Wednesday, June 17, 2009
The deadly traps laid by carnivorous plants
The usual order of events in nature is that plants manufacture their own food through photosynthesis, while animals either eat plants or other animals. But more than 500 plant species are exceptions to this order: they eat animals. All grow in soil or water that contains little or no nitrogen, an element vital for plant growth, and animals provide that missing nourishment.
Pitcher plants, for example, a family of climbing vines found throughout the tropics of the Old World, grow pitcher- or urn-shaped traps, some large enough to capture a rat. Although they occasionally digest small mammals or reptiles, they are designed for eating insects, which enter the trap under the illusion that it is a flower, attracted by its scent or by a supply of false nectar. Once over the pitcher's rim, they slither helplessly down to the bottom, where downward-pointing spikes prevent escape. The plant then secretes an acid and digestive enzymes to break down the body of its victim. The pitcher plant's distinctive lid serves as a lure to flying insects and as an umbrella to prevent the pitcher from filling up with rainwater. Some species, however, do let rainwater enter their pitchers, thus drowning their prey, instead of killing it with acid.
In mechanical terms, the most sophisticated traps of all are those of the bladderworts, a successful and varied group established throughout the world. They are mainly water plants, which, depending on their size, feed on anything from single-celled protozoans to small fish. Among their branching underwater leaves grow tiny bladders, each fitted with an inward-opening trap door. Sticking out from the entrance are hairs, and any creature brushing against these make the trap fly open. As water rushes in to fill the bladder, it sucks the hapless animal in with it. The bladder then uses special glands to extrqact the water, allowing the door to close again, and leaving the animal trapped.