Nepenthes thorelii has a large racemose inflorescence. The peduncle is 8 to 18 cm long, while the rachis is 50 to 70 cm long. Partial peduncles are one-flowered. Pedicels are up to 6 mm long and may or may not be shortly bracteate. The species has an indumentum of simple or branched white hairs between 0.3 and 0.4 mm long. The diploid chromosome number of N. thorelii is 78 according to a 1969 paper by Katsuhiko Kondo. However, a 1997 study by Günther Rudolf Heubl and Andreas Wistuba found the chromosome number to be 80, and this is now accepted as the diploid number of all species in the genus. Ecology Nepenthes thorelii is known with certainty only from Vietnam, where the type specimen was collected. Nepenthes thorelii occurs in seasonally dry savannah grassland from sea level to an elevation of 200 m. During the dry season it is believed to survive as a dormant rootstock while bushfires burn any above-ground vegetation. New shoots are produced annually with the advent of the wet season. Since the extent of its natural range is uncertain, N. thorelii is listed as Data Deficient on the 2006 IUCN Red List of Threatened Species based on an assessment carried out in 2000. Related species Nepenthes thorelii is closely related to the Indochinese species N. smilesii. These two taxa share several morphological features, such as narrow linear leaf blades and leaf bases that clasp the stem. In addition, N. smilesii may grow in similar habitats to N. thorelii. Distinguishing features of N. thorelii include the large rootstock that allows it to perennate through dry and wet seasons, the uncurled tendrils of upper pitchers, and its large inflorescence. However, the extent of the variation of the Indochinese species is unknown, making them difficult to circumscribe. Nepenthes thorelii is also allied to N. bokorensis of Cambodia, from which it differs in several aspects of vegetative morphology. Firstly, the lamina of N. bokorensis is sessile to sub-petiolate and only slightly decurrent down the stem, if at all. In comparison, N. thorelii has an amplexicaul leaf attachment and the base of the lamina is decurrent into two wings that extend up to 2.5 cm down the stem. In addition, N. thorelii has wholly ovoid lower pitchers, whereas those of N. bokorensis are only ovate in the basal third, becoming cylindrical above. In his Carnivorous Plant Database, taxonomist Jan Schlauer treats