Taxonomic revision of the Hoya mindorensis complex (Apocynaceae:-Asclepiadoideae)

Abstract

The Hoya mindorensis complex is revised, types for the names involved are selected, and a new classification of the taxa is proposed. Hoya erythrostemma is separated once again from Hoya mindorensis, Hoya elmeri is identified as an earlier name for Hoya mindorensis subsp. superba, and a further new species, Hoya rintzii, from Peninsular Malaysia, Sumatra and Borneo, is described. A key is provided as an aid to identification.

Full text

Taxonomic revision of the Hoya mindorensis complex (Apocynaceae:-Asclepiadoideae)
Michele Rodda
a
*, Nadhanielle Simonsson Juhonewe
b
and Sri Rahayu
c
a
The Herbarium, Singapore Botanic Gardens, Singapore;
b
National Research Institute of Papua New Guinea, Papua New Guinea;
c
Bogor Botanic Gardens, Indonesian Institute of Sciences, Bogor, Indonesia
(Received 13 February 2014; final version received 1 March 2014)
The Hoya mindorensis complex is revised, types for the names involved are selected, and a new classification of the taxa
is proposed. Hoya erythrostemma is separated once again from Hoya mindorensis,Hoya elmeri is identified as an earlier
name for Hoya mindorensis subsp. superba, and a further new species, Hoya rintzii, from Peninsular Malaysia, Sumatra
and Borneo, is described. A key is provided as an aid to identification.
Keywords: Borneo; Hoya elmeri;Hoya erythrostemma;Hoya mindorensis subsp. mendozae;Hoya mindorensis subsp.
superba;Hoya rintzii; Indonesia; Malaysia
Introduction
In preparation for a revision of the genus Hoya R.Br. the
careful study of species complexes is a necessary
preliminary step. We have already examined the applica-
tion of Hoya revoluta Hook.f., Hoya plicata Hook.f. and
the complex of related species (Rodda and Simonsson
Juhonewe 2013) and we here proceed with Hoya
mindorensis Schltr. and related species. The complex is
here defined as a group of closely allied glabrous climb-
ing species having globose inflorescences of flowers with
spreading or reflexed trilobed corolla lobes, presenting a
central acute lobe and two smaller round lateral lobes,
otherwise defined as ‘auricles’(Figure 1D). This group
of species was included by Burton (1996b) in the new
section Pachystelma C.M. Burton whose type species is
H. mindorensis.Hoya mindorensis was described from
Mindoro, Philippines (Schlechter 1906). Schlechter
considered it “remarkable in the two auricles that exist
between the corolla lobes”and with “rather unusual
pollinia”.
Merrill (1929) described Hoya elmeri Merrill based
on a specimen collected from Tawau, Sabah, Malaysia.
Elmer too noticed the presence of trilobed corolla lobes,
and was unable to highlight similarities with any other
previously published species. Shortly after, Kerr (1939)
described Hoya erythrostemma Kerr based on two collec-
tions from peninsular Thailand and Myanmar. Kerr com-
pared it with Hoya elliptica Hook. f. (1883: 58) from
which it differed due to larger acuminate leaves and vil-
lous corolla. Rintz (1978) later applied the name to spec-
imens collected in Peninsular Malaysia.
Kloppenburg (1991) considered H. erythrostemma a
synonym of H. mindorensis based on the common pres-
ence of the trilobed corolla lobes mentioned by Schlechter
(1906), very narrow, laterally compressed corona lobes
presenting a longitudinal ridge, and the distinctive and
unique pollinarium with spoon shaped translator arms
(caudicles). The synonymy was not readily accepted even
by the publishing author (Kloppenburg (1991,2005), and
contested by Burton (1996a). The name H. erythrostemma
was still used in Kress et al. (2003) and in Rahayu and
Wanntorp (2012) for specimens from Sumatra.
