Sol. ex J.Ellis
The Venus flytrap (also Venus's flytrap or Venus' flytrap), Dionaea muscipula, is a carnivorous plant native to subtropical wetlands on the East Coast of the United States. It catches its prey—chiefly insects and arachnids— with a trapping structure formed by the terminal portion of each of the plant's leaves and is triggered by tiny hairs on their inner surfaces. When an insect or spider crawling along the leaves contacts a hair, the trap closes if a different hair is contacted within twenty seconds of the first strike. The requirement of redundant triggering in this mechanism serves as a safeguard against a waste of energy in trapping objects with no nutritional value.
The Venus flytrap is a small plant whose structure can be described as a rosette of four to seven leaves, which arise from a short subterranean stem that is actually a bulb-like object. Each stem reaches a maximum size of about three to ten centimeters, depending on the time of year; longer leaves with robust traps are usually formed after flowering. Flytraps that have more than 7 leaves are colonies formed by rosettes that have divided beneath the ground.
The leaf blade is divided into two regions: a flat, heart-shaped photosynthesis-capable petiole, and a pair of terminal lobes hinged at the midrib, forming the trap which is the true leaf. The upper surface of these lobes contains red anthocyanin pigments and its edges secrete mucilage. The lobes exhibit rapid plant movements, snapping shut when stimulated by prey. The trapping mechanism is tripped when prey contacts one of the three hair-like trichomes that are found on the upper surface of each of the lobes. The trapping mechanism is so specialized that it can distinguish between living prey and non-prey stimuli such as falling raindrops; two trigger hairs must be touched in succession within 20 seconds of each other or one hair touched twice in rapid succession, whereupon the lobes of the trap will snap shut in about one-tenth of a second. The edges of the lobes are fringed by stiff hair-like protrusions or cilia, which mesh together and prevent large prey from escaping. (These protrusions, and the trigger hairs, also known as sensitive hairs, are probably homologous with the tentacles found in this plant’s close relatives, the sundews.) Scientists have concluded that the Venus flytrap is closely related to Drosera (sundews), and that the snap trap evolved from a fly-paper trap similar to that of Drosera.
The holes in the meshwork allow small prey to escape, presumably because the benefit that would be obtained from them would be less than the cost of digesting them. If the prey is too small and escapes, the trap will reopen within 12 hours. If the prey moves around in the trap, it tightens and digestion begins more quickly.
Speed of closing can vary depending on the amount of humidity, light, size of prey, and general growing conditions. The speed with which traps close can be used as an indicator of a plant's general health. Venus flytraps are not as humidity-dependent as are some other carnivorous plants, such as Nepenthes, Cephalotus, most Heliamphora, and some Drosera.
The Venus flytrap exhibits variations in petiole shape and length and whether the leaf lies flat on the ground or extends up at an angle of about 40–60 degrees. The four major forms are: 'typica', the most common, with broad decumbent petioles; 'erecta', with leaves at a 45-degree angle; 'linearis', with narrow petioles and leaves at 45 degrees; and 'filiformis', with extremely narrow or linear petioles. Except for 'filiformis', all of these can be stages in leaf production of any plant depending on season (decumbent in summer versus short versus semi-erect in spring), length of photoperiod (long petioles in spring versus short in summer), and intensity of light (wide petioles in low light intensity versus narrow in brighter light).
When grown from seed, plants take around four to five years to reach maturity and will live for 20 to 30 years if cultivated in the right conditions.
The plant's common name refers to Venus, the Roman goddess of love. The genus name, Dionaea ("daughter of Dione"), refers to the Greek goddess Aphrodite, while the species name, muscipula is Latin for "mousetrap".
Most carnivorous plants selectively feed on specific prey. This selection is due to the available prey and the type of trap used by the organism. With the Venus flytrap, prey is limited to beetles, spiders and other crawling arthropods. In fact, the Dionaea diet is 33% ants, 30% spiders, 10% beetles, and 10% grasshoppers, with fewer than 5% flying insects. Given that Dionaea evolved from an ancestral form of Drosera (carnivorous plants that use a sticky trap instead of a snap trap) the reason for this evolutionary branching becomes clear. Whilst Drosera consume smaller, aerial insects, Dionaea consume larger terrestrial bugs. From these larger bugs, Dionaea are able to extract more nutrients. This gives Dionaea an evolutionary advantage over their ancestral sticky trap form.
