N.dubia strongly resembles the striking N.inermis, but the difference is too large to unite these two species. N.inermis, like N.Lowii has only a rudiment of a peristome. N.dubia has a broad and flat one. There are, however, also differences in the other parts: the pitchers are less widely infundibuliform and the lid is not so narrow as in N.inermis. Perhaps N.dubia is a hybrid of N.inermis and another species with normal peristome and in that case N. Bongso could be the other parent species, the more so as the vegetative parts of N.inermis, N.dubia and N. Bongso are very similar, and between the other species of the gymnamphora-group intermediate forms often occur.
Renewed interest in Nepenthes in the latter half of the 20th century saw N.dubia become the subject of both confusion and taxonomic revision.
In an article published in 1973 on the Nepenthes of Borneo, Singapore, and Sumatra,[4] botanist Shigeo Kurata incorrectly identified specimens of a natural cross between N.inermis and N.talangensis as belonging to N.dubia.[8] Kurata would later describe this hybrid as a new species, N.pyriformis.[9]
In 1986, Mitsuru Hotta and Rusjdi Tamin included plant material belonging to N.dubia and N.inermis in their description of N.bongso.[3][8] However, N.bongso differs considerably in pitcher morphology from these species and is not easily confused with them.
However, subsequent authors have rejected this interpretation. Charles Clarke restored N.tenuis to species rank in Nepenthes of Sumatra and Peninsular Malaysia (2001), citing differences in pitcher morphology. Clarke also identified Kurata s.n. as the natural hybrid N. × pyriformis, which is similar to N.dubia, but can be distinguished on the basis of several stable characters. The hybrid has a wider pitcher lid that is never relfexed beyond 90 degrees and the pitcher cup is not appressed in the lower parts as in N.dubia. In addition, the mouth of N. × pyriformis is raised towards the back as opposed to being horizontal.[8]
Description
Nepenthes dubia is a climbing plant. The stem can reach 3 m in length and is 3 to 4mm in diameter. It is cylindrical or slightly angular.[2]Internodes are up to 10cm long.[8]
A rosette plant, showing the reddish stem of this species (left) and a closeup of a rosette pitcher (right)
Leaves are sessile and coriaceous.[2] The lamina is lanceolate-spathulate in form. It may be up to 10cm long and 2cm wide. It has an acute apex and is gradually attenuate towards the base, which clasps the stem for one third to a half of its circumference.[2] Around three pairs of longitudinal veins are present on the lamina, originating from the basal third of the midrib. Pinnate veins are oblique and irregularly reticulate, although they are not easily distinguishable.[2]Tendrils can be up to 15cm long and may or may not have a curl.[8]
Rosette and lower pitchers are rarely produced. They are narrowly infundibular in the lower two-thirds, becoming ovoid above, and are sharply contracted below the rim. Terrestrial pitchers are relatively small, reaching 5cm in height and 3.5cm in width. A pair of fringed wings (≤3mm wide) runs down the upper third of the pitcher beneath the rim. The glandular region covers the lower two-thirds of the inner surface. The pitcher mouth is round and slightly raised at the back. The peristome is cylindrical, up to 3mm wide, and bears indistinct teeth. The lid or operculum is ovate and slightly raised in the middle. It bears no appendages. An unbranched spur (≤4mm long) is inserted at the base of the lid.[8]
An upper pitcher
