Nine new species and one new subspecies of Hoya (Apocynaceae: Asclepiadoideae) from Borneo

Abstract

In the present paper we publish nine new species and one new subspecies from Borneo, Hoya ariffinii Rodda & S.Rahayu, H. boycei Rodda & S.Rahayu, H. curtisii King & Gamble subsp. collariata S.Rahayu & Rodda, H. dulitensis Rodda & S.Rahayu, H. kaikoeana S.Rahayu & Rodda, H. kapuasensis S.Rahayu & Rodda, H. kerangasensis Rodda & S.Rahayu, H. peltata Rodda & S.Rahayu, H. polypus S.Rahayu & Rodda and H. sangguensis S.Rahayu & Rodda. Five species are endemic to Kalimantan, two to Brunei and one to Sarawak. Only Hoya kerangasensis is found in Brunei, Sabah and Sarawak, and only H. sangguensis is also found outside Borneo in Peninsular Malaysia. With these new species the number of Hoya of Borneo reaches 85 species and four subspecies.

Full text

101
Gardens’ Bulletin Singapore 74(1): 101–129. 2022
doi: 10.26492/gbs74(1).2022-08
Nine new species and one new subspecies of Hoya
(Apocynaceae: Asclepiadoideae) from Borneo
M. Rodda1 & S. Rahayu2
1Singapore Botanic Gardens, National Parks Board,
1 Cluny Road, Singapore 259569, Singapore
rodda.michele@gmail.com
2Bogor Botanic Gardens, Research Center for Plant Conservation,
Botanic Gardens and Forestry, National Research and Innovation Agency,
Jln. Ir. H. Juanda 13, Bogor, Jawa Barat 16122, Indonesia
srirahayukrb@yahoo.com
ABSTRACT. In the present paper we publish nine new species and one new subspecies from
Borneo, Hoya arifnii Rodda & S.Rahayu, H. boycei Rodda & S.Rahayu, H. curtisii King &
Gamble subsp. collariata S.Rahayu & Rodda, H. dulitensis Rodda & S.Rahayu, H. kaikoeana
S.Rahayu & Rodda, H. kapuasensis S.Rahayu & Rodda, H. kerangasensis Rodda & S.Rahayu,
H. peltata Rodda & S.Rahayu, H. polypus S.Rahayu & Rodda and H. sangguensis S.Rahayu &
Rodda. Five species are endemic to Kalimantan, two to Brunei and one to Sarawak. Only Hoya
kerangasensis is found in Brunei, Sabah and Sarawak, and only H. sangguensis is also found
outside Borneo in Peninsular Malaysia. With these new species the number of Hoya of Borneo
reaches 85 species and four subspecies.
Keywords. Brunei, Indonesia, Kalimantan, Malaysia, Malesia, Sabah, Sarawak
Introduction
The genus Hoya R.Br., with an estimated 350–450 species, is the largest among Asian
Apocynaceae (Rodda, 2015; Endress et al., 2019) and is to be found over a large area
from the Himalayan foothills to Okinawa (Japan) and through Southeast Asia and
Malesia to Australia and the Fiji Islands.
The morphological diversity of Hoya, with particular emphasis on Bornean
species, was extensively discussed in Lamb et al. (2014) and Lamb & Rodda (2016).
Based on the available data (Pelser et al., 2011 onwards; Lamb & Rodda, 2016;
Cámara-Leret et al., 2020) the centres of diversity of Hoya are Borneo, New Guinea
and the Philippines. What is known of the diversity of the genus in the three areas,
however, differs markedly, leading to either an underestimation or an overestimation
of the number of species. At one extreme is New Guinea, which is the least explored
but has seen a recent increase in taxonomic research (Cámara-Leret et al., 2020;
Rodda & Simonsson, in press). In New Guinea there are 77 recognised Hoya taxa
(including ve subspecies), 19 of which have been published since 2017 due to recent
eld collections (Nathalie Simonsson spent six years in Papua New Guinea, 2010–
2016, and conducted extensive eld work) and extensive examination of specimens

Gard. Bull. Singapore 74(1) 2022
102
(Simonsson Juhonewe & Rodda, 2017; Rodda & Simonsson, in press). Six species
are considered doubtful either because of lack of type material or unavailability of
complete specimens (updated from Cámara-Leret et al., 2020; Rodda & Simonsson, in
press) but the other species are all fairly well known and unlikely to be synonymised. A
guidebook to Hoya of New Guinea is in preparation and will include short descriptions
and images of most species. More than 90% of the New Guinean taxa are considered
endemic and it is almost certain that with more intensive eld collections further
novelties will be collected and described.
At the other extreme is the Philippines, where Pelser et al. (2011 onwards) list as
many as 202 Hoya taxa, but state that ‘The number of Hoya species in the Philippines
is doubtlessly much lower than suggested by this account’. We agree that many names
will become synonyms once the genus is revised.
The diversity of Hoya in Borneo has been recently studied quite extensively
and publications include Lamb & Rodda (2016), an illustrated guidebook to Hoya of
Borneo including 71 species and one subspecies, with about 50% endemic taxa. Rodda
(2017) claried the types of all of these names. Since then, the number of taxa has
been steadily increasing, reaching 76 species and three subspecies (Rahayu & Rodda,
2021).
We here describe an additional nine species and one subspecies from Borneo.
Five were recently collected in Kalimantan, Indonesia, which is the largest yet the
most poorly explored part of Borneo; four are only known from one or few herbarium
specimens from Brunei, Sabah or Sarawak; and one is endemic to the heath forests
of Brunei. The number of Hoya taxa occurring in Borneo is therefore updated to 85
species and four subspecies. In preparation for this paper, specimens of Hoya were
examined in person or via loans from A, BM, BO, FI, K, L, LAE, MO, P, SING and
UC herbaria (Thiers, continuously updated). We are well aware that describing new
taxa based on one or very few collections, as is the case for most of the taxa described
here, only allows for a very incomplete understanding of the morphological variability
of each taxon. When and if new collections become available, species descriptions,
habitat and ecological information, and conservation assessments will be updated. We
are also aware that more collections of these taxa might narrow the morphological
discontinuities between similar taxa. Nonetheless, we do not consider any of the
taxa described here to be part of known variable species complexes such as Hoya
verticillata (Vahl) G.Don or Hoya camphorifolia Warb., and are therefore less likely
to be synonymised in the future. In this paper, the global conservation categories and
criteria follow IUCN Standards and Petitions Committee (2022).
Taxonomy
1. Hoya arifnii Rodda & S.Rahayu, sp. nov.
Similar to Hoya sammannaniana A.Lamb et al. in habit (terrestrial or hemi-epiphytic),
leaf texture (thinly coriaceous) and shape (lanceolate or elliptic), number and shape

