Carnivorous plant

A carnivorous plant is a plant that derives some or most of its nutrients (but not energy) by trapping and consuming animals, especially insects and other arthropods. Carnivorous plants usually 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.

Trapping mechanisms

There are five basic trapping mechanisms exploited by carnivorous plants. These are:

• Pitfall traps (pitchers), which trap prey in a rolled leaf that contains a pool of digestive enzymes;
• Flypaper traps, which trap prey using a sticky mucilage;
• Snap traps, which trap prey with rapid leaf movements;
• Bladder traps, which suck in prey with a bladder that generates an internal vacuum; and
• Lobster-pots, which use inward pointing hairs to force prey to move towards a digestive organ.

These traps may also be classified as active or passive. For example, there are both passive flypapers, like Triphyophyllum, which secrete mucilage, but whose leaves do not grow or move in response to prey capture; but there are also active flypapers, like most sundews, whose leaves undergo rapid growth, which aids in the retention and digestion of prey.

Pitfall traps

Pitfall traps have evolved independently on at least four occasions. The simplest pitfall traps are probably those of Heliamphora, the marsh (or sun) pitcher plant. In this genus, the leaves are quite clearly evolutionarily derived from a simple rolled leaf, whose margins have been sealed together. These plants live in areas of high rainfall (such as Mount Roraima), and consequently have a problem ensuring their pitchers do not overflow. To counteract this problem, natural selection has favoured the evolution of an overflow, similar to that of a bathroom sink: there is a small gap in the zipped up leaf margins that allows excess water to flow out of the pitcher.

Heliamphora is a member of the Sarraceniaceae, a New World family in the order Ericales (heathers and allies). Heliamphora is limited to South America, but the family contains two other genera, Sarracenia and Darlingtonia, which are endemic to Florida (for the most part) and California respectively. Sarracenia is the pitcher plant genus most commonly encountered in cultivation, and are usually known as trumpet pitchers. S. purpurea has a more cosmopolitan distribution.

In the genus Sarracenia, the problem of pitcher-overflow is solved by the possession of an operculum, which is essentially a flared leaflet that covers the opening of the rolled-leaf tube, and protects it from collecting rain. Possibly because of this improved waterproofing, species of Sarracenia secrete enzymes such as proteases and phosphatases into the digestive fluid at the bottom of the pitcher: Heliamphora relies on bacterial digestion alone. Darlingtonia californica, the cobra plant, uses a trick also found in Sarracenia psittacina and to a lesser extent in Sarracenia minor: the operculum is balloon-like, and almost seals the opening to the tube. This balloon-like chamber is pitted with areolae, which are chlorophyll-free patches through which light can penetrate. Insects (mostly ants) get into the chamber via the opening, which is underneath the balloon, and once inside, tire themselves out trying to escape from these false exits.

The second major group of pitcher plants are the monkey cups or tropical pitcher plants of the genus Nepenthes. In the seventy or so species of this genus, the pitcher is borne on the end of a tendril, which grows as an extension to the midrib of the leaf. Most species catch insects, although the larger ones, particularly N. rajah, will also occasionally take small mammals and reptiles. These pitchers represent a convenient source of food to small insectivores: N. bicalcarata possesses two sharp thorns that project from the base of the operculum over the entrance to the pitcher, which provide some protection from raids by freeloading mammals.

The pitfall trap has evolved in at least two other groups. Cephalotus follicularis, the Albany pitcher plant, is a small pitcher plant from Western Australia, with moccasin-like pitchers. In this species, the rim of the pitcher's opening (the peristome) is particularly pronounced, and both secretes nectar, and provides a thorny overhang to the opening, which prevents trapped insects from climbing out. The lining of most pitcher plants is very waxy, which provides a very dangerous footing for insects. The insects are often attracted by nectar bribes secreted by the peristome, and by bright flower-like anthocyanin patterning. In at least one species (Sarracenia flava), the nectar bribe is laced with coniine, a toxic alkaloid, which probably increases the efficiency of the traps.

The final carnivore with a pitfall-like trap is the bromeliad, Brocchinia reducta. Like most relatives of the pineapple, this species has an urn, formed from the tightly-packed waxy leaf bases of the strap-like leaves. In most bromeliads, water collects readily in this urn, and may provide habitats for frogs, insects and (more usefully from the plant's point of view), diazotrophic (nitrogen-fixing) bacteria. In Brocchinia, the urn is specialised as an insect-trap, with a population of digestive bacteria, and a loose, waxy lining.

