| Nepenthes dubia | |
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| An upper pitcher ofNepenthes dubia | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Order: | Caryophyllales |
| Family: | Nepenthaceae |
| Genus: | Nepenthes |
| Species: | N. dubia |
| Binomial name | |
| Nepenthes dubia | |
| Synonyms | |
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Nepenthes dubia/nɪˈpɛnθiːzˈduːbiə/ is a tropicalpitcher plantendemic to the Indonesian island ofSumatra, where it grows at an altitude of 1,600–2,700 mabove sea level.[6] Thespecific epithetdubia is theLatin word for "doubtful".
Nepenthes dubia was first collected on May 29, 1917[note a] byH. A. B. Bünnemeijer onMount Talakmau, at an altitude of around 1900 m above sea level.[2] Eleven years later,B. H. Danserformally describedN. dubia in his seminal monograph "The Nepenthaceae of the Netherlands Indies".[note b] Danser noted similarities betweenN. dubia and the closely relatedN. inermis and suggested that it might represent a naturalhybrid involving this species.[2][7] He wrote:[2]
N. dubia strongly resembles the strikingN. inermis, but the difference is too large to unite these two species.N. inermis, likeN. 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 inN. inermis. PerhapsN. dubia is a hybrid ofN. inermis and another species with normal peristome and in that caseN. Bongso could be the other parent species, the more so as the vegetative parts ofN. inermis,N. dubia andN. Bongso are very similar, and between the other species of thegymnamphora-group intermediate forms often occur.
Danser based his description on the specimen collected in 1917,Bünnemeijer 938. It consists of a portion of a climbing stem with upper pitchers. The specimen is deposited at theBogor Botanical Gardens (formerly the Herbarium of the Buitenzorg Botanic Gardens) inJava andHerbarium Lugduno-Batavum inLeiden,Netherlands.[2]Bünnemeijer 938 was later designated as thelectotype ofN. dubia byMatthew Jebb andMartin Cheek.[5]
Renewed interest inNepenthes in the latter half of the 20th century sawN. dubia become the subject of both confusion and taxonomic revision.
In an article published in 1973 on theNepenthes of Borneo, Singapore, and Sumatra,[4] botanistShigeo Kurata incorrectly identified specimens of a natural cross betweenN. inermis andN. talangensis as belonging toN. dubia.[8] Kurata would later describe this hybrid as a new species,N. × pyriformis.[9]
In 1986,Mitsuru Hotta andRusjdi Tamin included plant material belonging toN. dubia andN. inermis in their description ofN. bongso.[3][8] However,N. bongso differs considerably in pitcher morphology from these species and is not easily confused with them.
In 1997,Matthew Jebb andMartin Cheek published their monograph "A skeletal revision ofNepenthes (Nepenthaceae)", in which they referred toN. dubia specimens fromMount Talang (Kurata s.n. SING) and the mountains of theTjampo region nearPayakumbuh (Meijer 6949 L). The latter specimen is the holotype ofN. tenuis, which Jebb and Cheek treated in synonymy withN. dubia.[5]
However, subsequent authors have rejected this interpretation.Charles Clarke restoredN. tenuis to species rank inNepenthes of Sumatra and Peninsular Malaysia (2001), citing differences in pitcher morphology. Clarke also identifiedKurata s.n. as the natural hybridN. × pyriformis, which is similar toN. 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 inN. dubia. In addition, the mouth ofN. × pyriformis is raised towards the back as opposed to being horizontal.[8]
Nepenthes dubia is a climbing plant. The stem can reach 3 m in length and is 3 to 4 mm in diameter. It is cylindrical or slightly angular.[2]Internodes are up to 10 cm long.[8]
Leaves aresessile andcoriaceous.[2] Thelamina islanceolate-spathulate in form. It may be up to 10 cm long and 2 cm wide. It has an acute apex and is graduallyattenuate 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 themidrib.Pinnate veins are oblique and irregularlyreticulate, although they are not easily distinguishable.[2]Tendrils can be up to 15 cm long and may or may not have a curl.[8]
Rosette and lower pitchers are rarely produced. They are narrowlyinfundibular in the lower two-thirds, becoming ovoid above, and are sharply contracted below the rim. Terrestrial pitchers are relatively small, reaching 5 cm in height and 3.5 cm in width. A pair of fringed wings (≤3 mm 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. Theperistome is cylindrical, up to 3 mm wide, and bears indistinct teeth. The lid oroperculum is ovate and slightly raised in the middle. It bears no appendages. An unbranchedspur (≤4 mm long) is inserted at the base of the lid.[8]