103
New taxa of Hoya from Borneo
of owers (generally 1 open at a time, corolla campanulate-urceolate), but can be
distinguished in leaf surface (at in H. arifnii vs undulate with recurved margins in H.
sammannaniana), corolla tube-lobe ratio (tube > 2 times lobe length in H. arifnii vs
tube < 2 times lobe length in H. sammannaniana), corolla tube shape (broader at base
in H. arifnii vs broader at mouth in H. sammannaniana) and corona shape (broadly
conical in H. arifnii vs almost at-topped in H. sammannaniana). – TYPE: Brunei,
Belait, Bukit Sawat, Jln Labi km 13, Jln Sg Liang-Labi, entrance sub road behind
nursery, 4°34′32″N 114°30′19″E, 71 m, 23 November 2010, Arifn & Azlan BRUN
23277 (holotype BRUN [acc. no. B030492]; isotypes K n.v., SING n.v.). (Fig. 1, 2)
Climber or creeper, terrestrial or hemi-epiphytic, latex white in all vegetative parts,
glabrous. Stems cylindrical, slender, to 3 m long, 2–3 mm diam., internodes (1.5–)7–
12 cm long, older stems with pale peeling bark. Roots basal, no adventitious roots
observed. Leaves opposite or alternate, petiole channelled above, 3.5–10 mm long,
1–1.5 mm diam.; lamina at, lanceolate or elliptic, thinly coriaceous, 7–15 × 2–5
cm, apex acuminate, base attenuate; venation pinnate, secondary veins 4–6 each side,
tertiary venation reticulate; basal colleters not present. Inorescence one per node,
with one open ower at a time; peduncle extra-axillary, positively geotropic, terete, 2–7
cm long, 1–1.5 mm diam., older peduncles forming a rachis from previous owerings,
glabrous; pedicels liform, 10–15 mm long, 0.3–0.5 mm diam., glabrous. Flowers in
bud globose, 5-ridged, pale yellow, positively geotropic. Calyx 3–3.2 mm diam.; lobes
broadly deltate, 1–1.2 × 0.8–1.2 mm, apex acute, outside sparsely pubescent, inside
glabrous, ciliate; basal colleters one at each calyx lobe sinus, oblong, c. 0.15 mm
long. Corolla campanulate-urceolate, 11–12 mm long, 12–14 mm diam.; tube c. 15
mm long, basally broadly campanulate, then straight, narrowing towards the mouth,
glabrous; lobes broadly triangular, spreading at anthesis, 5–7 × 2–3 mm, apex obtuse,
glabrous. Gynostegium sessile. Corona staminal, conical, 3.5–3.8 mm high, 6–6.5
mm diam.; lobes rhomboid, 3–3.5 × 1.8–2.2 mm, inner process acuminate, ascending-
erect, outer processes spreading, apex rounded, with basal revolute margins. Anthers
triangular, with apical round membranous appendage c. 1 × 1 mm. Pollinia oblong,
500–600 × 200–250 µm, with pellucid margin, corpusculum rhomboid, 250–300 × c.
150 µm, caudicles attached to the middle part of the corpusculum, triangular, c. 120
µm long. Ovary conical, 2.2–2.4 mm long, glabrous. Fruit and seed not observed.
Distribution. Endemic to Brunei, where it appears to be locally common in the white
sands in Tutong and Belait Districts.
Habitat and ecology. Terrestrial or hemi-epiphytic in heath (kerangas) forest, white
sandy soils, at 15–71 m altitude. It appears to be owering throughout the year as
fertile specimens were collected in April, June, September and November.
Etymology. The species is named after Muhammad Arifn A. Kalat, formerly forest
botanist at the Brunei National Herbarium, the rst and type specimen collector.

Gard. Bull. Singapore 74(1) 2022
104
Fig. 1. Hoya arifnii Rodda & S.Rahayu. A. Habit. B. Flower (side view). C. Flower (side view
with part of corolla removed). D. Corona (from above). E. Pollinarium. All from Arifn et al.
BRUN 16429. Drawn by X.Y. Loh.

105
New taxa of Hoya from Borneo
Fig. 2. Hoya arifnii Rodda & S.Rahayu. A. Habit. B. Inorescence (side view). C. Flower,
showing staminal corona. D. Flower (from above). From Arifn & Azlan BRUN 23277. (Photos:
Muhammad Arifn A. Kalat)
Provisional IUCN conservation assessment. Endangered (EN B1ab(iii), B2ab(iii)).
The estimated EOO is 102 km2, the AOO is 12 km2. The species appears to be relatively
common in Tutong and Belait Districts but may be locally endemic as it has not been
recorded from elsewhere, and the lowland heath forest habitat is in decline in Borneo,
including in Brunei, as it is threatened by re and drought (Miyamoto et al., 2021).

Gard. Bull. Singapore 74(1) 2022
106
Additional specimens examined. BRUNEI: Belait: Bukit Sawat, Sungai Mau, Labi Road km
13, along road to plantation, 4°34′N 114°29′E, 15 m, 20 Jun 1996, Arifn et al. BRUN 16429
(BRUN [sheet no. B 005 684], K); Bukit Sawat, Sungai Mau, km 13 Jalan Labi-Sungai Mau,
3 Apr 2001, Arifn & Jangarun BRUN 19421 (BRUN [sheet no. B 005 762]). Tutong: Bt
Beruang / Udal, Jln Ternakan Kolam Udang, 4°42′55.3″N 114°37′33.5″E, 32 m, 18 Sep 2012,
Zwah Asano [partially illegible] BRUN 23970 (BRUN).
Notes. Hoya arifnii is an unusual species that often develops only one leaf at each
node and has long thin inorescence rachises (Fig. 2). As mentioned in the diagnosis,
it is most similar to Hoya sammannaniana, also endemic to Borneo. Other species
with thinly coriaceous leaves and generally one open ower at a time are Hoya
wongii Rodda et al., endemic to Borneo, Hoya wallichii (Wight) C.M.Burton from
Borneo, Peninsular Malaysia and Singapore, and H. mappigera Rodda & Simonsson
from Borneo and Peninsular Malaysia. They are all distinguished from Hoya arifnii
because their owers have a broadly campanulate corolla while H. arifnii has a
campanulate-urceolate corolla. The corolla of Hoya arifnii is somewhat similar to
that of Hoya vacciniiora O.Schwartz, from Gunung Mulu (Sarawak) which is so far
only found in montane forest and its leaves are thick and eshy, while H. arifnii has
thinly coriaceous leaves.
2. Hoya boycei Rodda & S.Rahayu, sp. nov.
Similar to Hoya peltata Rodda & S.Rahayu in lamina shape (broadly elliptic to round)
and size (8–17 mm long), but can be distinguished in lamina pubescence and base
shape (pubescent and papillose, base rounded to attenuate in H. boycei vs smooth
and sparsely pubescent to glabrescent, base rounded to peltate in H. peltata), and
inorescence and ower size (inorescence many-owered, pedicels 18–40 mm long,
corolla 7–9 mm diam. in H. boycei vs 1–2-owered, pedicels 1–3 mm long, corolla
c. 6 mm diam. in H. peltata). – TYPE: Brunei, Belait, Ulu Ingei, Bukit Batu Patam,
lower slopes near Sungei Ingei, 4°5′N, 114°42′E, 70 m, 11 June 1989, Boyce, Wong,
Dranseld & Dranseld 305 (holotype BRUN [sheet no. B005714]; isotype K [spirit
bottle no. 56265.000]). (Fig. 3)
Climber, likely epiphytic, latex colour unknown. Stems cylindrical, slender, to 1 m
long, 0.8–1.5 mm diam., pale green, papillose and pubescent, internodes 1–3 cm
long. Roots adventitious, occurring below the nodes. Leaves: petiole terete, 1–3 mm
long, 0.8–2 mm diam., papillose and pubescent; lamina broadly elliptic to round, very
coriaceous when dry, likely very eshy when fresh, 8–17 × 6–13 mm, mid green above,
almost white below, papillose and pubescent on both surfaces, apex acute to obtuse,
often mucronulate, base rounded to attenuate; venation pinnate, secondary veins
inconspicuous; basal colleter one at lamina base, ovoid, c. 0.2 mm long. Inorescence
one per node, pseudo-umbelliform, consisting of 3–10 owers; peduncle extra-
axillary, tropism unknown but likely positive, terete, 4.5–6 cm long, 0.8–1 mm diam.,
papillose and pubescent; pedicels liform, 18–40 mm long, 0.3–0.4 mm diam., white

107
New taxa of Hoya from Borneo
Fig. 3. Hoya boycei Rodda & S.Rahayu. A. Habit. B. Flower (from above, with attened corolla
lobes). C. Flower (side view). D. Pollinarium. All from Boyce et al. 305. Drawn by X.Y. Loh.