Mechanism of trapping
The mechanism by which the trap snaps shut involves a complex interaction between elasticity, turgor and growth. In the open, untripped state, the lobes are convex (bent outwards), but in the closed state, the lobes are concave (forming a cavity). It is the rapid flipping of this bistable state that closes the trap, but the mechanism by which this occurs is still poorly understood. When the trigger hairs are stimulated, an action potential (mostly involving calcium ions—see calcium in biology) is generated, which propagates across the lobes and stimulates cells in the lobes and in the midrib between them. It is hypothesized that there is a threshold of ion buildup for the Venus flytrap to react to stimulation. The acid growth theory states that individual cells in the outer layers of the lobes and midrib rapidly move 1H+ (hydrogen ions) into their cell walls, lowering the pH and loosening the extracellular components, which allows them to swell rapidly by osmosis, thus elongating and changing the shape of the trap lobe. Alternatively, cells in the inner layers of the lobes and midrib may rapidly secrete other ions, allowing water to follow by osmosis, and the cells to collapse. Both of these mechanisms may play a role and have some experimental evidence to support them.
If the prey is unable to escape, it will continue to stimulate the inner surface of the lobes, and this causes a further growth response that forces the edges of the lobes together, eventually sealing the trap hermetically and forming a 'stomach' in which digestion occurs. Digestion is catalysed by enzymes secreted by glands in the lobes.
Oxidative protein modification is likely to be a predigestive mechanism of the Dionaea muscipula. Aqueous leaf extracts have been found to contain quinones such as the naphthoquinone plumbagin that couples to different NADH-dependent diaphorases to produce superoxide and hydrogen peroxide upon autoxidation. Such oxidative modification could rupture animal cell membranes. Plumbagin is known to induce apoptosis, associated with the regulation of Bcl-2 family of proteins. When the Dionaea extracts were preincubated with diaphorases and NADH in the presence of serum albumin (SA), subsequent tryptic digestion of SA was facilitated. Since the secretory glands of Droseraceae contain proteases and possibly other degradative enzymes, it may be that the presence of oxygen-activating redox cofactors function as extracellular predigestive oxidants to render membrane-bound proteins of the prey (insects) more susceptible to proteolytic attacks.
The carnivorous diet is a very specialized form of foliar feeding, and is an adaptation found in several plants from soil poor in nutrients. Their carnivorous traps were evolutionarily selected to allow these organisms to survive their harsh environments.
The "snap trap" mechanism so characteristic of Dionaea is shared with only one carnivorous plant genus, Aldrovanda. This relationship was thought to be coincidental, more precisely convergent evolution, for most of the 20th century - some phylogenetic studies even suggested that the closest living relative of Aldrovanda was the sundew. It was not until 2002 that a molecular evolutionary study conducted by Ken Cameron, from the University of Wisconsin, indicated that Dionaea and Aldrovanda did in fact share a most recent common ancestor by analysis of combined nuclear and chloroplast DNA sequences.
Another study, this time appearing in the New Phytologist, presented evidence for the evolution of snap traps of Dionaea and Aldrovanda from a flypaper trap like D. regia, based on molecular data. The molecular and physiological data implies that Dionaea and Aldrovanda snap traps evolved from the flypaper traps of a common ancestor with the Drosera; the living evidence of a link between Drosera and Dionaea is D. regia and its remnant characteristics.
In this evolutionary model, pre-adaptations to evolution into snap-traps were identified in several species of Drosera, such as rapid leaf and tentacle movement. The model proposes that plant carnivory by snap-trap evolved from the flypaper traps driven by increasing prey size. Bigger prey provides increasingly higher nutritional value, but large insects can easily escape the sticky mucilage of flypaper traps; the evolution of snap-traps would prevent escape and kleptoparasitism (theft of prey captured by the plant before it can derive any benefit from it), and would also permit a more complete digestion.
