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0 Center for Ecological Research, Kyoto University, Kamitanokami-hirano, Otsu 520-2113, Japan
Received for publication April 29, 1999. Accepted for publication October 26, 1999.
ABSTRACT
Pollination ecology of three Durio species, D. grandiflorus, D. oblongus, and D. kutejensis (Bombacaceae), was studied in a lowland dipterocarp forest in Sarawak, Malaysia, during a peak flowering period when at least 305 species of plants bloomed in 1996. Durio has been reported to be pollinated by bats in Peninsular Malaysia. However, my observations of flower visitors and pollination experiments indicated that two species, D. grandiflorus and D. oblongus, were pollinated by spiderhunters (Nectariniidae) and that the other species, D. kutejensis, was pollinated by giant honey bees and bats as well as birds. Hand-pollination experiments showed that all three species were obligate outbreeders. A resource limitation in fruit production was suggested. The former two species were visited only by spiderhunters, and the bagged flowers that were opened for animal visitors only at night bore no fruit, while those that were opened only during the day bore fruits, at comparable fruiting ratios to open pollination. Durio kutejensis was observed to be visited by giant honey bees, birds, and bats at different times of day, and three series of bagged experiments that exposed the flowers to animal visitors at different times of day bore fruits at a comparable ratio to open-pollination.
Key Words: bat • Bombacaceae • Durio • giant honey bees • peak flowering period • pollination • spiderhunters
Lowland tropical rainforests in west Malesia are characterized by a high diversity of tree species (Whitmore, 1984
; Richards, 1996
) and the phenomenon of supra-annual mass flowering (Ashton, Givnish, and Appanah, 1988
; Appanah, 1993
). Several studies in the peak flowering period in 1996 in Lambir Hills National Park, Sarawak, Malaysia, were reported in Momose et al. (1998)
and Sakai et al. (1999a, b)
. The reproductive phenology of 576 individual plants representing 305 species in 56 families was monitored for 53 mo during 1992–1996. Among 527 effective reproductive events during 43 mo, 57% were concentrated in a peak flowering period of 10 mo in 1996, and 35% of species only flowered during that period (Sakai et al., 1999b
). During the 53 mo, flower visitors of 270 plant species were observed or collected. Plants pollinated by social bees comprised the largest number of species (32%) followed by beetle-pollinated plants (20%) (Momose et al., 1998
).
The number of plants identified as vertebrate-pollinated was relatively small; 19 species in seven families were pollinated by birds (7%), four species in three families by bats (1.5%), and one species by squirrels (Momose et al., 1998
) compared to the data obtained in La Selva, Costa Rica, where 14.9% of plants were pollinated by hummingbirds and 10% by bats (Kress and Beach, 1994
).
The pollination system of Durio has been reported as typical bat-pollination (Start, 1974
; Start and Marshall, 1976
; Gould, 1978
; Faegri and van der Pijl, 1979
; Whitmore, 1992
), but the observations were confined to Peninsular Malaysia and the most intensely studied species was the cultivated durian, Durio zibethinus. Twenty-eight species of Durio are distributed widely in the Indochina Peninsula, Malay Peninsula, Sumatra, Borneo, and Philippines (Kochummen, 1972
). Thus, very limited studies have been done on wild durians, especially in Borneo, which is regarded as center of diversity of the genus.
MATERIALS AND METHODS
Study site and plant species
The study site was in a primary lowland dipterocarp forest in Lambir Hills National Park, Sarawak, Malaysia (4°20'N, 113°50'E, altitude 150–250 m). A Canopy Biology Plot was demarcated in August 1992 for monitoring plant phenology and for the observation of plant-animal interactions (Yumoto et al., 1998
). The canopy observation system consisted of two tree towers connected by >300 m of aerial walkways set within the main forest canopy (Inoue et al., 1995
).
The genus Durio (Bombacaceae) is well known for its edible fruits, with 28 species in Malesia (Kochummen, 1972
), especially in Borneo. Nine species were recorded in a 52-ha forest dynamics research plot in Lambir Hills National Park (LaFrankie, Tan, and Ashton, 1995
), and 15 species are present in Sarawak overall (Ashton, 1988
).
