Carnivorous Plants

Plants are supposed to be food, not eaters. Yet in bogs, swamps and wet rock faces around the world grow hundreds of species that catch, kill and digest animals, from gnats to the occasional frog. They are not monsters, and none could trouble a human, but they are a striking example of how far evolution will go when the ordinary rules fail.

Why eat animals?

Carnivorous plants almost all live in the same kind of place: wet, sunny and desperately poor in nutrients. Bogs are acid and waterlogged; the bacteria that normally recycle dead material cannot work in them, so nitrogen and phosphorus, which every plant needs to build proteins and DNA, are nearly absent. Sunlight is plentiful, so the plants can make sugar, but they cannot grow without the minerals. Insects, however, are packed with them. A plant that could catch an insect and dissolve it would have a supply of fertiliser that its neighbours lacked. Over millions of years, several unrelated groups of plants did exactly that, evolving traps from ordinary leaves. The same solution arose independently at least six times, which suggests the pressure to find it was strong.

Pitfall traps

The simplest trap is a pit. Pitcher plants grow leaves rolled into tall cups part filled with liquid. The rim is coloured and glazed with nectar to lure insects, and either slippery with wax or lined with downward-pointing hairs, so a visitor that leans in to feed loses its footing and slides into the pool below. There it drowns, and enzymes secreted by the plant, sometimes helped by bacteria living in the fluid, dissolve its soft parts for the leaf walls to absorb. The pitchers of the tropical genus Nepenthes hang from vines in the forests of South East Asia; the largest, on the mountains of Borneo, hold over a litre and have been found digesting rats and shrews. Some species have evolved past hunting: one Bornean pitcher serves as a toilet for tree shrews, which lick nectar from its lid and defecate into the bowl.

Flypaper

Sundews cover their leaves with hundreds of stalked glands, each tipped with a glistening droplet of glue. The droplets look like dew and attract insects seeking water. Once an insect touches one it is stuck, and the more it struggles the more droplets it contacts. Over minutes to hours the leaf slowly curls around the prey, pressing it against more glands, and digestive enzymes go to work. Butterworts do the same with flat, greasy leaves, trapping tiny flies and even pollen.

Snap traps

The Venus flytrap, which grows wild only in a small area of the Carolinas in the United States, is the most famous carnivorous plant and one of only two with a true snap trap. Each leaf ends in two lobes hinged along the middle, fringed with teeth and carrying a few stiff trigger hairs inside. An insect walking across the lobe brushes a hair. One touch does nothing; the plant waits. A second touch within about twenty seconds, which rules out a falling raindrop or a speck of dust, fires the trap. The lobes change shape from convex to concave and snap together in about a tenth of a second, the teeth interlocking into a cage. If the prey keeps struggling, the trap seals tight and floods with digestive fluid; after a week or so it reopens, the husk blows away, and the trap is ready again, though each one can only close a few times before it dies. The plant counts touches and keeps a short memory, with no nerves or brain of any kind.

Suction

The fastest hunters of all are nearly invisible. Bladderworts float in ponds or creep through wet soil, carrying hundreds of tiny bladders, each a millimetre or two across. The plant pumps water out of each bladder until its walls cave inward under suction, and a trapdoor holds the vacuum. When a water flea or mosquito larva touches the trigger hairs by the door, it opens, water rushes in and the prey with it, and the door shuts again. The whole event takes less than a millisecond, faster than any animal's strike.

Why it matters

Carnivorous plants show that the division between plants and animals is a matter of habit, not law. Given the right pressures, a leaf can become a stomach and a plant can count. They also show how fragile specialised life can be. Their bogs and wetlands are among the fastest-disappearing habitats in the world, drained for farming and peat, and a great many species, including the Venus flytrap in the wild, are now threatened. The plants that learned to eat animals are being eaten, in their turn, by the loss of the only places they can live.