Kloppenburg (2005) published a subspecies of
H. mindorensis,H. mindorensis subsp. superba Klop-
penb. from cultivated material originating in the Philip-
pines, that differs from the typical species by having
larger flowers, coronal lobe inner processes not reaching
the centre of the flower therefore leaving the anthers
exposed, and corona lobes not horizontal. A second sub-
species, H. mindorensis subsp. mendozae Kloppenb. and
Ferreras was published in 2014 (Kloppenburg and
Ferreras 2014). Compared with H. mindorensis ssp.
superba the corona lobes of the new subspecies were
described as horizontal and reaching the centre of the
flower and not curving downward at the outer apex and
slightly spaced in the centre of the flower; flowers and
pollinia were also described as smaller.
The present study is based on the examination of
herbarium materials of Hoya in A, BK, BKF, BM, BO,
FI, K, KEP, L, P, SAN, SAR, SING, UC and US Her-
baria and living materials growing in Singapore Botanic
Gardens Research Collection and Bogor Botanical Gar-
dens, Indonesia. The study aims to (i) revise the species
in the H. mindorensis complex, (ii) typify the names
involved, and (iii) describe the new species Hoya rintzii
Rodda, Simonsson and S. Rahayu.
Results
Kloppenburg’s(
1991) synonymization of H. ery-
throstemma with H. mindorensis was based upon the
*Corresponding author. Email: rodda.michele@gmail.com
© 2014 Dipartimento di Biologia, Università di Firenze
Webbia: Journal of Plant Taxonomy and Geography, 2014
Vol. 69, No. 1, 39–47, http://dx.doi.org/10.1080/00837792.2014.900261

examination of “live flowers”and “several herbarium
sheets”, but the photographs provided in his
publication were of flowers from two cultivated plants
confusingly distinguished as either H. mindorensis or
H. erythrostemma, from Ramos & Edano 45454 (UC),
and from the illustration of H. erythrostemma in Rintz
(1978). No vouchers were mentioned for the cultivated
plants, and Ramos & Edano 45454 (UC) is potentially a
mixed specimen bearing vegetative parts of a different
species (as annotated on the specimen by C.M Burton).
We are therefore unable to verify most of the original
materials used. The illustration of H. erythrostemma in
Rintz (1978) depicts instead the new species H. rintzii
(see below).
When Burton (1996a) proposed once again a
separation between the two species, she compared
H. erythrostemma with a specimen belonging to what
was later classified by Kloppenburg (2005)as
H. mindorensis ssp. superba, and not with specimens of
H. mindorensis.
Our examination of specimens shows that
H. erythrostemma and H. mindorensis bear strong simi-
larities in general flower morphology. Flowers are held
by 15–20 mm long pedicels, and form globular
inflorescences, 3–4 cm in diameter; the corolla is
spreading to reflexed, 10–14 mm diameter when
flattened, and the lobes are trilobed, very broad, c.3.5 ×
5.5–6 mm, the lateral lobes round, the central lobe acute,
revolute. The corolla is always pubescent especially in
the basal portion of the corolla lobe, but with variation
in the type of pubescence, from hispid to densely hirsute.
The corona lobes observed from above are laterally com-
pressed (Figure 2E), 2.5 to 3 times as long as broad, and
their inner processes are swollen and nearly touching in
the centre of the flower, above the stigma head. The
anther appendages do not extend above the inner corona
lobe processes. The basal margins of the corona lobes
are revolute, meeting in the centre from the tip of the
outer corona process almost to the filament tube (Fig-
ure 2F). Laterally, the corona lobes are triangular with an
acute outer tip. Greatly variable is the flower colour,
from light yellow and pink to almost black; corona and
corolla may be of different colours as also observed in
Widiarsih et al. (2012).
Two taxa can be separated based on leaf and pollina-
ria morphology and on distribution area. Specimens from
the Philippines have generally shiny, dark green leaves
with inconspicuous secondary venation (Figure 3L,M),
Figure 1. Hoya rintzii.(A) Inflorescence. (B) Flower, lateral view. (C) Flower, top view. (D) Flower from below, with evident
trilobed corolla lobes. Photographed under fluorescent light. (E) Corona from below. (F, G) Calyx (from M. Rodda MR12-H149
(SING)).
40 M. Rodda et al.

whereas specimens from Thailand (Figure 3N,O) have
lighter green leaves, with a much lighter abaxial side,
and with conspicuous secondary venation on the adaxial
side anastomosing near the lamina margin.