Upper pitchers are generally larger, growing to 8 cm in height and 4 cm in width. They gradually arise from the ends of the tendrils, forming a 5 to 10 mm wide curve.[2] They are tubular to infundibular in the lower parts with laterally appressed pitcher walls. As inN. 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 orinvolute peristome (≤4 mm wide) with indistinct teeth spaced 0.25 to 0.5 mm apart.[2] The inner portion of the peristome accounts for around 45% of its total cross-sectional surface length.[10] The lid is narrowlycuneate, without appendages, and up to 4 cm long and 0.7 cm 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 5 mm long.[2]
Nepenthes dubia has aracemoseinflorescence that is distinctly short and compact. Thepeduncle may be up to 8 cm long. Therachis grows to 10 cm in length, although it is usually shorter in female inflorescences.Pedicels arebracteolate and up to 8 mm long.Sepals are oblong-lanceolate and up to 3 mm long.[8]
Most parts of the plant are virtuallyglabrous. Inflorescences sometimes bear a sparseindumentum 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]
Nepenthes dubia was for a long time thought to beendemic toMount Talakmau, but it was recently found on a second mountain, where it grows at lower elevations of around 1,600 mabove sea level.[11] On Mount Talakmau, the species occurs at an altitude of 1,800–2,700 m. At lower elevations on Talakmau, it growsepiphytically inmontane forest. However, above around 2,400 m, vegetation is very stunted and hereN. dubia also grows terrestrially on clumps of moss.[8]
In its natural habitat,N. dubia is sympatric withN. gymnamphora,N. izumiae,N. jamban, andN. lingulata.[8][12] The stunted upper montane forest which these species inhabit is dominated byferns of the generaDipteris andDicranopteris. Trees rarely exceed 3 m in height and fewNepenthes plants emerge above the vegetation.[8]
Due to its extremely localised distribution,N. dubia is listed asCritically Endangered on the2006 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 ofN. inermis. Because of this, wild populations ofN. dubia have not suffered from over-collection.[8]
Nepenthes dubia produces thick,mucilaginous pitcher liquid similar to that found in related species such asN. inermis. The pitchers ofN. inermis function not only aspitfall traps but also asflypaper traps, with the sticky inner walls trapping flying insects above the surface of the fluid. A similar trapping method may be employed byN. dubia.[8]Nepenthes likeN. jamban also use this method withmucilaginous pitcher fluid.
Nepenthes dubia belongs to a group of closely related montane Sumatran species that includesN. flava,[13]N. inermis,N. jacquelineae,N. jamban,[12]N. talangensis, andN. tenuis. These species are characterised by infundibular upper pitchers and highly viscous pitcher fluid.
Nepenthes dubia is thought to be most closely related toN. 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 ofN. 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 ofN. tenuis,Joachim Nerz andAndreas Wistuba included a table of morphological characteristics that distinguish it from related species, includingN. 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 acladistic analysis of theNepenthes species of Sumatra andPeninsular Malaysia using 70 morphological characteristics of eachtaxon. The following is part of the resultantcladogram, showing "Clade 1", which has 51% bootstrap support. Its most strongly supported subclade is the sister pair ofN. inermis andN. dubia, having 95% support.[8]
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A single mature female plant ofN. dubia ×N. izumiae grows along the summit trail onMount Talakmau. It produces infundibular upper pitchers that are yellowish-green in colouration. The pitchers are relatively small, reaching only around 10 cm in height. As inN. dubia, the stem and tendrils are purplish-red. The lamina is green with a red midrib.Nepenthes dubia ×N. izumiae differs most obviously fromN. dubia in having an ovate lid that is never reflexed beyond 180 degrees.[8] This hybrid is listed asN. dubia ×N. singalana inCharles Clarke'sNepenthes of Sumatra and Peninsular Malaysia, sinceN. izumiae is very closely related toN. singalana and was only described as a distinct species in 2003.[15]
In 2009, Adrian Y. Wartono observed a putative cross betweenN. dubia andN. jamban in an area where these two species grew withN. lingulata andN. rhombicaulis.[16]
Among natural populations ofN. jacquelineae, botanistAndreas Wistuba observed a single plant that appears to represent a natural cross betweenN. dubia andN. jacquelineae.[11][17]
Folia mediocria sessilia, lamina lanceolato-spathulata, nervis longitudinalibus utrinque c. 3, basi attenuata 1/3-2/3 caulis amplectente, vagina 0;ascidia rosularum etinferiora 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.