Flypaper traps

The flypaper traps are those whose trapping mechanism is based on a sticky mucilage, or glue. The leaf of flypaper traps is studded with mucilage-secreting glands, which may either 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.

In the genus Pinguicula, the mucilage glands are quite short (sessile), and the leaf, whilst shiny (giving the genus its common name of 'butterwort'), does not look particularly carnivorous. However, this belies the fact that the leaf is an extremely effective trap of small flying incests (such as fungus gnats), whose surface responds to prey by relatively rapid growth: this may involve rolling of the leaf blade (to prevent rain from splashing the prey off the leaf surface), or 'bowling' of the surface under the prey.

The sundews (Drosera) consist of over 100 species of active flypapers, whose mucilage glands are borne at the end of long tentacles, which frequently grow fast enough in response to prey (thigmotropism) to aid the trapping process. The tentacles of D. burmanii are capable of bending 180° in only a minute or so. Sundews are extremely cosmopolitan, and are found on all the continents except the Antarctica mainland. They are probably at their most diverse in Australia, the home of the large subgroup of pygmy sundews, such as D. pygmaea. These species are so dependent on insect sources of nitrogen that they generally lack the enzyme (nitrate reductase), which most plant require to assimilate soilborne nitrate into organic chemicals.

Closely related to Drosera is the Portuguese dewy pine, [[Drosophyllum], which differs from the sundews in being passive: the leaves are incapable of rapid movement or growth. Unrelated, but similar in habit, are the Australian rainbow plants ([[Byblis_(plant)|Byblis). Drosophyllum is unusual amongst carnivores in that it grows under near-desert conditions: almost all other carnivores are either bog plants or grow in moist tropical areas.

The final flypaper is Triphyophyllum peltatum. This plant is usually encountered as a liana, however, in its juvenile phase, the plant is carnivorous: this may be related to a requirement for specific nutrients for flowering.

Snap traps

There are only two snap-traps, which are believed to have had a similar common ancestor. These are the Venus flytrap (Dionaea muscipula]]) and the waterwheel plant (Aldrovanda). Aldrovanda is aquatic, and specialised in catching small aquatic invertebrates; Dionaea is terrestrial and catches mostly flies. The traps are very similar: they have leaves whose terminal section is formed into two lobes, hinged along the midrib. Trigger hairs inside the trap lobes are sensitive to touch. When the trigger hairs are touched, stretch-gated ion channels in the membranes of cells at the base of the trigger hair open, generating an action potential, which propagates to cells in the midrib. These cells respond by pumping out potassium ions. Water follows by osmosis, and the cells in the midrib collapse, allowing the lobes (which are held under tension) to snap shut. This whole process takes less than a second. In the Venus flytrap, spurious closure (in response to raindrops and blown-in debris) is prevented by the leaf having a simple memory: for the lobes to shut, two stimulations are required, roughly between one half and 30 seconds apart.

The snapping of the leaves is a case of thigmonasty. Further stimulation of the lobe's internal surfaces by the struggling insects causes the lobes to grow together, sealing the lobes hermetically, and forming a stomach in which digestion occurs over a period of one to two weeks. Leaves can be reused three or four times before they become unresponsive to stimulation.

Bladder traps

Bladder traps are exclusive to the genus Utricularia, or bladderworts. These possess bladders, which pump ions out of their interiors. Water follows the ions by osmosis, and this generates 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) that touch these hairs deform the door by lever action: this releases the vacuum, and sucks the invertebrate into the bladder, where it is digested. Many species of Utricularia are terrestrial, growing on waterlogged soil, and their trapping mechanism is triggered in a different manner.

Lobster-pot traps

Lobster pots are found in Sarracenia psittacina, and more elegantly, in Genlisea, the corkscrew plants. In these plants, which appear to specialise in aquatic protozoa, a Y-shaped modified leaf allows entrance, but not exit. This is achieved by inward-pointing hairs. Prey items entering the spiral entrance that coils around the arms of the 'Y' are forced to move inexorably towards a 'stomach' where they are digested.

Borderline carnivores

To be a fully fledged carnivore, a plant must attract, kill, and digest prey; and it must benefit from absorbing the products of the digestion (mostly amino acids and ammonium ions). There are a number of plants which fail on one or more of these counts: whether these count as carnivorous plants is a matter of definition. From this, it should be obvious that there is a spectrum of carnivory: from 'non-carnivores' like cabbages, through borderline carnivores, and then through the true carnivores, from unspecialised and simple, like Heliamphora, to extremely specialised and complex, like the Venus flytrap.