Gard. Bull. Singapore 74(1) 2022
108
speckled pink, glabrous. Calyx c. 3 mm diam., calyx lobes deltate, c. 1.5 × 0.5 mm,
apex acuminate to rounded, glabrous, sparsely ciliate; basal colleters not visible on
the available material. Flower tropism unknown but likely positive. Corolla rotate
with reexed lobes or rotate with spreading lobes, 7–9 mm diam. when attened,
very pale pink; tube 1.5–2 mm long, outside glabrous, inside pubescent, glabrescent
towards the centre; lobes deltate, 2.5–3 × 2.5–2.8 mm, apex acute, outside glabrous,
inside pubescent. Gynostegium sessile. Corona staminal, c. 2 mm high, 3–3.5 mm
diam., brown; lobes with a distinct inner and outer process, inner process incumbent
on style head, broadly elliptic, 1–1.2 × 0.5–0.6 mm, apically acute, basally almost
rounded, outer processes spreading, bilobed, each lobe 1.5–1.7 × 0.3–0.4 mm, with
basal revolute margins. Anthers triangular, with apical round membranous appendage
c. 0.7 × 0.6 mm. Pollinia oblong, c. 350 × 400 µm, with pellucid margin, corpusculum
rhomboid, c. 150 × 100 µm, caudicles attached to the centre of the corpusculum,
c. 100 µm long. Ovary narrowly conical, 0.3–0.5 mm long, c. 0.3 mm at the base,
glabrous. Fruit and seed not observed.
Habitat and ecology. Mixed dipterocarp forest, on a gentle slope. Belait series
sandstones.
Etymology. Named after British botanist Peter Boyce, the rst and type specimen
collector and a specialist on Araceae.
Provisional IUCN conservation assessment. Data Decient (DD). Hoya boycei
is known only from the type specimen and its distribution and population size are
insufciently known. The type locality, Bukit Batu Patam, is within the Labi Forest
Reserve and very close to the border with Sarawak, Malaysia. Although Brunei still
retains suitable habitat for Hoya boycei, the neighbouring forest in Malaysia is not
protected and at risk of logging and conversion to oil palm plantation.
Notes. The available material of Hoya boycei is in bud close to anthesis, therefore the
corolla shape cannot be veried, and the size of mature owers may be slightly larger.
Based on observation of similar species, the shape of the corolla is likely rotate with
reexed lobes (as shown in Fig. 3), or less likely with spreading lobes. Hoya boycei is
one of the few species from Borneo belonging to one of the few monophyletic sections
of the genus, Hoya section Acanthostemma, characterised by revolute corolla lobes,
corona lobes with bilobed outer process apices, and pollinaria with broad, spathulate
caudicles. The small, broadly elliptic to round leaves of Hoya boycei are also found
in H. peltata (published here), which however has much smaller owers borne in
1–2-owered inorescences. The inorescences with long slender pedicels, the corolla
rotate with reexed lobes or spreading lobes, pubescent inside, and the corona with
prominent bilobed outer processes make Hoya boycei similar to H. beccarii Rodda &
Simonsson. The leaves of Hoya beccarii, however, are elliptic to narrowly lanceolate,
4–10 × 2–3.5 cm.

109
New taxa of Hoya from Borneo
3. Hoya curtisii King & Gamble subsp. collariata S.Rahayu & Rodda, subsp. nov.
Similar to Hoya curtisii King & Gamble subsp. curtisii in habit (epiphytic creeper
growing tightly attached to host tree trunk), leaf shape (round or broadly ovate), leaf
thickness (very eshy, to 3 mm thick), peduncle size (sessile, to 5 mm long), peduncle
and ower tropism (negative), corolla type (rotate with reexed lobes), and presence
of annular corona below staminal corona, but can be distinguished by the corolla
pubescence (glabrous throughout in subsp. collariata vs pubescent inside in subsp.
curtisii), and corona lobe shape (oblong, erect, laterally attened in subsp. collariata
vs sub-spherical, apically attened in subsp. curtisii). – TYPE: Indonesia, Central
Kalimantan, Barito Selatan, Telang Anrau near Sanggu, lowland, wetland and heath
forest, 27 April 2021, Rahayu 1337 (holotype BO). (Fig. 4)
Epiphytic creeper growing tightly attached to host tree trunk, with white latex in all
vegetative parts. Stem 1–2 mm diam., pubescent, internodes 0.5–4 cm long. Roots
adventitious, produced all along the stem. Leaves: petiole terete, 1–5 mm long, 0.5–1
mm diam., sparsely pubescent; lamina round or broadly ovate, stiff and very eshy,
0.7–1.5 × 0.7–1 cm, 1–3 mm thick, pale to mid green above, turning maroon or
yellowish when exposed to bright light, sometimes with small grey markings, sparsely
pubescent, pale green, sparsely pubescent to glabrescent underneath, base round,
apex acute; basal colleters not observed on the available material. Inorescence 4–7
cm diam., convex and bearing 5–20 owers; peduncle sessile, generally negatively
geotropic, forming a rachis c. 3 mm diam.; pedicels 2.8–3 cm long, c. 1 mm diam.,
pinkish white, glabrous. Calyx lobes broadly triangular, c. 1.5 × 1 mm, apex acute,
pubescent outside, glabrous inside; basal colleters one at each calyx lobe sinus,
oblong, c. 0.1 mm long. Flowers negatively geotropic. Corolla rotate with reexed
lobes, c. 1.5 cm diam. when attened; tube 1–2 mm long, white inside, pinkish outside,
glabrous; lobes oblong, c. 7 × 4 mm, white inside, pinkish outside, glabrous. Corona
annular at the base of staminal corona, c. 1 mm wide, white, glabrous. Gynostegium
sessile. Corona staminal, erect, c. 4 mm high, 4 mm diam., white; lobes oblong, erect,
c. 4 mm tall, c. 1.5 mm wide, with a distinct inner and outer process, inner process
oblong, erect with a rounded tip, outer process erect, widest at the base, progressively
narrowing and becoming laterally attened towards the apex, tip round. Anthers ovate,
with apical round membranous appendage c. 0.7 × 0.7 mm. Pollinia oblong, 900–1100
× 200–300 µm, with pellucid margin, corpusculum ovate, 150–200 × 90–120 µm,
caudicles attached to the middle part of the corpusculum, spathulate, 100–150 µm
long. Ovary broadly conical, with a narrower apical part, c. 2 mm long, c. 1 mm diam.
at base, glabrous. Fruit and seed not seen.
Distribution and ecology. This subspecies is only known from Central Kalimantan,
Buntok, where it grows in lowland heath forest. Other species of Hoya found in the
same area are Hoya mitrata Kerr., Hoya elmeri Merr. and Hoya scortechinii King &
Gamble.

Gard. Bull. Singapore 74(1) 2022
110
Fig. 4. Hoya curtisii King & Gamble subsp. collariata S.Rahayu & Rodda. A. Habit. B.
Inorescence (from above). C. Inorescence (side view). From Rahayu 1337. (Photos: Abdul
Lalang)

111
New taxa of Hoya from Borneo
Etymology. The name Hoya curtisii subsp. collariata (from the Latin collariatus =
with a collar) refers to the annular corona present at the base of the staminal corona.
Provisional IUCN conservation assessment. Data Decient (DD). Hoya curtisii subsp.
collariata is known only from the type specimen and its distribution and population
size are insufciently known. The site where it was collected is a community forest
which is facing quite intensive logging activities. At the time of collection only a small
population was observed (fewer than 50 individuals). However, exploration of suitable
habitats in neighbouring areas is needed before the conservation assessment of Hoya
curtisii subsp. collariata can be updated.
Notes. Hoya curtisii subsp. curtisii, is a widespread but rarely collected taxon from
Thailand, Peninsular Malaysia, Borneo and the Philippines. The similarities with
Hoya curtisii subsp. collariata extend to vegetative morphology, making them almost
indistinguishable when dry, but they might be separated on lamina colour when
fresh: the leaves of H. curtisii subsp. collariata are pale to mid green above, turning
maroon or yellowish when exposed to bright light above, with few grey markings,
while the leaves of H. curtisii subsp. curtisii are mid to dark green, turning maroon
when exposed to strong light, with prominent grey markings. As mentioned in the
diagnosis, the tropism of the inorescence, type of corolla (but not pubescence), and
presence of an annular corona are all shared features of the two subspecies. They may
be distinguished in corolla pubescence and corona lobe shape. The erect, laterally
attened corona lobes of Hoya curtisii subsp. collariata bear strong similarities to
those of three other species with negatively geotropic inorescences: Hoya greenii
Kloppenb. from the Philippines, Hoya mitrata, a widespread Sundaland species, and
H. oreostemma Schltr. from New Guinea. They have upright, laterally attened corona
lobes but they are climbers with long internodes (generally > 5 cm long), linear-
lanceolate to elliptic, coriaceous leaves > 5 cm long and are therefore easily separated
from Hoya curtisii subsp. collariata which is a shorter creeper with short internodes
(0.5–4 cm long) and small laminas (0.7–1.5 × 0.7–1 cm).
4. Hoya dulitensis Rodda & S.Rahayu, sp. nov.
Similar to Hoya cumingiana Decne in the non-twining habit with owering stems
with tightly placed leaves, relatively short peduncles (0.5–1.5 cm in H. dulitensis, to 3
cm in H. cumingiana) and owers with reexed corolla lobes. It can be distinguished
by the leaf shape, base and thickness (narrowly elliptic, base attenuate to acute,
very thick and coriaceous in H. dulitensis vs broadly elliptic to round, base round to
cordate, stify chartaceous in H. cumingiana), and ower colour (dark crimson in
H. dulitensis vs greenish white to yellow corolla, yellow to purplish brown corona
in H. cumingiana). – TYPE: Malaysia, Sarawak, Mount Dulit ridge, c. 1230 m, 9
September 1932, anonymous native collector in Richards 1637 (holotype K; isotype
OXF n.v.). (Fig. 5)

Gard. Bull. Singapore 74(1) 2022
112
Fig. 5. Hoya dulitensis Rodda & S.Rahayu. A. Habit. B. Flower (top view). C. Flower (side
view). D. Pollinarium. All from Richards 1637. Drawn by X.Y. Loh.

