A Plant That Moves Like None Other

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Slender pitcher plant oozing nectar. Credit: Ulrike Bauer

Slender pitcher plant oozing nectar. Credit: Ulrike Bauer

If you wish to watch a plant move, you’d need to set your camera to time-lapse mode. Whether you gaze at plants on window sills arching towards sunlight or sunflowers tracking the path of the sun with their heads, these movements are too slow for human eyes to perceive.

Some plant movements are so rapid, you need to watch these in slow motion. Seed pods explode shooting seeds into the air. Others discharge spores and pollen. These are usually one-time events. Once a pod has exploded, releasing its pent-up energy, it can’t be put back together.

However, researchers have discovered one species of plant that can move fast again and again. The slender pitcher plant uses the energy of falling raindrops to move faster than we can see. Since it doesn’t use its own energy, it can move for as long as the downpour lasts. No other plant in the known botanical world has this ability.

Pitcher plants trap insects, but they don’t usually move. They grow in habitats so poor that they get nutrients such as nitrogen and phosphorus by consuming insects. To catch them, some leaves undergo a makeover to become hollow pitchers. They produce nectar to attract prey and a viscous fluid inside the pitchers to drown and digest their quarry.

On dry days, ants, beetles, and flies scurry around safely, drinking nectar and inviting their mates to join. However, when the collar-like rim around the mouth of the pitcher gets wet, insects slide without traction like car tyres on slick wet roads. The creatures fall into the pitcher. The waxy surface of the pitcher’s inner wall prevents them from escaping. A flap-like growth acts as a lid that appears to prevent the pitcher from flooding during rain.

The slender pitcher plant, a species found throughout Southeast Asia, has another trick up its pitcher. During a tropical rainstorm in Brunei Darussalam, in the island of Borneo, Ulrike Bauer of the University of Bristol, U.K., observed a beetle seeking shelter under its lid. The insect lost its footing and ended up in the pitcher. On sunny days, however, Bauer had observed ants crawling under the lid without difficulty. She wondered if rain played a role in helping the plant trap the beetle.

Crematogaster ants – ants swarming over a slender pitcher plant. Credit: Ulrike Bauer

Crematogaster ants – ants swarming over a slender pitcher plant. Credit: Ulrike Bauer

Bauer and her team collected 12 slender pitcher plants and brought them to a field laboratory. The researchers simulated rain using intravenous drip system used in hospitals, releasing water drops from a height of 40 cm. They rigged high speed cameras and sensitive laser equipment to observe the plant’s behaviour.

When raindrops drum on the lid, it flaps up and down on its flexible hinge like a springboard. The free end of the lid vibrates so fast, it cannot be seen with the naked eye. Such rapid movement causes ants to lose their footing and fall into the pitcher below.

Numerous species of pitcher plants throughout tropical Southeast Asia get buffeted by tropical rainstorms. The slender pitcher plant appears to be the only one to take advantage of the situation. While the pitchers are the primary traps in other species, this unique species has turned the lid appendage into a weapon.

The lids of slender pitcher plants are stiffer, smaller, and lighter than other species, so they vibrate faster. On closer examination, Bauer found the underside of the lid was coated with highly specialised wax crystals. None of the other pitcher plants have lids with a slippery coating.

“We thought it would be very slippery for insects, just like other wax crystal coatings in plants that reduce the grip of insects,” says Bauer. “So we wondered if ants could walk on it. We were very surprised that they could walk without difficulty, even upside down. So we scratched our heads. We thought what if the wax only makes the lid a little bit more slippery, so that insects fall off more easily when the lid is shaken by wind or rain.”

To test how crucial the pitcher lid was to capturing insects, the researchers neutralised it. First, they painted it over with liquid silicon rubber that cures to form a smooth, rubbery surface. Insect feet can easily find purchase on it and the plant couldn’t catch as many insects.

In another trial, the researchers used a soft cloth and carefully wiped the lid clean of its wax layer. This too favoured insects and handicapped the plant.

Although the waxy veneer on the lid resembles the lining of the pitcher’s inner wall, it is structurally different. It allows insects to walk when the lid is still, but when the lid is pummelled by rain, they can’t get a grip on it.

Armed with the lid’s treacherous springboard-like vibration and slippery surface, the slender pitcher plant lures its prey by producing 3.4 times more nectar than other pitcher plants. Typically, pitcher plants ooze nectar outside the pitcher, above and below the lid, and along the inner margin of the pitcher’s mouth. But the slender pitcher plant exudes most of its nectar under the lid. It offers a free buffet on dry days, winning the trust of unsuspecting insects.

When numerous insects seek refuge from the rain, the unique lid becomes a trap, shaking off as many as 40% of the insects swarming around the underside of the lid into the pitcher below.

The Venus flytrap, another carnivorous plant, snaps shut when an insect gets caught in its tentacles. Bauer says, “It’s 10 times slower than the slender pitcher plant’s lid vibration.” The Venus flytrap uses different water pressures in two layers of tissue to open and close its traps. Pumping water from one tissue to another takes time and effort, restricting the number of times the plant can activate its traps.

Since the slender pitcher plant’s lid movement is externally driven, the plant isn’t limited by energy or time. It can move rapidly as long as it rains.

“It’s always a great buzz when you realise you discovered something new,” says Bauer. “There were a number of ‘eureka’ moments in the lead up to this paper. One such moment was when we discovered the lid is so perfectly adapted to the trapping purpose that it only responds in a very specific way to raindrop impacts. Our measurements showed that the material properties of the lid ensure a large effective trapping area, and minimum loss of [raindrop] energy. It was around that time when we realised we really had discovered a new kind of plant movement.”

“We used to think of carnivorous plant traps as either ‘active,’ movement-employing, or ‘passive,’ motionless traps,” Bauer says. “Now there is a trap that moves quickly but passively. It does not fit in with the conventional categories.”

The study was published on October 5, 2015, in the Proceedings of the National Academy of Science.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.

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