Pollinaria of the two taxa are of similar size and gen-
eral shape, but Philippine specimens have elliptic pollinia
with a round base and apex, lacking pellucid margins,
connected to the almost round retinaculum by laterally
Figure 2. Comparison of flowers of Hoya rintzii, Hoya mindorensis and Hoya elmeri.Hoya rintzii:(A) Flower, lateral view. (B)
Flower, top view. (C) Corona from below (from R.E. Rintz, RER61, KEP). Hoya mindorensis:(D) Flower, lateral view. (E) Corona
lobe from above. (F) Corona lobe from below (from M. Rodda MR12-H106 (SING)). Hoya elmeri:(G) Flower, lateral view. (H) Cor-
ona lobe from above. (I) Corona lobe from below (from M. Rodda MR429, SING). (Drawings by Michele Rodda).
Webbia: Journal of Plant Taxonomy and Geography 41

concave oblong caudicles c.120 × 60 μm (Figure 3E),
whereas Thai specimens have oblong pollinia with a
round base and obliquely truncate apex and a clearly
distinguishable pellucid edge, connected to the ovoid
retinaculum by laterally convex winged caudicles
c.140 × 120 μm (Figure 3C,D). Based on these consistent
vegetative and reproductive differences, and the disjunct
distribution area (Figure 4) we propose to separate once
again H. mindorensis and H. erythrostemma.
Hoya mindorensis subsp. mendozae was only com-
pared by Kloppenburg & Ferreras (2014)toH. mindorensis
ssp. superba. The differences stated between the two taxa
are in agreement with those listed by Kloppenburg
(2005) to separate H. mindorensis ssp. superba from
H. mindorensis. As the size of the corolla, the morphology
of the corona lobes and calyx lobes, and the size of the
pollinia are within the range of variation observed for
H. mindorensis the subspecies is synonymized.
Hoya mindorensis subsp. superba specimens are
indistinguishable from H. elmeri, and if sterile are easily
confused with H. mindorensis because the distinctive
characters lie in the size of the flowers and in the mor-
phology of the corona lobes and of the pollinarium,
while the foliage is easily confused with that of
H. mindorensis. The flowers are held by (1.5)2–2.5 cm
long pedicels, and form globose inflorescences up to 6
cm in diameter. The corolla is spreading to reflexed, his-
pid to densely hirsute, slightly back-curved, up to 1.6 cm
Figure 3. Pollinaria and leaves comparison in the Hoya mindorensis complex. (A–G) Pollinaria. Hoya rintzii:(A) Rehydrated (from
R.E. Rintz RER61, KEP). (B) Fresh (from M. Rodda MR12-H149, SING). Hoya erythrostemma:(C) Rehydrated (from M. Rodda
MR291, SING). (D) Fresh (from M. Rodda MR287, SING). Hoya mindorensis:(E) Fresh ( from M. Rodda MR12-H106, SING). Hoya
elmeri:(F) Left fresh, right, rehydrated (from M. Rodda MR429, SING). (G) Rehydrated from A.D.E.Elmer 20652, SING). The dif-
ferent size of the pollinia is due to dehydration, as already observed when comparing fresh and dry pollinia of Hoya burmanica
(Rodda and Simonsson 2012). (H–O) Leaves (all fresh). Hoya rintzii:(H) Adaxial. (I) Abaxial ( from L. Gokusing, AL2315/2012,
SAN). (J) Adaxial. (K) Abaxial (from M. Rodda MR12-H149, SING). Hoya mindorensis:(L) Adaxial. (M) Abaxial (from M. Rodda
MR12-H106, SING). Hoya erythrostemma (from M. Rodda MR291, SING): (N) Adaxial. (O) Abaxial. (Photographs by Michele
Rodda).