Upper pitchers are generally larger, growing to 8cm in height and 4cm in width. They gradually arise from the ends of the tendrils, forming a 5 to 10mm wide curve.[2] They are tubular to infundibular in the lower parts with laterally appressed pitcher walls. As in N.inermis, there is almost no gap between the walls in mature pitchers. The upper part of the pitcher is widely infundibular throughout. Wings are reduced to ribs in upper pitchers. The inner surface of the pitcher is covered with numerous small, slightly depressed glands, occurring at a density of 600-900 per square centimetre;[2] it lacks a waxy zone.[10] The pitcher mouth is ovate, horizontal, and acute towards the lid.[2] It bears a cylindrical or involute peristome (≤4mm wide) with indistinct teeth spaced 0.25 to 0.5mm apart.[2] The inner portion of the peristome accounts for around 45% of its total cross-sectional surface length.[10] The lid is narrowly cuneate, without appendages, and up to 4cm long and 0.7cm wide. It is rounded at the apex, gradually attenuate towards the base, and bears numerous small round or elliptical glands on its undersurface.[2] Characteristically, it is almost always reflexed beyond 180 degrees relative to the pitcher mouth.[8] In upper pitchers, the unbranched spur is 3 to 5mm long.[2]
Nepenthes dubia has a racemoseinflorescence that is distinctly short and compact. The peduncle may be up to 8cm long. The rachis grows to 10cm in length, although it is usually shorter in female inflorescences. Pedicels are bracteolate and up to 8mm long. Sepals are oblong-lanceolate and up to 3mm long.[8]
Most parts of the plant are virtually glabrous. Inflorescences sometimes bear a sparse indumentum of simple hairs. Caducous brown hairs are present on developing pitchers.[2][8]
The stem, inflorescence and tendrils are characteristically purplish-red in most plants. The lamina is green, often with a red midrib. Pitchers generally range in colour from light green to yellow throughout, although orange and red forms are also known to exist.[8] Danser described the colour of herbarium specimens as "fallow-dun, here and there blackish".[2]
Ecology
A young climbing plant in mossy forest at around 1600m
Nepenthes dubia was for a long time thought to be endemic to Mount Talakmau, but it was recently found on a second mountain, where it grows at lower elevations of around 1600m above sea level.[11] On Mount Talakmau, the species occurs at an altitude of 1800–2700m. At lower elevations on Talakmau, it grows epiphytically in montane forest. However, above around 2400 m, vegetation is very stunted and here N.dubia also grows terrestrially on clumps of moss.[8]
Numerous plants growing among stunted ridgetop vegetation
In its natural habitat, N.dubia is sympatric with N.gymnamphora, N.izumiae, N.jamban, and N.lingulata.[8][12] The stunted upper montane forest which these species inhabit is dominated by ferns of the genera Dipteris and Dicranopteris. Trees rarely exceed 3 m in height and few Nepenthes plants emerge above the vegetation.[8]
Due to its extremely localised distribution, N.dubia is listed as Critically Endangered on the 2006 IUCN Red List of Threatened Species.[1]N.dubia occurs at several locations along the only permanent trail up Mount Talakmau. However, the upper pitchers of this species bear a peristome, and so are not as desirable as those of N.inermis. Because of this, wild populations of N.dubia have not suffered from over-collection.[8]
Carnivory
Nepenthes dubia produces thick, mucilaginous pitcher liquid similar to that found in related species such as N.inermis. The pitchers of N.inermis function not only as pitfall traps but also as flypaper traps, with the sticky inner walls trapping flying insects above the surface of the fluid. A similar trapping method may be employed by N.dubia.[8] Nepenthes like N.jamban also use this method with mucilaginous pitcher fluid.
Related species
Profile view of an upper pitcher, showing the lid reflexed beyond 180 degrees
Nepenthes dubia belongs to a group of closely related montane Sumatran species that includes N.flava,[13]N.inermis, N.jacquelineae, N.jamban,[12]N.talangensis, and N.tenuis. These species are characterised by infundibular upper pitchers and highly viscous pitcher fluid.
