Danger​— Killer Plants!

By Awake! writer in Britain

IT IS commonly known that animals feed on plants. But did you know that some plants feed on animals? There are some 550 identified species of these carnivorous, or insectivorous, plants, and more are being discovered. These extraordinary plants can photosynthesize, but those that grow in poor soil lack essential nutrients, such as nitrates. Insects provide important food supplements for these hungry plants.

Each plant has its own method of snaring prey. Some have traps or trapdoors, while others lure their victims into slippery pitfalls or onto sticky pads from which escape is impossible. Let us take a closer look at carnivorous plants.

Traps and Trapdoors

Probably the best-known carnivorous plant is the Venus flytrap, which grows to a height of about 12 inches [30 cm]. Found in peat bogs in the Carolinas of North America, the Venus flytrap has shiny, brightly colored leaves that are edged with nectar-producing glands​—a temptation that is difficult for insects to resist! Therein lies the danger, for the Venus flytrap contains three trigger hairs in the middle of each leaf lobe. When an unwary creature touches them, the leaves close. Like the jaws of a steel trap, spines around the edge of the leaves interlock to prevent the prey from escaping.

If the trap closes on something that was blown into it by the wind​—a piece of dry twig, for example—​the plant will release it in a couple of days. But when the sensory organs on the leaf detect the presence of nitrogenous substances, enzymes are released to break down the captured insect’s body so that the plant can absorb the nutrients. This process takes from 10 to 35 days, depending on the size of the prey.

Interestingly, if just one trigger hair is disturbed, perhaps by a raindrop, the trap will not close. It is only when two or more trigger hairs are touched​—even after an interval of up to 20 seconds—​that the plant reacts. The speed at which the trap closes is dependent on temperature and sunlight. In certain conditions it closes at a speed of a thirtieth of a second.

The snares of some plants move even faster than that. Consider the bladderwort, primarily an aquatic plant, with underwater leaves. The leaves contain numerous bladders, each of which is equipped with a trapdoor and several long hairs. When a small creature, such as a water flea, touches the hairs, the trapdoor opens. Because the water pressure inside the bladder is lower than that outside, the tiny victim is sucked in and the trapdoor closes behind it. This can happen in as little as a thirty-fifth of a second!

Slippery Pitfalls

Among the largest of the carnivorous plants are the hanging pitcher plants. Some, such as those found in Southeast Asia, are vines that grow to the tops of trees. These contain traps that can hold up to two quarts [2 L] of liquid to catch creatures as large as frogs. Some are even reputed to have caught rats. But how do the traps work?

Each leaf of the pitcher plant resembles a jug, or pitcher, and has a lid to keep out the rain. An insect is lured by the bright color and abundant nectar of the pitcher, but the rim has a slippery surface. When the insect attempts to get the nectar, it loses its footing and slides into the pool of fluid at the bottom. Downward-pointing hairs inside the leaf prevent the insect from escaping. In addition, the nectar of some pitcher plants contains a narcotic that stupefies the prey.

The pitcher plant with the most striking appearance is surely the cobra plant, which grows in the mountains of California and Oregon in the United States. The pitcher closely resembles a cobra rearing up to strike. An insect finds its way into the  mouth of the plant but then becomes confused by light that seems to be shining through small windows. The insect keeps flying toward the light in a futile attempt to escape. Exhausted, it falls into the liquid at the base of the plant and drowns.

Domestic and Commercial Value

Butterworts have sticky leaves that attract the sciarid fly and the whitefly. These insects are pests affecting both commercial greenhouse plants and domestic houseplants. Man-made flytraps are effective but indiscriminate, as they also entrap bees and hover flies. The butterworts’ stickiness is superior. It holds only the tiny pests.

 North American pitcher plants are now gaining popularity with gardeners. Their beautiful flowers and elegant leaf shapes compare favorably with other plants, and they are easy to cultivate. They also consume flies in enormous numbers. Indeed, each clump of leaves can trap thousands of flies in a season. Bees are not at risk, as it appears that they are not attracted by the plants. But how are the flowers fertilized without trapping the insects that pollinate them? The flowers appear first, while the pitchers are still growing. By the time the pitchers come to maturity, the flowers have died and the pollinators have moved on.

An easy-to-grow plant, which can tolerate a wide range of temperatures, is Australia’s fork leaf sundew. “It is an ideal plant to have if mosquitoes prevent you from sitting outside in the evening,” says Chris Heath, carnivorous plant specialist at the Walworth Garden Farm Environmental Education Centre in London. “Plant it in a hanging basket where its shiny droplets of sticky fluid will attract mosquitoes.” Any that brush against a leaf are held fast by its sticky hairs, which curve inward and press the insect against the surface of the leaf.

Preserving Predator Plants

Sad to say, many carnivorous plants grow in habitats that are being destroyed by man. For example, the hanging pitcher plant of Southeast Asia is endangered because of slash-and-burn agriculture. Wetlands in other areas are being drained for development. Some species have already become extinct. *

Would you like to grow a carnivorous plant? There is no need to take them from the wild, for many can readily be obtained from suppliers who propagate them from seed or from tissue cultures. Instructions for growing them are simple: Keep the plant wet at all times with rainwater. Also, carnivorous plants thrive in sunshine, but species from temperate zones are best kept cool during the winter season. Patience is required, as it may take up to three years for some plants to reach maturity. At least there is no need to supply them with food. After all, they find their own!

[Footnote]

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▪ Fungi Food

The tiniest carnivorous plants are fungi that snare microscopic soil-dwelling nematode worms. Some of these fungi have sticky knobs on stalks, but others have three-celled nooses only a thousandth of an inch [2.5 cm] thick, which tighten around any nematode worm that unwittingly crawls through them. As soon as the worm is trapped, it is invaded by fungus filaments, and death quickly follows. These fungi are being studied for their pest control potential, since nematodes cause hundreds of millions of dollars of crop damage every year.

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▪ Insects Fight Back!

Not all insects succumb to the wiles of insectivorous plants. The blowfly, for example, has an appendage on each foot. Like a climber’s grappling iron, these enable the insect to clamber up the down-curved hairs of the pitcher plant. When the blowfly’s eggs hatch, the maggots feed on the decaying insects. Then, as soon as they are ready to pupate, they cut holes in the pitcher and escape. Caterpillars of a small moth, in their turn, cover offending pitcher-plant hairs with a web. Some spiders craftily spin their webs in the upper part of pitcher plants so as to have first call on any inquisitive insects, and at least one kind of spider has a special skin that allows it to seek shelter in the plant’s digestive juices if it is threatened.

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Hanging pitcher-plant leaf

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Venus flytrap

Leaf on left shows a trapped fly; leaf on right shows trigger hairs

[Credit Line]

Plants: Copyright Chris Heath, Kentish Town City Farm, London

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Flower and immature pitcher leaf of cobra plant

[Credit Line]

Copyright Chris Heath, Kentish Town City Farm, London

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North American pitcher plant

Its flower is about the size of an orange

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Butterwort

Sciarid flies and whiteflies are trapped on its sticky leaves

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Leaves of the North American pitcher plant

Inset: A fly feeds on nectar that causes it to become drowsy

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Fork leaf sundew

An insect is trapped by the leaf’s sticky hairs

[Credit Line]

Plants: Copyright Chris Heath, Kentish Town City Farm, London