Among Sulawesi Hoya species similar to H. brevialata and H. pallilimba in habit (prostrate and pendant epiphyte), lamina shape (convex round to elliptic) and inflorescence type (positively geotropic, convex) but separated because both H. brevialata and H. pallilimba have smaller flowers (c. 5 vs. 8–10 mm in diameter in H. isabelchanae) with a finely pubescent corolla (vs. setose corolla in H. isabelchanae).

Indonesia, Sulawesi, Gorontalo, Gunung Boliohutu, 400 m, 23 Apr 2002, S.M. Scott 02-116, grown on at the Royal Botanic Garden Edinburgh (Acc no. 20021229), Sep 2012, C.E. Berthold 0013 (holotype: E; isotype: SING)

Epiphytic climber with white latex in all vegetative parts. Stems slender, prostrate, pendant, internodes (2)4–6(–10) cm × 1–1.5 mm, dull green or brown, pubescent when young, rarely almost glabrous when mature; adventitious root sparsely produced along the stem and just under the nodes where they are usually paired. Leaves petiolate; petiole recurved, round, 4–6(–8) × ca. 1.5 mm, pale green, pubescent; lamina orbicular-ovate (to elliptic), convex, fleshy and stiff (1.5–)2–4(–7) × (1–)1.5–2.5 cm, base cuneate (round), apex obtuse (round), pale to mid-green green above, with or without grey spots, pubescent on young leaves only, paler green underneath, pubescent; penninerved, secondary veins obscure; colleters (one) two at each lamina base, triangular to ovate 0.2–0.4 × 0.3–0.5 mm. Inflorescence positively geotropic, pseudo-umbellate, slightly concave; peduncle (1–)2–4(–7) cm × 1.5–2 mm in diameter, dull green to brown, pubescent when young; rachis indeterminate. Flowers 10–15 each inflorescence; pedicel variable in length, the internal ones ca. 8 mm long, the external ones 2.5–3 cm × 1.2–1.5 mm in diameter, bright green, glabrous. Calyx lobes triangular, 1.2–1.6 × 1–0.8 mm wide, apex round, light green or brownish, glabrous; basal colleter one in each calyx lobe sinus, ovate, 300–400 × ca. 100 µm. Corolla revolute, 8–10 mm in diameter, ca. 16 mm when flattened; corolla lobes basally fused, tube 3–4 mm long, pomegranate red, from almost glabrous at the base to thinly pubescent becoming setose towards the distal part of the inner side of the tube, glabrous outside, lobes broadly ovate, 5–6 × 4–5 mm, pomegranate red with a paler edge, inside setose with a glabrous tip, outside glabrous. Corona staminal, 7–8 mm in diameter, 3–3.5 mm high; corona lobes ovate, ca. 3.5 × 2 mm, slightly convex above, underneath sulcate, inner process apex acuminate, outer process apex divided in an upper round part and a lower bifid part, upper part cream yellow with a pinkish inner process tip, lower part and bilobed outer process reddish. Anthers with apical translucent appendages, broadly triangular, c. 1.2 × 1.2 mm. Pollinia oblong, 250–300 × 130–150 µm, base obliquely truncate, apex round, sterile edge all along the outer edge of the pollinium; corpusculum oblong, 120–140 × 50–60 µm; caudicle broad, spathulate, 180–200 × 100 µm at the widest point. Style-head 5 angled in cross section, c. 2.5 mm in diameter, with 5 lobes alternating with the stamens, style-head apex mamillate, ca. 1 × 0.5 mm broad at the base; ovary ovoid, shortly beaked, 1.5–1.7 mm long, each carpel ca. 0.7 mm wide at the base, pale green, glabrous. Fruit and seed not observed.

Hoya isabelchanae is named after Isabel Claire Chan Yuen Ching, late daughter of Elisabeth Chan, Singaporean patron of botanical research and a gardener with an interest in Hoya.

Hoya isabelchanae is only known from the base of Gunung Boliohutu, Sulawesi, where it was collected as a sterile cutting in 2002 and brought into cultivation at the Royal Botanic Garden Edinburgh where it regularly blooms in a heated greenhouse from May to October. The species was collected in primary forest and it was growing in shaded but exposed area on a decaying tree 12 m tall. A further collection is widely available in cultivation under Gerard Paul Shirley number GPS10161 and 7-35 http://www.paulshirleysucculents.nl/shop_hoyas.htm [accessed on 24 June 2016]. This accession is apparently also from Sulawesi but no further collection information is available.

The only localised specimen of Hoya isabelchanae is the type collection. No information is available on the extent of the wild population in Sulawesi and the threats to its habitat therefore its conservation status is Data Deficient (DD) (IUCN 2014). Hoya isabelchanae is at present in cultivation at the Royal Botanic Gardens Edinburgh (type collection, acc. no. 20021229) and at the Singapore Botanic Gardens (unlocalised collection, vouchered as M. Rodda MR573).

Hoya isabelchanae belongs to Hoya section Acanthostemma (Blume) Kloppenburg whose members have revolute corolla lobes, bilobed outer corona lobes and pollinaria with broad, spathulate caudicles. With a corolla 8–10 mm in diameter, H. isabelchanae is one of the largest flowered species in Acanthostemma, only comparable with Hoya benchaii Gavrus et al. (corolla 9–12 mm in diameter) Hoya kloppenburgii T.Green (10–12 mm), Hoya rundumensis (T.Green) Rodda & Simonsson (7–10 mm) and Hoya sigillatis T.Green ssp. sigillatis (7–10 mm), all from Borneo. All these can be separated from H. isabelchanae because their corolla is puberulent while H. isabelchanae has a setose corolla (Fig. 1).

Figure 1. 

Hoya isabelchanae photographed in cultivation in Thailand (reference specimen M. Rodda MR573, SING). A Branch and inflorescence, side view B Inflorescence, from underneath C A single flower, front view D A single flower, lateral view E Corolla from underneath with calyx removed F Calyx G Anther H Pollinarium. (Photographs by M. Rodda).

Figure 2. 

Leaves of Hoya isabelchanae photographed in cultivation in Singapore (reference specimen M. Rodda MR573, SING). A Larger leaf, from above B Larger leaf, from underneath C Smaller leaf, from above D Smaller leaf, from underneath (Photographs by M. Rodda).

Among Sulawesi Acanthostemma members, H. isabelchanae is vegetatively similar to H. brevialata and H. pallilimba, that make large clumps of prostrate and pendant stems and have convex round to elliptic laminas. They also have similar positively geotropic convex inflorescences. However H. isabelchanae can be separated from H. brevialata and H. pallilimba because it has much larger flowers, both H. brevialata and H. pallilimba have flowers c. 5 mm across while H. isabelchanae has flowers 8–10 mm in diameter. The corona of H. isabelchanae has almost flat lobes while the coronas of H. brevialata and H. pallilimba have the inner lobe held much higher than the outer lobe process. Further, the corolla of H. brevialata and H. pallilimba is finely pubescent while the corolla of H. isabelchanae is setose.

Indonesia, Sulawesi, (live collection numbers 7-35 and GPS10161), grown in Thailand, Ratchaburi, 23 Mar 2014, M. Rodda MR573 (SING).