Pollination of three Durio species, D. grandiflorus (Mast.) Kost et Soegeng, D. oblongus Mast., and D. kutejensis (Hassk.) Becc., was studied from 14 May to 24 June 1996 during a general flowering period. Two trees of D. grandiflorus [22 cm in dbh (diameter at breast height) and 13 m in height, 20 cm in dbh and 10 m in height] were situated near the Park headquarters and along the waterfall trail from the headquarters to the Operation Raleigh Tower, respectively. A tree of D. oblongus (32 cm in dbh and 25 m in height) was located near the canopy walkway system. Two trees of D. kutejensis (28 cm in dbh and 12 m in height, and 25 cm in dbh and 10 m in height) were in a secondary stand near an Iban (Sea Dayak) settlement. Aluminum ladders were erected to observe the flowers and flower visitors, as well as to conduct bagging experiments of these trees.
Observation of flowers and nectar sampling
I observed flower anthesis. Nectar sampling by micro-capillary and injection syringe was done just after flower opening, and thereafter at intervals of 4–6 h over 24 h, depending on the available number of flowers. Ten flowers were taken for each sampling. Flowers from which nectar was sampled were not bagged, thus permitting unhindered animal visitation. The volume of nectar was measured by micro-capillary (up to 10 µL) or injection syringe (up to 10 mL). Sugar concentration was estimated by refractometer (BS-R70, Bellingham & Stanley Ltd., UK) as sucrose equivalent. Nectar samplings were done during 3–4 May for D. grandiflorus, 26–27 May for D. oblongus, and 18–19 May for D. kutejensis.
Observation of flower visitors
Flower visitors were observed using binoculars and torchlight, if necessary. A mist net was used to catch flower visitors to D. kutejensis. For each species, observation was carried out for at least 24 h over a full day. Foraging animals were identified by binoculars or by trapping. Observation of flower visitors was done during 14–15 April on D. grandiflorus, 28–29 May on D. oblongus, and 20–21 May on D. kutejensis. Additional night observations were made during 16–19 April and 30 May–1 June for D. grandiflorus and D. oblongus, respectively.
The scientific names of birds followed MacKinnon and Phillips (1993)
. The bill length (upper mandible length) and the total length of the observed spiderhunters were measured for the specimens collected in Sarawak Museum. These specimens were collected in Sarawak, mainly around Kuching area.
Pollination experiments
To examine the breeding system of Durio species and the contribution of animal visitors to effective pollination, six experiments were performed on trees: (1) open—flowers were left exposed permitting unhindered animal visitation; (2) untreated, bagged—flowers were bagged before each flower bloomed; (3) self-pollinated (geitonogamous)—flowers were bagged before each flower bloomed and were hand-pollinated with geitonogamous pollen from different flowers on the same tree collected just before the hand-pollination; (4) cross-pollination—flowers were bagged before each flower bloomed and were hand-pollinated with pollen from the flowers of a different tree collected just before the hand-pollination; (5) open at daytime—flowers were bagged before each flower bloomed and were exposed permitting unhindered animal visitation only at daytime (0500–1800); (6) open at night—flowers were bagged before each flower bloomed and were exposed permitting unhindered animal visitation only at night (2000–0300). Flowers were bagged by tetron bags (TORAY, tetron, number 9000, Japan). In hand-pollination, pollen was transferred using Chinese calligraphy brushes. One tree was used for bagging experiments, and another only as pollen donor, for both D. grandiflorus and D. kutejensis. For D. oblongus, cross-pollination treatments could not be done because the neighboring tree was not available within at least 1 km in distance. For D. kutejensis, giant honey bees were observed to visit a short time after dawn, so flower bags were opened during 0900–1800 in the daytime experiment, and an experiment (7), open in the early morning, was added in which flowers were bagged before each flower bloomed and exposed to permit unhindered animal visitation only in the early morning (0500–0700).
