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Fatal Attraction
National Geographic, March 2010

A hungry fly darts through the pines in North Carolina. Drawn by what seems like the scent of nectar from a flowerlike patch of scarlet on the ground, the fly lands on the fleshy pad of a ruddy leaf. It takes a sip of the sweet liquid oozing from the leaf, brushing a leg against one tiny hair on its surface, then another. Suddenly the fly's world has walls around it. The two sides of the leaf are closing against each other, spines along its edges interlocking like the teeth of a jaw trap. As the fly struggles to escape, the trap squeezes shut. Now, instead of offering sweet nectar, the leaf unleashes enzymes that eat away at the fly's innards, gradually turning them into goo. The fly has suffered the ultimate indignity for an animal: It has been killed by a plant.

The swampy pine savanna within a 90-mile radius of Wilmington, North Carolina, is the one place on the planet where Venus flytraps are native. It is also home to a number of other species of carnivorous plants, less famous and more widespread but no less bizarre. You can find pitcher plants with leaves like champagne flutes, into which insects (and sometimes larger animals) lose themselves and die. Sundews envelop their victims in an embrace of sticky tentacles. In ponds and streams grow bladderworts, which slurp up their prey like underwater vacuum cleaners.

There is something wonderfully unsettling about a plant that feasts on animals. Perhaps it is the way it shatters all expectation. Carl Linnaeus, the great 18th-century Swedish naturalist who devised our system for ordering life, rebelled at the idea. For Venus flytraps to actually eat insects, he declared, would go "against the order of nature as willed by God." The plants only catch insects by accident, he reasoned, and once a hapless bug stopped struggling, the plant would surely open its leaves and let it go free.

Charles Darwin knew better, and the topsy-turvy ways of carnivorous plants enthralled him. In 1860, soon after he encountered his first carnivorous plant--the sundew Drosera--on an English heath, the author of Origin of Species wrote, "I care more about Drosera than the origin of all the species in the world." He spent months running experiments on the plants. He dropped flies on their leaves and watched them slowly fold their sticky tentacles over their prey. He excited them with bits of raw meat and egg yolk. He marveled how the weight of just a human hair was enough to initiate a response. "It appears to me that hardly any more remarkable fact than this has been observed in the vegetable kingdom," he wrote. Yet sundews ignored water drops, even those falling from a great height. To react to the false alarm of a rain shower, he reasoned, would obviously be a "great evil" to the plant. This was no accident. This was adaptation.

Darwin expanded his studies from sundews to other species, eventually recording his observations and experiments in 1875 in a book, Insectivorous Plants. He marveled at the exquisite quickness and power of the Venus flytrap, a plant he called "one of the most wonderful in the world." He showed that when a leaf snapped shut, it formed itself into "a temporary cup or stomach," secreting enzymes that could dissolve the prey. He noted that a leaf took more than a week to reopen after closing and reasoned that the interlocking spines along the margin of the leaf allowed undersized insects to escape, saving the plant the expense of digesting an insufficient meal. Darwin likened the hair-trigger speed of the Venus trap's movement--it snaps shut in about a tenth of a second--to the muscle contraction of animals. But plants don't have muscles and nerves. So how could they react like animals?

Today biologists using 21st-century tools to study cells and DNA are beginning to understand how these plants hunt, eat, and digest--and how these bizarre adaptations arose in the first place. After years of study, Alexander Vol­kov, a plant physiologist at Oakwood University in Alabama, believes he has figured out the Venus flytrap's secret. "This," Volkov declares, "is an electrical plant."

When an insect brushes against a hair on the leaf of a Venus flytrap, the bending triggers a tiny electric charge. The charge builds up inside the tissue of the leaf but is not enough to stimulate the snap, which keeps the Venus flytrap from reacting to false alarms like raindrops. A moving insect, however, is likely to brush a second hair, adding enough charge to trigger the leaf to close.

Volkov's experiments reveal that the charge travels down fluid-filled tunnels in a leaf, which opens up pores in cell membranes. Water surges from the cells on the inside of the leaf to those on the outside, causing the leaf to rapidly flip in shape from convex to concave, like a soft contact lens. As the leaves flip, they snap together, trapping an insect inside.

The bladderwort has an equally sophisticated way of setting its underwater trap. It pumps water out of tiny bladders, lowering the pressure inside. When a water flea or some other small creature swims past, it bends trigger hairs on the bladder, causing a flap to open. The low pressure sucks water in, carrying the animal along with it. In one five-hundredth of a second, the door swings shut again. The cells in the bladder then begin to pump water out again, creating a new vacuum.

Many other species of carnivorous plants act like living flypaper, snagging animals on sticky tentacles. Pitcher plants use yet another strategy, growing long tube-shaped leaves into which insects fall. Some of the largest have pitchers up to a foot deep and can consume a whole frog or even a rat unlucky enough to fall into them. Sophisticated chemistry helps make the pitcher a death trap. Nepenthes rafflesiana, a pitcher plant that grows in jungles on Borneo, produces nectar that both lures insects and forms a slick surface on which they can't get a grip. Insects that land on the rim of the pitcher hydroplane on the liquid and tumble in. The digestive fluid in which they fall has very different properties. Rather than being slippery, it's gooey. If a fly tries to lift its leg up into the air to escape, the fluid holds on tenaciously, like a rubber band.