113
New taxa of Hoya from Borneo
Climber, epiphytic, latex white. Stems of two types: climbing, with adventitious roots,
internodes 5–12 cm long, sterile; non climbing, without adventitious roots, internodes
1.5–3 cm long, fertile, both types cylindrical, stout to slender, 1.5–5 mm diam., pubescent
turning glabrescent when mature. Leaves: petiole terete, channelled above, 4–8 mm
long, 1.2–2 mm diam., pubescent; lamina elliptic, very thick and coriaceous, 2.5–6.5 ×
1–2.5 cm, dark green, glabrous above, very sparsely pubescent below, margin recurved,
sparsely ciliate, apex rounded, base attenuate to acute; venation pinnate, secondary
veins indistinct; basal colleters not observed on the available material. Inorescence
one per node, pseudo-umbelliform, convex, consisting of 9–20 owers; peduncle
extra-axillary, tropism unknown, likely positive, terete, 0.5–1.5 cm long, 1–1.5 mm
diam., older peduncles forming a rachis from previous owerings, pubescent; pedicels
liform, 23–28 mm long, c. 0.5 mm diam., glabrous. Calyx 2.5–3 mm diam., calyx
lobes oblong, 0.8–1.1 × 0.4–0.5 mm, apex rounded or acute, dull red, outside sparsely
pubescent, inside glabrous, ciliate; basal colleters one at each calyx lobe sinus, ovate,
c. 0.1 mm long. Corolla rotate with reexed lobes, 6–8 mm diam. when attened, dark
crimson; tube 1–1.2 mm long, outside glabrous, inside densely pubescent, glabrescent
towards the centre; lobes ovate, 3–4 × 1.8–2.2 mm, apex acute, outside glabrous, inside
densely pubescent. Gynostegium sub-sessile. Corona staminal, conical, 1.8–2 mm
high, 3.5–4 mm diam., cream coloured; lobes broadly elliptic, 2–2.2 × 0.9–1.1 mm,
with a distinct inner and outer process, inner process acuminate, erect, c. 0.5 mm long,
outer processes spreading, apex rounded, with basal revolute margins. Anthers ovate,
with apical round membranous appendage c. 0.7 × 1 mm. Pollinia oblong, 400–450
× 200–250 µm, with pellucid margin, corpusculum rhomboid, 180–200 × 130–150
µm, caudicles attached to the middle part of the corpusculum, clavate, c. 100 µm long.
Ovary conical, 1–1.2 mm long, glabrous. Fruit and seed not observed.
Distribution. The species is only known from two collections on Mount Dulit in
Sarawak.
Habitat and ecology. Montane mossy forest at c. 1230 m.
Etymology. The epithet ‘dulitensis’ refers to the type locality, Mount Dulit.
Provisional IUCN conservation assessment. Data Decient as it is known only from
two collections on Mt Dulit, both from 1932. More exploration of suitable habitats in
neighbouring areas is needed to update the conservation assessment of Hoya dulitensis.
Additional specimen examined. MALAYSIA: Sarawak: Mount Dulit ridge, 6 Sep 1932, Synge
420 (K, OXF n.v.).
Notes. A species characterised by a non-twining habit, producing fertile stems with
tightly placed leaves, making it similar to Hoya cumingiana. The type specimen only
has non-climbing stems, whereas Synge 420 has some stems with more widely placed

Gard. Bull. Singapore 74(1) 2022
114
leaves and producing adventitious roots all along.
Hoya dulitensis can be separated from H. cumingiana even when sterile because
they have different leaves (elliptic, base attenuate to acute, very thick and coriaceous in
H. dulitensis vs broadly elliptic to round, base round to cordate, stify chartaceous in
H. cumingiana) and they occur in different habitats (montane forest for H. dulitensis,
while H. cumingiana is from lowland forests). Both species occur in Borneo, but Hoya
cumingiana is also found in Java and the Philippines.
5. Hoya kaikoeana S.Rahayu & Rodda, sp. nov.
Similar to Hoya scortechinii King & Gamble in lamina shape (lanceolate) and
corolla lobes (reexed), but can be distinguished on ower orientation (negatively
geotropic in H. kaikoeana vs ageotropic in H. scortechinii) and size of pollinium and
corpusculum (pollinium 900–1000 × 300–400 µm, corpusculum 250–300 × c. 200 µm
in H. kaikoeana vs pollinium 530–560 × 190–210 µm, corpusculum 370–400 × 160–
180 µm in H. scortechinii). – TYPE: Indonesia, West Kalimantan, Sanggau, lowland,
heath forest, 23 May 2021, Rahayu 1341 (holotype BO). (Fig. 6)
Climber, epiphytic, with white latex. Stems slender, c. 1 mm diam., glabrous,
internodes 8–10(–21) cm. Roots adventitious, occurring all along the stem. Leaves:
petiole terete, c. 1 cm long, c. 2 mm diam., glabrous; lamina lanceolate, succulent,
(9–)15–20 × 1.5–2.5 cm, mid green to maroon or yellowish when exposed to bright
light, immature leaves maroon-red, glabrous, base round, apex acute or acuminate;
basal colleters present, c. 1.5 mm long. Inorescence one per node, convex, consisting
of 6–10 owers; peduncle negatively geotropic or laterally held, 2–10 cm long, 1.2–
1.6 mm diam., glabrous; pedicels c. 20 × 1 mm, pubescent. Calyx lobes deltate, c. 2
× 1 mm, apex acute, pubescent outside, glabrous inside; basal colleters one at each
calyx lobe sinus, oblong, c. 0.1 mm long. Flowers negatively geotropic. Corolla
reexed, c. 1.5 cm diam. when attened; tube 2–3 mm long, inside pale yellow to red,
basally pubescent, outside very pale yellow to pale pink, glabrous; lobes triangular, c.
7 × 4 mm, apex acute, yellow with orange tips, yellowish red or red, margin slightly
recurved. Gynostegium sessile. Corona staminal, 3.5–4 mm high, c. 5 mm diam.,
white with pink centre or fully red; lobes erect, broadly ovate, 3.5–4 × 2.5–3 mm,
inner process a c. 0.5 mm long acute upcurved tip, outer process apex rounded, with
basal revolute margins. Anthers ovate, with apical round membranous appendage c.
0.7 × 0.7 mm. Pollinia oblong, 900–1000 × 300–400 µm with a thin pellucid margin,
corpusculum dark brown, guitar-shaped, 250–300 × c. 200 µm, caudicles attached
to the base of the corpusculum, c. 100 µm long. Ovary oblongoid, c. 1.5 × 0.5 mm,
glabrous. Fruit and seed not observed.
Distribution and ecology. This species is only known from the lowland, heath forest
habitat in Sanggau and Kapuas Hulu, West Kalimantan. It was found growing near
specimens of Hoya scortechinii and Hoya corneri Rodda & S.Rahayu.

