The term carnivorous plant often conjures up images from Sci-fi movies of some unsuspecting explorer deep in a remote jungle being snatched up by a monstrous, man-eating plant, never to be seen again (at least in that movie). While such depictions might be helpful in selling movie tickets, they place an unfortunate, sensationalistic stigma on what is a very interesting group of plants with some rather remarkable adaptations that aid in their survival.
To be classified as carnivorous, a plant must attract, capture, kill and digest its prey for the purpose of using its nutrients in its metabolic processes. The process of digestion suggests the secretion of enzymes (proteases, ribonucleaces, phosphatases). Some plants such as the bromeliads benefit nutritionally from the decomposition of animal life in their vases or cups. However, since they do not secrete enzymes to aid in the decomposition process they are not considered to be carnivorous.
Although widespread in nature, carnivorous plants are very rare. Plant scientists estimate there are between 250,000 and 400,000 species of flowering plants on earth, yet only about 200 have been found that are carnivorous. Most inhabit areas such as bogs where the soil is thin and nutrient availability very poor. Water and sunlight are plentiful, however. Adaptations that allow these plants to derive their nutrients by means other than from the soil are obvious benefits to their survival.
Carnivorous plants employ a number of different techniques to capture their prey. These include traps that are pitfall, snap (or mechanical), flypaper, bladder, or lobster cage in nature or action. Plants with pitfall traps utilize rolled leaves whose margins are sealed to form a vessel (trap) filled with liquid to capture their prey. Slippery hairs, that point downward into the vessel, line its interior surface that is coated with wax. Insects are attracted to the leaves by nectar-secreting glands located along the rim. Upon landing, insects slip into the vessel and are unable to escape. As they attempt to climb out, their feet become coated with wax making them heavy and cumbersome. Soon the victim slips into a watery grave.
The pitcher plant (Heliamphora sp.) is a good example of a carnivorous plant with this type of trapping mechanism. The liquid in the pitfall trap contains digestive enzymes secreted by the plant to digest its prey once it is entrapped. Pitcher plants are native to areas with high amounts of rainfall, which posses an interesting problem relative to the trap overflowing. The latter would allow the prey to escape. To counter this, pitcher plants have a small gap in their rolled and zipped leaves that acts much like the overflow of a bathroom sink.
Plants with snap mechanisms use rapid leaf movement to snare their prey. The Venus flytrap (Dionaea muscipula) is an excellent example of a plant with such a mechanism. This unusual plant catches insects by means of two wing-like projections at the ends of its leaves.
These trap-like leaf parts are brightly colored on the inside to attract unsuspecting insects and have teeth-like projections along their edges, which helps to hold the prey as it closes. As the insect lands on these traps and moves into them, small hairs trigger them to snap together within a fraction of a second. Interestingly, traps contains only six trigger hairs each, two of which must be contacted simultaneously by the prey in order for the trap to close. Plant scientists still are uncertain what causes them to close so quickly.
One might wonder how the plant knows whether or not it has captured anything worth consuming when the trap snaps shut. The answer once again involves the six trigger hairs. Trapped prey will continue to thrash about inside the trap further stimulating the trigger hairs. This prompts the plant to close the trap even tighter and to begin the digestion process. If an inanimate object falls into the trap and triggers it to close, the trigger hairs will not be further stimulated and the trap will reopen within about 12 hours.
Some plants such as the sundews (Drosera sp.) secrete sticky, glue-like substances that act like flypaper to trap their prey. Leaves of this genus range from slender and elongate to broad and spatulate in shape. Each leaf is covered with numerous tubular hairs that contain glands on their ends, which secrete a sticky, glue-like substance that has the appearance of dew as it sparkles in the sun and attracts insects. Once entrapped by the sticky dew, the prey is pulled close to the surface of the leaf where digestive enzymes aid in absorbing nutrients from it.
Bladders are trapping mechanisms found exclusively on plants belonging to the genus Utricularia, which contains the bladderworts. Many of the latter are aquatic in nature although there are terrestrial species. The bladders on bladderworts have small openings equipped with a hinged door. They physically suck in prey by creating a vacuum within the bladder whenever prey (usually aquatic organisms since the bladders are submerged) activate a trigger mechanism.
Some plants, such as the corkscrew plants (Genlisea), possess chambers that are easy for prey to enter, but hard to leave. Such plants are classified as having lobster cage trapping mechanisms and are most often aquatic in nature. Corkscrew plants contain a Y-shaped modified leaf with inward-pointing hair. Prey (aquatic protozoa) enter the leaf through a spiral structure that wraps around the two arms of the Y. Eventually, the prey is forced to move toward the bottom (leg) of the Y, which posses a stomach-like appendage that digests the prey.
Growing carnivorous plants is an interesting but rewarding challenge for the plant enthusiast. Venus flytrap, pitcher plant and sundew are some of the more common types grown indoors. Since these are bog plants in nature, they need constant moisture and high humidity. A closed terrarium provides the best growing environment for them in the average home. They also need an acid, organic growing medium such as sphagnum peat or sphagnum moss. A mixture of two parts sphagnum peat, two parts sphagnum moss and one part clean sand is a suitable growing medium for many of the carnivorous plants.
The need these plants have for acid conditions precludes the use of hard, alkaline water. Collected rain water, de-ionized or distilled water should be used exclusively. Fortunately, in a closed terrarium carnivorous plants will require little additional water once established. Fertilization also should be kept to a minimum. Feeding them an occasional insect is helpful; a few small insects per month should suffice. Avoid the temptation of using meat (e.g., hamburger) instead of insects. If catching insects is not to your liking, then fertilize them with a nutrient solution one-fourth the recommended concentration about every two months during the spring and summer only.
Carnivorous plants need bright light for optimum growth and color development. Setting a closed terrarium in a sunny window is likely to cause overheating in which case the terrarium must be opened to prevent temperatures from building up. A more practical solution to the need for bright light might include using fluorescent artificial light positioned as close to the terrarium as possible without touching it. Fluorescent lights do not emit significant amounts of heat, but will provide the plants with adequate amounts of radiant energy for growth and development.
In spite of the fact carnivorous plants consume insects, certain species of the latter can be problematic. Watch for scale, mealybug, aphid, thrips and spider mite. If present, hand removal or isopropyl alcohol applied with a cotton swab are advised since carnivorous plants are quite sensitive to most pesticides.
In closing, perhaps we should address the scenario described in the opening paragraph of this article and question whether or not there are carnivorous plants that pose a danger to humans. A relative of the pitcher plant belonging to the genus Nepenthes has the distinction of being the largest known carnivorous plant. It possesses vessel-like traps 12 inches in length on vines that might be 50 feet long. Although its traps have been known to devour frogs and an occasional small rodent, the greatest danger they pose to humans is from tripping over them as one hikes through the rainforests of Southeast Asia, their native habitat.
REVISED: September 30, 2015