Carnivorous plant

Nepenthes mirabilis in flower, growing on a road cut in Palau

Carnivorous plants (sometimes called insectivorous 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 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.^[1]

True carnivory is thought to have evolved in at least ten separate lineages of plants, and these are now represented by more than a dozen genera in five families. These include about 625 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.

Trapping mechanisms

The primitive pitchers of Heliamphora chimantensis are an example of pitfall traps.

Five basic trapping mechanisms are found in carnivorous plants.

1. Pitfall traps (pitcher plants) trap prey in a rolled leaf that contains a pool of digestive enzymes or bacteria.
2. Flypaper traps use a sticky mucilage.
3. Snap traps utilize rapid leaf movements.
4. Bladder traps suck in prey with a bladder that generates an internal vacuum.
5. 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, aiding in the retention and digestion of prey.

Pitfall traps

Main article: Pitcher plant

Pitfall traps are thought to have evolved independently on at least four occasions. The simplest ones are probably those of Heliamphora, the sun pitcher plant. In this genus, the traps are clearly derived evolutionarily from a simple rolled leaf whose margins have sealed together. These plants live in areas of high rainfall in South America 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—a small gap in the zipped-up leaf margins 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 the Southeastern United States (with the exception of one species) and California respectively. S. purpurea subsp. purpurea (the northern pitcher plant) has a more cosmopolitan distribution, found as far north as Canada.

Sarracenia is the pitcher plant genus most commonly encountered in cultivation, because it is relatively hardy and easy to grow.

Darlingtonia californica: note the small entrance to the trap underneath the swollen "balloon" and the colourless patches that confuse prey trapped inside.

In the genus Sarracenia, the problem of pitcher overflow is solved by an operculum, which is essentially a flared leaflet that covers the opening of the rolled-leaf tube and protects it from rain. Possibly because of this improved waterproofing, Sarracenia species secrete enzymes such as proteases and phosphatases into the digestive fluid at the bottom of the pitcher; Heliamphora relies on bacterial digestion alone. The enzymes digest the proteins and nucleic acids in the prey, releasing amino acids and phosphate ions, which the plant absorbs.

Darlingtonia californica, the cobra plant, possesses an adaptation 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, chlorophyll-free patches through which light can penetrate. Insects, mostly ants, enter the chamber via the opening underneath the balloon. Once inside, they tire themselves trying to escape from these false exits, until they eventually fall into the tube. Prey access is increased by the "fish tails", outgrowths of the operculum that give the plant its name. Some seedling Sarracenia species also have long, overhanging opercular outgrowths; Darlingtonia may therefore represent an example of neoteny.

Brocchinia reducta: a carnivorous bromeliad

The second major group of pitcher plants are the monkey cups or tropical pitcher plants of the genus Nepenthes. In the hundred or so species of this genus, the pitcher is borne at 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, 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, providing some protection from raids by freeloading mammals.

The pitfall trap has evolved independently in at least two other groups. The Albany pitcher plant Cephalotus follicularis is a small pitcher plant from Western Australia, with moccasin-like pitchers. The rim of its pitcher's opening (the peristome) is particularly pronounced (both secrete nectar) and provides a thorny overhang to the opening, preventing trapped insects from climbing out. The lining of most pitcher plants is covered in a loose coating of waxy flakes, which are slippery for insects, prey that 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 also found in hemlock, which probably increases the efficiency of the traps by intoxicating prey.

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

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.

Pinguicula gigantea with prey. The insect was too large and was able to escape.

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 appear carnivorous. However, this belies the fact that the leaf is an extremely effective trap of small flying insects (such as fungus gnats), and its surface responds to prey by relatively rapid growth. This thigmotropic growth may involve rolling of the leaf blade (to prevent rain from splashing the prey off the leaf surface) or dishing of the surface under the prey to form a shallow digestive pit.