Pollination experiments were done during 19–21 April on D. grandiflorus, 24–27 May on D. oblongus, and 19–25 May for D. kutejensis. Fruits were checked 30–35 d after the experiments.
RESULTS
Flower characteristics of three Durio species
Characteristics of three Durio species are shown in Table 1. The flower color of D. grandiflorus and D. oblongus is white, while D. kutejensis is bright red. The flowers of D. oblongus and D. kutejensis are cup-shaped, while that of D. grandiflorus is rotate (Fig. 1). The base of the flower of D. grandiflorus is constricted tightly, so that a long slender bill or proboscis is needed to suck nectar. In D. oblongus, the style and filaments of flowers form a tough tube. Petals and sepals of all three species are thick, but the petals of D. grandiflorus were thinner compared to the other two species.
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Daily pattern of nectar availability
Daily changes of nectar volume and concentration are shown in Fig. 2. Nectar concentration of D. grandiflorus was high at daytime and low at midnight. Nectar volume at daytime was larger with greater variation than at night when no animal was observed to visit (Fig. 2).
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Nectar concentration of D. kutejensis changed only slightly during the 24-h observation period and was the highest at midnight (Fig. 2); variation among flowers was the least. Nectar volume increased monotonously once the flower had opened. A wide range of nectar volume was recognized throughout 24 h except just after the flowers opened.
Animal visitors on flowers
Flowers of D. grandiflorus were visited mainly by two species of spiderhunters, Arachnothera robusta and A. chrysogenys (Nectariniidae). The number of visits by A. chrysogenys was much more than that by A. robusta. Especially after 0900, only A. chrysogenys was observed to visit flowers (Fig. 3a). Spiderhunters were observed to be dusted with pollen on the forehead and bill. Besides birds, the Malay bird-wing butterfly (Troides amphrysus, Papilionidae) was observed to forage on flowers in the evening, and stingless bees (Trigona spp., Apidae) hovered around the flowers but never sucked nectar or collected pollen. No bat was recorded during four nights of observations.
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The flower visitors to D. kutejensis were much more diverse (Fig. 5). At 0530, giant honey bees (Apis dorsata, Apidae) began to forage on flowers and to collect pollen. Nearly 50 individuals of A. dorsata were counted on a flower in one half-hour period. The number of A. dorsata decreased during 0700–0900 and increased again at 0900. After 0900, giant honey bees began to collect nectar. Stingless bees (Trigona spp., Apidae) began to visit flowers at 0600, mainly foraging on nectar. Also, a small number of A. dorsata visited flowers immediately after anthesis at 1630. Birds also visited the flowers of D. kutejensis. Spiderhunters, Arachnothera robusta (four times), A. flavigaster (six times), and A. longirostra (once) were observed to visit flowers, and plain sunbirds, Anthreptes simplex (Nectariniidae) also visited flowers. But the most frequent bird visitor was the orange-bellied flower pecker, Dicaeum trigonostigma (Dicaeidae). During 1100–1400, only D. trigonostigma visited flowers, while spiderhunters were recorded in the morning and in the evening and the sunbirds were only in the evening. All birds touched the stigma and anthers when they sucked nectar from flowers. A male individual of D. trigonostigma was caught by mist net, and pollen was observed on the forehead. From 2000 to 2200, the cave nectar bat, Eonycteris spelaea (Pteropodidae), was observed, and an individual was caught by mist net. Pollen was collected on the forehead of the captured bat.
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DISCUSSION
Pollen limitation or resource limitation
All Durio species in this study were self-incompatible. The cultivated durian, D. zibethinus, is usually self-incompatible (Yaacob and Subhadrabandhu, 1995
), and wild durian, D. griffithii in Peninsular Malaysia is also reported as self-incompatible (Ha et al., 1988
).
