Similar to Hoya uncinata Teijsm. and Binn as both species have clear exudate, deeply lobed rotate corolla and corpusculum of the pollinarium almost as large as the pollinium. The flattened corolla is smaller (1.5–1.7 cm in diameter) with ovate lobes in H. uncinata, while the corolla of H. ruthiae is 1.8–2.2 cm in diameter with narrowly lanceolate lobes.

Malaysia, Sabah, Lahad Datu, Bukit Baturong, on limestone, 7 July 2000, Kiew R RK5029 (SING, holotype; barcode SING0077484).

Delicate lithophytic climber with clear exudate in all vegetative parts; all vegetative parts glabrous. Leafy stems cylindrical, slender, up to 4 mm in diameter, dark brown or grey, with membranaceous peeling bark; internodes 5–15 cm long. Petioles terete, fleshy, 5–15 × 1.5–3 mm in diameter, lamina lanceolate, fleshy, (5–)7–15 × 1.5–3 cm, apex acute-acuminate, base cuneate, light green above turning red in bright light with numerous grey spots, lighter green underneath; penninerved, secondary veins obscure. Inflorescences pseudo-umbelliform or globular, 3–4 cm in diameter, 4–20 flowered; peduncles persistent, extra-axillary, terete, 1–3 cm × 1.5–2.5 mm in diameter, glabrous; pedicels 10–15 mm × 0.5–0.8 mm in diameter, glabrous. Buds conical with a 5-ridged base, ca. 10 × 6 mm. Calyx lobes triangular, white-pink 1.3–1.5 × 0.7–1 mm, apex rounded, glabrous; basal colleters 1 in each sepal sinus, ovoid. Corolla rotate, deeply lobed, 1–1.5 cm in diameter, 1.8–2.2 cm when flattened, white tinged pink, thinly and minutely pubescent inside, outside glabrous, tube 1.5–2 mm long; corolla lobes narrowly lanceolate with a triangular acuminate apex, 9–10 × 3–4 mm, laterally revolute, lobe tips recurved. Gynostegium stalked, corona column conical 1–1.2 × ca. 2 mm diam, glabrous; corona staminal, 2.5–3 mm high, 6–7 mm in diameter, fleshy, yellow with a purple centre; corona lobes laterally compressed, ovate above, with revolute margins beneath, 2.8–3.2 mm × 1–1.2 mm, inner process erecto-patent, linear with an acute tip, as high as the anthers, outer process round. Anthers ovate, 650–750 × 300–400 µm, with apical round membranaceous appendage as high as the style-head apex. Pollinia oblong, with obliquely truncate apex and round base and evident pellucid margin, 550–630 × 150–200 μm; caudicles attached at the base of the retinaculum, elongate, ca. 150 µm long, corpusculum 600–650 × 250–300 μm; style-head 5-angled in cross section, with 5 spreading lobes alternating with the stamens; style-head apex columnar, 1–1.2 mm long, ca. 0.5 mm broad at the base, apex conical; ovary linear, ca. 2 mm long, each carpel ca. 0.5 mm wide at the base. Fruits and seeds unknown.

This species is named after Ruth Kiew (1946–), tropical botanist based at the Forest Research Institute Malaysia and author of numerous publications on Begonia L. and Gesneriaceae, among others.

Hoya ruthiae is only known from Bukit Baturong, Sabah, Malaysia, where it was found growing on limestone.

The distribution area, population size and possible threats to the habitat of H. ruthiae are not known, as it is only known from the type specimen and unlocalised cultivated material. It is therefore considered Data Deficient (DD) (IUCN 2014).