The leaf of a Drosera capensis bending in response to the trapping of an insect

The sundew genus (Drosera) consists 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 can bend 180° in a minute or so. Sundews are extremely cosmopolitan and are found on all the continents except the Antarctic mainland. They are most diverse in Australia, the home to the large subgroup of pygmy sundews such as D. pygmaea and to a number of tuberous sundews such as D. peltata, which form tubers that aestivate during the dry summer months. These species are so dependent on insect sources of nitrogen that they generally lack the enzyme nitrate reductase, which most plants require to assimilate soil-borne nitrate into organic forms.

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

Recent molecular data (particularly the production of plumbagin) indicate that the remaining flypaper, Triphyophyllum peltatum, a member of the Dioncophyllaceae, is closely related to Drosophyllum and forms part of a larger clade of carnivorous and non-carnivorous plants with the Droseraceae, Nepenthaceae, Ancistrocladaceae and Plumbaginaceae. This plant is usually encountered as a liana, but in its juvenile phase, the plant is carnivorous. This may be related to a requirement for specific nutrients for flowering.

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.^[2]

The traps are very similar, with leaves whose terminal section is divided into two lobes, hinged along the midrib. Trigger hairs (three on each lobe in Dionaea, many more in the case of Aldrovanda) inside the trap lobes are sensitive to touch. When a trigger hair is bent, stretch-gated ion channels in the membranes of cells at the base of the trigger hair open, generating an action potential that propagates to cells in the midrib.^[3] These cells respond by pumping out ions, which may either cause water to follow by osmosis (collapsing the cells in the midrib) or cause rapid acid growth.^[4] The mechanism is still debated, but in any case, changes in the shape of cells in the midrib allow the lobes, held under tension, to snap shut,^[3] flipping rapidly from convex to concave^[5] and interring the prey. 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 leaves having a simple memory: for the lobes to shut, two stimuli are required, 0.5 to 30 seconds apart.

The snapping of the leaves is a case of thigmonasty (undirected movement in response to touch). Further stimulation of the lobe's internal surfaces by the struggling insects causes the lobes to grow together towards the prey (thigmotropism), 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

The tip of one stolon of Utricularia vulgaris, showing stolon, branching leaf-shoots, and transparent bladder traps

Genlisea violacea traps and leaves

Bladder 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.

Outside of Genlisea, features reminiscent of lobster-pot traps can be seen in Sarracenia psittacina, Darlingtonia californica, and, some horticulturalists argue, Nepenthes aristolochioides.

Borderline carnivores

Main article: Protocarnivorous plant

To be a fully fledged carnivore, a plant must attract, kill, and digest prey;^[6][7] and it must benefit from absorbing the products of the digestion (mostly amino acids and ammonium ions).^[8] To many horticulturalists, these distinctions are a matter of taste. There is a spectrum of carnivory found in plants: from completely non-carnivorous plants like cabbages, to borderline carnivores, to unspecialised and simple traps, like Heliamphora, to extremely specialised and complex traps, like that of the Venus flytrap.

Roridula gorgonias: a borderline carnivore that gains nutrients from its "prey" via the droppings of a predatory bug

The borderline carnivores include Roridula and Catopsis berteroniana. Catopsis is a borderline carnivorous bromeliad, like Brocchinia reducta. However, unlike the phosphatase of B. reducta, C. berteroniana has not been shown to produce digestive enzymes.^[9] In these pitfall traps, prey simply fall into the urn, assisted by the waxy scales located on the rim. Roridula has a more intricate relationship with its prey. The plants in this genus produce sticky leaves with resin-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 species of assassin bug (genus Pameridea), which eat the trapped insects. The plant benefits from the nutrients in the bugs' faeces.^[10]

A number of species in the Martyniaceae (previously Pedaliaceae), such as Ibicella lutea, have sticky leaves that trap insects. However, these plants have not been shown conclusively to be carnivorous.^[11] Likewise, the seeds of Shepherd's Purse,^[11] urns of Paepalanthus bromelioides,^[12] bracts of Passiflora foetida,^[13] and flower stalks and sepals of triggerplants (Stylidium)^[14] appear to trap and kill insects, but their classification as carnivores is contentious.