42 M. Rodda et al.

in diameter when flattened, and the lobes are trilobed,
very broad, 4–5 × c. 6 mm, the lateral lobes round, the
central lobe acute, revolute. The corona lobes are nearly
ovoid when observed from above (Figure 2H) twice as
long as broad, and the narrow inner process of the lobes
do not meet in the centre of the flower but otherwise,
above the stigmatic head, there is a gap occupied by the
anther appendages, which extend above the inner corona
lobe process (Figure 2G). The basal processes of each
lobe, observed from below the corona, are revolute and
meet in the centre of the lobe for the outer two-thirds of
the lobe length, then they split, forming thickened mar-
gins surrounding a more-or-less conspicuous hollow
space around the filament tube (Figure 2I), (however,
this is not as conspicuous in the type of H. elmeri) giv-
ing to the lobe the typical bulbous base in the inner pro-
cess, that abruptly narrows into a constricted outer lobe
process with a round tip. Flowers are usually red in col-
our, sometimes orange. The pollinaria present a massive
almost round retinaculum c.450 μm in diameter con-
nected by short caudicles to elliptic pollinia lacking a
pellucid edge (Figure 3F,G). Based on these differences,
we propose to apply for this taxon the name H. elmeri.
However, due to the overlapping distribution area of
H. mindorensis and H. elmeri and the scarcity of speci-
mens available in herbaria it is possible that new collec-
tions will show transition between the two species.
Specimens identified as H. erythrostemma from
Peninsular Malaysia, almost exclusively collected by
Richard E. Rintz in the 1970s and illustrated in his revi-
sion (Rintz 1978), from Sumatra (Rahayu and Wanntorp
2012), a recent collection from Sabah (Gokusing
AL2315/2012, SAN) and a living collection from Central
Kalimantan in Bogor Botanic Garden (Acc. No.
B2013030021) instead were found to belong to a differ-
ent undescribed species, validated here as H. rintzii.
Hoya rintzii can be easily separated from H. elmeri,
H. erythrostemma and H. mindorensis on both vegetative
and flowering morphology (see Key). Leaves are elliptic
to ovate with an obtuse apex, with conspicuous venation,
sometimes close to those of H.erythrostemma
(Figure 3H–K), but easily separated because in the latter
species the lamina apex is acute or acuminate. Flowers are
held by 8–11 mm long pedicels forming inflorescences
usually with a maximum 25 mm diameter (exceptionally
35 mm). The corolla is strongly reflexed, 5–6 mm diame-
ter, with typically trilobed lobes 2.2 × 4–4.5 mm, inter-
nally puberulent, except along the edge (Figure 1D).
Flowers are almost invariably ivory white or light yellow,
with the exception of the inner process of the corona lobes
that may be purple-tinged. The pollinaria are similar in
shape and size to those of H. erythrostemma, but the reti-
naculum is smaller, globose, and the caudicles are shorter
(Figure 3A,B).
Figure 4. Distribution map of Hoya mindorensis (circles), Hoya erythrostemma (squares), Hoya elmeri (stars), and Hoya rintzii
(triangles).
Webbia: Journal of Plant Taxonomy and Geography 43

Key to the species
1a. Corolla hispid to densely hirsute, inflorescences
diameter > 3 cm, lamina apex acute or acuminate
………............................................................................. 2
1b. Corolla puberulent, inflorescences diameter < 2.5 (3.5)
cm, lamina apex round…………….....…... 1. Hoya rintzii
2a. Corona lobes twice as long as broad, inflorescences 5–6
cm diam…………………………............. 2. Hoya elmeri
2b. Corona lobes laterally compressed, 2.5 to 4 times as
long as broad, inflorescences 3–4 cm diam. ………...... 3
3a. Secondary leaf venation inconspicuous, pollinia with-
out pellucid edge……....………....... 3. Hoya mindorensis
3b. Secondary leaf venation evident, pollinia with distinct
pellucid edge..………………..…4. Hoya erythrostemma
Taxonomy
1. Hoya rintzii Rodda, Simonsson & S. Rahayu, sp. nov.
Diagnosis
Hoya rintzii is similar to H. erythrostemma because both
species have generally elliptic leaf lamina with conspicu-
ous venation. However, H. rintzii leaves have a round
apex whereas H. erythrostemma leaves have an acute
apex; pedicels, inflorescences and flowers of H. rintzii
are smaller, the corolla puberulent and not hispid to
densely hirsute, and the pollinaria have smaller, globose
retinacula and shorter caudicles.
Type: Malaysia, Selangor, Sungai Langat, 3 Jun 1976,
R.E. Rintz RER61* (holo KEP!, isotype K!) (Figures 1,
2and 3).
