Date of Publication: 8 August 2010
© National University of Singapore
Alvin Francis S. L. Lok, W. F. Ang and Hugh T. W. Tan
This paper documents the distribution and status of Pomatocalpa diffusum Breda (Fig. 1) in Singapore. Pomatocalpa is
(Comber, 1990; Seidenfaden & Wood, 1992), but has recently been revised and determined to comprise of only 13
species and six subspecies (Watthana, 2007).
The genus is rather widely distributed and can be found from India to Malesia, eastwards through New Guinea, the
Seidenfaden & Wood, 1992; Watthana, 2007). The highest number of congeners are found in Thailand (six species) and
Peninsular Malaysia-Singapore (six species), followed by Borneo (five species), and Sumatra (five species), while
China, Taiwan, Bangladesh, the lesser Sunda Islands, Vanuatu, and Fiji only have one species each, indicating that the
centre of diversity of Pomatocalpa lies within the boundaries of Thailand, and Peninsular Malaysia-Singapore
Pomatocalpa is derived from the Greek pomataos meaning flask or cup and calpe meaning pitcher, which refers to the
flask- or cup-shaped labellum (lip). The genus was established by Jacob Gijsbertus Samuël van Breda in 1829, who first
described a single species (Pomatocalpa spicatum) based on a specimen collected by Kuhl, and van Hassel from Java
(Breda, 1829; Watthana, 2007). However, the holotype designated by Breda at Leiden was lost, thus leading Pearce and
Cribb (2002) to designate the illustration rendered by Breda (Breda, 1829: pl. 15) as the lectotype. The second species
Pomatocalpa diffusum was described by Breda (1830) in Genera et Species Orchidearum, Fascicle IV, which was for
many years thought to have existed as a single copy and thus was not accepted as being effectively published until van
Steenis-Kruseman & Veldkamp (1991) discovered a second copy of the book and reinstated the taxon.
The genus was initially disregarded and overlooked (Seindenfaden, 1988) until Smith (1912) reinstated the genus,
amongst many others. Smith later supplied a preliminary list of 29 taxa, which he admitted, was crude owing to
insufficient descriptions. On top of this, the genus Pomatocalpa was for many years erroneously placed in the subtribe
Sarcanthinae Benth., as first noted by Rasmussen (1985), because the subtribe was based on an illegitimate name,
Sarcanthus Lindl. Seindenfaden (1988) later proposed that Pomatocalpa be placed in the subtribe Aeridinae Pfitzer,
which has since been universally accepted (Watthana, 2007).
Pomatocalpa species are typically monopodial, with the stem apex growing continuously and producing distichously
alternating leaves while the base of the stem gradually dies away. The inflorescences are axillary and the aerial roots
penetrate the leaf sheaths (Watthana, 2007). Congeners can be broadly classified into two main groups. The first group
consists of small- to medium-sized, fan-shaped plants with 2–11 leaves and short internodes, while the second group
consists mainly of large, rambling plants with more than 10 leaves and long internodes. In the second group, the roots
are produced all along the length of the stem, to secure the plant firmly to its substrate. Pomatocalpa diffusum is
belongs to the second group.
Pomatocalpa species are generally found in evergreen rain forest, but have also been reported in other tropical habitats
such as mangrove forest, scrub, rocky sea shores, as well as isolated trees in open sites, and cultivated areas. Most
species are lowland plants up to an altitude of 750 m with only a few species being recorded above 1,000 m. The
flowers are probably bee-pollinated, as van der Pijl and Dodson (1966), and Jones (1981) have observed bees (Trigona
sp.) with dark pollinaria of Pomatocalpa macphersonii attached to their heads. Congeners also seem to have a
phorophyte preference for Myristica and Syzygium species (Mursidawati et al., 1999) and were observed to generally
grow on the lower main tree trunk instead of the high branches of the crown which is preferred by other genera
Fig. 1. Pomatocalpa diffusum growing midway on the main trunk and low branches of a Cratoxylum formosum tree just outside the
SAF Nee Soon Range I. Scale bar = 10 cm. (Photograph by: Alvin Francis Lok Siew Loon).
