Nepenthes rhombicaulis is a climbing tropical pitcher plant endemic to Sumatra, Indonesia. It was first collected in 1972 by Shigeo Kurata and formally described in 1973. The species name is derived from Latin: rhombicus meaning “rhomboid” and caulis meaning “stem,” referring to the distinctive rhomboid cross-section of its stem internodes. It is notable for its extreme climbing ability and unique pitcher placement.
 
Habitat Overview: Climate and Soil
This species grows in montane rainforest environments at elevations between 1,500 and 1,900 meters above sea level. The climate is cool, humid, and cloud-covered, with frequent rainfall. N. rhombicaulis typically grows in the understory, where its pitchers are often buried in leaf litter, suggesting an adaptation to trap subterranean insects.
 
Natural Range and Distribution
Nepenthes rhombicaulis is mainly known from a few mountains in the Toba region of North Sumatra, including Mount Pangulubao. Its distribution is limited and fragmented, and it is listed as Vulnerable under IUCN criteria. The species is sympatric with N. gymnamphora, meaning they share the same habitat and sometimes grow side by side.
 
Physical Characteristics
This species is a vigorous climber, with stems reaching up to 35 meters in length, making it one of the longest in the genus. Internodes are up to 20 cm long and 1 cm in diameter, typically rhomboid in cross-section. The lower pitchers are adapted for ground-level trapping, often hidden beneath leaf litter. Upper pitchers were once thought to be absent but have since been observed, though rarely. Pitchers are typically green to reddish, with a simple peristome and a lid that may or may not have a small appendage.
 
Similar Species and Differentiation
Nepenthes rhombicaulis is very similar to N. gymnamphora in both morphology and growth habit. It also resembles N. hirsuta from Borneo. However, it can be distinguished by its rhomboid stem shape, extreme climbing length, and pitcher placement strategy. The occasional presence of a lid appendage has been debated, with some experts suggesting it may be a developmental anomaly