Types of Carnivorous Plants
The Venus Flytrap, Dionaea muscipula, is probably the favorite of all Carnivorous Plants. The traps consist of two clamshell-like halves, lined with many stout guard hairs and minute nectar glands. In their resting position the traps are held open at 45°-60°. A healthy plant may have from 3 to 12 or more traps. The interior surface of each leaf is filled with microscopic digestive glands, giving the leaf surface a fine bumpy appearance. Each inner leaf half usually has three small delicate trigger hairs in a triangular pattern. Most of the time, for the trap to close, any one of these trigger hairs must be touched twice, or any two hairs touched one after the other. In very warm temperatures one touch may be enough. The longer the period between the two stimuli the slower the closure. The best closing response seems to happen when both stimuli occur within 40 seconds. Each trap will close about 10 times before it will no longer respond.
The time delay for trap closure seems to have some survival advantage. A single stroke caused by a brisk breeze or raindrop will not make the plant spend unnecessary closure energy on non-food. The double stroke will often allow prey to enter the center of the trap and be better positioned for capture. If the trap captures non-organic matter or misses its prey, it will open usually within 24 hours. A trap may take 3 to 5 days to digest completely entrapped prey.
The initial step of trap closure is generally quite fast. The trap seems to slam shut like a bear trap and lock the small prey in a barred tomb. The slower, secondary step of closure seals the tomb as the trap margins press tightly together. The trap becomes a flattened, stomach-like pouch. The struggling motion of the prey and the release of amino acids increase digestive activity. Total digestion takes about 3 to 5 days depending upon the size and nutrient content of the prey. Afterwards, the trap will reopen and the dried insect remains are released. Larger prey may be digested in several closures. Very small prey can escape through the tiny spacing between the outer guard hairs and save the plant from inefficient digestion. The complete sealing of the trap and the onset of digestion require the constant stimulation of live prey. An inanimate piece of cheese or hamburger, though organic, does not wiggle and will result in the reopening of the trap, leaving this "meal" undigested.
The exact mechanism of trap closure is not understood completely. Observation has shown that a stimulated trigger hair releases a small action potential or electrical current across the leaf. By itself, this single current is generally too weak to cause trap closure. However, a second stimulus soon enough after will increase the current to a level sufficient to close the trap. Closure actually appears to be from the rapid growth in the outer surface of the cells lining the trap exterior. This is probably the fastest cell growth known.
The trap does not close on a hinge like a bear trap. The trap acts more like a spring door. An open trap has a concave outer surface and a convex inner surface. During closure, the rapid growth of the cells on the exterior causes these surface features to reverse. The trap quickly closes and the guard hairs snare the prey.
Venus Flytraps grow in the wild in a very limited area of the Cape Fear River Basin, near Wilmington, NC. They grow in moist, sandy, acid savannahs among the grasses, sedges and native orchids. Venus Flytraps will tolerate short periods of drought and flooding. They are among the first plants to grow back after a fire. In fact, a rapid surface fire in the autumn can actually benefit Venus Flytraps, by burning out competing shrubs and trees. The Venus Flytrap is rather hardy climatically. It can be grown outdoors in more northern latitudes, although in some areas it may require protective winter cover by mulching.
The origin of the name Venus Flytrap is obscure. It was first described botanically in 1768 by John Ellis in England. Many believe "Venus" either refers to its beautiful flower (Venus, the goddess of love and beauty) or its alien-like feeding habit. An alternative explanation of the use of the term "venus" is that John Solander, who was the first to scientifically describe the plant, was preoccupied with his expedition to chronicle the transit of Venus. This was a major international, scientifically significant event. Solander had little time to fully dedicate studies of the Venus flytrap, and hastily jotted short notes about the plant. It was known in colonial days as "tipitiwitchet." "Aphrodite’s Mousetrap" is another one of its names. Some locals call them "meadow clams." All these names hint at the plant’s curious nature.
Active traps can also be found in bladderworts (Utricularia). These plants are either aquatic or terrestrial. Aquatic varieties are often found as mats of plants floating in quiet, acid ponds and bogs. The terrestrial forms are found growing in damp, sandy, acid soils. Bladderworts are often overlooked and only noticed when they are in flower. These rootless plants form branching feathery whorls. The bulbous traps are generally quite small, less than an 1/8 inch and are attached to the fine branches. The trap contains a small opening with a hinged lid surrounded by many branched plant hairs. While at rest, fluid inside the trap is slowly absorbed, resulting in decreased water pressure inside the trap. When a small prey brushes against one of the sensitive trigger hairs, an electrical action potential opens the trap door and the prey is "sucked" into the trap. Digestion takes several days. Bladderworts can ingest a large number of mosquito larva and are ecologically valuable.
The Water Wheel, Aldrovanda vesiculosa, is another Carnivorous Plant with active traps. It is less known and difficult to obtain. It grows much like the bladderworts. It consists of a rootless stem with leaves arranged like spokes on a wheel. Each whorl consists of 8 leaves about a 1/2 inch long with traps at the tips that closely resemble the Venus Flytrap. There are about 40 trigger hairs. It grows near the surface of freshwater, acid swamps in tropical and temperate regions. Its range extends from Europe and Africa to Japan and Australia.
