dark brown, or even black, although the latter is commonly lighter on its lower surface. The pigmentation of the upper pitchers is generally similar to that of the lowers, although typically lighter. The describing authors observed that rosette pitchers typically have a dark brown peristome, whereas that of upper pitchers is often orange to dark red. Herbarium specimens have an almost black stem. Dried leaves are light brown above with a dark brown underside, whereas pitchers are blackish-brown with dark brown blotches and an almost black lid. No infraspecific taxa of N. rigidifolia have been described. Ecology Nepenthes rigidifolia is endemic to Sumatra, where it has been recorded from a single small area near Sidikalang in Karo Regency, North Sumatra province. The only known population of this species grows on a road bank. The describing authors counted 24 mature plants at the type locality. Subsequent field observations suggest that the population has since dwindled in number, with perhaps only two plants remaining. Nepenthes rigidifolia grows terrestrially in lower montane forest and scrub. Young plants have been observed growing as lithophytes. The species has an altitudinal distribution of 1000–1600 m above sea level. It grows sympatrically with N. spectabilis and a natural hybrid between these species has been recorded. Nepenthes rigidifolia has been evaluated as critically endangered on the IUCN Red List. This agrees with an informal assessment carried out in 2006 by Hernawati and Pitra Akhriadi, who also classified the species as critically endangered based on the IUCN criteria and using field data collected by the "Nepenthes Team" of Andalas University. Since it does not grow within a protected area, N. rigidifolia is threatened by habitat loss due to land clearing, land conversion, road broadening, and clearing of road banks. Other threats include forest and shrub fires as well as poaching by plant collectors. If the type locality is the site of the last remaining population of this species then it is one of rarest Nepenthes known and "threatened with imminent extinction". In Pitcher Plants of the Old World, Stewart McPherson wrote that field studies are "urgently required" to properly assess its conservation status. In 2010, the Rare Nepenthes Collection was established with the aim of conserving N. rigidifolia and three other critically endangered Nepenthes species: N. aristolochioides, N. clipeata, and N. khasiana. Related species The pitchers of N. rigidifolia resemble those of N. bongso to a degree, although their