Carnivorous plants may be subdivided into 2 major groups; those with passive traps and those with active traps. For some of these traps the actual method of insect decomposition involves digestive enzymes produced by the plant and bacterial decay within the trap. A classic passive trap is the "pitfall trap" of pitcher plants, including Darlingtonia and Sarracenia of the Sarraceniaceae, and Nepenthes of the Nepenthaceae, where an insect falls into a vase-like modified leaf. Downward-pointing hairs on the slippery walls prevent the insect from crawling out, and the hapless victim ultimately drowns in a pool of digestive enzymes at the bottom. Other well-known passive traps are the "flypaper" or adhesive traps of sundews (Drosera, Droseraceae) and butterworts (Pinguicula, Lentibulariaceae). In both of these unrelated genera, the leaves are covered with sticky, gland-tipped hairs (Drosera) or a sticky (viscid) layer of mucilage (Pinguicula) which entangle the hopeless, struggling victim.

In active traps a rapid plant movement takes place as an integral part of the trapping process. Probably the best known active trap is the Venus' flytrap (Dionaea muscipula, Droseraceae), one of the most astonishing plants in the world. A relative of the sundews (Drosera), this remarkable species belongs to the Sundew Family (Droseraceae). Its native habitat in all the world is a narrow strip of coastal land approximately 10 miles (16 km) wide and 100 miles (160 km) long in North Carolina and adjacent South Carolina. Its generic name is a modification of Dione, the Greek name for Venus. When triggered by an insect, the leaf blade folds closed along its midrib bringing the two halves together. Three bristle-like hairs near the middle of the upper side of the leaf blade are sensitive to touch and cause the blade to snap shut. Touching one hair will not trigger the closing mechanism. Only when one hair is touched twice or two hairs are touched in succession will the leaf blade fold closed. This strategy generally prevents an inanimate object (such as pebbles or small sticks) from activating the trap. A fringe of stiff hairs around the edge of the blade become interlocked (intermeshed) when the blade folds closed, thus trapping the insect like bars in a jail cell. The action of this remarkable mechanism involves a rapid loss of turgor pressure within the leaf cells on the upper side of the leaf. Digestive enzymes from glands on the leaf surface break down the proteins of the imprisoned victim, and the plant gets a supplemental source of nitrogen.

But why would some insectivorous plants need an additional supply of nitrogen, particularly when they are living in organically-rich bogs? The answer to this question may involve the pH of the water and soil which is too acidic for nitrifying bacteria that convert ammonia from protein decay into nitrite and nitrate ions. This important bacterial process is called nitrification. The nitrite and nitrate ions made available by the bacteria are readily absorbed by the roots of plants. If the nitrification process is impaired, there could actually be a shortage of these nitrite and nitrate ions; hence, the carnivorous plants have evolved a mechanism to obtain a supplemental supply of nitrogen.

The mechanism responsible for the rapid closure of the leaf (within 1/2 second) has generally been explained as being due to rapid loss of turgor pressure in the upper epidermal cells. Recent studies indicate that the pressure loss may be in the layer of mesophyll cells underlying the upper epidermis. When these cells suddenly become flaccid, the leaf folds upward along the midrib. A decrease in ATP (adenosine triphoshate) is associated with each closure, suggesting that biochemical energy is also involved. Repeated stimulation of the trap by touching the trigger hairs too frequently will noticeably fatigue the trap. Apparently the ATP supply is exhausted, and not enough time has elapsed for sufficient ATP regeneration.

The only carnivorous plant with a true "trapdoor" is the remarkable bladderwort (Utricularia). This little submersed aquatic plant has one of nature's most precise and delicate traps, and certainly the most rapid. Thousands of minute bladders are attached to feathery submersed branchlets by tiny stalks. Some authorities consider these finely divided branchlets to be modified leaves. The flattened, pear-shaped bladders range in diameter from 2 millimeters (the size of a pinhead) to about 4 millimeters (the size of a BB). At one end is an opening and a flap of tissue which forms the door. The door hangs down from the top of the entrance like a garage door, except it opens inward. Support tissue and a mucilage coating around the door frame helps to seal the door and prevent water from entering the bladder. The door opening is surrounded by several bristly hairs that resemble the antennae of a tiny crustacean or insect. Numerous, tiny glands inside the bladder absorb most of the internal water and expel it on the outside. As a result, a partial vacuum is produced inside the bladder and the pressure on the outside becomes greater than inside. This causes the walls to squeeze inward and explains their slightly concave appearance.