Nepenthes dubia is thought to be most closely related to N.inermis. It shares with this species the general morphology of its pitchers and leaves. It differs in having a well-developed peristome, more glands on the underside of the lid, and the lid being reflexed by more than 180 degrees. The lid of N.dubia is unique in this respect and is almost always reflexed beyond this angle, unless surrounding objects prevent it from assuming such a position.[8]
In their description of N.tenuis, Joachim Nerz and Andreas Wistuba included a table of morphological characteristics that distinguish it from related species, including N.dubia:[14]
Character
N.bongso
N.dubia
N.talangensis
N.tenuis
Shape of upper pitchers
tubulate - infundibulate
tubulous in the lower part, infundibulate above the middle
tubulous to narrow infundibuliform in the lower half, ovate in the upper half
wide infundibulate, contracted below the mouth
Lid
orbiculate
narrow cuneate
broad-ovate
very narrow elliptical
Length/width ratio of upper pitchers
3.3
1.9
2.3
1.75
In 2001, Charles Clarke performed a cladistic analysis of the Nepenthes species of Sumatra and Peninsular Malaysia using 70 morphological characteristics of each taxon. The following is part of the resultant cladogram, showing "Clade 1", which has 51% bootstrap support. Its most strongly supported subclade is the sister pair of N.inermis and N.dubia, having 95% support.[8]
A putative natural hybrid between N.dubia and N.jacquelineae
A single mature female plant of N.dubia × N.izumiae grows along the summit trail on Mount Talakmau. It produces infundibular upper pitchers that are yellowish-green in colouration. The pitchers are relatively small, reaching only around 10cm in height. As in N.dubia, the stem and tendrils are purplish-red. The lamina is green with a red midrib. Nepenthes dubia × N.izumiae differs most obviously from N.dubia in having an ovate lid that is never reflexed beyond 180 degrees.[8] This hybrid is listed as N.dubia × N.singalana in Charles Clarke's Nepenthes of Sumatra and Peninsular Malaysia, since N.izumiae is very closely related to N.singalana and was only described as a distinct species in 2003.[15]
In 2009, Adrian Y. Wartono observed a putative cross between N.dubia and N.jamban in an area where these two species grew with N.lingulata and N.rhombicaulis.[16]
Among natural populations of N.jacquelineae, botanist Andreas Wistuba observed a single plant that appears to represent a natural cross between N.dubia and N.jacquelineae.[11][17]
Notes
a.^ Bünnemeijer reached the base of Mount Talakmau and set up camp there on April 21, 1917, at 400 m altitude. This was followed by a stay at 1060 m on the mountain's northwestern slope on May 6, at 1300 m on May 11, at 1900 m on May 21, and finally on the plateau, at an elevation of 2780 m, between June 1 and June 4, during which he climbed the summit.[18][19]
b.^ The original Latin description of N.dubia reads:[2]
Folia mediocria sessilia, lamina lanceolato-spathulata, nervis longitudinalibus utrinque c. 3, basi attenuata 1/3-2/3 caulis amplectente, vagina 0; ascidia rosularum et inferiora ignota; ascidia superiora parva, parte inferiore tubulosa v. leviter ventricosa, supra medium infundibuliformia, costis 2 prominentibus; peristomio fere horizontali, operculum versus acuto, applanato, 2-4 mm lato, costis 1/2-1/4 mm distantibus, dentibus 0; operculo anguste cuneato, facie inferiore plana; inflorescentia ignota; indumentum parcum, iuventute tomentum fuscum, denique deciduum.
Related Research Articles
Nepenthes aristolochioides is a tropical pitcher plant endemic to Sumatra, where it grows at elevations of 1800–2500 m above sea level. It has an extremely unusual pitcher morphology, having an almost vertical opening to its traps. It is critically endangered by overcollection.
Nepenthes bongso is a tropical pitcher plant endemic to Sumatra, where it has an altitudinal distribution of 1000–2700 m above sea level. The specific epithet bongso refers to the Indonesian legend of Putri Bungsu, the spirit guardian of Mount Marapi.
Nepenthes eymae is a tropical pitcher plant endemic to Sulawesi in Indonesia, where it grows at elevations of 1,000–2,000 m (3,300–6,600 ft) above sea level. It is very closely related to N. maxima, from which it differs in its wine glass-shaped upper pitchers.
Nepenthes lamii is a tropical pitcher plant endemic to New Guinea, where it grows at an altitude of up to 3520 m above sea level, higher than any other Nepenthes species. Although once confused with N. vieillardii and previously regarded as conspecific with the closely related N. monticola, it is now recognised as a distinct species.
Nepenthes eustachya is a tropical pitcher plant endemic to Sumatra, where it grows from sea level to an elevation of 1600 m. The specific epithet eustachya, formed from the Greek words eu (true) and stachys (spike), refers to the racemose structure of the inflorescence.
Nepenthes sumatrana is a tropical pitcher plant endemic to the Indonesian island of Sumatra, after which it is named.
Nepenthes longifolia is a tropical pitcher plant endemic to Sumatra, where it grows at elevations of between 300 and 1100 m above sea level. The specific epithet longifolia, formed from the Latin words longus (long) and folius (leaf), refers to the exceptionally large leaves of this species.
Nepenthes jacquelineae is a tropical pitcher plant endemic to the Indonesian island of Sumatra. Due to its unique pitcher morphology, it is considered to be one of the most spectacular Nepenthes species native to the island.
Nepenthes talangensis is a tropical pitcher plant endemic to Sumatra, where it grows in upper montane forest at elevations of 1800–2500 m above sea level.