There are several carnivorous fungi with active traps. Arthrobotrys obligiospora, Dactyllela bebbicoides and Pleurotus ostreatus have mycelium that form lariat-like nooses which can entrap small worms or nematodes. Another fungus, Zoophagus insidians, has hyphae that quickly swell and trap the small biting victim. The common Oyster Mushroom, Pleurotus ostriatus, is also carnivorous.
Carnivorous Plants with semi-active traps include the sundews (Drosera) and butterworts (Pinguicula). These generally have numerous sticky glands covering the upper leaf surfaces acting like flypaper. After capture, the leaf margins may roll over to cover the prey.
The sundews are truly beautiful Carnivorous Plants. Their green and red leaves are covered with glistening droplets of mucilage that sparkle in the sunlight. They are generally perennial herbaceous plants. Germination by seed is easy. Leaf shape ranges from circular to linear and any combination in between. The leaves are flat and have numerous tentacles or stalked glands on their upper surfaces. There are two types of stalked glands. Those on the leaf margins are longer and help entrap the prey. Those near the center of the leaf are short and also help trap prey, but mainly they secrete digestive fluids. Neighboring tentacles and those in contact with the prey all move towards the victim. Movement of these tentacles results when cells on the far side of the stalk grow faster, thus bending the stalk towards the prey. An auxin (plant hormone) is involved in this cell growth.
There are almost a 100 species of sundews. They are usually quite abundant in appropriate habitats. In some areas the ground is carpeted with these beautiful, red, glistening plants. Some species form winter buds or hibernacula in autumn.
The butterworts form a rosette with stalkless leaves. The thin glistening leaves are elongated, narrow and generally flat with curled leaf edges. The inner surface is covered with tiny glands and feels greasy to the touch. Their genus name Pinguicula comes from the Latin word pinguis, meaning fat. The plant is usually a pale yellow-green. The butterworts are generally inconspicuous except during the flowering season, and they resemble other plants growing in the same area. Butterworts capture their prey using the flypaper-like method. Small insects become ensnared in the mucilage covered leaves. The margins slowly curl over the animal in about two hours, after which digestion takes place.
There is a parasitic fungus with sticky passive traps, Endocahleus asteroides. The fungus grows very sticky branches that can break off and attach to a passing host. There it slowly grows into its victim, absorbing nutrients and eventually killing it. The fungus then grows more of the sticky branches and waits for another victim to pass by.
There is an interesting variety of passive trap Carnivorous Plants including: pitcher plants, the Cobra Lily, Nepenthes, even the seeds of the Shepherd’s Purse and some Bromeliads.
Pitcher plants have prominent, decorative trap leaves that are basically tubular and usually topped with a hood. Pitcher plants have evolved several interesting methods to trap prey. Bright coloring and the secretion of nectar lure prey to the pitcher opening. The nectar is often intoxicating and disorients the insect. The slick upper inner lining of the tube causes most insects to fall into the trap. There, stiff downward pointing hairs allow the prey to make a one-way journey deeper into the ever narrowing tube. In some species digestive glands secret enzymes which digest the prey. In other species bacterial action breaks down the victim. In either case the prey is digested and its nutrients are absorbed. The foul smell of the decomposing prey may be helpful in luring other prey.
The digestive fluids do not digest all creatures. Some protozoa and insect larvae can survive and even breed within the traps. The inside of a living pitcher plant can be a complex little ecosystem of algae, fungi, bacteria, protozoa, and various resistant insect larva, including some species of mosquitoes, flies and moths.
Sarracenia purpurea is the provincial flower of Newfoundland, Canada.
There are a large number of pitcher plants and they are found in nearly every carnivorous habitat. Darlingtonia californica, the Cobra Lily, is restricted in range to northern California and southern Oregon. Cephalotus follicularis, or the Western Australian Pitcher Plant is found in a very restricted area of southwestern Australia. This is a very interesting little pitcher plant with small traps looking like slippers with teeth. The Heliamphora or Sun Pitchers of the high cool plateaus or Tepuis of Venezuela are a more primitive, but quite lovely pitcher plants. The Nepenthes, Hanging Pitcher Plants or Monkey Pitcher Plants are orchid-like pitchers native to Southeast Asia.
The seeds of Shepherd’s Purse, Capsella bursa-pastoris, are carnivorous even though the plant itself is not. The seeds secrete chemicals which seem to attract a large variety of small invertebrates. The seeds release toxins which kill the prey and then enzymes which digest them. This creates a nutrient rich area around the seed which gives the emerging plant an advantage over its competitors.
Another passive trap carnivore is Genlisea. This is a submerged aquatic plant that blossoms above water. The plant has flat leaves and a dangling trap, often described as resembling a spiraling lobster trap. Insects entering the trap can only go one way, further into the plant, where they are digested. Genlisea is less than an inch long.
There are several South American bromeliads which have recently been shown to have a carnivorous habit. Brocchinia reducta and B. hechtioides are terrestrial species growing in poor soils. Their vase-like rosette of leaves functions in a similar fashion to the pitcher plants. Other related carnivorous species include Catopsis berteroniana and Paepalanthus bromelioides.