Description
Epiphytic laticiferous glabrous vine; leafy stems twining,
up to 10 m long, cylindrical, 3–5 mm in diameter; older
stems lignified, with copious adventitious roots; inter-
nodes 5–20 cm long. Leaves opposite, petiolate; lamina
elliptic to ovate (oblong), fleshy, coriaceous when dry,
5–10(17) × 3–6 cm, adaxial surface dark green, with a
few grey markings, abaxial surface light green, apex
round (occasionally acute), base attenuate, three to five
pairs of secondary veins, branching from the midrib at
30–45°, lighter in colour than lamina on abaxial side,
not visible on adaxial side; petiole (5)8–15 × 2.5–4 mm;
inflorescences perennial, occurring singly at the nodes,
interpetiolar, negatively geotropic when young, becoming
positively geotropic in subsequent flowerings,
umbelliform, convex to globular 20–25 (35) mm in
diameter, up to 30-flowered; peduncle terete 0.5–1.5 (9)
cm × 2–3 mm, commonly bearing scars of previous
flowerings; pedicels terete, (6)8–11 × 0.6–0.8 mm, papil-
lose; buds star-shaped, white; calyx lobes obovate with a
rounded apex, pink-red, 0.8–1.2 × 0.7–1 mm, alternating
with single basal colleters, margin irregular; corolla cam-
panulate, strongly reflexed, cream white, 5–6mm in
diameter, 7–8 mm when flattened, lobes trilobate, small,
2.2 × 4–4.5 mm, lateral lobes round, central lobe acute,
revolute, abaxially glabrous, puberulent adaxially exclud-
ing the margins; corona staminal, fleshy, white or light
yellow, with or without a more or less pronounced pink,
red or dark maroon centre, 2–3 mm high, 5–6mm in
diameter; corona lobes laterally compressed, three to four
times as long as broad, ridged above, with basal revolute
margins, 2.5–3 mm long, 2.3–2.5 mm high, outer process
acute, inner process apiculate; pollinaria erect, 550–
400 × 450 μm; pollinia oblong, 400–450 × 150–200 μm
with a round base and an obliquely truncate apex and a
conspicuous pellucid edge, retinaculum nearly round,
200–220 μm in diameter, caudicles 70–100 μm long;
ovary bottle-shaped, 1.3–1.5 mm long. Fruits and seeds
not observed.
Etymology
Hoya rintzii is named after Richard Edward Rintz
(1945–), who collected the type specimen and the major-
ity of the other known specimens of the taxon.
Distribution and ecology
The exact distribution of H. rintzii is unknown. Its
known localities are scattered across a broad area includ-
ing Peninsular Malaysia, Sumatra and Borneo (Figure 4),
suggesting that the species may be widespread but not
common throughout the distribution area. The species
was collected from sea level to 500 m above sea level.,
along rivers and often draping trees. As documented by
Rintz (1978) it can be commonly found as an epiphytic
climber on trees overhanging streams and rivers in the
lowlands in Peninsular Malaysia.
Conservation status
Most of the original collecting localities of Rintz, on the
outskirts of Kuala Lumpur, have probably been lost to
development. The species is nonetheless common in cul-
tivation and recorded over a broad distribution area,
therefore the conservation status is assessed as Least
Concern (LC) (IUCN 2013).
2. Hoya elmeri Merrill, Univ Calif Publ Bot 15: 258.
1929
Type: Malaysia, Sabah, Tawau, October 1922 to March
1923, A.D.E.Elmer 20652* (lecto here designated A!,
barcode A00076433, isolectotypes BM!, MICH!, MO!,
NY!, P!, SING!, UC!, Z!) (Figures 2and 3).
44 M. Rodda et al.

(=) Hoya mindorensis subsp. superba Kloppenb. syn.
nov. Fraterna 18(3): 1–4. 2005 Type: Philippines, Luzon,
Dept. village (unlocalized, possibly incorrectly recorded
locality), cultivated in USA, Oregon, Central Point,
2005, A. Wayman s.n.* (holotype UC!).