115
New taxa of Hoya from Borneo
Fig. 6. Hoya kaikoeana S.Rahayu & Rodda. A. Leaves (left one immature). B. Inorescence
(side view). C. Inorescence (from above). All from Rahayu 1341. (Photos: Ewaldus Heny)
Etymology. The species is named after Kaiko, daughter of Ewaldus, a plant collector
from Sanggau, West Kalimantan who found this new species at about the same time as
his wife was giving birth.
Provisional IUCN conservation assessment. Data Decient as it is known only
from two localities in Central Kalimantan and its distribution and population size
are insufciently known. The habitat at the type locality is mixed use forest which
is disturbed by logging and land conversion to oil palm plantation. The population
at the type locality appears to be very small. The population at Kapuas Hulu (West
Kalimantan), the district next to Sanggau, is larger but not well documented and not
yet vouchered. More exploration of suitable habitats in neighbouring areas is required
before updating the conservation assessment for Hoya kaikoeana.

Gard. Bull. Singapore 74(1) 2022
116
Notes. The owers of Hoya kaikoeana are negatively geotropic, making it easily
distinguishable from the otherwise morphologically similar H. scortechinii, which has
ageotropic inorescences. In Borneo, the only other species with negatively geotropic
owers are Hoya curtisii, H. elliptica Hook.f., H. fauziana Rodda et al. subsp. angulata
Rodda et al., and H. mitrata. Hoya kaikoeana can be separated from H. curtisii, H.
elliptica and H. mitrata even when sterile on leaf morphology (H. mitrata leaves form
cabbage-shaped megadomatia, the leaves of H. curtisii subsp. collariata and H. curtisii
subsp. curtisii are round or broadly ovate and < 2 cm long, H. elliptica has elliptic
leaves to 6 cm long, while H. kaikoeana leaves are lanceolate (9–)15–20 cm long).
Hoya fauziana subsp. angulata also has lanceolate leaves just like H. kaikoeana, but
the two species can be separated on the orientation of the corolla lobes which are
reexed in H. kaikoeana and inrolled (making the corolla look turban shaped) in H.
fauziana subsp. angulata. Further, Hoya kaikoeana is found in lowland heath forests
while H. fauziana subsp. angulata is from hill forests.
6. Hoya kapuasensis S.Rahayu & Rodda, sp. nov.
Similar to Hoya jiewhoeana Rodda et al. in indumentum (stem, petiole and lamina
are pubescent), leaf texture (coriaceous) and shape (elliptic to oblong), inorescence
shape (convex to spherical in H. kapuasensis) and corolla pubescence (tube inside
very nely pubescent, more densely towards the centre, outside sparsely pubescent),
and ovary indumentum (pubescent), but can be distinguished in inorescence size
(3–6 cm diam. in H. kapuasensis vs 8–10 cm in Hoya jiewhoeana), pedicel length
(1.5–2.2 cm in H. kapuasensis vs 2.5–3 cm in Hoya jiewhoeana), and corona lobe
outer apex (held almost horizontal (slightly raised) in H. kapuasensis vs erect in Hoya
jiewhoeana). – TYPE: Indonesia, West Kalimantan, Kapus Hulu, lowland, 25 January
2021, Rahayu 1333 (holotype BO). (Fig. 7)
Climber, epiphytic, latex white. Stems cylindrical, up to 5 mm diam., pubescent
turning glabrescent when old, internodes 5–10(–20) cm long. Roots adventitious,
rarely produced along the stem. Leaves: petiole terete, 1.5–2.5 cm long, c. 3 mm
diam., pubescent; lamina elliptic to oblong, 5–14 × 3.5–6.5 cm, coriaceous, apex
apiculate, base rounded or acute, upper surface rough, pubescent to glabrescent, lower
surface densely velvety short pubescent; venation pinnate, secondary veins 3–5 each
side; basal colleters present, c. 1 mm diam. Inorescence convex to spherical, 3–6 cm
diam., consisting of up to 12 owers (to 25 in cultivation); peduncle extra-axillary, one
per node, ageotropic, terete, 4–5 cm long, c. 3 mm diam., pubescent, older peduncles
forming a rachis from previous owerings; pedicels 1.5–2.2 cm long, c. 1 mm diam.,
cream or pinkish white, pubescent. Calyx lobes triangular, 1.8–2.2 × 1–1.2 mm,
cream or pinkish white, apex acute or round, glabrous adaxially, pubescent abaxially;
colleters one at each calyx lobe sinus, ovoid. Flowers ageotropic. Corolla rotate,
1.8–2 cm diam. when attened; tube 6–7 mm long, inside glossy white-cream, very
nely pubescent, more densely towards the centre, outside white to pale pink at the
base, shiny and sparsely pubescent; lobes elliptic-ovate, 7–8 × 5–6 mm, inside white-

117
New taxa of Hoya from Borneo
Fig. 7. Hoya kapuasensis S.Rahayu & Rodda. A. Inorescence, front view. B. Inorescence,
from below. C. Flower (front view). D. Flower (side view). E. Pedicel, calyx and ovary. F.
Pollinarium. All from Rodda MR 2188. (Photos: A–E, S. Somadee; F, M. Rodda)

Gard. Bull. Singapore 74(1) 2022
118
cream, very nely pubescent to glabrescent, outside white to very pale pink, shiny and
sparsely pubescent, margin recurved, apex acuminate. Gynostegium sessile. Corona
staminal, c. 3 mm high, 6.5–8 mm diam., cream; lobes ovate, 2.5–3 mm × c. 1.5 mm,
inner process oblong, shortly upcurved reaching just below the style head, apex acute,
pinkish, outer process slightly raised, apex rounded. Anthers ovate, with apical round
membranous appendage c. 1 × 1.2 mm. Pollinia oblong, 680–720 × 250–300 µm, with
round apex and base and evident pellucid margin, corpusculum oblongoid, 400–500 ×
280–320 µm, caudicles triangular, attached to the mid-section of the corpusculum, c.
120 µm long. Ovary conical, 1.5–2 mm long, c. 0.7 mm wide at the base, pubescent,
apex round. Fruit and seed not observed.
Distribution and ecology. This species is only known from Kapuas Hulu in West
Kalimantan, Indonesia, where it was found in lowland heath forest. Other species of
Hoya growing nearby are Hoya mitrata, Hoya elmeri and Hoya scortechinii.
Etymology. The name Hoya kapuasensis refers to the type locality in Kapuas Hulu,
West Kalimantan, Indonesia.
Provisional IUCN conservation assessment. Data Decient. Only known from Kapuas
Hulu, West Kalimantan where it occurs in a local community forest which is now
also a protected forest. The population size was estimated at the time of collection as
fewer than 50 individuals. It was also found in nearby logged forests. However, more
exploration of suitable habitats in neighbouring areas is needed before updating the
conservation assessment for Hoya kapuasensis.
Additional specimen examined. Cultivated in Thailand, Nov 2021, Somadee in Rodda MR 2188
(SING).
Notes. Hoya kapuasensis is most similar to H. jiewhoeana but when sterile it is also
similar to H. hamiltoniorum A.Lamb et al. as the three species have pubescent stems
and leaves and coriaceous, elliptic (to oblong) leaves. Hoya kapuasensis can be
distinguished from H. hamiltoniorum in corolla morphology (rotate in H. kapuasensis
vs salverform in H. hamiltoniorum). Hoya hamiltoniorum and H. jiewhoeana occur
in montane forests and are so far only known from Sabah, Malaysia, while Hoya
kapuasensis occurs in lowland forests and is so far only known from West Kalimantan.
7. Hoya kerangasensis Rodda & S.Rahayu, sp. nov.
Similar to Hoya fauziana Rodda et al. in leaf shape (lanceolate), ower colour (pink
or dull violet) but can be distinguished in lamina indumentum (sparsely pubescent
in H. kerangasensis vs glabrous in H. fauziana), secondary venation (secondary
veins prominent and arching in H. kerangasensis vs hardly visible, not arching in
H. fauziana) and corolla lobe shape (reexed in H. kerangasensis vs revolute in H.
fauziana). – TYPE: Brunei, Temburong, Bukit Retak, montane forest, 3 May 1992,
Ibrahim Abdullah s.n. (holotype BRUN [sheet no. B005727]). (Fig. 8)

119
New taxa of Hoya from Borneo
Fig. 8. Hoya kerangasensis Rodda & S.Rahayu. A. Habit. B. Flower (top view). C. Flower
(side view). D. Pollinarium. All from Ibrahim Abdullah s.n. Drawn by X.Y. Loh.