Plants of Durio bear very large fruits. Wet masses of fruits recorded in Lambir Hills National Park were as follows: D. graveolens, 757. 5 g; D. kutejensis, 406.2 g; D. griffithii, 353.1 g; D. oblongus, 302.3 g and D. grandiflorus, 295.1 g (T. Yumoto, unpublished data). It is reasonable to assume that there is severe resource limitation in fruit production of Durio species, although there was no relationship between wet mass of a fruit and fruit-set ratio among the three species observed.
Pollination experiments showed that flowers that were allowed open-pollination received enough pollen from other individuals in at least two species, because the fruit-set ratios of open-pollination and that of artificial cross-pollination were not significantly different. It strongly suggests that the low fruit-set ratio was due to resource limitation, but not to pollen limitation when the flowers were permitted foraging by animals.
Example of bird-pollination in Lambir Hills National Park
Nineteen species in seven families were reported as bird-pollinated in Lambir Hills National Park, and 13 species among them were mainly pollinated by spiderhunters (Momose et al., 1998
). Four species of spiderhunters, Arachnothera robusta, A. longirostra, A. flavigaster, and A. chrysogenys, have been recorded visiting flowers in Lambir Hills National Park, and only A. robusta and A. longirostra were observed before the peak flowering period in 1996.
Studies on pollination of the Loranthaceae (Yumoto, Itino, and Nagamasu, 1997
) and the Zingiberaceae (Sakai, Kato, and Inoue, 1999
) in Lambir Hills National Park proved that A. robusta never forages in the understory where A. longirostra frequently does. But it is difficult to explain by the vertical habits of spiderhunters why A. chrysogenys was a predominant visitor to D. grandiflorus but rarely to D. oblongus, and vice versa for A. robusta, because the height of the flowers of both species was almost the same. One possible reason may be the territoriality of spiderhunters. Most spiderhunters establish their territory around the flowering plants (Yumoto, Itino, and Nagamasu, 1997
), and the order of dominance from the strongest to the weakest is A. flavigaster, A. robusta, A. chrysogenys, and A. longirostra. Flowers of D. grandiflorus provided less nectar, compared to D. oblongus. So, as a resource for nectar, D. oblongus was likely more valuable than D. grandiflorus. These Durio species shared a common flowering period, thus we deduce that the dominant species, A. robusta and A. flavigaster, foraged on the nectar-rich resource more often than the less dominant species A. chrysogenys.
Bird-pollination vs. bat-pollination
Observations of animal foraging on flowers showed that D. grandiflorus and D. oblongus were only visited by birds and that no bats visited these flowers. Moreover, effective pollination was shown by pollination experiments to occur at daytime, not at night. Daytime nectar volume data showed large variations, which suggested that the consumption of nectar by animal visitors varied among flowers.
Different patterns were observed on D. kutejensis. Giant honey bees, birds, and bats were observed to forage on flowers at different times of day and night, and pollination experiments showed that effective pollination occurred at all times. These results suggested that all these three, giant honey bees, bird, and bats, contributed to effective pollination.
The typical characteristics of ornithophily and chiropterophily pollination syndromes are contrasting. Ornithophilous flowers are characterized by vivid colors, absence of odor, a deep tube or spur, and diurnal anthesis; chiropterophilous flowers are characterized by whitish or creamy color, strong odor, large-mouthed single flowers or brush inflorescence, and nocturnal anthesis. The characteristics of flowers of the three Durio species observed are intermediate between ornithophily and chiropterophily. The single flowers of D. grandiflorus are white, faint smelling, and large-mouthed with diurnal anthesis: two of these characteristics are typical of ornithophilous flowers and two are typical of chiropterophilous ones. Three characteristics of D. oblongus, white color, strong odor, and large-mouthed flowers, are typical of chiropterophily. Durio kutejensis also has flowers intermediate between chiropterophily and ornithophily; both diurnal and evening anthesis, a very strong odor, and a large mouth are characteristics of the former, while red color is characteristic of the latter.