Proposed evolutionary history
Carnivorous plants are generally herbs, and their traps primary growth. They generally do not form readily fossilizable structures such as thick bark or wood. As such, there's no fossil evidence of the steps that would link Dionaea and Aldrovanda, or with their common ancestor with Drosera. Despite that, it's possible to extrapolate an evolutionary history based on phylogenetic studies of both genera. So, the researchers proposed a series of steps that would ultimately result in the complex snap trap mechanism:
- Larger insects usually walk over the plant, instead of flying to it, and are more likely to break free from sticky glands alone. Therefore, a plant with wider leaves, like Drosera falconeri, must have adapted to move the trap and its stalks in directions that maximized its chance of capturing and retaining such prey - in this particular case, longitudinally. Once adequately "wrapped", escape would be more difficult.
- Then, evolutionary pressure selected the plants with shorter response time, in a manner similar to Drosera burmannii or Drosera glanduligera. The faster the closing, less reliant on the flypaper model the plant would be.
- As the trap became more and more active, the energy demanded to "wrap" the prey increased. Therefore, plants that could somehow differentiate between actual insects and random detritus/rain droplets would be in advantage, thus explaining the specialization of inner tentacles into trigger hairs.
- Ultimately, as the plant relied more in closing around the insect rather than gluing them, the tentacles so evident in Drosera would lose its original function altogether, becoming the "teeth" and trigger hairs — an example of natural selection hijacking pre-existing structures for new functions.
- Completing the transition, at some point in its evolutionary history the plant developed the depressed digestive glands found inside the trap, rather than using the dews in the stalks, further differentiating it from the Drosera genus.
The Venus flytrap is found in nitrogen- and phosphorus-poor environments, such as bogs and wet savannahs. Small in stature and slow growing, the Venus flytrap tolerates fire well, and depends on periodic burning to suppress its competition. Fire suppression threatens its future in the wild. It survives in wet sandy and peaty soils. Although it has been successfully transplanted and grown in many locales around the world, it is found natively only in North and South Carolina in the United States, specifically within a 60-mile radius of Wilmington, North Carolina. One such place is North Carolina's Green Swamp. There also appears to be a naturalized population of Venus flytraps in northern Florida as well as an introduced population in western Washington. The nutritional poverty of the soil is the reason that the plant relies on such elaborate traps: insect prey provide the nitrogen for protein formation that the soil cannot. The Venus flytrap is not a tropical plant and can tolerate mild winters. In fact, Venus flytraps that do not go through a period of winter dormancy will weaken and die after a period of time.
Venus flytraps are popular as cultivated plants, but have a reputation for being difficult to grow. Successfully growing these specialized plants requires recreating a close approximation to the plant's natural habitat.
Healthy Venus flytraps will produce scapes of white flowers in spring; however, many growers remove the flowering stem early (2–3 inches), as flowering consumes some of the plant's energy, and reduces the rate of trap production. If healthy plants are allowed to flower, successful pollination will result in the production of dozens of small, shiny black seeds.
Plants can be propagated by seed, although seedlings take several years to mature. More commonly, they are propagated by division in spring or summer.
Venus flytraps are by far the most commonly recognized and cultivated carnivorous plant. They are sold as houseplants and are often found at florists, hardware stores and supermarkets. During the past ten years or so large quantities of cultivars (cultivated varieties) have come into the market through tissue culture of selected genetic mutations. It is through tissue culture that great quantities of plants are raised for commercial markets.
Currently, there are estimated to be more than 3–6 million plants in cultivation compared to only 35,800 plants remaining in nature. Several prominent plant conservationists suggest that the plant be labeled as Vulnerable. Precise data on the distribution of population sizes in 1992 from the Office of Plant Protection suggests a more dire state for the species. Every size class in red is slated for eventual extinction with the green ones persisting longer. Smaller populations may go extinct for stochastic reasons and, since small population are more numerous in nature now and contribute more to the total number of plants remaining in the species, most of this unique and remarkable carnivorous plant species may be going extinct soon. Note that the figure of 35,800 plants in 1992 is over 20 years old and may not accurately reflect the current situation.
In alternative medicine
Venus flytrap extract is available on the market as herbal remedy, sometimes as the prime ingredient of a patent medicine named "Carnivora". According to the American Cancer Society, these products are promoted in alternative medicine as a treatment for a variety of human ailments including HIV, Crohn's disease and skin cancer, but "available scientific evidence does not support the health claims made for Venus flytrap extract".
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