Gard. Bull. Singapore 74(1) 2022
120
Climber, likely epiphytic, latex white. Stems cylindrical, 1.5–2.5 mm diam., sparsely
pubescent, internodes 15–20 cm long. Roots adventitious, sparsely produced along the
stems. Leaves: petiole channelled above, 5–7 mm long, 1.5–2 mm diam., pubescent;
lamina lanceolate, very coriaceous when dry, likely succulent when fresh, 4–11 ×
0.8–3 cm, sparsely pubescent above, turning glabrescent, sparsely pubescent along
midvein only below, apex acuminate, base cuneate; venation pinnate, secondary veins
prominent and arching; basal colleters 3–4, ovoid, 0.2–0.3 mm long. Inorescence
one per node, pseudo-umbelliform, consisting of 8–20 owers; peduncle extra-
axillary, terete, tropism unknown, likely positive, 3.5–5 cm long, 1–1.5 mm diam.,
pubescent; pedicels liform, 25–30 mm long, c. 0.3 mm diam., glabrous. Calyx c. 4
mm diam., calyx lobes deltate, 1–1.3 × 0.6–0.8 mm, apex rounded to acute, outside
sparsely pubescent, inside glabrous; basal colleters one at each calyx lobe sinus, ovate,
c. 0.2 mm long. Flowers tropism unknown, likely positive, pink or dull violet. Corolla
rotate with reexed lobes, 12–14 mm diam. when attened; tube c. 1.2 mm long,
outside glabrous, inside sparsely pubescent, glabrescent towards the centre; lobes
elliptic, 5–5.5 × 3–3.5 mm, apex obtuse to acute, outside glabrous, inside sparsely
pubescent. Gynostegium shortly stalked, stalk cylindrical, c. 0.5 × 1.5 mm, glabrous.
Corona staminal, concave, 2.5–3 mm high, 4–4.5 mm diam.; lobes broadly ovate,
2.5–2.7 mm tall × 1.2–1.4 mm diam., inner process deltate, erect, c. 7 mm tall, c. 0.5
mm wide, outer processes raised, forming an acute angle with the inner process, apex
rounded, with basal revolute margins. Anthers ovate, with apical round membranous
appendage c. 1 × 1.2 mm. Pollinia oblong, 600–650 × 250–270 µm, with pellucid
margin, corpusculum rhomboid, c. 150 × 200 µm, caudicles attached to the lower half
of the corpusculum, perpendicular to corpusculum, c. 150 µm long. Ovary conical, c.
1.5 mm long, glabrous. Fruit and seed not observed.
Distribution. This species is known from the type locality in Brunei, Temburong,
Bukit Retak, from one collection from Sarawak (de Vogel 980186) and one from
Sabah (Huisman et al. 8578).
Habitat. From information on the specimens of Huisman et al. 8578 the species is
found in low open kerangas forest 5–10 m tall with much undergrowth of orchids and
herbs. The habitat is indicated as montane forest on the type specimen.
Etymology. The epithet ‘kerangasensis’ refers to the habitat where the species was
collected. Originally from the Iban language and meaning ‘land which cannot grow
rice’, kerangas refers to heath forests.
Provisional IUCN conservation assessment. Endangered (EN B1ab(iii), B2ab(iii)).
The estimated EOO is 1800 km2, the AOO is 12 km2. The species occurs in a protected
habitat in Brunei but the two other localities in Sabah and Sarawak do not lie within
protected areas and are at risk of habitat destruction.

121
New taxa of Hoya from Borneo
Additional specimens examined. MALAYSIA: Sarawak: Kelabit Highlands, 1998, De Vogel
et al. 980186 (L, spirit). Sabah: along trail Long Pa Sia to Long Samado, c. 2 km from the
Sarawak border, 4°20′N 115°41′E, 1400 m, 25 Oct 1986, Huisman et al. 8578 (L [L0275605]).
Notes. Few species have so far been found in montane heath forest in Borneo. The only
one similar to Hoya kerangasensis is H. fauziana. The two species can be separated
on secondary venation (secondary veins prominent and arching in H. kerangasensis
vs hardly visible, not arching in H. fauziana) and corolla lobe shape (reexed in H.
kerangasensis vs revolute in H. fauziana).
8. Hoya peltata Rodda & S.Rahayu, sp. nov.
Similar to Hoya minutiora Rodda & Simonsson in leaf shape (broadly elliptic
to round), peduncle length (3.5–10 cm) and ower size c. 6 mm diam., but can be
distinguished in leaf base (peltate, rounded in H. peltata vs not peltate, rounded-
truncate (subcordate) in H. minutiora), number of owers per inorescence (1–2
in H. peltata vs 5–10 in H. minutiora) and corolla diameter (spread out) (c. 6 mm
in H. peltata vs c. 4 mm in H. minutiora). – TYPE: Indonesia, North Kalimantan,
Nunukan, N of Tarakan, low elevation, November 1953, Meijer 1973 (holotype K;
isotype BO n.v.). (Fig. 9)
Climber, epiphytic, latex colour unknown. Stems cylindrical, non-twining, 0.8–1
mm diam., sparsely pubescent to glabrescent, internodes (0.5–)1–2 cm long. Roots
adventitious, occurring just below the nodes. Leaves: petiole terete, 1–1.5 mm long,
c. 0.5 mm diam., sparsely pubescent to glabrescent; lamina broadly elliptic to round,
peltate, thinly coriaceous when dry, likely very eshy when fresh, 5–10 × 5–10 mm,
very smooth and sparsely pubescent to glabrescent above, sparsely pubescent and
papillose below, margin sparsely ciliate, apex acute to obtuse, often mucronulate,
base rounded; venation pinnate, secondary veins inconspicuous; basal colleters not
observed on the available material. Inorescence one per node, consisting of 1–2
owers; peduncle extra-axillary, tropism unknown but likely positive, terete, 7–10 cm
long, 0.7–1 mm diam., sparsely pubescent to glabrescent, older peduncles forming a
rachis from previous owerings; pedicels liform, 1–3 mm long, 0.2–0.3 mm diam.,
glabrous. Calyx not present on available material. Flower red-brown-violet, tropism
unknown but likely positive. Corolla rotate with slightly reexed spreading lobes,
c. 6 mm diam. when attened; tube c. 1.5 mm long, basally forming an irregularly
lobed raised annulus surrounding the stalk, outside glabrous, inside pubescent; lobes
narrowly triangular, 2.5–2.7 × 1.4–1.6 mm, apex rounded, outside glabrous, inside
pubescent. Gynostegium stalked, stalk cylindrical, 0.7–0.9 × 0.5–0.7 mm, sparsely
pubescent. Corona staminal, c. 1.5 mm high, 2.8–3 mm diam.; lobes broadly oblong,
1.8–2 × c. 0.5 mm, with a distinct inner and outer process, inner process acuminate,
erect, outer processes downcurved, long-acuminate, acute. Anthers ovate, with apical
round membranous appendage c. 0.5 × 0.6 mm. Pollinia oblong, c. 220 × 120 µm,
with pellucid margin, corpusculum rhomboid, c. 150 × 80 µm, caudicles attached

Gard. Bull. Singapore 74(1) 2022
122
Fig. 9. Hoya peltata Rodda & S.Rahayu. A. Habit. B. Flower (side view). C. Pollinarium. All
from Meijer 1973. Drawn by X.Y. Loh.

