Carnivorous plants are plants that derive some or most of their nutrients (but not energy) from trapping and consuming animals or protozoans, typically insects and other arthropods.
Carnivorous plants (sometimes called insectivorous plants) are plants that get some or most of their nutrients (but not energy) from trapping and eating animals or protozoans. Most of the time they trap insects and other arthropods. Because they get some of their food from animals, carnivorous plants can grow in places where the soil is thin, or poor in nutrients. This is especially true for soils with little nitrogen, such as acidic bogs and rock outcrops. Charles Darwin wrote the first well-known book on carnivorous plants in 1875.
This ability of plants to catch animals, as described above, is called true carnivory. It probably evolved in at least ten separate groups of plants. There are more than twelve genera in five families. These include about 625 species that attract and trap prey, produce digestive enzymes, and use the resulting nutrients. In addition to these, there are more than 300 species in several genera that show some but not all of these characteristics. These are usually called protocarnivorous plants.
There are five different mechanisms of trapping that can be found in plants:
- Pitfall traps (pitcher plants) trap prey in a rolled leaf that contains a pool of digestive enzymes or bacteria.
- Flypaper traps use a sticky mucilage.
- Snap traps use rapid leaf movements.
- Bladder traps suck in prey with a bladder that generates an internal vacuum.
- Lobster-pot traps force prey to move towards a digestive organ with inward-pointing hairs.
These traps may be active or passive, depending on whether movement aids the capture of prey. For example, Triphyophyllum is a passive flypaper that secretes mucilage, but whose leaves do not grow or move in response to prey capture. Meanwhile, sundews are active flypapers whose leaves undergo rapid growth, which helps the capture and digestion of prey.
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Carnivorous plants appear adapted to grow in places where the soil is thin or poor in nutrients, especially nitrogen, such as acidic bogs and rock outcroppings. Charles Darwin wrote the first well-known treatise on carnivorous plants in 1875.
True carnivory is thought to have evolved independently six times in five different orders of flowering plants, and these are now represented by more than a dozen genera. These include about 630 species that attract and trap prey, produce digestive enzymes, and absorb the resulting available nutrients. Additionally, over 300 protocarnivorous plant species in several genera show some but not all these characteristics.
Pitcher plants are carnivorous plants whose prey-trapping mechanism features a deep cavity filled with liquid known as a pitfall trap. It has been widely assumed that the various sorts of pitfall trap evolved from rolled leaves, with selection pressure favouring more deeply cupped leaves over evolutionary time. However, some pitcher plant genera (such as Nepenthes) are placed within clades consisting mostly of flypaper traps: this indicates that this view may be too simplistic, and some pitchers may have evolved from flypaper traps by loss of mucilage.
Flypaper traps: The flypaper trap is based on a sticky mucilage, or glue. The leaf of flypaper traps is studded with mucilage-secreting glands, which may be short and nondescript (like those of the butterworts), or long and mobile (like those of many sundews). Flypapers have evolved independently at least five times.
Snap traps: The snap traps of Dionaea muscipula close rapidly when triggered to trap prey between two lobes.The only two active snap traps—the Venus flytrap (Dionaea muscipula) and the waterwheel plant (Aldrovanda vesiculosa)—are believed to have had a common ancestor with similar adaptations. Their trapping mechanism has also been described as a "mouse trap" or "man trap", based on their shape or rapid movement. However, the term snap trap is preferred as other designations are misleading, particularly with respect to the intended prey. Aldrovanda is aquatic and specialised in catching small invertebrates; Dionaea is terrestrial and catches a variety of arthropods, including spiders.