The airtight door is hinged to allow easy entry; but like a door, it cannot be forced open from within. Special trigger hairs near the lower free edge of the door cause it to open. When a minute aquatic organism touches or hits one of these extremely sensitive hairs, the hair acts as a lever, multiplying the force of impact and bending or distorting the very pliable door. This breaks the watertight seal and, since the bladder contains a partial vacuum, the hapless victim is sucked in. The whole trapping process occurs within 15 to 20 milliseconds (about 1/60 of a second), roughly the speed of a daylight camera shutter setting. Bladder extracts from some species of bladderworts indicate that enzymes secreted by the plant may be involved in the digestion process.

Any discussion of carnivorous plants would be incomplete without mentioning the amazing predatory fungi that actually capture and devour they prey. These organisms are technically not plants since they belong to the Kingdom Fungi, but they are nonetheless quite remarkable. The predatory fungi belong to the Phylum (Division) Zygomycota. In some mycology books they are placed in the Class Zygomycetes. The zygomycetes include a number of microscopic fungi that attack bread, dead flies and moving animals. You have probably seen the web-like filaments and black sporangia of black bread mold, especially if you allow freshly-baked bread (without preservatives) to get moldy. Other references place these fungi in the Class Deuteromycetes (Imperfect Fungi) because their sexual cycle is not fully understood; therefore, it is difficult to place them in a definite fungal class. The visible body of these fungi consists of a mass of intricately branched filaments, collectively referred to as a mycelium. Several predatory species in the genus Dactylaria attack minute nematodes called eelworms, and another fascinating species (Dactylella tylopaga) attacks microscopic amoebas in the soil.

It is hard to imagine a filamentous fungus that actually lassos its prey, but this is the case in certain species of Dactylaria. Some of the filamentous strands of this fungus form a loop which serves as an animal trap. Minute nematodes (called eelworms) slither into the loop, hoping to eat the fungus. As the eelworm touches the fungus, the loop tightens and captures the struggling eelworm. When the victim finally dies, the fungus penetrates the eelworm body and proceeds to digest and absorb it. Exactly what triggers this lassoing mechanism has been the subject of considerable scientific speculation. It undoubtedly involves a chemical reaction between the eelworm body and the fungus. Just as researchers made attempts to fool Venus' flytraps with probes made of glass, wood or metal, they have also tried to trick the fungus. But to no avail, the fungal noose refused to tighten up. Other species of these unusual fungi catch their prey with hundreds of sticky, adhesive pads, similar to the glue-like, gland-tipped hairs and sticky leaves of sundews and butterworts.

A fascinating article about a rare and little-known member of the carnivorous Bladderwort Family (Lentibulariaceae) called Genlisea appeared in Nature Volume 392 (2 April 1998) by Wilhelm Barthlott, Stefan Porembski, Eberhard Fischer and Bjorn Gemmel. Since the time of Darwin it has been postulated that the specialized leaves of this unusual plant are traps for catching small prey, and finally after more than a century it was proven. Genlisea species are rare in the wild and occur mainly in nutrient-poor white sands and moist rock outcrops in South America and tropical Africa. They form a small rosette about 3 centimeters in diameter, with linear or spatulate leaves. The yellow or violet flowers, similar to those of the closely-related bladderwort, are borne on an inflorescence up to 20 centimeters tall. When a rosette of leaves and rhizome are dug up, pale bundles of slender, root-like organs up to 15 centimeters long are revealed. Since the plant is rootless, these organs are actually subterranean modified leaves lacking chlorophyll. Each leaf consists of a long hollow strand that divides into 2 spirally twisted arms or branches (like a long inverted Y). The whole structure (called a trapping leaf) hangs downward in the water or wet soil. Just above the fork (toward the rhizome) of each trapping leaf is a widened portion called the bulb or bladder. The arms are also hollow with an average inside diameter of 200 micrometers. At each twist along the spiral arms is a slit-like opening about 400 micrometers wide and 180 micrometers high. Protozoans (including at least 9 documented species of ciliates) are attracted to the arms by chemicals secreted by the plant. The hapless protozoans swim into the slit-like openings where they become trapped and digested by the plant. Their escape is blocked by rows of special inwardly-pointing hairs which line the slits. Glands between the rows of hairs presumably secrete digestive enzymes, and the inviting entrances are definitely a one-way trip for the protozoans. This is the first documented case of a flowering plant that actually captures and digests microscopic protozoans. It is also the first example of a carnivorous plant secreting a chemical as a lure. Thus, 125 years after Charles Darwin's initial postulations, the puzzle of Genlisea's feeding behavior is finally solved.