Nepenthes tenuis is a tropical pitcher plant endemic to the Indonesian island of Sumatra. The species was first collected in 1957, from a remote mountain in the western part of the island. It remained undescribed until 1994, and was only rediscovered in the wild in 2002. Prior to this, N. tenuis was known solely from a single photograph and dried herbarium specimen.
Nepenthes mikei is a tropical pitcher plant endemic to Sumatra. It is characterised by its black mottled lower and upper pitchers. The species is closely related to N. angasanensis and N. tobaica.
Nepenthes inermis is a tropical pitcher plant endemic to the Indonesian island of Sumatra. The specific epithet inermis is Latin for "unarmed" and refers to the upper pitchers of this species, which are unique in that they completely lack a peristome.
Nepenthes izumiae is a tropical pitcher plant endemic to Sumatra, where it grows in montane forest at 1700–1900 m above sea level. It appears to be most closely related to N. lingulata and N. singalana.
Nepenthes rhombicaulis is a tropical pitcher plant endemic to Sumatra. The specific epithet rhombicaulis is formed from the Latin words rhombicus, meaning "rhomboid", and caulis, "stem". It refers to the cross-sectional shape of the stem internodes.
Nepenthes ovata is a tropical pitcher plant endemic to Sumatra. The specific epithet ovata is Latin for "ovate" and refers to the shape of the lower pitchers.
Nepenthes spectabilis is a tropical pitcher plant endemic to Sumatra, where it grows at elevations of between 1400 and 2200 m above sea level. The specific epithet spectabilis is Latin for "visible" or "notable".
Nepenthes jamban is a tropical pitcher plant endemic to northern Sumatra. The specific epithet jamban is the Indonesian word for "toilet" and refers to the shape of the pitchers.
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Nepenthes lingulata is a tropical pitcher plant endemic to northern Sumatra. The species is characterised by the highly developed appendage present on the underside of the lid. The specific epithet lingulata is derived from the Latin word lingula, meaning "small tongue", and refers to this unique morphological feature.
Nepenthes flava is a tropical pitcher plant endemic to northern Sumatra, where it grows in montane forest at 1800–2200 m above sea level.
1 2 (in Indonesian) Tamin, R. & M. Hotta 1986. Nepenthes di Sumatera: The genus Nepenthes of the Sumatra Island. In: M. Hotta (ed.) Diversity and Dynamics of Plant Life in Sumatra: Forest Ecosystem and Speciation in Wet Tropical Environments. Part 1: Reports and Collection of Papers. Kyoto University, Kyoto. pp. 75–109.
1 2 Kurata, S. 1973. Nepenthes from Borneo, Singapore and Sumatra. The Gardens' Bulletin Singapore26(2): 227–232.
↑ (in Japanese and English) Kurata, S. 2001. スマトラ島およびミンダナオ島産ウツボカズラの２新種(英文). [Two new species of Nepenthes from Sumatra (Indonesia) and Mindanao (Philippines).] The Journal of Insectivorous Plant Society52(2): 30–34.
1 2 Bauer, U., C.J. Clemente, T. Renner & W. Federle 2012. Form follows function: morphological diversification and alternative trapping strategies in carnivorous Nepenthes pitcher plants. Journal of Evolutionary Biology25(1): 90–102. doi:10.1111/j.1420-9101.2011.02406.x
↑ Clarke, C.M., T. Davis & R. Tamin 2003. Nepenthes izumiae (Nepenthaceae): a new species from Sumatra. Blumea48(1): 179–182. doi:10.3767/000651903X686141
Meimberg, H., A. Wistuba, P. Dittrich & G. Heubl 2001. Molecular phylogeny of Nepenthaceae based on cladistic analysis of plastid trnK intron sequence data. Plant Biology3(2): 164–175. doi:10.1055/s-2001-12897
Meimberg, H. & G. Heubl 2006. Introduction of a nuclear marker for phylogenetic analysis of Nepenthaceae. Plant Biology8(6): 831–840. doi:10.1055/s-2006-924676
Meimberg, H., S. Thalhammer, A. Brachmann & G. Heubl 2006. Comparative analysis of a translocated copy of the trnK intron in carnivorous family Nepenthaceae. Molecular Phylogenetics and Evolution39(2): 478–490. doi:10.1016/j.ympev.2005.11.023
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