Pomatocalpa diffusum was previously known by synonyms such as Cleisostoma cumingii Rchb. f., Cleisostoma
Schltr., and Sarcanthus cumingii (Rchb. f.) J. J. Sm. (Watthana, 2007). Pomatocalpa diffusum plants are mainly
rambling although compact plants are encountered from time to time (Figs. 1, 2). Their stems have been known to reach
5 m long, with a stem diameter of 1–1.3 cm and internodes of up to 4.6 cm long. The leaves are scattered evenly along
the stem, with tubular leaf sheaths tightly embracing the stem for their entire length. The leaf blade is 15–23 cm ×
as many as 200 flowers (Fig. 2). Each branch is 2.4–5.8 cm long, bearing 15–50 flowers, with basal branches sometimes
producing second-order branches. The peduncle is 4–23 cm long with brownish-purple spots. The terminal rachis is 2–
6.4 cm long, and the side branch rachides 0.2–4.3 cm long and usually ridged. The non-resupinate flowers are dirty-
yellow with reddish-brown borders on the sepals and petals and are about 1.2 cm in diameter (Fig. 3). The dorsal sepal
is 3.0–4.2 × 0.9–1.6 mm, obovate-oblong, obtuse and three-veined. The laterals are 2.3–3.8 × 1.2–1.8 mm, obovate to
obovate-oblong, slightly oblique, acute to obtuse and three-veined. The petals are 2.1–3.9 × 0.7–1.5 mm, obovate-
oblong and sometimes falcate, obtuse and three-veined. The labellum is complex with the mid-lobe producing a less
than 90° abaxial angle to the spur, which is broadly-ovate, acute sometimes obtuse and strongly recurved. The
labellum’s side lobes are obliquely triangular, subacute to acute. The spur is 2.4–3.5 mm long and 1.3–1.7 mm in
diameter, bucket-shaped, dorsoventrally compressed, and slightly inflated.
Pomatocalpa diffusum is quite widely distributed, ranging from Thailand, Peninsular Malaysia, Singapore, Borneo,
Sumatra, Java, Bali, Sulawesi (Watthana, 2007), and possibly the Philippines (Comber, 2001). This species usually
grows as an epiphyte on tree trunks and low branches in shaded as well as open areas in lowland tropical rain forest,
mangrove forest, dipterocarp forest, beach forest and solitary trees in villages up to an altitude of 300 m (Watthana,
Fig. 2. Cultivated Pomatocalpa diffusum specimen flowering. Scale bar = 5 cm. (Photograph by: Alvin Francis Lok Siew Loon).
Fig. 3. Close-up of Pomatocalpa diffusum flowers showing non-resupination. Scale bar = 0.5 cm. (Photograph by: Alvin Francis Lok
J. S. Goodenough
Fig. 4. Pomatocalpa diffusum growing with other orchidaceous epiphytes and Platycerium coronarium on the trunk and branches of a
scrub (Comber, 2001). Other phorophytes include cultivated Lagerstroemia, Garcinia and Tamarindus species trees
In Singapore this species is nationally critically endangered (Tan et. al., 2008; Chong et. al., 2009) and was last
sterile specimens were first observed in Feb.2010 growing on a Cratoxylum formosum tree outside the SAF Nee Soon
Range I (Fig. 4), with other native orchid species including the nationally critically endangered Bulbophyllum sessile
(Fig. 5), nationally endangered Bulbophyllum vaginatum (Fig. 6), and Polystachya concreta which was previously
thought to be nationally extinct. In mid-Apr.2010, specimens collected from the host phorophytes in Feb.2010 started
developing inflorescences and flowered in cultivation in May 2010 (Fig. 2), which also coincided with the flowering of
wild specimens on the host phorophyte in-situ (Figs. 1, 6). This may have indicated that the flowering of the plants in
cultivation were natural and not caused by stress.
Interestingly, only a single Cratoxylum formosum phorophyte seemed to be festooned with a wealth of epiphytes, while
especially for the orchids. Possible explanations could include physical factors such as the highly scaly and fissured
bark of Cratoxylum formosum which probably helps retain water in an increasingly desiccating Singapore environment
and also provide a better foothold for orchid roots to penetrate. Other biotic reasons for phorophyte preference might
also include symbiotic relationships with Crematogaster ant species which are found under the bark of the entire tree as
well as around the root systems of the orchids and within the shield fronds of the Platycerium coronarium growing on
the same tree. Crematogaster ant infestation may help play a part in plant herbivory protection as well as from
mechanical damage from Macaca fasicularis (long-tailed macaques), which are usually found foraging on all other
surrounding trees, but hardly ever on this specific Cratoxylum formosum individual. The last possible reason is that the
fissured bark of this Cratoxylum formosum may be extensively infected with orchid mycorrhizae and combined with the
moisture-retaining characteristics of the bark; create a more ideal habitat for orchid seed germination.
Fig. 6. Pomatocalpa diffusum growing and flowering in-situ with
Alvin Francis Lok Siew Loon).
Fig. 5. The critically endangered Bulbophyllum sessile growing
on the same tree as Pomatocalpa diffusum. Scale bar = 1 cm.
(Photograph by: Alvin Francis Lok Siew Loon).
We would like to express our gratitude to the Chief Executive Officer and staff members of the National Parks Board
(SING), as well as for granting us permission to survey the Central Catchment Nature Reserve.
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