Faegri and van der Pijl (1979)
pointed out that the Bignoniaceae and Bombacaceae have some species that are intermediate between bird-and bat-pollination; Bombax malabaricum (Gossampinus heptaphylla) is ornithophilous, but incompletely so with open, red, cup-shaped, diurnal flowers. Its sister species B. valetonii is chiropterophilous with bat-smelling flowers. Baum (1995)
discovered that two species of section Brevitubae in Adansonia (Bombacaceae) are pollinated by nocturnal mammals (fruit bats and lemurs) and that five species in section Longitubae are pollinated by long-tongued hawk moths. These studies proved clearly that the traditional typological thinking with regard to pollination syndromes is a gross oversimplification and potentially misleading, particularly as it applies to distinctions between bird-and bat-pollination.
The genus Durio may include species intermediate between ornithophily and chiropterophily. Of course, we need a molecular phylogeny for Durio to understand the evolution of floral types within the genus, in relationship to floral visitors. However, the basic characteristics of Durio flowers are still more chiropterophilous than ornithophilous, or, there may be a trend from chiropterophily to ornithophily. Especially in D. oblongus, the pattern of nectar production is apparently nocturnal, and diurnal bird visitors only rarely made contact with anthers and stigma. Durio oblongus could have been able to compensate to some degree, using what is perhaps an alternative diurnal pollinator.
These three species, as well as other Durio species in Lambir Hills National Park, had not been observed in flower since 1992; they only bloomed and fruited during the peak flowering period of 1996. Momose et al. (1998)
and Sakai et al. (1999b)
noted that completely chiropterophilous plants, such as Parkia singularis, P. speciosa (Leguminosae), Fagraea racemosum (Loganiaceae), and Ganua beccariana (Sapotaceae), were observed to bloom only during the peak flowering period in 1996, while some ornithophilous plants of the Loranthaceae and the Zingiberaceae bloomed before the peak flowering period. During an 18-mo stay in Lambir Hills National Park before the peak flowering period in 1996, I never saw the cave nectar bat, Eonycteris spelaea, but the nectarivorous spiderhunters and sunbirds were always observed. Start (1974)
estimated that E. spelaea forage >38 km from the cave in which they roost. From Lambir Hills National Park, there is a limestone area at Niah Cave National Park, 50–70 km distant. During the peak flowering period in 1996, I observed > 200 individuals of E. spelaea fly in the same direction (from south to north) at 1830–1900 every evening. It may be possible for E. spelaea to visit the forest in Lambir from Niah Cave when many nectar sources become available. Both Lambir Hills National Park and Niah Cave National Park have isolated natural vegetation surrounded by oil palm plantations. The availability of wide-ranging nectarivorous bats is less stable now than nectarivorous birds in Lambir Hills National Park because the natural vegetation is fragmented. We also discovered a new pollination syndrome, squirrel pollination in Lambir Hills National Park in Ganua (=Madhuca, Sapotaceae), which was previously believed to be bat-pollinated (Yumoto, Momose, and Nagamasu, 1999
). Even in D. oblongus, penduliflory, which is especially suited to bat-pollination and limits the accessibility to birds and other mammal, has been lost. The reduced availability of nectivorous bats could lead to bird-pollination or pollination by other animals, rather than bat-pollination.
FOOTNOTES
1 The author thanks Prof. Kazuhiko Ogino (Ehime University) and Prof. Tamiji Inoue (Kyoto University) for their encouragement and suggestions; Dr. Lee Hua Seng and Mr. Abdul Abang Hamid (Forest Department Sarawak) for their administrative management; Dr. Kuniyasu Momose, Dr. Shoko Sakai, and other members of Sarawak Canopy Biology Project for helping with field survey; Dr. Lee Moi Ung (Sarawak Museum) for permitting me to measure the bird specimen; and Prof. Peter S. Ashton (Harvard University) and Dr. Ian M. Turner (Singapore Botanic Garden) for critical reading on the manuscript. This study is partly supported by a Japan Ministry of Education, Science, Culture and Sport Grand-in-Aid for International Scientific Research (number 04041067) and is approved by State Secretary, Sarawak and Director of Forests, Sarawak by reference number of 80/PKM/1335/5/79. 
2 E-mail address: yumoto{at}ecology.kyoto-u.ac.jp
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