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article imageCarnivorous pitcher plant uses raindrops to eat ants

By Megan Hamilton     Oct 7, 2015 in Environment
One species of pitcher plant has a sneaky secret. Falling raindrops provide it with enough power to fling insects to their doom, biologists have discovered.
The raindrops trigger rapid vibrations in the lid of the plant's jug-shaped leaves.
This catapults the ants off of the lid and into the pitcher trap below, where the creatures drown and are consumed by digestive juices, BBC News reports.
The researchers used high-speed cameras and laser vibration measurements, and their findings were published in the journal PNAS. In using the instruments, Dr. Ulrike Bauer and her colleagues from the University of Bristol recorded extremely rapid movement in the lid of the pitcher leaf after it was struck by a raindrop.
The raindrop caused it to wobble like a stiff spring, she noted.
"You have a raindrop hitting the surface and that causes it to move down, fast. Then because of this spring property, it moves to a certain point and springs back."
"You get an oscillation, very similar to when you put a ruler on the edge of your desk and flick the end down with your finger," she said.
This pitcher plant species, Nepenthes gracilis is native to Asia, and it's uniquely equipped to trap its prey using raindrops. The lid of the plant is lined with slippery wax crystals and it vibrates much more quickly than that of a related species. These two features appear to be crucial to the plant's prey-trapping abilities, Science News reports. When the researchers removed the wax coating or attached a slick N. gracilis lid beneath that of another pitcher plant species, few, if any ants fell to their deaths when the scientists used simulated rainfall.
In one way or another, all plants move, but their movements are very slow and can't really be observed without time-lapse photography, according to a press release from the University of Bristol. Sunflowers turn to follow the path of the sun, and potted plants in a window turn their leaves towards the light. Plants also have tiny pores on their leaves that open and close constantly so that the plants may breathe.
Some plants, however, can react with lightning speed. A Venus flytrap snaps shut on unsuspecting insects in a mere fraction of a second. Other plants utilize catapult mechanisms that shoot seeds, spores or pollen into the air. In some extreme cases, these are launched at almost half the speed of sound.
The pitcher plant doesn't have to bother with all that. It exploits the energy of falling raindrops to drive its trapping movement. It gets the movement for free, in other words. And this newly discovered pitcher plant movement is too fast to be seen with the naked eye. It's actually an order of magnitude faster than the aforementioned flytrap.
Whoa.
"Having a fast movement in a plant is unusual in itself, but having a fast movement that doesn't require the plant to invest any energy - it just requires it to build the structure - that's something quite surprising," Bauer said, per BBC News.
The plant's uniqueness places it in its own carnivorous category; it doesn't belong with "active" carnivorous plants, such as flytraps, and it also doesn't belong with motionless "passive" insect eaters - the vast majority that are made up of other pitcher plants.
It's that stiff upper lid that's the key to the pitcher's rain-powered trap. The team studied another species--one that primarily catches ants using only the slippery rim of its pitcher, and they found it had a more flexible lid.
That flexibility means that vibrations from falling raindrops were concentrated at the tip; similar to the motion of a springboard used in competitive diving, Bauer said.
"It concentrates the acceleration at the very tip. If you try to jump off the middle of a diving board, it's not very effective," she said. "That's why divers walk to the very edge."
Concentrating the action at the very tip isn't so great for the pitcher plant if it's going to fling ants into the depths below.
"The pitcher wants to maximize the area where insects fall from that surface," Bauer noted. The stiff leaves of N. gracilis are perfect for this, because it spreads out the movement, making things much more difficult for its prey.
Worldwide there are three families of pitcher plants, Encyclopedia.com reports. Insects are lured inside the plant by nectar and sometimes bright coloration, and once inside, they are prevented from leaving by sharp bristles. The trapped insects eventually drown and are digested by plant enzymes and possibly bacteria collected in rainwater.
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