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Stranger than fiction: Nature’s flesh-eating plants

Posted in Nature, Plants on Monday, 28 November 2011

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This edited article about plants originally appeared in Look and Learn issue number 864 published on  5 August  1978.

Carnivorous plants, picture, image, illustration

A group of canivorous plants by Theo Carreras

Plants make their own food from air and sunshine, the chlorophyll which is present in their leaves, and a plentiful supply of water and minerals from the soil. But what of the plants that catch animals and use them as food?

Carnivorous plants, as they are called, normally have green leaves with which they make their food, but often they grow in bogs and marshes or in poor soils from which it is difficult for them to get enough minerals to live on. When this is the case, the plants trap animals and use them as a source of food.

All carnivorous plants possess a bait and a trap. Obviously a plant can never go in search of prey in the way that a fox or a shrew can. The carnivorous plant makes the prey come to it.

It attracts and lures animals to itself by colour, smell or sweet nectar. When the animal has been attracted by this bait, it is caught by the plant’s trap.

The usual prey is small animals – ants, flies, spiders, fleas – but one or two plants can catch much larger prey; as big as mice or small birds.

But apart from possessing a bait and a trap, the carnivorous plant must be able to do three things, all of which are vital to it once its prey is caught. First of all, it must dissolve solid animals; it must absorb the dissolved matter into itself by digesting it; and finally, it must re-set the trap.

Once the animal is caught, the plant pours digestive juices over it, which changes the animal’s soft parts into liquid to form a kind of soup, leaving only the bony skeleton undigested. The soup is then absorbed into the plant, to become food which helps the plant to survive.

But it is the carnivorous plant’s trap that is the most fascinating thing to study. There are four main kinds. One, the sticky-surface trap entangles its victim in the sticky heads of the leaf tentacles. The victim, having landed on the leaf, is firmly caught and tentacles on one side immediately bend over it. Not all sticky-surface traps have tentacles, some have a leaf surface covered with gum.

Then there is the jug-of-water trap, which is quite a large, jug-like container, called a pitcher, of water. The fly is lured to the edge of the pitcher and if it falls in, the steep, slippery surface inside will prevent it from crawling back out again. A pitcher is part of the leaf and is attached to its tip by a long tendril.

Probably the most famous of all plant traps is the snap trap. The unsuspecting victim licking nectar on the surface of the open trap is taken unawares when it accidentally touches some of the trigger hairs inside. As soon as these are touched, the trap will snap shut with lightning speed and the victim is caught behind the interlocking spiked edges of the leaf.

Finally, there is the suction trap, which has a door that opens when a victim touches it. The victim is quickly sucked in and the door closes.

If you want to watch a sticky-surface trap in action, try to find a butterwort in your nearest hilly countryside. It has a small purple flower on its central stem. When an animal – a spider or an ant – crawls on to a leaf of the butterwort, the sticky gum will cling to its legs and body. The victim will soon begin to struggle.

The more it struggles, the more firmly it will become stuck, because its struggling action sets in motion small glands on the leaf surface, allowing them to produce more gum and more digestive juices. The curled edges of the leaf will then slowly turn upwards so that, as more juices are poured out, they carry the animal towards the middle of the leaf.

The jug-of-water or pitcher trap is a deadly, effective form of trap which can range in size from tiny pitchers that can catch only small insects, to enormous ones that can trap lizards and even frogs. One of the most interesting is the pitcher of the cobra plant found in America. This plant has no other leaves, just groups of pitchers. The victim crawls up the outside of the pitcher, happily licking away the nectar, blissfully unaware that when it reaches the end of its journey, it will find itself walking through a little hole into the top dome of the pitcher.

Light shines through the transparent, non-green patches of the dome, and the animal cannot find the one hole through which it has entered – and through which it can escape. Eventually, it slips down over the rough hairy surface and drowns in the fluid at the bottom of the pitcher.

The smallest plant traps are some tiny balloon-shaped sacks called bladders which are found on bladderworts. These plants grow all over the world, and the best known of them are the water bladderworts. These have fine, hair-like green leaves and bladders on their trailing stems. Aquatic animals trying to escape from their pursuers often swim among these leaves, only to find themselves trying to escape again. For round the entrance to each bladder there are bunches of bristles. As soon as they are touched, the bristles cause the door to open and swing quickly inwards. The unsuspecting victim is sucked in with the rush of water, and then the door swings back and closes. Inside the trap, digestive juices are poured from the glands in the walls and in a day or two the victim will be dissolved. The trap walls then absorb both the digested animal and the water.

No study of plant traps would be complete without mention of the Venus flytrap. This small plant is no more than 150 mm across and grows naturally only in a few bogs and swamps of North America, but the lightning speed of its snap trap has given it world-wide fame.

Each plant has a circle of small leaves with a trap held out at the end of each one.

If you look closely at the illustration, you will see the magnified leaves of a Venus flytrap with its three stiff bristles on each half leaf. The bristles are the most sensitive part of the leaf and as soon as they are touched the trap is triggered.

A fly can walk safely on the outer, green edge, sipping the nectar which is made there, but as soon as it wanders on to the red-tinged area and stumbles against the bristles, it is doomed. The trap swings shut in about a quarter of a second.

After the trap has snapped round a suitable victim, the sides of the closed trap start to press firmly together. This squashes the trapped animal, and within half-an-hour the soft animal will be dead and ready for digestion.

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