The production of specific prey-digesting enzymes (proteases, ribonucleases, phosphatases, etc.) is sometimes used as a criterion for carnivory. However, this would probably discount Heliamphora^[15] and Darlingtonia,^[16] both of which appear to rely on the enzymes of symbiotic bacteria to break down their prey but are generally considered as carnivores. However, discounting the enzyme-based definition leaves open the question of Roridula. There is no reason why a plant's possession of symbiotic bacteria that allow it to benefit from trapped prey should allow the plant to be considered carnivorous, whilst possession of symbiotic bugs should not.

Evolution

Artist's restoration of Archaeamphora longicervia, the earliest known carnivorous plant

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.

Still, much can be deduced from the structure of current traps. Pitfall traps are quite clearly derived from rolled leaves. The vascular tissues of Sarracenia is a case in point. The keel along the front of the trap contains a mixture of leftward- and rightward-facing vascular bundles, as would be predicted from the fusion of the edges of an adaxial (stem-facing) leaf surface. Flypapers also show a simple evolutionary gradient from sticky, non-carnivorous leaves, through passive flypapers to active forms. Molecular data show the Dionaea–Aldrovanda clade is closely related to Drosera,^[17] but the traps are so dissimilar that the theory of their origin—very fast-moving flypapers became less reliant on glue—remains rather speculative.

There are over a quarter of a million species of flowering plants. Of these, only around five hundred are known to be carnivorous. True carnivory has probably evolved independently at least ten times; however, some of these "independent" groups probably descended from a recent common ancestor with a predisposition to carnivory. Some groups (the Ericales and Caryophyllales) seem particularly fertile ground for carnivorous preadaptation, although in the former case, this may be more to do with the ecology of the group than its morphology, as most of the members of this group grow in low-nutrient habitats such as heath and bog.

It has been suggested that all trap types are modifications of a similar basic structure—the hairy leaf.^[18] Hairy (or more specifically, stalked-glandular) leaves can catch and retain drops of rainwater, especially if shield-shaped or peltate, thus promoting bacteria growth. Insects land on the leaf, become mired by the surface tension of the water, and suffocate. Bacteria jumpstart decay, releasing from the corpse nutrients that the plant can absorb through its leaves. This foliar feeding can be observed in most non-carnivorous plants. Plants that were better at retaining insects or water therefore had a selective advantage. Rainwater can be retained by cupping the leaf, leading to pitfall traps. Alternatively, insects can be retained by making the leaf stickier by the production of mucilage, leading to flypaper traps.

The pitfall traps may have evolved simply by selection pressure for the production of more deeply cupped leaves, followed by "zipping up" of the margins and subsequent loss of most of the hairs, except at the bottom, where they help retain prey.

The lobster-pot traps of Genlisea are difficult to interpret. They may have developed from bifurcated pitchers that later specialised on ground-dwelling prey; or, perhaps, the prey-guiding protrusions of bladder traps became more substantial than the net-like funnel found in most aquatic bladderworts. Whatever their origin, the helical shape of the lobster pot is an adaptation that displays as much trapping surface as possible in all directions when buried in moss.

The traps of Catopsis berteroniana are unlikely to have descended from a hairy leaf or sepal.

The traps of the bladderworts may have derived from pitchers that specialised in aquatic prey when flooded, like Sarracenia psittacina does today. Escaping prey in terrestrial pitchers have to climb or fly out of a trap, and both of these can be prevented by wax, gravity and narrow tubes. However, a flooded trap can be swum out of, so in Utricularia, a one-way lid may have developed to form the door of a proto-bladder. Later, this may have become active by the evolution of a partial vacuum inside the bladder, tripped by prey brushing against trigger hairs on the door of the bladder.

Flypaper traps include the various true flypapers and the snap traps of Aldrovanda and Dionaea. The production of sticky mucilage is found in many non-carnivorous genera, and the passive glue traps in Byblis and Drosophyllum could easily have evolved.

