Carnivorous Plants

Carnivorous Plants

Plants that trap and digest tiny animals have fascinated people for centuries. It was known by 1790 that sundews, pitcher plants, and the Venus's-flytrap could catch insects. This interest led Thomas Jefferson to collect Venus'sflytraps near Charleston, South Carolina, for study. A century later, Charles Darwin referred to the Venus's-flytrap as one of the most wonderful plants in the world. More recently, certain adventurous, twentieth-century Hollywood movies depicted man-eating plants as inhabiting mysterious tropical jungles. Carnivorous plants, in fact, are relatively small and do not live in dark swamps and jungles, and the largest animal ever found trapped in one of the plants was a small rat. Carnivorous plants catch mostly insects, and hence are often referred to as insectivorous plants.

Carnivorous plants are defined as plants that attract, catch, digest, and absorb the body juices of animal prey (referred to as the carnivorous syndrome). The major types of carnivorous plants are sundews, pitcher plants, butterworts, bladderworts, and the unique Venus's-flytrap. More than 150 different types of insects have been identified as victims, but also arachnids (spiders and mites), mollusks (snails and slugs), earthworms, and small vertebrates (small fish, amphibians, reptiles, rodents, and birds) are known to have been caught.

Many different kinds of plants have insect-attracting structures such as colorful leaves and flower parts and produce sweet sugar secretions (like nectar). Others may ensnare and kill small animals using sticky hairs, thorns, cupped leaves, poisonous liquids, or a combination of these tactics. In some cases it is known that the juices of dead animals can be absorbed through the surfaces of plant leaves. However, only true carnivorous plants have the ability to obtain nutrients from animal prey.

It is known that carnivorous plants can survive without catching prey. However, botanists believe that the added nutrition derived from carnivory helps the plants grow faster and produce more seeds, thus allowing the plants to survive better and spread into new areas. In general, carnivorous plants grow in poor soils where nitrogen, phosphorus, and potassium are lacking.

MAJOR CARNIVOROUS PLANT GROUPS
Genus	Common Name	Number of Species (approximate)	Geographical Distribution
Sarracenia	Trumpet pitcher plant	10	Southeastern United States, with one species extending across Canada
Darlingtonia	California pitcher plant	1	Northern California and adjacent Oregon
Heliamphora	South American pitcher plant	5	Venezuela, Guyana, Brazil
Nepenthes	Tropical pitcher plant	75	Southeast Asian tropics, from Australia, Malaysia, and India to Madagascar
Cephalotus	Australian pitcher plant	1	Western Australia
Drosera	Sundew	110	Worldwide, especially South Africa and Australia
Dionaea	Venus's-flytrap	1	Southeastern North Carolina and adjacent South Carolina
Pinguicula	Butterwort	60	Mostly Northern Hemisphere
Utricularia	Bladderwort	200	Worldwide

They obtain these nutrients, especially nitrogen and phosphorus, from their prey, and they are quickly absorbed through the leaf surface and transported throughout the plant. Although carnivorous plants do absorb nutrients from a weak fertilizer, for instance, high concentrations of fertilizer, as are suitable for garden crops and houseplants, normally kill carnivorous plants.

Habitats

There are more than 450 different species of carnivorous plants found in the world. At least some occur on every continent except Antarctica. They are especially numerous in North America, southeastern Asia, and Australia. Carnivorous plants typically live in wet habitats that are open and sunny, with nutrient-poor soils having an acidic pH . They do not like competition from other plants, and thus seem to thrive in the nutrient-poor habitats where other types of plants do not grow very well. These plants may be found in wet meadows in the southeastern United States or in peat-moss bogs in northern North America and Eurasia. Some are true aquatics, growing in the quiet waters of ponds and ditches around the world. Still others grow on wet, seeping, rocky cliffs or moist sand. In many cases they grow in places that have periodic fires that act to cut down on competition, keep their habitats open, and release nutrients into the soil.

Types of Traps

The traps of carnivorous plants are always modified leaves. They may be active or passive in their mechanism. Active traps have sensitive trigger hairs and moving parts, such as the sticky, glue-tipped hairs that cover the leaves of sundews. The paired leaf blades of the Venus's-flytrap snap shut like jaws when trigger hairs are touched inside. The aquatic bladderworts have little inflated pouches that suck in microscopic animals and mosquito larvae. The passive traps of terrestrial butterworts consist of flat leaves covered with a greasy, sticky surface that are effective at catching crawling insects much like flypaper traps flies. The elegant pitcher plants have passive pitfall traps that are hollow, tubular leaves. Insects fall in, die, and sink to the bottom to be digested. The hoods on most pitcher plants keep out rain-water and prevent prey from flying out. In many cases, especially in pitcher plants that hold water, bacteria may aid in digesting prey. It is also known that several species of mites and fly larvae live inside the trumpet leaves of pitcher plants, without themselves being harmed, and help break down prey for digestion. Pitcher plants may be terrestrial, growing in clumps of erect pitcher leaves (such as the Sarracenia pitcher plants) in the North American temperate zone, or they may grow as sprawling vines in the Malaysian tropics, with pitchers hanging from the tips of their flat leaf blades (such as the Nepenthes pitcher plants).

Microscopic glands are present on each type of trap. They are specialized cells that perform various jobs. They may act as receptors to detect the presence of prey, or they may secrete digestive fluids to dissolve the animal bodies with only the outer shell of chitin of arthropods (insects, spiders, and their relatives) remaining undigested. The glands also absorb the products of digestion, taking them into the leaves of the plant. For example, the sticky hairs of the sundew trap the insect and slowly curve over to press the victim onto the leaf surface where digestive juices are secreted and nutrients absorbed.

While a variety of carnivorous plants are scattered around the world, the area with the most numerous types is the Green Swamp Nature Preserve in southeastern North Carolina (Brunswick County). Occurring in this area are four species of Sarracenia, four species of Drosera, ten species of Utricularia, three species of Pinguicula, and the single species of Dionaea.

see also Evolution of Plants; Interactions, Plant-Insect; Peat Bogs; Wetlands.

T. Lawrence Mellichamp

Bibliography

Cheers, Gordon. A Guide to Carnivorous Plants of the World. New York: Collins Publishers, 1992.

D'Amato, Peter. The Savage Garden: Cultivating Carnivorous Plants. Berkeley, CA: TenSpeed Press, 1998.

Lloyd, Francis E. The Carnivorous Plants. First printed in 1942. New York: Dover Publications, 1976.

Schnell, Donald E. Carnivorous Plants of the United States and Canada. Winston-Salem, NC: John F. Blair Publisher, 1976.