The following is an excerpt from an article by Carl Zimmer, entitled “Fatal Attraction” that comes from the March 2010 issue of National Geographic.
“A hungry fly darts through the pines in North Carolina. Drawn by what seems like the scent of a 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 a sweet nectar, the leaf unleashes enzymes that eat 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.
There is something wonderfully unsettling about a plant hat 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 accidence, he reasoned, and once a hapless bug stopped struggling, the plan 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 Origins of Species wrote, ‘I care more about Drosera than the origin of all 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.
Today biologists using 21st century tolls 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, belives 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 rapidly flip in shape from convex to concave, like a soft contact lens. As the leaves flip, they snap together, trapping an insect inside.
(Aaron Ellison is a senior ecologist at the Harvard Forest in central Massachusetts and Nicholas Gotelli is an ecologist from teh University of Vermont) Ellison and Gotelli are trying to figure out what evolutionary forces pushes 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-harvestingenzymes. 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.”