Bladder traps: The tip of one stolon of Utricularia vulgaris, showing stolon, branching leaf-shoots, and transparent bladder traps
Genlisea violacea traps and leavesBladder traps are exclusive to the genus Utricularia, or bladderworts. The bladders (vesicula) pump ions out of their interiors. Water follows by osmosis, generating a partial vacuum inside the bladder. The bladder has a small opening, sealed by a hinged door. In aquatic species, the door has a pair of long trigger hairs. Aquatic invertebrates such as Daphnia touch these hairs and deform the door by lever action, releasing the vacuum. The invertebrate is sucked into the bladder, where it is digested. Many species of Utricularia (such as U. sandersonii) are terrestrial, growing on waterlogged soil, and their trapping mechanism is triggered in a slightly different manner. Bladderworts lack roots, but terrestrial species have anchoring stems that resemble them. Temperate aquatic bladderworts generally die back to a resting turion during the winter months, and U. macrorhiza appears to regulate the number of bladders it bears in response to the prevailing nutrient content of its habitat.
Lobster-pot traps: A lobster-pot trap is a chamber that is easy to enter, and whose exit is either difficult to find or obstructed by inward-pointing bristles. Lobster pots are the trapping mechanism in Genlisea, the corkscrew plants. These plants appear to specialise in aquatic protozoa. A Y-shaped modified leaf allows prey to enter but not exit. Inward-pointing hairs force the prey to move in a particular direction. Prey entering the spiral entrance that coils around the upper two arms of the Y are forced to move inexorably towards a stomach in the lower arm of the Y, where they are digested. Prey movement is also thought to be encouraged by water movement through the trap, produced in a similar way to the vacuum in bladder traps, and probably evolutionarily related to it.
A protocarnivorous plant (sometimes also paracarnivorous, subcarnivorous, or borderline carnivore), according to some definitions, traps and kills insects or other animals but lacks the ability to either directly digest or absorb nutrients from its prey like a carnivorous plant. The morphological adaptations such as sticky trichomes or pitfall traps of protocarnivorous plants parallel the trap structures of confirmed carnivorous plants.
The evolution of carnivorous plants is obscured by the paucity of their fossil record. Very few fossils have been found, and then usually only as seed or pollen. Carnivorous plants are generally herbs, and their traps primary growth. They generally do not form readily fossilisable structures such as thick bark or wood. The traps themselves would probably not be preserved in any case.
Carnivorous plants are widespread but rather rare. They are almost entirely restricted to habitats such as bogs, where soil nutrients are extremely limiting, but where sunlight and water are readily available. Only under such extreme conditions is carnivory favoured to an extent that makes the adaptations obvious.
Cultivation: Cultivated Nepenthes rajah and other species.Although different species of carnivorous plants have different requirements in terms of sunlight, humidity, soil moisture, etc., there are commonalities.
Most carnivorous plants require rainwater, or water that has been distilled, deionised by reverse osmosis, or acidified to around pH 6.5 using sulfuric acid.
Common tap or drinking water contains minerals (particularly calcium salts) that will quickly build up and kill the plant. This is because most carnivorous plants have evolved in nutrient-poor, acidic soils and are consequently extreme calcifuges. They are therefore very sensitive to excessive soil-borne nutrients. Since most of these plants are found in bogs, almost all are very intolerant of drying. There are exceptions: tuberous sundews require a dry (summer) dormancy period, and Drosophyllum requires much drier conditions than most.
Carnivorous fungi or predaceous fungi are fungi that derive some or most of their nutrients from trapping and digesting microscopic or other minute animals. More than 200 species have been described, belonging to the phyla Ascomycota, Mucoromycotina, and Basidiomycota. They usually live in soil and many species trap or stun nematodes (nematophagous fungus), while others attack amoebae or collembola. Fungi that grow on the epidermis, hair, skin, nails, scales or feathers of living or dead animals are considered to be dermatophytes rather than carnivores. Similarly fungi in orifices and the digestive tract of animals are not carnivorous, and neither are internal pathogens. Neither are insect pathogens that stun and colonize insects normally labelled carnivorous if the fungal thallus is mainly in the insect as does Cordyceps, or if it clings to the insect like the Laboulbeniales.
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