PNH, where the first set of Merrill specimens was
deposited, was destroyed during the Second World War,
following Japanese invasion. The type of H. elmeri did
not survive the fire, but many duplicates are still found
in herbaria worldwide. We select as lectotype a duplicate
found in A, which bears a leafy shoot and well-preserved
flowers.
Distinguishing features
Leaf lamina elliptic to ovate (oblong), fleshy, coriaceous
when dry, (5)11–15 × 5–6.5 cm, adaxial surface dark
green, abaxial surface light green, apex acute (acuminate),
base attenuate (acute), secondary veins lighter in colour
than lamina, barely visible on abaxial side, not visible on
adaxial side. Inflorescences convex to globular 5–6cm in
diameter; pedicels (1.5)2–2.5 cm; corolla red or orange,
8–11 mm in diameter, up to 1.6 cm when flattened, hispid
to densely hirsute inside; lobes trilobed, very broad,
4–5 × c.6 mm; corona staminal, red or purple (orange),
fleshy, 7–8 mm diameter; corona lobes nearly ovoid when
observed from above about twice as long as broad, inner
process not meeting in the centre, basal part bulbous,
outer lobe constricted terminating in a round tip, lower
than inner lobe; when observed laterally presenting a
swollen part attached to the anthers and a narrower ovoid
outer lobe; pollinia elliptic 500–700 × 250–350 μm,
pellucid edge not present, caudicles oblong c.130 ×
70 μm, retinaculum nearly round, c.450 μm diameter.
Distribution and ecology
Hoya elmeri occurs in Sabah (Malaysia) and Luzon
(Philippines) (Figure 4). It is common in Sabah along
streams (L. Gokusing pers. comm.)
Conservation status
The conservation status of H. elmeri cannot be formally
assigned as yet and is considered as Data Deficient (DD)
(IUCN 2013). Just a handful of specimens are present in
herbaria, many more from undocumented localities are
present in the nursery and hobbyist trade, suggesting that
the species may be rather common but under-represented
in herbaria.
3. Hoya mindorensis Schltr., Philipp. J. Sci. 1(Suppl. 4):
303. 1906
Type: Philippines, Mindoro, Baco River, April–May
1905, R.C. McGregor 332*, (lecto here designated B!,
barcode B100277216) (Figures 2and 3)
Hoya mindorensis Schltr. subsp. mendozae Kloppenb. &
Ferreras, syn. nov. Hoya New 2(1): 34. 2014. Type:
Philippines, Luzon, Laguna, Nagcarlan, Mt. Mabilog, 2
October 2012, G. Mendoza GM45* (holo PUH! sheet
number 14650).
Despite the destruction by fire in a bombing raid on the
night of March 1–2 1943 of Berlin Herbarium, 125
sheets of Asclepiadaceae were on loan and escaped the
fire (Hiepko 1978). Among these specimens is R.C.
McGregor 332, mentioned by Schlechter in the publica-
tion of H. mindorensis. This specimen bears an auto-
graph sketch of a dissected flower as is common in
specimens examined by Schlechter and is therefore
selected as lectotype.
Distinguishing features
Leaf lamina lanceolate-elliptic (ovate), fleshy, coriaceous
when dry, (5)9–15 × 3–6 cm, adaxial surface dark green,
shiny, abaxial surface light green, apex acute (acumi-
nate), base acute (attenuate), secondary veins lighter in
colour than lamina, inconspicuous on abaxial side, not
visible on adaxial side. Inflorescences convex to globular
3–4 cm in diameter; pedicels 15–20 mm; corolla spread-
ing to reflexed very variable in colour including green,
lilac, purple, red, yellow and almost black 7–11 mm in
diameter, 10–14 mm when flattened, from hispid to den-
sely hirsute inside; corolla lobes trilobed, very broad,
c.3.5 × 5.5–6 mm; corona staminal, fleshy, variable in
colour including lilac, purple, red and almost black, 6–7
mm diam.; corona lobes laterally compressed, 2.5 to 3
times as long as broad, inner processes swollen and
nearly touching in the centre of the flower, above the
stigma head; outer process tip round; laterally triangular;
pollinia elliptic 450–550 × 230–270 μm, pellucid edge
not present, caudicles oblong, laterally concave c.120 ×
60 μm, retinaculum nearly round, c.300 μm diameter.
