Nepenthes rafflesiana

Two pitchers, two diets

Jack named Nepenthes rafflesiana in 1821 ^[1] from a plant collected in Singapore ^[2]. Several field studies since have compared its two kinds of pitcher ^{[8, 9, 10]}.

Lower pitchers form on the ground rosette of young plants. Upper pitchers form on the leaves of climbing stems. A developmental study found that the two follow separate growth programs from the start, rather than one being a modified version of the other ^[10]. In a 1996 field study in Borneo, upper pitchers caught more flying insects than lower ones, although ants were the largest prey group in both. Both kinds show ultraviolet patterns around the main nectaries, and only the upper pitchers produce a fragrance that draws flying insects ^[8]. A later analysis of their scent found that upper pitchers give off more volatile compounds and a wider range of them, including some typical of sweet floral scents. They attract a group of flower-visiting insects that never turn up at the lower pitchers ^[9].

The pitchers also change as the plant ages. In typical plants from Brunei, each new pitcher has a smaller waxy zone than the last, until the wax disappears at the end of the juvenile stage. Pitcher shape shifts from elongated to ovoid along the way ^[11]. In the field, Jebb and Cheek recognise the species by its stalked leaves and by its peristome, which rises into a long, flattened neck that is broadest just below the lid. The lid is often notched or blunt at the tip, with nectar glands mostly near its edge ^[2].

How the trap works

Wax is not the main weapon. Field experiments that disabled one pitcher part at a time showed that the typical form relies mainly on its peristome, the slippery wet rim ^[12]. Its fluid is viscoelastic: when stretched between the fingers it forms threads, and both this fluid and the fluid of the elongate form held on to more ants than plain water did ^{[12, 13]}. In a comparison of seven lowland Nepenthes in Brunei, N. rafflesiana had among the most acidic fluid, near pH 2. It secreted visible nectar, smelled sweet and caught one of the broadest ranges of prey, including several orders of flying insects ^[13].

A new pitcher is not fully armed on the day it opens. Over the first two weeks it becomes more fragrant, secretes more nectar and gets a more wettable peristome, and natural prey capture picks up three to six days after opening ^[14]. The plant also manages the fluid level. Freshly opened pitchers are about half full. Pitchers flooded with extra water returned to an intermediate level within two to three days, and pitchers given a smaller volume of concentrated fluid secreted more within two days. Intermediate levels gave the highest capture rates ^[15].

The bat pitcher that became a separate species

For much of the twentieth century a second Bornean plant was lumped into N. rafflesiana. Researchers in Brunei used informal names such as “elongata” or “elongate form” for it ^[16]. Its aerial pitchers are up to four times longer than those of typical N. rafflesiana, give off less scent and catch insects at rates up to seven times lower. Instead, Hardwicke’s woolly bats roost inside them, and their droppings supply an estimated third of the plant’s leaf nitrogen ^[17]. In 2013 Scharmann and Grafe reinstated the name N. hemsleyana for this plant. The two species differ in the waxy zone: N. hemsleyana keeps or extends it through life and keeps its elongate pitchers, while N. rafflesiana loses the wax and turns ovoid. Upper leaves also differ, narrower in N. hemsleyana and broader on longer stalks in N. rafflesiana ^[16]. Cheek and Jebb use the name N. hemsleyana for the bat-roosting plant ^[18], and WCVP accepts it as a species ^[19]. The variety name elongata, which has no located type, remains on WCVP’s list of N. rafflesiana synonyms ^{[1, 2]}.

Hybrids with neighbours are known. N. × hookeriana, the cross with N. ampullaria, is widespread but scarce ^[2], and in Brunei crosses between N. hemsleyana and N. rafflesiana have been found only in disturbed habitats ^[16].

Where it grows

Jebb and Cheek record it from Sumatra, Peninsular Malaysia and Borneo. It is one of the most abundant species in northern Borneo but rather rare on the Sumatran and Malay mainland, though common on offshore islands such as the Riau archipelago and Singapore. It grows from open ground to shady forest, often in weedy roadside regrowth, from sea level to 1000 m ^[2]. IUCN assessed it for the Red List in 2018; see the assessment for its category ^[3]. It falls under the CITES Appendix II listing for the genus ^[4].

In cultivation

ICPS names N. rafflesiana with N. bicalcarata and N. truncata as good lowland species for beginners growing in a house ^[7]. As a lowland plant it wants warmth all year; NC State Extension gives 21 to 35 °C (70 to 95 °F) for the lowland group ^[6]. Plants in a house dry out quickly, so ICPS suggests a heavier mix of about three parts long-fibered Sphagnum to one part perlite ^[7]. See the genus page for the lowland and highland groups, and terrariums and humidity if your air is dry.

The ICPS seed guide illustrates its sowing method with N. rafflesiana: seed sprouting on chopped live Sphagnum, one-year-old seedlings in live Sphagnum, and a three-year-old plant raised from seed. Once seedlings have pitchers large enough, the guide suggests trying a piece of dried bloodworm in one or two pitchers first ^[20]. See sowing seeds and feeding carnivorous plants. Climbing stems can be cut up for stem cuttings.