CARNIVOROUS PLANTS

by: Dr. Thomas Ombrello Biology Department Union County College Cranford, New Jersey

Capable of capturing and digesting insects and other very small animals, the carnivorous (also called insectivorous) plants form a peculiar assemblage of the world's flora. There are at least 450 species of carnivorous plants belonging to 6 botanical families. Habitats range from dry, almost desert conditions to standing or slowly f lowing water. Altitudes extend from near sea level to about 9,000 feet. Some carnivorous plants are rootless and free floating, others are rooted in soil, and a few are woody shrubs.

While legends abound of man-eating plants of tremendous size, none of these have been substantiated. Carnivorous plants do not pose any threat to man, but we are threatening their existence. Many species are on the verge of extinction due to widespread collecting and habitat destruction. Over-collecting and landfill operations on swampy areas are endangering several species in the United States. As a protective measure, the Endangered Species Act of 1973 has restricted the interstate commercial use of most carnivorous plants, and in many states you can be fined for removing them from their native habitat.

Carnivorous plants have evolved some amazing adaptations in order to thrive in the environments in which they are found. There are still many unanswered questions as to why these plants developed the ability to derive nutrients from captured animals. One hypothesis is that the plants supplement their mineral-poor intake from the soil with the organic compounds from digested animals. For example, it is well known that many of the carnivorous plants grow best in swamps or bogs where available nitrogen is scarce. This does not appear to be the complete explanation, however, since it has been well documented that carnivorous plants can grow indefinitely in their native habitats without capturing any animals at all.

The following are brief descriptions of the major -groups of carnivorous plants:

Pitcher Plants (Sarracenia spp.)

The Sarracenia pitcher plants consist of 7 or 8 species and a considerable number of subspecies and hybrids. Native to the bogs and swamps of Eastern North America, they have hollow, tubular leaves which form urn, or trumpet-shaped receptacles in which insects are captured. The upper portion of the pitcher, which itself may measure up to a foot in length, is colored red and purple, resembling a bright flower. The colors plus the nectar the plant produces attract insects to an opening at the top with a very slippery lip. When insects crawl over the lip, they slide into the receptacle which contains water. It is difficult if not impossible for the insects to crawl out because the side walls of the pitcher are lined with stiff, sharp hairs, all pointing downwards. After the insects have exhausted themselves in repeated attempts to climb out, they fall into the liquid where enzymes secreted by the plant digest the soft parts of the animals. Older leaves can become partially filled with the exoskeletons of insects. The digestive process may also be aided by the bacteria that reside in the pitcher. Ants form a large part of the pitcher plant's prey although larger victims such as flies, wasps, crickets and even spiders are occasionally caught.

Sundews (Drosera spp.)

Sundews have practically a worldwide distribution of over 90 species. The few that grow in North America are distributed from Alaska to northern California and in the Great Plains region as well as on the Atlantic coastal plain.

The characteristic form of American sundews is a rosette of leaves up to 2 inches in diameter and often much smaller. When growing in strong light these little plants usually take on a rich red color.

Each leaf has an elliptical or almost circular blade supported by a slender leaf stalk. The blade is supplied with numerous hairs or tentacles, each of which has a large gland at its outer end. Each gland produces a droplet of mucilage which has the appearance of a small dew drop. This can be seen even in hot dry weather. When a small insect comes in contact with the tentacle, various glands produce enzymes, an acid, an antiseptic which inhibits bacterial action, and more mucilage. Within a few seconds the tentacles begin to bend toward the animal and gradually press it firmly into the secreted mixture. After 24-48 hours, digestion of the soft tissue is complete. The tentacles extend and the hard parts of the insect blow away or are washed off by rain. The leaf is then ready for another juicy victim.

Venus' Flytrap (Dionaea muscipula)

The Venus' flytrap is probably the most remarkable of all the carnivorous plants because of its intricate trapping mechanism. This rare plant grows on the coastal plain of North and South Carolina and is found nowhere else in the world.

The plant grows from an underground rhizome ("bulb") and appears as a rosette of leaves up to 6 inches in diameter. The leaf stalk is broad and flat and carries out most of the plant's photosynthesis. The leaf blade, on the other hand, acts as the trap. Every blade is hinged in the middle and each of the two halves bears glands for secreting nectar and 3 sensitive trigger hairs. If an insect happens to touch 2 of these hairs or perhaps brushes against a hair twice as it walks across an open trap, the leaf halves fold together and the spines which line the margins of the blade fold together like interlocking fingers in less than half a second. While the insect is held tight in this fashion, enzymes, formic acid, and an antiseptic are secreted and the organic materials of the animal are digested and absorbed by the plant. After a few days the leaf reopens and the insect's exoskeleton is blown or washed away. A healthy leaf which closes without making a catch is likely to open within a few hours.

Butterworts (Pinguicula spp.)

Butterworts are distributed throughout the Northern Hemisphere, and about 30 species have been identified. They are found in temperate or cool temperate regions on wet soils.

These plants take the form of rosettes of yellow-green leaves measuring up to 2 inches or more in diameter. The leaves lie close to the ground and have upturned edges. Numerous glands in the leaf blade secrete a mucilage-like substance in which small insects become trapped This secretion, along with digestive enzymes and an antiseptic increase in amount as the insect struggles to free itself from the leaf. At about the same time, the margins of the leaf infold, producing a shallow cup in which the victim disintegrates. The antiseptic properties of the butterwort's secretions are so effective that many alpine peoples use the leaves to treat open wounds.

Cobra Plant (Darlingtonia spp.)

Growing in the wet meadows and marshes of western Oregon and northern California, the cobra plant (cobra lily) gets its common names from the fang-like projections from the hood of its modified leaves that resemble the pitchers of pitcher plants.

Insects are attracted to the slit-like opening on the side of the pitcher by bright colors and nectar. Once the insect enters the darkened chamber, it has difficulty in retracting its steps, for the entrance is screened from the outside light by the overhanging hood. Furthermore, the insect is distracted by the light shining through thin, transparent places in the roof which resemble skylights. After repeated attempts to escape, the insect falls exhausted into the liquids of the receptacle where it is eventually digested through the activities of bacteria. Any attempts to scale the walls of the receptacle are repelled by extremely slippery places and by numerous down-curved hairs. The prey found in these pitchers are usually ants, beetles, and flies with an occasional small frog or toad.

Bladderworts (Utricularia spp.)

With an almost worldwide distribution, most bladderworts are aquatic but a few are terrestrial in moist locations. Species car be found floating free in quiet or slow-moving fresh water over most of eastern North America. Small insects and other animals are trapped in the special bladder- like modified leaves of this frail, intricately branched plant. These tiny bladder-like traps are furnished with valve-like lids that open inwards when their pressure sensitive hairs are triggered by contact. Any very small animal that makes contact with the hairs is sucked into the trap due to the negative pressure that builds up behind the lid. Once trapped inside the bladder, digestion soon takes place. The trap is reset after a few days and is ready for another victim.

RESOURCES

Tippo, 0. and W.L. Stern. 1977. Humanistic Botany. Norton Co. New York. 605 pp.

Carolina Biological Supply Co., 1965, 1966. Carolina Tips. Burlington, N.C.