The active glue traps use rapid plant movements to trap their prey. Rapid plant movement can result from actual growth, or from rapid changes in cell turgor, which allow cells to expand or contract by quickly altering their water content. Slow-moving flypapers like Pinguicula exploit growth, but the Venus flytrap uses such rapid turgor changes that glue became unnecessary. The stalked glands that once made it and which are so evident in Drosera have become the teeth and trigger hairs—an example of natural selection hijacking preexisting structures for new functions.

Recent taxonomic analysis^[19] of the relationships within the Caryophyllales indicate that the Droseraceae, Triphyophyllum, Nepenthaceae and Drosophyllum, whilst closely related, are embedded within a larger clade that includes non-carnivorous groups such as the tamarisks, Ancistrocladaceae, Polygonaceae and Plumbaginaceae. Interestingly, the tamarisks possess specialised salt-excreting glands on their leaves, as do several of the Plumbaginaceae (such as the sea lavender, Limonium), which may have been co-opted for the excretion of other chemicals, such as proteases and mucilage. Some of the Plumbaginaceae (e.g. Ceratostigma) also have stalked, vascularised glands that secrete mucilage on their calyces and aid in seed dispersal and possibly in protecting the flowers from crawling parasitic insects. These are probably homologous with the tentacles of the carnivorous genera. Perhaps carnivory evolved from a protective function, rather than a nutritional one. The balsams (such as Impatiens), which are closely related to the Sarraceniaceae and Roridula, similarly possess stalked glands.

The only traps that are unlikely to have descended from a hairy leaf or sepal are the carnivorous bromeliads (Brocchinia and Catopsis). These plants use the urn—a fundamental part of a bromeliad—for a new purpose and build on it by the production of wax and the other paraphernalia of carnivory.

Ecology and modelling of carnivory

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.

The archetypal carnivore, the Venus flytrap, grows in soils with almost immeasurable nitrate and calcium levels. Plants need nitrogen for protein synthesis, calcium for cell wall stiffening, phosphate for nucleic acid synthesis, and iron for chlorophyll synthesis. The soil is often waterlogged, which favours the production of toxic ions such as ammonium, and its pH is an acidic 4 to 5. Ammonium can be used as a source of nitrogen by plants, but its high toxicity means that concentrations high enough to fertilise are also high enough to cause damage.

Drosophyllum lusitanicum is one of the few carnivorous plants to grow in dry, alkaline soil.

However, the habitat is warm, sunny, constantly moist, and the plant experiences relatively little competition from low growing Sphagnum moss. Still, carnivores are also found in very atypical habitats. Drosophyllum lusitanicum is found around desert edges and Pinguicula valisneriifolia on limestone (calcium-rich) cliffs.^[20]

In all the studied cases, carnivory allows plants to grow and reproduce using animals as a source of nitrogen, phosphorus and possibly potassium.^[21][22][23] However, there is a spectrum of dependency on animal prey. Pygmy sundews are unable to use nitrate from soil because they lack the necessary enzymes (nitrate reductase in particular).^[24] Common butterworts (Pinguicula vulgaris) can use inorganic sources of nitrogen better than organic sources, but a mixture of both is preferred.^[21] European bladderworts seem to use both sources equally well. Animal prey makes up for differing deficiencies in soil nutrients.

Plants use their leaves to intercept sunlight. The energy is used to reduce carbon dioxide from the air with electrons from water to make sugars (and other biomass) and a waste product, oxygen in the process of photosynthesis. Leaves also respire, in a similar way to animals, by burning their biomass to generate chemical energy. This energy is temporarily stored in the form of ATP (adenosine triphosphate), which acts as an energy currency for metabolism in all living things. As a waste product, respiration produces carbon dioxide.

For a plant to grow, it must photosynthesise more than it respires. Otherwise, it will eventually exhaust its biomass and die. The potential for plant growth is net photosynthesis, the total gross gain of biomass by photosynthesis, minus the biomass lost by respiration. Understanding carnivory requires a cost-benefit analysis of these factors.^[8]

In carnivorous plants, the leaf is not just used to photosynthesise, but also as a trap. Changing the leaf shape to make it a better trap generally makes it less efficient at photosynthesis. For example, pitchers have to be held upright, so that only their opercula directly intercept light. The plant also has to expend extra energy on non-photosynthetic structures like glands, hairs, glue and digestive enzymes.^[25] To produce such structures, the plant requires ATP and respires more of its biomass. Hence, a carnivorous plant will have both decreased photosynthesis and increased respiration, making the potential for growth small and the cost of carnivory high.