Many carnivorous plants have special glands that secrete enzymes powerful enough to penetrate the hard exoskeleton of insects so they can absorb nutrients from inside their prey. But the purple pitcher plant, which lives in bogs and infertile sandy soils in much of North America, enlists other organisms to digest its food. It is home to an intricate food web of mosquito larvae, midges, protozoans, and bacteria, many of which can survive only in this unique habitat. The animals shred the prey that fall into the pitcher, and the smaller organisms feed on the debris. Finally, the pitcher plant absorbs the nutrients released by the feeding frenzy. "Having the animals creates a processing chain that speeds up all the reactions," says Nicholas Gotelli of the University of Vermont. "And then the plant dumps oxygen back into the pitcher for the insects. It's a tight feedback loop."

Pitcher plants grow by the thousands in the bogs at the Harvard Forest in central Massachusetts. One late spring day Aaron Ellison took me on a tour, stopping from time to time to watch patiently as I pulled a sinking leg out of the muck. "You haven't had a real bog experience till you're up to your crotch in it," said Ellison, a senior ecologist at the forest. Little orange flags fluttered across the bogs. Each one marked a pitcher plant impressed into the service of science. In the distance a student was feeding flies to the flagged plants. The researchers raise these insects on food spiked with unusual forms of carbon and nitrogen so they can later harvest the pitcher plants and measure how much of each element from the flies has been absorbed into the plants. Because pitcher plants grow slowly (they can live for decades), the experiments can take years to yield results.

Ellison and Gotelli are trying to figure out what evolutionary forces pushed these plants toward a taste for meat. Carnivorous plants clearly benefit from eating animals; when the scientists feed pitcher plants extra bugs, the plants get bigger. But the benefits of eating flesh are not the ones you might expect. Carnivorous animals like ourselves use the carbon in protein and the fat in meat to build muscles and store energy. Carnivorous plants instead draw nitrogen, phosphorus, and other critical nutrients from their prey in order to build light-harvesting enzymes. Eating animals, in other words, lets carnivorous plants do what all plants do: grow by grabbing energy directly from the sun.

Alas, they do a lousy job of it. Carnivorous plants turn out to be very inefficient at converting sunlight into tissue. That's because they have to use a lot of energy to make the equipment they need to catch animals--the enzymes, the pumps, the sticky tentacles, and so on. A pitcher or a flytrap cannot carry out much photosynthesis because, unlike plants with ordinary leaves, they do not have flat solar panels that can grab lots of sunlight. Ellison and Gotelli suspect that only under special conditions do the benefits of carnivory outweigh the costs. The poor soil of bogs, for example, offers little nitrogen and phosphorus, so carnivorous plants enjoy an advantage there over plants that obtain these nutrients by more conventional means. Bogs are also flooded with sunshine, so even an inefficient carnivorous plant can carry out enough photosynthesis to survive. "They're stuck, and they're making the best of it," says Ellison.

Evolution has repeatedly made this trade-off. By comparing the DNA of carnivorous plants with other species, scientists have found that they evolved independently on at least six separate occasions. Some carnivorous plants that look nearly identical turn out to be distantly related. Both kinds of pitcher plants--the tropical genus Nepenthes and the North American Sarracenia--grow deep pitcher-shaped leaves and employ the same strategy for capturing prey. Yet they evolved from different ancestors.
In several cases scientists can see how complex carnivorous plants evolved from simpler ones. Venus flytraps, for example, share an ancestor with Portuguese sundews, which only make passive flypaper glands on their stems. They share a more recent ancestor with Drosera sundews, which not only make flypaper glands but can also curl their leaves over their prey. Venus flytraps appear to have evolved an even more elaborate version of this kind of trap, complete with jawlike leaves.

Unfortunately, the adaptations that enable carnivorous plants to thrive in marginal habitats also make them exquisitely sensitive to environmental changes. Agricultural runoff and pollution from power plants are adding extra nitrogen to many bogs in North America. Carnivorous plants are so finely tuned to low levels of nitrogen that this extra fertilizer is overloading their systems. "They eventually burn themselves out," says Ellison.

Humans also threaten carnivorous plants in other ways. The black market trade in exotic carnivorous plants is so vigorous now that botanists are keeping the location of some rare species a secret. Venus flytraps are being poached from North Carolina by the thousands to be sold at roadside stands. The North Carolina Department of Agriculture has been dabbing wild Venus flytraps with harmless dye that's normally invisible but glows in UV light so that inspectors who come across Venus flytraps for sale can quickly determine if the plants were raised in a greenhouse or poached from the wild. But even if the poaching of carnivorous plants can be halted (a very big if), they will continue to suffer from other assaults. Their habitat is disappearing, to be replaced by shopping centers and houses. Fires are being suppressed, allowing other plants to grow quickly and outcompete the Venus flytraps. Good news, perhaps, for flies. But a loss for all who delight in the sheer inventiveness of evolution. 

Copyright 2010 Carl Zimmer
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