The borderline carnivores of most interest are Roridula and Catopsis. Catopsis is a borderline carnivorous bromeliad, like Brocchinia; however, Roridula is more interesting. The plants in this genus produce sticky leaves with mucilage-tipped glands, and look extremely similar to some of the larger sundews. However, they do not directly benefit from the insects they catch. Instead, they form a mutualistic symbiosis with a species of bug (Pameridea), which eat the trapped incests: the plant benefits by absorbing nutrients from the bugs faeces.

Evolution

Elucidating the evolution of carnivorous plants is made difficult by the paucity of their fossil record. Very few fossils have been found, and almost all are either seed or pollen. However, much can be deduced from the structure of current traps. Pitfall traps are clearly derived from rolled leaves (the vascular tissues of Sarracenia show this quite clearly), and flypapers show a simple evolutionary gradient from sticky, non-carnivorous leaves, through passive flypapers to active forms. The snap-traps are thought to be derived from very fast-moving flypapers which became less reliant on glue.

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Ecology

Carnivorous plants are widespread but rather rare: there are only about 500 species, out of about 250,000 flowering plants. 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.

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Classification

The classification of all flowering plants is currently in a state of flux. In the Cronquist system, the Droseraceae and Nepenthaceae were placed in the order Nepenthales, based on the radial symmetry of their flowers, and their possession of insect-traps. The Sarraceniaceae was placed either in the Nepenthales, or in its own order, the Sarraceniales. The Byblidaceae, Cephalotaceae, and Roridulaceae were placed in the Saxifragales; and the Lentibulariaceae in the Scrophulariales.

In more modern classification, such as that of the Angiosperm Phylogeny Group, the families have all been retained, but have been redistributed amongst several disparate orders. The current recommendations are shown below (only carnivorous genera are shown):

Dicots

• Caryophyllales, (carnation order)
  • Dioncophyllaceae
    • Triphyophyllum, (a tropical liana)
  • Droseracaeae, (sundew family)
    • Aldrovanda, (waterwheel plant)
    • Dionaea, (Venus flytrap)
    • Drosera, (sundews)
    • Drosophyllum, (Portuguese dewy pine)
  • Nepenthaceae, (tropical pitcher-plant family)
    • Nepenthes, (tropical pitcher plants or monkey-cups, including Anurosperma)

• Ericales (heather order)
  • Roridulaceae
    • Roridula, (a borderline carnivore)
  • Sarraceniaceae (trumpet pitcher family)
    • Sarracenia, (North American trumpet pitchers)
    • Darlingtonia, (cobra plant)
    • Heliamphora, (sun or marsh pitchers)

• Lamiales, (mint order)
  • Byblidaceae
    • Byblis, (rainbow plants)
  • Lentibulariaceae, (bladderwort family)
    • Pinguicula, (butterworts)
    • Genlisea, (corkscrew plant)
    • Utricularia, (bladderworts, including Polypompholyx, the fairy aprons or pink petticoats)

• Oxalidales, (wood sorrel order)
  • Cephalotus, (Albany pitcher plant)

Monocots

• Poales, (grass order)
  • Bromeliales, (bromeliad or pineapple family)
    • Brocchinia, (a terrestrial bromeliad)
    • Catopsis, (a borderline carnivore)

Cultivation

Although different species of carnivorous plants have different requirements in terms of sunlight, humidity, soil moisture, etc., there are commonalities.

Most carnivorous plants require rain water, or water that has been distilled, deionised by reverse osmosis, or acidified 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 mush drier conditions than most.

Outdoor-grown carnivorous plants generally catch more than enough insects to keep themselves properly fed. Insects may be fed to the plants by hand to supplement their diet; however, carnivorous plants are generally unable to digest large non-insect food items; bits of hamburger, for example, will simply rot, and this may cause the trap, or even the whole plant, to die. A carnivorous plant that catches no insects at all will not die, but its growth will be impaired. In general, these plant are best left to their own devices: after underwatering with tap-water, the commonest cause of Venus flytrap death is prodding the traps to watch them close.

Most carnivorous plants require bright light, and most will look better under such conditions, as this encourages them to synthesise red and purple anthocyanin pigments. Nepenthes and Pinguicula will do better out of full sun, but most other species are happy in direct sunlight.