The most striking feature of H. ruthiae is its lack of coloured latex. Other species lacking coloured latex are the type species of the genus Hoya carnosa R.Br. and several morphologically similar taxa (Rodda and Simonsson Juhonewe 2012). All these have rotate corollas with thickly pubescent lobes within and rhomboid corona lobes with narrow inner and outer corona lobe processes. In Borneo only Hoya monetteae T. Green belongs to this group. Hoya ruthiae, as mentioned above, is morphologically very similar to Hoya uncinata, another non-laticiferous species known to occur in Java and Sumatra. Both species have deeply lobed rotate corollas, laterally compressed corona lobes and corpusculum of the pollinarium almost as large as the pollinium. Hoya ruthiae has larger flowers (1.8–2.2 cm in diameter when flattened vs. 1.5–1.7 cm) and the corolla lobes are narrowly lanceolate (vs. ovate in H. uncinata). The corona lobes of both species are ovate but in H. ruthiae the outer process is round while in H. uncinata it terminates in an incurved membranaceous apiculate appendage. Flowers of H. uncinata are illustrated in Fig. 2.

Figure 1. 

Hoya ruthiae photographed from Rodda M. MR606 (SING) prior to pressing A Flower, lateral view with two corolla lobes removed B Corolla, underneath C Corolla and corona, top view D Corona, underneath E Pedicel, calyx and ovaries F Pollinarium with twin pollinia. (Photographs by M. Rodda)

Figure 2. 

Hoya ruthiae photographed from Rodda M. MR606 (SING) prior to pressing A Inflorescence D, E, F, G Two leaves (D, F adaxial surface E, G abaxial surface). Hoya uncinata photographed from Rodda M MR607 (SING) prior to pressing B Flower, lateral view C Flower, top view. (Photographs by M. Rodda)

Unlocalised (nursery origin), Cultivated in Thailand, Ratchaburi Prov., Ratchaburi, 23 March 2014, Rodda M MR606 (SING).

Similar to Hoya aeschynanthoides Schltr. as both species have bilobed outer corona lobes, but differing in the size and colour of the flowers (3.5–4 mm in diameter, pink corolla, yellow and red corona lobes for H. bakoensis; ca. 5 mm in diameter and white for H. aeschynanthoides) and in the habit, that in H. aeschynanthoides is decumbent while H. bakoensis is a weak twining climber.

Malaysia, Borneo, Sarawak, Bako National Park, along Tajor Trail, 20 March 2015, Rodda M MR1042b (SING, holotype; SAR, KEP, isotypes)

Epiphytic climber with white exudate in all vegetative parts. Stems slender, weakly twining upward, internodes (1 mm–)2–7(–10) cm × 0.7–1.5(–2) mm, dull green, sparsely pubescent when young, scabrous; adventitious root sparsely produced along the stems and just under the nodes where they are paired. Leaves petiolate; petiole straight or recurved, 4–10 × 1–1.5 mm, dark green to maroon, sparsely pubescent; lamina ovate (lanceolate) (1.5–)2–4(–5) × 1–2.8 cm, base attenuate (acute), apex apiculate (cuspidate), dark green above, slightly pubescent on young leaves only, lighter green underneath with occasionally a slightly darker midrib and edge, glabrous, margin occasionally ciliate; penninerved, secondary veins obscure; colleters one at each lamina base, triangular to ovate 0.1–0.3 × 0.3–0.5 mm. Inflorescence pseudo-umbelliform, slightly convex, 10–15 flowered; peduncle 4–6 cm × 1–1.5 mm in diameter, dull green to brown, pubescent; rachis indeterminate (–7) × ca. 2 mm in diameter. Pedicel 3–7 × 0.7–0.9 mm in diameter, pale green with pink spots, papillose. Calyx lobes triangular, 0.7–1 × 0.5–0.8 mm, apex acute or round, light green with pink edge, sparsely ciliate; basal colleter one in each calyx lobe sinus, ovate, 150–200 × 80–100 µm. Corolla revolute, 3.5–4 mm in diameter, ca. 6 mm when flattened; corolla lobes basally fused, tube 1.2–1.5 mm long, lobes triangular, 2–3 × 2.4–2.7 mm, pink, pubescent inside, outside glabrous. Corona staminal, 2.8–3 mm in diameter, 0.9–1.1 mm high; corona lobes oblong, 1.2–1.4 × 0.7–0.8 mm, convex above, underneath sulcate with inrolled margins, outer process apex bifid, light yellow, inner process elongate, red, with a yellow round tip. Anthers broadly triangular, 400–450 × 550–650 µm, with apical triangular membranaceous appendage. Pollinia clavate, 200–250 × 90–110 µm, narrowing towards the base, apex obliquely truncate, with evident pellucid margin; corpusculum oblong, constricted in the middle, 80–100 × 50–60 µm; caudicle broad, spathulate, hyaline, 110–130 × 30–45 µm at the widest. Style-head 5 angled in cross section, with 5 lobes alternating with the stamens, style-head apex round, 200–300 µm long, ca. 0.5 mm broad at the base; ovary ovate with a narrow tip, 0.8–1 mm, each carpel ca. 0.3 mm wide at the base, light green. Fruit (unripe) a single linear follicle, 10 cm × 2 mm (widest), seed (unripe) narrowly lanceolate, 3–4 mm long, winged, long comose.