Distribution and ecology
Hoya mindorensis is so far known to occur in the Philip-
pines and Borneo (Figure 4). The taxon, as for many
other Hoya species, has been collected along rivers and
streams in the lowlands. It is scarcely represented in
herbaria, but at least in Sabah it appears to be common
(L. Gokusing, pers. comm.)
Conservation status
As indicated for H. elmeri the conservation status of
H. mindorensis is Data Deficient (DD) (IUCN 2013)as
the species is under-represented in herbaria.
4. Hoya erythrostemma Kerr., Kew Bulletin 460. 1939.
Florae Siamensis Enumeratio (1951: 36)
Webbia: Journal of Plant Taxonomy and Geography 45

Type: Thailand, Tasan, C. B. Kloss 6909* (lecto desig-
nated here, K!) (Figures 2and 3).
The type of H. erythrostemma was indicated by Kerr
(1939)as“Kloss 6909”. Since no herbarium was men-
tioned a lectotype needs to be selected. We could only
find a duplicate at K bearing an autograph determination
as H. erythrostemma by Kerr dated November 1938,
making the specimen a suitable lectotype. The paratype
listed by Kerr (Parkinson 1680, from Thebyu Chaung,
South Tenasserim) could not be located in the herbaria
consulted and it may be extant in DD, a herbarium that
we have been unable to contact.
Distinguishing features
Leaf lamina elliptic-lanceolate, fleshy, coriaceous when
dry, (5)10–15 × 2–5 cm, adaxial surface light green, abax-
ial surface lighter green, apex acute-acuminate, base
acute–almost round, secondary venation conspicuous on
abaxial side, anastomosing near the lamina margin, not
visible on adaxial side. Inflorescences convex to globular
3–4 cm in diameter; pedicels 15–20 mm; corolla spreading
to reflexed, variable in colour including green, lilac, pur-
ple, red, white and yellow, 7–10 mm in diameter, 10–14
mm when flattened, from hispid to densely hirsute inside;
corolla lobes trilobed, very broad, c.3.5 × 5.5–6 mm; cor-
ona staminal, fleshy, variable in colour lilac, purple, red,
white and yellow, 6–7.5 mm in diameter; corona lobes
observed from above laterally compressed, 2.5 to 3 times
as long as broad, inner processes swollen and nearly
touching in the centre of the flower, above the stigma
head; outer process tip round; when observed laterally tri-
angular shaped; pollinia oblong, 450–550 × 230–270 μm,
base round, apex obliquely truncate, pellucid edge present,
caudicles laterally convex, winged, c.140×120 μm, pellu-
cid edge evident, retinaculum c.300 × 220 μm.
Distribution and ecology
Hoya erythrostemma is endemic to Thailand (Figure 4).
Contrary to what was stated in Rintz (1978) we could
not find any specimen proving its occurrence in
Peninsular Malaysia, but the specimen Middleton, et al.
4160 (E) was collected in Thailand and is in close prox-
imity with the Malaysian border where it also probably
occurs but still awaits to be collected. The species, like
H. mindorensis, has been frequently sighted along rivers
and streams in the lowlands (S. Somadee, pers. comm.).
Conservation status
For the same reasons indicated for H. mindorensis the
conservation status is Data Deficient (DD) (IUCN 2013).
Additional specimens examined
(*) indicates specimens mapped in Figure 4 (
+
) indicates
specimens mapped in Figure 4 but not examined.
