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Environment

Protozoan-Killing Plants

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Genlisea, with its delicate yellow and violet flowers and tiny green leaves, looks a bit like a snapdragon. But it's a puzzling plant. It lives on nutrient-poor soils where other plants would starve.

Darwin was the first to suggest that Genlisea was carnivorous in 1875. Since it had no roots, little chlorophyll, and long, hollow subterranean leaves lined with microscopic slits, carnivory seemed the plant's only option. The problem was that no one knew what Genlisea ate.

Wilhelm Barthlott, a botanist at the University of Bonn, has recently discovered that the plant's strange subterranean leaves consume perhaps thousands of protozoans daily. Genlisea is the only protozoan-eating plant known.

Barthlott realized that the holes on Genlisea's underground foliage were roughly the same size as protozoans and would make excellent traps. "So we exposed the plants to solutions with protozoans, and the protozoans were attracted like magnets to the holes," he says. Genlisea's leaves, which spread out like tendrils across a wide area, send out chemical lures for their prey. Once the protozoans detect the chemicals, they move directly into the holes and disappear.

To show that the plants ate the protozoans, Barthlott tagged some protozoans with a radioisotope and fed them to Genlisea. Within two days he detected the radioactivity in the plant's cells. "Each leaf has a hollow stomachlike structure. They digest the protozoans chemically," says Barthlott. Barthlott believes Genlisea evolved from a plant similar to the butterwort--a simple insectivore that catches bugs with its sticky leaves. Over the past 20 million years, Genlisea has developed a structure and a diet perfectly suited for moist, nutrient-poor environments. "It's really an evolutionary sideshow. It's overspecialized," says Barthlott.

All 18 known species live either in pure quartz sand in South American rain forests or along the base of rock outcrops in central Africa. Protozoans are one of the few substantial sources of nutrients in these areas. By tapping into this resource, the plant lives free of competitors. Barthlott's next goal is to figure out how the plants digest their food and what chemicals make up their protozoan perfume: "There is still so much we don't know."

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