The species is named after the collection locality of the holotype, Bako National Park (Sarawak, Malaysia).

Hoya bakoensis is only known from Bako National Park, where it is common along Tajor Trail in moist, shady kerangas heath forest near a stream. The species is strictly epiphytic and it was found mostly germinating from the opening of small ant nests located inside hollow tree trunks (Fig. 4). The plants observed were forming small clumps of weakly climbing stems near the germinating point or more rarely were climbing towards the tree canopy potentially in response to low light.

Figure 3. 

Hoya bakoensis photographed in the field from the type plant Rodda M MR1042b (SING) prior to pressing. A inflorescence, frontal view B Inflorescence, side view C A single flower, front view D Revolute margins of the corolla lobes, calyx and pedicel E, F Corolla, side view G Flower, longitudinal section H Pedicel, calyx and ovary I Pollinarium with twin pollinia. (Photographs by M. Rodda)

Hoya bakoensis is locally common and well protected inside Bako National Park. Its conservation status is therefore Least Concern (LC) (IUCN 2014).

Hoya bakoensis is one of the 11 Bornean Hoya species in Hoya section Acanthostemma (Bl.) Kloppenb. Acanthostemma species are characterised by revolute corolla lobes, bilobed outer corona lobes and pollinaria with broad, spathulate caudicles. The other Bornean species in Acanthostemma are H. aeschynanthoides, Hoya acicularis, T. Green, Hoya beccarii Rodda & Simonsson, Hoya kloppenburgii T. Green, Hoya minutiflora Rodda and Simonsson, Hoya pubera Bl., Hoya revoluta Wight ex Hook.f., Hoya rundumensis (T. Green) Rodda and Simonsson, Hoya sigillatis T. Green and Hoya waymaniae Kloppenb.

Most of these species can be easily separated from H. bakoensis because their flowers are larger (>5 mm in diameter) or their inflorescence is markedly concave (in H. beccarii and H. revoluta). As mentioned in the diagnosis H. aeschynanthoides has white, only slightly larger flowers than H. bakoensis (ca. 5 vs. 3.5–4 mm in diameter). Additionally the bilobed outer lobes of H. aeschynanthoides appear to be less pronounced than those of H. bakoensis, but this observation is based solely on the examination of the drawing attached to the type of H. aeschynanthoides as only small buds are present on the specimen while no other specimens have been found at present. The flowers of H. pubera are also comparable in size with those of H. bakoensis, but they are yellow-orange, the corolla lobes are only apically revolute and the corona is more markedly raised in the centre. Lastly, H. minutiflora has the smallest flowers among all Bornean Acanthostemma (2.6–2.8 mm in diameter).

Figure 4. 

Hoya bakoensis in situ in Bako National Park (Sarawak, Malaysia) A Habitat, kerangas heath forest B Mature plant rooted inside the trunk of the host plant where an ant nest is located C Seedlings germinating from the opening of an ant nest in a hollow trunk.

Figure 5. 

Hoya bakoensis leaves photographed in the field from Rodda M MR1042b (A, B, E, F) and Rodda M MR1042a (C, D) (SING) prior to pressing. A, C, E Adaxial side B, D, F Abaxial side.

Malaysia, Borneo, Sarawak, Bako National Park, along Tajor Trail, 20 March 2015, Rodda M MR1042a (KEP, SAR, SING)