandro sp. nov. ( Figs. 1 – 2 )

Argostemma separatum is similar with A. diversifolium by having free stamens in star-shaped corolla but can be distinguished by its indumentum on stem (puberulous to villous vs glabrous), smaller leaf blades (2.7 – 5.2 × 1.5 – 2 vs 15 × 7 cm) with trichomes on both sides, shape and apex of stipule (ovate, acuminate vs lanceolate, bifid), inflorescence (1–4 vs 19–28 flowered), shape and indumentum of calyx (gamosepalous and strigose vs lanceolate and glabrous), red tips on corolla lobes, and color of anthers (yellow vs pale blue).

Type:— PHILIPPINES. Oriental Mindoro: Municipality of Baco, Mt. Halcon, Aranas et al. 14-728, 13° 18.97 ’N, 121° 04.08 E, 9 April 2014, (holotype: USTH [ USTH013118 ]; isotype: PNH) .

Lithophythic single-stemmed herbs, 5 – 10 cm tall. Stem erect, unbranched, internodes 2.5 to 3 cm long, fleshy, subterete, puberulous to villous. Stipules persistent, ovate, 3 – 7 × 3 – 5 mm, glabrous, acuminate at apex, margin entire. Leaves anisophyllous, decussate; petioles 0.5 – 2 cm, glabrescent to villous; leaf blades 2.7 – 5.2 × 1.5 – 2 cm, ovate, lanceolate or elliptic oblong, with brochidodromous venation, attenuate to acuminate at apex, cuneate to obtuse at base, drying membranous, repand at margin, with trichomes on both sides but more conspicuous adaxially; midrib distinct, lateral veins 7 – 8 pairs. Inflorescences terminal, cymose, 1 – 4 flowered; peduncles 1 – 1.5 cm, pilose; bracts triangular, 1 – 3 × 1.5 – 2 mm, persistent, glabrescent to glabrous; pedicels 1 – 1.5 cm. Calyx gamosepalous, strigose; hypanthium infundibuliform. Corolla lobes 5, white with red colored tips, 6 × 3 – 5 mm, glabrous, broadly lanceolate, acute to attenuate at apex, with palmate venation. Stamens 5, free, 5 – 6 mm long, inserted at base of corolla tube; filaments short, straight; anthers yellow, 5.5 – 7 mm long. Ovary inferior, bilocular, globose; style 7 – 8 mm long, glabrous; stigma exserted, 1 – 2 mm long, club-shaped. Fruit cupuliform, 2 – 3 × 1 – 1.5 mm, densely villous, capped with persistent calyx, opening by apical pore. Seeds flat, numerous, small (ca. 200 µm).

Etymology:—The specific epithet is based on its free stamens, a distinct trait among the other Philippine Argostemma .

Distribution and Habitat:—This species is only known from its type locality at Mt. Halcon Baco, Oriental Mindoro, thriving on boulders beside streams and shaded areas at elevations of 450 to 500 m asl, growing with bryophytes and ferns.

Phenology:— Argostemma separatum has been observed to be flowering from March to June, and fruiting from April to August.

Discussion: — Argostemma separatum is morphologically similar to A. diversifolium from Thailand by its free stamens in star-shaped corolla, the only two species recorded in the whole genus with this type of flower. It differs by having shorter (up to 10 cm) puberulous to villous stems, smaller (2.7 – 5.2 cm) anisophyllous leaf blades covered with trichomes on both sides, attenuate to acuminate at apex, cuneate to obtuse at base, ovate and not bilobed stipules, fewer flowered cymose inflorescences (1–4) with shorter (up to 1.5 cm) pilose peduncle, gamosephalous and strigose calyx, red tips in corolla lobes, and yellow anthers up to 7 mm long. These morphological characters warrant the appraisal of a new Argostemma species. Table 1 provides a detailed comparison between A. diversifolium and A. separatum .

Conservation Status:— Argostemma separatum was only collected at the type locality, and the distribution range of this species remains unknown. Thus, the conservation status of A. separatum is Data Deficient (DD) based on the IUCN (2019) categories. Although several individuals of A. separatum were found in moist evergreen undisturbed forest, it is susceptible to the effects of human activities since indigenous communities inhabit the area.

Figures

FIGURE 1. Field photographs of Argostemma separatum. (A) Habit showing its lithophytic nature; (B) Flower showing free stamens in star-shaped corolla with red apex. Photos taken by D.R.Aranas.

FIGURE 2. Argostemma separatum (drawn from the type specimen)—A. habit; B. adaxial surface of leaf. C. top view of open flower showing free stamens; D. open flower showing exserted stigma; E. free stamens; F. corolla lobe; G. calyx; H: style. Scale bars: A & B, 1 cm; C–H, 2mm. Illustrated by N. Diego.