Nepenthes rhombicaulis

Species of pitcher plant from Sumatra

Nepenthes rhombicaulis /n//ᵻ/'/p//ɛ//n//θ//iː//z// /,/r//ɒ//m//b//ᵻ/'/k//ɔː//l//ɪ//s/ 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 rhombicaulis was first collected by Shigeo Kurata on March 29, 1972, on Mount Pangulubao at an altitude of between 1700 and 1900 m above sea level. The species was mentioned by name in a 1972 issue (volume 26, number 10, page 44) of The Heredity. It was formally described by Kurata the following year in The Gardens' Bulletin Singapore. Kurata's illustration of the type specimen shows a small apical appendage on the underside of the pitcher lid. However, Matthew Jebb and Martin Cheek pointed out that this feature is not present in the isotype held in Singapore. Kurata suggested that the appendage might be a developmental defect and of little significance. Observations made at the type locality by Charles Clarke and Ch'ien Lee seem to confirm this; while some wild plants exhibit this appendage, most do not. In 1993, Bruce Salmon postulated that the lower pitchers of N. rhombicaulis are specially adapted to trapping subterranean insects. His observations were published in the Carnivorous Plant Newsletter. Rudolf Schmid Höllinger reported observing upper pitchers of N. rhombicaulis on Mount Pangulubao in 1993. He published his findings the following year in the Carnivorous Plant Newsletter. Charles Clarke also reported finding one small upper pitcher in 1995. Prior to this, it was thought that the upper stem of N. rhombicaulis was only used for climbing and did not produce pitchers. Nepenthes rhombicaulis grows terrestrially in dense, shady montane forest. It is usually found in lower montane forest above steep slopes, but has also been recorded from upper montane forest. It is one of the few Nepenthes species that are common in the understory. In its natural habitat, N. rhombicaulis occurs sympatrically with N. flava, N. ovata, N. spectabilis, and N. tobaica. Natural hybrids with all of these species have been recorded. Due to its somewhat restricted distribution, N. rhombicaulis is listed as Vulnerable on the 2006 IUCN Red List of Threatened Species.

Nepenthes rhombicaulis is very similar to N. gymnamphora in both morphology and growth habit. It is sympatric with this species on Mount Pangulubao. Nepenthes rhombicaulis also bears a close resemblance to N. hirsuta from Borneo. 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 resultant cladogram placed N. rhombicaulis in an unresolved polytomy at the base of the Montanae/Nobiles clade, together with N. benstonei.

At least four natural hybrids involving N. rhombicaulis have been recorded and at least three of the parent species (excluding N. rhombicaulis) occur on Mount Pangulubao. N. flava × N. rhombicaulis
? N. gymnamphora × N. rhombicaulis
N. ovata × N. rhombicaulis
N. rhombicaulis × N. spectabilis
N. rhombicaulis × N. tobaica