Carnivores mostly live in bogs, and those that do not are generally tropical. Hence, most require high humidity. On a small scale, this can be achieved by placing the plant in a wide saucer containing pebbles that are kept permanently wet. Small Nepenthes species grow well in large terraria.

Many carnivores are temperate, and although most are intolerant of hard frosts, many can be kept outside in temperate climates for the majority of the year. The main exception to this are Nepenthes sp., which are tropical, and require temperatures from 20 to 30°C to thrive.

Carnivorous plants mostly live in bogs, and require appropriate nutrient-poor soil. Most appreciate a 3:1 mixture of Sphagnum peat to sharp sand (coir is an acceptable, and more ecofriendly substitute for peat). Nepenthes will grow in orchid compost, or in pure Sphagnum moss.

Ironically, carnivorous plants are themselves susceptible to infestation by parasites such as aphids or mealybugs. Although small infestations can be removed by hand, larger infestations necessitate use of an insecticide. Isopropyl alcohol (rubbing alcohol) is effective as a topical insecticide. Diazinon is an excellent systemic insecticide that is tolerated by most carnivorous plants. Malathion and Acephate (Orthene) have also been reported as tolerable by carnivorous plants.

Although insects can be a problem, by far the biggest killer of carnivorous plants (besides human maltreatment) is grey mould (Botrytis cinerea). This thrives under warm, humid conditions, and can be a real problem in winter. To some extent, temperate carnivorous plants can be protected from this pathogen by ensuring that they are kept cool in winter, and that any dead leaves are removed promptly. If this fails, a fungicide is in order.

The easiest carnivorous plants for beginners are those from the cool temperate zone. These plants will do well under cool greenhouse conditions (minimum 5°C in winter, maximum 25°C in summer) if kept in wide trays of acidified or rain water during summer, and kept moist during winter:

• Drosera capensis, the Cape sundew: attractive strap-leaved sundew, pink flowers, very tolerant of maltreatment.
• Drosera binata, the fork-leaved sundew: large, Y-shaped leaves.
• Sarracenia flava, the yellow trumpet pitcher: yellow, attractively veined leaves, yellow flowers in spring.
• Pingicula grandiflora, the common butterwort: beautiful lilac flowers in spring, hibernates as a bud (hibernaculum) in winter. Fully hardy.
• Pingicula moranensis, the Mexican butterwort: pink flowers, non-carnivorous leaves in winter.
• Darlingtonia californica, the cobra lily: dramatic leaves, purple and lime-green flowers, likes to be watered with large amounts of cooled water during summer.

Venus flytraps will do well under these conditions, but is actually rather difficult to grow: even if treated well, it will often succumb to grey mould in winter unless well ventilated. Some of the lowland Nepenthes are very easy to grow, as long as they are provided with relatively constant, hot and humid conditions.

Pop culture

A fanciful carnivorous plant with an insatiable appetite was the central theme of the comedic play, Little Shop of Horrors, made from a more serious 1960s movie of the same name.

The triffids presented in John Wyndham's book The Day of the Triffids are plants which can uproot themselves, move, and can kill with a poisonous, whip-like tail. The book leaves open the question of whether the triffids are intelligent.

The film Attack of the Killer Tomatoes is a campy movie about tomatoes that for some reason eat people. It is an intentional spoof on 50s monster movies.

A large floral plant consumed a young woman in Madagascar in 1878, as witnessed by Dr Carl Liche, or so he reported in the September 26 1920 issue of The American Weekly. The woman was supposed to have been a member of the Mkodos, a little known but cruel tribe. The woman was pictured in an accompanying artwork. In 1925 the same paper offered another carnivorous plant story, of a tree species on Mindanao, in the Philippines. There is no evidence that either of these plants is more than a fanciful story.

External links

• The Carnivorous Plant FAQ
• Carnivorous Plant discussion forum
• Carnivorous Plant Database, provides an up to date, searchable database of all the published species of carnivorous plant
• Carnivorous Plant Nurseries
• The Carnivorous Plant Society, has a 24/7 live link with a forum.
• The Carnivorous Plant Society, the UK Carnivorous Plant Society has information on carnivorous plants and tips on cultivation.
• International Carnivorous Plant Society, provides an extensive FAQ and links to the Carnivorous Plant Web Ring.
• Insectivorous Plants, by Charles Darwin