Nepenthes rigidifolia

Species of pitcher plant from Sumatra

Nepenthes rigidifolia /n//ᵻ/'/p//ɛ//n//θ//iː//z// /,/r//ɪ//dʒ//ɪ//d//ᵻ/'/f//oʊ//l//i//ə/ is a critically endangered tropical pitcher plant endemic to Sumatra, where it grows at elevations of 1000–1600 m above sea level. The specific epithet rigidifolia is formed from the Latin words rigidus (rigid) and folia (leaves), and refers to the plant's stiff, coriaceous leaf blades. Instead, the species was formally described later that year as N. rigidifolia by Pitra Akhriadi, Hernawati, and Rusjdi Tamin. Two years later, Hernawati and Akhriadi covered the species in their book, A Field Guide to the Nepenthes of Sumatra, and assessed its conservation status. The holotype of N. rigidifolia—Nepenthes Team (Hernawati, P. Akhriadi & I. Petra) NP 354—was collected on December 11, 2003, near Sidikalang in Karo Regency, North Sumatra, at an elevation of 1000–1500 m. It is deposited at the Herbarium Universitas Andalas (ANDA) of Andalas University in Padang, West Sumatra. An isotype is held at Herbarium Bogoriense (BO) in Java. is cylindrical and up to 1.3 cm in diameter. It can attain a length in excess of 10 m.

The development of the indumentum is variable in this species. Some plants are completely glabrous, while others bear a covering of orange to brown hairs (≤1 mm long) on the stem, pitchers, tendrils, and portions of the inflorescence. Nepenthes rigidifolia grows terrestrially in lower montane forest and scrub.

The pitchers of N. rigidifolia resemble those of N. bongso to a degree, although their colouration is closer to that of N. spectabilis. Nepenthes rigidifolia differs from N. bongso, N. ovata and related species in having mostly ovoid upper pitchers (compared to infundibular in the others), distinctly thick and coriaceous laminae, and a narrower, cylindrical peristome with very short teeth. In addition, the lower pitchers of N. bongso are considerably larger than those of N. rigidifolia. While recognising N. rigidifolia as a valid species in his Carnivorous Plant Database, taxonomist Jan Schlauer suggests that it may be conspecific with N. densiflora. Terrestrial pitchers of N. rigidifolia may superficially resemble those of N. spectabilis, but it can be distinguished from this species on the basis of its trap colouration, upper pitcher shape (largely ovate in N. rigidifolia versus predominantly cylindrical in N. spectabilis), thinner leaves, and branched spur.

Only one natural hybrid involving N. rigidifolia (with N. spectabilis) is known. This cross, which is only known from a single open rocky outcrop, may now outnumber N. rigidifolia at the type locality (as observed by Stewart McPherson in 2007). The hybrid differs from N. rigidifolia in having narrower pitchers with a strongly infundibular base and distinct hip around the middle. On the other hand, the pitchers of this hybrid are broader than those of N. spectabilis and have an expanded peristome, as well as a markedly shorter spur. Charles Clarke wrote of this cross: "Unlike N. ovata × N. spectabilis (from Gunung Pangulubao), this hybrid seems to combine the more attractive characteristics of both parent species, producing large, beautifully coloured pitchers". The richly coloured lower pitchers of N. rigidifolia × N. spectabilis may superficially resemble those of N. macfarlanei. However, since that species is confined to Peninsular Malaysia, the two taxa are not easily confused.