Diagnosis: — Gastrodia isabelensis is close to G. callosa Smith (1931: 248) but differs in having longer (10–16 vs. 1–8 cm) inflorescences, broadly ovate to semiorbicular (vs. ovate to ovate-deltoid) sepal lobes, ovate-orbicular (vs. ovate to subdeltoid) petal lobes and lips with mauve (vs. reddish brown) basal calli, semiorbicular (vs. shortly ligulate) epichiles and greenish brown (vs. whitish) apical keels.

Type: — SOLOMON ISLANDS. Isabel: Santa Isabel Island, Mt. Kobinitu, 600–800 m, 17 July 2014, T. C.Hsu et al. SITW05535 (holotype: BSIP!; isotype: TNM! [sheet no. BS04783], image available at http://siflora.nmns.edu.tw/specimen/ SITW05535 -TNM-001/).

Morphology: — Herbs perennial, terrestrial, leafless, achlorophyllous. Roots few, slender, often extending from rhizome apex during anthesis. Rhizome tuberous, fusiform, 4–7 cm long, 4–8 mm in diameter, yellowish brown to dark brown, ±villous, scaly; scales verticillate, linear-lanceolate, 1–2 mm long. Inflorescence erect, 1–3-flowered; peduncle 10–16 cm long, ca. 1.5 mm in diameter, glabrous, pale brown; sterile bracts 3–4, sheath-like, membranous, translucent, 4–5 mm long. Bracts membranous, glabrous, ovate, brown, apex acute, 3–4 × 2–3 mm. Pedicel and ovary ca. 5 mm long, slightly verruculose. Flowers ascending, not widely opening. Perianth tube ca. 10 × 10 mm, obovate-orbicular in top view, very fleshy, spongy, 5-lobed, with sepals fused for ca. 2/3 length with petals and lateral sepals fused for ca. 4/5 length with each other; abaxially pale orange, verruculose, slightly papillose toward apex; abaxially whitish near base, pale orange toward apex, with 2 large, orange, rugose calli. Lobe of dorsal sepal very broadly ovate, ca. 2.5 × 7.0 mm, concave, apex rounded. Lobes of lateral sepals semiorbicular, ca. 3.5 × 7.5 mm, concave, apices rounded, margins entire. Lobes of petals ovate-orbicular, ca. 2 × 2 mm, bases slightly contracted, apices rounded and slightly erose, margin subentire. Lip adnate to distal part of column-foot, free from perianth tube, entire, ca. 5.5 × 4 mm, tripartite; hypochile ca. 1 mm long, with two transversely ellipsoid, sessile, mauve, nectared calli; mesochile ovaterectangular, pale orange, glabrous, surface with 6 very obscure longitudinal ridges; epichile somewhat undifferentiated with mesochile, semiorbicular, creamy yellow tinged with orange, ca. 1.5 × 3 mm, with 2 prominent, ca. 0.8 mm long, greenish brown keels. Column semicylindrical, white, ca. 6 × 2.5 mm, ventrally canaliculate; column-foot short but well developed; column wings prominent, yellowish brown, with long narrow tips superior to anther; rostellum well developed, membranous; stigma basal, shield-shaped; anther ovate-hemispheric, ca. 1 mm long and wide; pollinia 2, white, sectile; viscidium minute. Capsule erect, ellipsoid, pale reddish brown, 18–25 mm long, 4–6 mm in diameter, verruculose; fruiting pedicel elongate after pollinated, up to 3–10 cm long. Seeds fusiform, ca. 2 mm long.

Distribution and habitat:— Gastrodia isabelensis is endemic to the Solomon Islands. So far only a single population is found in Santa Isabel Island of Isabel Province, where this species grows among deep leaf litter in rain forests at 600–800 m elevation.

Phenology:— Gastrodia isabelensis flowers from June to July and fruits from July to August.

Conservation:— Only a single wild population with 5 mature individuals (2 flowering and 3 fruiting) of Gastrodia isabelensis are so far observed. Considering the fact that most mountainous areas in the Santa Isabel Island have not been thoroughly investigated and the wild population of such mycoheterotrophic plants are generally difficultly discovered, G. isabelensis is tentatively evaluated as “Data Deficient” according to the IUCN Red List Categories and Criteria (IUCN 2012).

Etymology:— Gastrodia isabelensis is named from its type locality, Santa Isabel Island.

Additional specimens examined:— SOLOMON ISLANDS. Isabel: Santa Isabel Island, Mt. Kobinitu, 600–800 m, 16 July 2014, Hsu et al. SITW05462 (BSIP!, TAIF!, TNM!).

Taxonomic remarks:— Gastrodia isabelensis is characterized by its 10–16 cm long inflorescence, broadly ovate to semiorbicular sepal lobes, ovate-orbicular petal lobes and lips with mauve basal calli, semiorbicular epichiles and greenish brown, very elevated and sharp apical keels.