H. rintzii
Malaysia, Sabah, Tawau, nr. Kalabakan, sg. Serudong,
19 June 2012, flowered in cultivation at Kipandi Park, L.
Gokusing, AL2315/2012* (SAN!); Selangor, Sungai
Batu, 11 July 1976, 500 ft. [152 m], R.E. Rintz, RER88*
(L!); ibid, Sungai Langat, 2 June 1976, R.E. Rintz
RER62* (KEP!); ibid, Sungai Semenyik, 30 May 1976,
R.E. Rintz RER53* (KEP!); Ibid, Sungai Langat, 12 June
1976, R.E. Rintz RER68* (KEP!); ibid, Sungai Langat,
12 June 1976, R.E. Rintz RER64* (KEP!); Ibid, Sungei
Batu, 11 July 1976, R.E. Rintz RER92* (KEP!); Pahang,
Krau Wildlife Reserve, cultivated at Forest Research
Institute Malaysia, 1 December 2003, Y.Y. Sam
FRI47239* (KEP!); Indonesia, Sumatra, Alas River
Valley, vicinity of Ketambe Research Station, 10 June
1978, W.J.J.O. de Wilde and B.E.E. de Wilde-Duyfrjes
18053* (L!,BO!), Ibid, Lorzing 16034a (BO!);
Cultivated at Singapore Botanic Gardens (Acc. No.
20123475), horticultural origin, 22 September 2012,
M. Rodda MR12-H149 (SING!).
Cultivated specimens examined
Indonesia, Sumatra, Jambi, TN Bukit Tigapuluh, culti-
vated at Bogor Botanical Gardens (Acc. No.
B20001180*); Central Kalimantan, cultivated at Bogor
Botanical Gardens (Acc. No. B2013030021*).
H. mindorensis
Indonesia, Kalimantan, Long Sungai Barang, 22 Mar.
1994, 750 m, J.V.V Van Valkenburg 1407* (K!, L!);
Malaysia, Sabah, Kampung Melangkap Tomis, 1 Octo-
ber 1995, L. Lugas 996* (K!); Philippines, Dumaguete,
Island of Negros, June 1908, A.D.E. Elmer 10267* (E!);
Mindoro, Baco River, Cultivated in Singapore Botanic
Gardens, 24 August 2012, M. Rodda, MR12-H106*
(SING!)
H. erythrostemma
Myanmar, South Tenasserim, Thebyu Chaung, Parkin-
son 1680
+
(not located, possibly in DD) Thailand,
Nakhon Sawan, Khao Rum, February 1922, 1500 ft.
[457 m] E. Smith 579* (K!); Nakhon Sri Thammarat,
Kiriuong, Jap Charung, 29 July 1957, 400 m, J. Smithi-
nand 769* (K!); Songkhla, Nam Tok Ton Nga Cham, 27
March 1997, P.C. Boyce 1214* (K!); Ibid, Haad Yai, Dton
Nga Chan Reserve, 9 May 1986, 50 m, J. F. Maxwell
86–300* (L!); Satun, Khuan Don, Wang Prachan,
Thale Ban National Park, Tam Pliu Waterfall, 08 March
2006, D. Middleton, V. Chamchamroon, S. Lindsay, K.
Pattarahirankanok, S. Sirimongkol 4160* (E!);
Cultivated in Singapore Botanic Gardens (acc. No.
20123439), horticultural origin, 4 February 2013
M. Rodda MR287 (SING!); Cultivated in Singapore
Botanic Gardens (acc. No. 20060276), horticultural origin,
17 June 2011, M. Rodda MR291 (SING!).
46 M. Rodda et al.

Hoya elmeri
Philippines, Tabayas, Casiguran, May–June 1925,
M. Ramos and G. Edano 45454*(flowers only) (UC!);
Luzon, Dept Village (unlocalized, possibly incorrectly
recorded locality), cultivated in Singapore Botanic
Gardens (Acc. No. 20130115, IML 774), 4 October
2013, M. Rodda MR429* (SING!);
Acknowledgements
This study is part of an ongoing research project on the system-
atics of Marsdenieae. Financial support has been received from
the National Parks Board Singapore to Rodda and from Helge
Ax:son Johnsons Stiftelse to Simonsson Juhonewe. We would
like to thank the curators of the herbaria mentioned in the text
for allowing access to the collections and for providing high-
quality images of herbarium specimens; David Goyder for
providing excellent photographs of R.E. Rintz RER61 (K), an
anonymous benefactor for donating a living specimen of H.
mindorensis, Yap Kim Fatt for donating living specimen of
H. rintzii, Anthony Lamb for the fruitful discussion on H. rintzii,
Linus Gokusing and Surisa Somadee for providing information
on the native habitat of H. elmeri,H. erythrostemma and
H. mindorensis, and two anonymous reviewers for their
valuable comments on the manuscript.
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