123
New taxa of Hoya from Borneo
towards the middle of the corpusculum, spathulate, c. 50 µm long. Ovary not present
on available material. Fruit and seed not observed.
Distribution and ecology. Only found in Indonesia, North Kalimantan, Nunukan, north
of Tarakan, in dipterocarp forest at low elevation.
Etymology. The epithet ‘peltata’ refers to the leaves which have a characteristic peltate
base.
Provisional IUCN conservation assessment. Data Decient as it is known only from
the type specimen collected 48 years ago and its distribution and population size are
not known.
Notes. A most unusual species with small broadly elliptic to round leaves growing
tightly adpressed to the host tree bark but producing very long peduncles 7–10 cm
bearing very small owers (c. 6 mm diam. when attened). It is somewhat similar
to Hoya minutiora but readily separated based on leaf base, corolla diameter and
number of owers in each inorescence. Other species with very small owers also
occurring in Borneo are Hoya ignorata T.B.Tran et al. and H. corymbosa Rodda &
Simonsson (< 2.8 mm diam. and < 7 mm diam. when attened, respectively), but
both species are small epiphytic shrubs and their owers are borne in multi-owered
inorescences with peduncles < 2 cm long, and are therefore easily separated from H.
peltata. The leaves of Hoya peltata are similar to those of H. boycei in shape (broadly
elliptic to round) and size (8–17 mm long), but can be distinguished on the lamina base
shape and pubescence (base rounded to attenuate, pubescent and papillose in H. boycei
vs base rounded, peltate, smooth and sparsely pubescent to glabrescent in H. peltata),
inorescence and ower size (inorescence many-owered on pedicels 18–40 mm
long, corolla 7–9 mm diam. in H. boycei vs 1–2 owered on pedicels 1–3 mm long,
corolla c. 6 mm diam. in H. peltata).
9. Hoya polypus S.Rahayu & Rodda, sp. nov.
Similar to Hoya ruthiae Rodda as both have clear latex, lanceolate corolla lobes and
a corpusculum that is about as large as the pollinium. They can be distinguished on
corolla shape (with an urceolate tube in H. polypus vs with almost completely free
lobes in H. ruthiae), and corona size and shape (conical, 4–5 mm high, c. 5 mm diam.
in H. polypus vs almost at topped (except apex of inner processes), 2.5–3 mm high,
6–7 mm diam. in H. ruthiae). – TYPE: Indonesia, East Kalimantan, Berau, lowland
heath forest, 5 November 2021, Rahayu 1355 (holotype BO). (Fig. 10)
Climber, epiphytic, with clear latex. Stem slender, 1–2 mm diam., glabrous, internodes
5–15 cm long. Roots adventitious, produced sporadically along the stems. Leaves:
petiole terete, 5–15 mm long, 2–4.5 mm diam., sometimes thicker than stems; lamina
elliptic, eshy, 7–10 × 3.5–4 cm, apex acute-acuminate, base round to acute, pale to mid

Gard. Bull. Singapore 74(1) 2022
124
Fig. 10. Hoya polypus S.Rahayu & Rodda. Inorescence. From a cultivated plant. (Photo:
Umar)
green above, venation pinnate; basal colleters ovoid, 0.5–1 mm long. Inorescences
globular, 3–4 cm diam., 4–10-owered; peduncles extra-axillary, terete, positively
geotropic, 5–10 cm long, 1–1.5 mm diam., glabrous; pedicels c. 1.5 cm long, c. 1.5
mm diam., white, glabrous. Calyx lobes triangular, white-pink, c. 2 mm long, c. 1 mm
wide, apex acute, glabrous; colleter 1 in each calyx lobe sinus, oblong, c. 0.15 mm
long. Flowers ageotropic. Corolla salverform, c. 2 cm diam.; tube urceolate, 5.5–7 ×
5.5–7 mm, cream-white; lobes narrowly lanceolate, laterally recurved, 9–11 × 3–4.5
mm, apex acuminate. Gynostegium stalked, stalk conical, c. 1 × 2 mm, glabrous.
Corona staminal, conical, 4–5 mm high, c. 5 mm diam., white; lobes with a distinct
outer and inner process, outer process elliptic in outline, 3–4 × c. 1.5 mm, concave
and spoon-like, inner process erect, with a middle acute ridge pointing outwards and
an acuminate apex pointing inwards. Anthers ovate, with apical round membranous
appendage 1–1.5 × 0.9–1.2 mm. Pollinia oblong, with evident pellucid margin, c. 500
× 150–180 µm, corpusculum 500–600 × 200–250 µm, caudicles attached at the base
of the corpusculum, oblongoid. Ovary linear, c. 3 mm long, each carpel c. 1 mm wide
at the base, glabrous. Fruits and seeds not observed.

125
New taxa of Hoya from Borneo
Distribution and ecology. This species is known from lowland mixed dipterocarp
forest in East Kalimantan.
Etymology. The name Hoya polypus refers to the ower shape, with an urceolate
corolla and long, narrow corolla lobes reminiscent of the shape of a polyp.
Provisional IUCN conservation assessment. Critically Endangered (CR D). The
habitat at the type locality is mixed dipterocarp forest which was being logged at
the time of collection. The population size at the time of collection was fewer than
40 individuals. Ex-situ conservation is particularly needed for this species, as well
as more exploration of suitable habitats in neighbouring areas to better assess the
distribution area and update the conservation assessment.
Notes. Hoya polypus belongs to the H. uncinata Teijsm. & Binn. complex which is
dened as including species with clear latex, a deeply lobed rotate corolla and the
corpusculum of the pollinarium almost as large as the pollinium itself (Rodda & Rahayu,
2018). The complex includes Hoya uncinata from Java and Sumatra, H. ruthiae from
Borneo (Sabah), and H. corneri from Thailand, Peninsular Malaysia and Borneo (West
Kalimantan). All species in the complex known so far have narrow corolla lobes and
a very short tube (< 3 mm long). Hoya polypus instead has an urceolate tube that
makes it easy to distinguish from all other species in the complex. The corona instead
is most similar to that of Hoya corneri (lobes ovate, laterally compressed and slightly
convex above) and the corolla lobes are most similar to those of H. ruthiae which are
lanceolate.
10. Hoya sangguensis S.Rahayu & Rodda, sp. nov.
Similar to Hoya caudata Hook.f. in leaf shape, texture and margin (lamina (ovate to)
elliptic, (6–)9–20 × 4–7 cm, weakly succulent, apex acuminate (to cuspidate), base
rounded or cordate, margin entire or undulate), and corolla and corona colour (corolla
white or slightly yellowish (to pale pink in H. caudata), corona red, sometimes red
and white in H. caudata). They can be distinguished in corolla shape (shallowly
campanulate in H. sangguensis with tube about as long as lobes vs rotate in H. caudata
with tube shorter than lobes), staminal corona lobes shape (inner process upcurved,
acute tip, outer process upcurved, rounded in H. sangguensis vs inner process raised,
acuminate, outer process spreading, rounded in H. caudata) and anther appendages
(ovate, just covering style head in H. sangguensis vs linear and extending well beyond
style head in H. caudata). – TYPE: Indonesia, Central Kalimantan, Buntok, Sanggu,
lowland, wetland and heath forest, 27 October 2021, Rahayu 1353 (holotype BO).
(Fig. 11)

Gard. Bull. Singapore 74(1) 2022
126
Fig. 11. Hoya sangguensis S.Rahayu & Rodda. A–C. Inorescence (from below). D.
Inorescence (from underneath). A, B from Rahayu 1353; C, D from a cultivated plant in
Kedah, Peninsular Malaysia. (Photos: A, B, Abdul Lalang; C, D, Faizal Azmi)

