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The strange world of the carnivorous plant

Actual botany is not like this. Rank Organisation/Allied Artists

Ever since their discovery, carnivorous plants have fascinated scientists and spurred the imagination of artists, writers and filmmakers. While the Puya chilensis cactus at the Royal Horticultural Society Garden in Wisley, Surrey, that recently blossomed for the first time in 15 years has been breathlessly reported as a “sheep-eating” cactus (they are merely entangled in its velcro-like spines), the reality of carnivorous plants is often considerably stranger than fiction.

Triffids and monster plants exist only in novels and horror movies, but real carnivorous plants have evolved an impressive arsenal of tricks and traps that are absolutely deadly to insects, and sometimes even small vertebrates. Using these, carnivorous plants can survive in places where nutrients are scarce and other plants struggle to thrive. Their high diversity (more than 600 species in five different plant orders) and global distribution from Arctic tundra to equatorial rainforests testifies to the success of their evolutionary niche.

The startling methods they use to trap their prey range from the millimetre-sized suction traps of bladderworts, to those with pitchers - leaves shaped like hollow containers filled with liquid to drown and digest prey - such as the Sarracenia pitchers, to the archetypal Venus fly trap.

Some species employ sophisticated trapping mechanisms, including some of the fastest known movements in plants. Others simply wait for prey to get caught on sticky secretions or slip into pitfall traps. But recent scientific discoveries have shown that there is more to those “simple” mechanisms than one might think.

The Venus flytrap - a fast mover.

The interior of many Asian Nepenthes pitchers is lined with a conspicuous crystalline wax coating that renders it extremely slippery for insects. Scanning electron micrographs have revealed a dense array of minute, upright wax platelets that create an extremely fine surface roughness that minimises the available contact area for the adhesive pads of insects’ feet. Insect pads do not stick well to rough surfaces – much like Sellotape does not stick well to a sandpaper surface - and they slip.

An ant collects sweet nectar from the glands of the N. rafflesiana pitcher plant, positioned so that the ants stand dangerously close to the mouth of the trap. Ulrike Bauer

Likewise, the roughness of the waxy inside of the pitcher wall is too fine for insects’ claws to grab hold, and the waxy crystals are attached to brittle stalks that break off easily, making it even harder for the insect to find a footing. Largely discovered in the first half of the 20th century, this has been widely accepted as the modus operandi of Nepenthes pitcher traps.

But in early 2003, a young German PhD student was caught in a tropical downpour while observing a trail of ants running across a Nepenthes bicalcarata pitcher in the peat swamp forests of Brunei, North Borneo. The ants were collecting nectar from the collar-like upper rim of the pitcher, known as the peristome - a structure thought to purely serve prey attraction. As soon as the rain started, the ants suddenly started to slip on the peristome, and dozens of them ended as prey in the pitcher. Unlike most other plant surfaces, the peristome is highly wettable. Water droplets spread across the surface and form a continuous thin film on which insects hydroplane like a car tyre on a wet road. The effect is so striking that it has inspired engineers to design artificial anti-adhesive surfaces based on it.

Danger: slippery when wet. Source: Holger F. Bohn.

The past ten years have seen an unprecedented boom of scientific interest in pitcher plants, leading to the description of a staggering 38 new species and the discovery of further astonishing trapping mechanisms.

One of the strangest strategies has been described for Nepenthes gracilis, which uses the impact of falling rain drops to catapult prey into the trap, or captures insects seeking shelter from the elements under the pitcher lid. Other species have given up carnivory entirely and instead engage in a mutual relationship with tree shrews or mountain rats.

Caught short: a tree shrew snacks at a Nepenthes lowii and relieves himself - providing a nitrogen-rich meal for the plant. Ulrike Bauer

Here, in these so-called “tree shrew lavatories”, the plant offers sugary secretions to the mammal which in turn leaves its excrements in the pitcher, fertilising the plant with precious nitrogen. Similarly, Nepenthes hemsleyana benefits from the faeces of tiny woolly bats (Kerivoula hardwickii) that roost inside the pitchers.

Maybe the most remarkable case of mutualism, however, is found in Nepenthes bicalcarata: the pitchers of this bizarre plant are inhabited by a specially adapted ant species (Camponotus schmitzi) which can not only negotiate the wet peristome with ease but also swim and dive in the pitcher fluid, hunting for aquatic mosquito and fly larvae.

Nepenthes bicalcarata, the only known example of a carnivorous plant inside which a species of ant exclusively lives and hunts for food, residing in leaves specially adapted for them. The ants even drag large prey items out from the pitcher to eat (see inset). Ulrike Bauer

By killing the larvae that feed from the same nutrient bounty that would feed the plant, the ants reduce the loss of nutrients to the plants. The ants also clean the peristome which keeps it slippery, and protect the developing pitcher buds by aggressively attacking particular herbivorous weevils trying to dine on them. The plant, in turn, offers nesting space for the ants in specialised hollow tendrils.

The biology of most of the 140 currently described Nepenthes species is still virtually unstudied, with many more astonishing secrets likely to be discovered. Sadly, the rapid progress of land development poses a serious threat to many lowland species as their natural habitats get converted into plantations, settlements and industrial parks. Mountain species are less threatened by habitat loss but instead suffer from poaching for local horticultural markets, and from a warming climate from which there is no escape.

Their incredibly sophisticated adaptations have enabled pitcher plants to survive in some of the most hostile and nutrient-deficient environments such as poisonous soils, acidic peat bogs and even sheer rock faces. But in the face of bulldozers and man-made forest fires, they are fully at our mercy.

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