127
New taxa of Hoya from Borneo
Climber, epiphytic, with white or yellow latex. Stems slender, cylindrical, up to 3
mm diam., sparsely pubescent when young, turning glabrescent with rough grey
surface when mature, internodes (4–)7–10(–20) cm long. Roots adventitious
produced all along the stem. Leaves: petiole terete, 1–1.5 cm long, 2–2.5 mm diam.,
green or purplish, glabrous; lamina elliptic, 9–20 × 4–7 cm, weakly succulent, apex
acuminate, base round or cordate, margin entire or very slightly undulate, darker in
colour compared to the rest of lamina, mid green turning red when exposed to the
bright light, glabrous; venation pinnate, secondary veins 3–5 each side; basal colleters
present, ovoid, c. 0.6 mm long. Inorescence at, consisting of (2–)4–10 owers;
peduncle extra-axillary, positively geotropic, terete, slender, 3–10 cm long, 1–1.5 mm
diam., green to brown, pubescent, older peduncles forming a rachis from previous
owerings; pedicels 2–2.5 cm long, 1–1.5 mm diam., white or very pale pink-green,
sparsely pubescent. Calyx lobes triangular, 4–5 × 2.5–3 mm, apex acute or narrowly
rounded, sparsely pubescent outside, glabrous inside; basal colleters one at each calyx
lobe sinus, narrowly triangular, c. 0.6 mm long. Flowers positively geotropic. Corolla
rotate, ranging from shallowly campanulate (young owers) to slightly reexed with
slightly curly lobes (older owers), (2.5–)3–3.5 cm diam. when attened (> 1.8 cm
diam. when dry); tube 5–7 mm long, white to cream, glabrous to sparsely pubescent;
lobes elliptic-ovate, 7–10 × 7–10 mm, margin at to recurved, apex acuminate and
revolute at the tip, inner surface white or slightly yellowish at the base, covered
with dense white hairs, hairs longer at margin, to 3.5 mm long, outer surface cream,
glabrous. Gynostegium subsessile. Corona staminal, 3–3.5 mm high, 5–8 mm diam.,
red; lobes ovoid, c. 3.5 × 2 mm, slightly translucent or glassy, inner process upcurved,
apex acute, outer process upcurved, held at an angle of 30° to 80°, apex rounded, with
basal revolute margins. Style head convex, white, covered by the anther appendages.
Anthers triangular, membranous, appendage c. 2.8 × 1.5 mm. Pollinia subrectangular,
c. 500 × 400 µm, with truncate apex and rounded base, pellucid margin not observed,
corpusculum ellipsoid, c. 700 × 500 mm, caudicles short, attached towards the middle
of the corpusculum. Ovary oblongoid, c. 1 mm long, c. 0.5 mm wide at the base. Fruit
and seed not observed.
Distribution and ecology. This species is only known from lowland heath forest in
Central Kalimantan (Buntok). It was found in an area where Hoya mitrata, H. elmeri
and H. scortechinii have also been sighted. A living collection, shown in Fig. 11C,
D (not yet vouchered) was made in Peninsular Malaysia, Kedah (Faizal Azmi, pers.
comm.).
Etymology. The name Hoya sangguensis refers to the type locality in Sanggu, Buntok,
Central Kalimantan.

Gard. Bull. Singapore 74(1) 2022
128
Provisional IUCN conservation assessment. Data Decient as it is known only from
two localities more than 1500 km apart. In Sanggu the species was found in a local
community forest. Plants were also spotted but not collected in nearby Ekeng and
Majundre, all areas affected by logging. More exploration of suitable habitats in
neighbouring areas and in Kedah, Malaysia is needed to better assess the distribution
area and update the conservation assessment.
Notes. Hoya sangguensis belongs the H. caudata complex, also known as Hoya sect.
Peltostemma Schlechter (Schlechter, 1916). The species in this group are generally
characterised by upright corona lobes (forming a conical corona), long (often linear)
anther appendages extending well above the style-head, and pollinaria with well-
developed caudicle wings (Kidyoo, 2016; Rahayu & Rodda, 2021). Hoya sangguensis
is however not a typical member of this group as it does not have particularly elongated
anther appendages and the corona is not conical. The leaves are, however, almost
indistinguishable from those of Hoya caudata, with a slightly undulate margin and
covered in grey specks on the upper surface, the inorescence is at, like in all other
members of Hoya sect. Plocostemma, the corolla is white with long hairs around the
margins like in H. caudata and H. agellata Kerr, and the corona is red, a colour
predominant in the other species of Hoya sect. Plocostemma.
The corona lobes of Hoya sangguensis are quite variable in shape and angle,
with the outer processes held at 30° (in the Kedah collection) to 80° angle (in the
Kalimantan collection). This is likely just part of the species variation, as observed
in other species such as Hoya krusenstierniana Simonsson & Rodda in New Guinea.
Hoya sangguensis can be separated from all other members of Hoya sect.
Peltostemma because it lacks the elongated anther appendages and because its owers
are particularly large (> 2.5 cm diam.), whereas the second largest-owered species,
H. caudata, has owers not more than 1.5 cm diam. when attened.
ACKNOWLEDGEMENTS. This study is part of an on-going research project on the systematics
of Hoya. Michele Rodda received support from the National Parks Board (Singapore), which
sponsored numerous herbarium trips to Asian and European herbaria. We would like to
thank the curators of the herbaria mentioned in the text for allowing access, loans, and/or for
providing high quality images of herbarium specimens. We thank Anas Saruli for providing the
sample and type specimen of Hoya kapuasensis; Umar (Berau, East Kalimantan) for providing
the sample and type specimen of Hoya polypus; Apri Susanto and Abdul Lalang (Sanggu)
for providing the sample and type specimen of Hoya sangguensis; Umpu Kakah Maayan and
Abdul Lalang for providing the sample and type specimen of Hoya curtisii subsp. collariata;
Ewaldus Heny (Sanggau) for the sample and type specimen of Hoya kaikoeana; Muhammad
Arifn A. Kalat, Surisa Somadee and Faizal Azmi for providing images of Hoya arifnii, H.
kapuasensis and H. sangguensis respectively.

129
New taxa of Hoya from Borneo
References
Cámara-Leret, R., Frodin, D.G., Adema, F., Anderson, C., Appelhans, M.S., Argent, G.,
Guerrero, S.A., Ashton, P., Baker, W., Barfod, A.S. et al. (2020). New Guinea has the
world’s richest island ora. Nature 584: 579–583.
Endress, M.E., Meve, U., Middleton, D.J. & Liede-Schumann, S. (2019 [‘2018’]). Apocynaceae.
In: Kadereit, J.W. & Bittrich, V. (ed.) The Families and Genera of Vascular Plants 15.
Flowering Plants. Eudicots. Apiales, Gentianales (except Rubiaceae), pp. 207–411.
Switzerland: Springer Nature.
IUCN Standards and Petitions Committee (2022). Guidelines for Using the IUCN Red List
Categories and Criteria. Version 15. Prepared by the Standards and Petitions Committee.
Available from http://iucnredlist.org/resources/redlistguidelines.
Kidyoo, M. (2016). Hoya phuwuaensis (Apocynaceae: Asclepiadoideae), a new species from
Northeastern Thailand. Phytotaxa 282(3): 218–224.
Lamb, A. & Rodda, M. (2016). A Guide to Hoyas of Borneo. Kota Kinabalu: Natural History
Publications (Borneo).
Lamb, A., Gavrus, A., Emoi, B. & Gokusing, L. (2014). The hoyas of Sabah, a commentary
with seven new species and a new subspecies. Sandakania 19: 1–89.
Miyamoto, K., Aiba, S.I., Aoyagi, R. & Nilus, R. (2021). Effects of El Niño drought on tree
mortality and growth across forest types at different elevations in Borneo. Forest Ecol.
Managem. 490: 119096.
Pelser, P.B., Barcelona, J.F. & Nickrent, D.L. (ed.) (2011 onwards). Co’s Digital Flora of the
Philippines. http://philippineplants.org. Accessed 15 Dec. 2021.
Rahayu, S. & Rodda, M. (2021). Hoya buntokensis (Apocynaceae, Asclepiadoideae), a new
myrmecophytic species and Hoya wallichii subsp. tenebrosa, a new subspecies from
Borneo (Kalimantan, Indonesia). Blumea 66: 236–241.
Rodda, M. (2015). Two new species of Hoya R.Br. (Apocynaceae, Asclepiadoideae) from
Borneo. PhytoKeys 53: 83–93.
Rodda, M. (2017). Index of names and types of Hoya (Apocynaceae: Asclepiadoideae) of
Borneo. Gard. Bull. Singapore 69: 33–65.
Rodda, M. & Rahayu, S. (2018). A revision of the Hoya uncinata complex (Apocynaceae,
Asclepiadoideae), with description of a new species. Phytotaxa 383: 252–258.
Rodda, M. & Simonsson, N. (in press). Contribution to a revision of Hoya (Apocynaceae:
Asclepiadoideae) of Papuasia. Part II: nine new species, one new subspecies. Blumea.
Schlechter, R. (1916). Neue Asclepiadaceen von Sumatra und Celebes. Beih. Bot. Centralbl.,
Abt. 2. 34(2): 1–18.
Simonsson Juhonewe, N. & Rodda, M. (2017). Contribution to a revision of Hoya (Apocynaceae:
Asclepiadoideae) of Papuasia. Part I: ten new species, one new subspecies and one new
combination. Gard. Bull. Singapore 69: 97–147.
Thiers, B. (continuously updated). Index Herbariorum: A global directory of public herbaria
and associated staff. New York Botanical Garden. http://sweetgum.nybg.org/science/
ih/. Accessed 25 May 2022.