Being carnivorous allows the plant to grow better when the soil contains little nitrate or phosphate. In particular, an increased supply of nitrogen and phosphorus makes photosynthesis more efficient, because photosynthesis depends on the plant being able to synthesise very large amounts of the nitrogen-rich enzyme RuBisCO (ribulose-1,5-bis-phosphate carboxylase/oxygenase), the most abundant protein on Earth.

It is intuitively clear that the Venus flytrap is more carnivorous than Triphyophyllum peltatum. The former is a full-time moving snap-trap; the latter is a part-time, non-moving flypaper. The energy "wasted" by the plant in building and fuelling its trap is a suitable measure of the carnivory of the trap.

Modelling carnivory in plants: gross photosynthesis, respiration and net photosynthesis as a function of the plant's investment in carnivorous adaptations. Non-zero optimum carnivory occurs in brightly lit habitats with very limiting soil nutrients.

Using this measure of investment in carnivory, a model can be proposed.^[8] Above is a graph of carbon dioxide uptake (potential for growth) against trap respiration (investment in carnivory) for a leaf in a sunny habitat containing no soil nutrients at all. Respiration is a straight line sloping down under the horizontal axis (respiration produces carbon dioxide). Gross photosynthesis is a curved line above the horizontal axis: as investment increases, so too does the photosynthesis of the trap, as the leaf receives a better supply of nitrogen and phosphorus. Eventually another factor (such as light intensity or carbon dioxide concentration) will become more limiting to photosynthesis than nitrogen or phosphorus supply. As a result, increasing the investment will not make the plant grow better. The net uptake of carbon dioxide, and therefore, the plant's potential for growth, must be positive for the plant to survive. There is a broad span of investment where this is the case, and there is also a non-zero optimum. Plants investing more or less than this optimum will take up less carbon dioxide than an optimal plant, and hence growing less well. These plants will be at a selective disadvantage. At zero investment the growth is zero, because a non-carnivorous plant cannot survive in a habitat with absolutely no soil-borne nutrients. Such habitats do not exist, so for example, Sphagnum absorbs the tiny amounts of nitrates and phosphates in rain very efficiently and also forms symbioses with diazotrophic cyanobacteria.

Modelling carnivory in plants: gross photosynthesis, respiration and net photosynthesis as a function of the plant's investment in carnivorous adaptations. An optimum carnivory of zero occurs in poorly lit habitats with abundant soil nutrients.

In a habitat with abundant soil nutrients but little light (as shown above), the gross photosynthesis curve will be lower and flatter, because light will be more limiting than nutrients. A plant can grow at zero investment in carnivory; this is also the optimum investment for a plant, as any investment in traps reduces net photosynthesis (growth) to less than the net photosynthesis of a plant that obtains its nutrients from soil alone.

Carnivorous plants exist between these two extremes: the less limiting light and water are, and the more limiting soil nutrients are, the higher the optimum investment in carnivory, and hence the more obvious the adaptations will be to the casual observer.

The most obvious evidence for this model is that carnivorous plants tend to grow in habitats where water and light are abundant and where competition is relatively low: the typical bog. Those that do not tend to be even more fastidious in some other way. Drosophyllum lusitanicum grows where there is little water, but it is even more extreme in its requirement for bright light and low disturbance than most other carnivores. Pinguicula valisneriifolia grows in soils with high levels of calcium but requires strong illumination and lower competition than many butterworts.^[26]

In general, carnivorous plants are poor competitors, because they invest too heavily in structures that have no selective advantage in nutrient-rich habitats. They succeed only where other plants fail. Carnivores are to nutrients what cacti are to water. Carnivory only pays off when the nutrient stress is high and where light is abundant.^[27] When these conditions are not met, some plants give up carnivory temporarily. Sarracenia spp. produce flat, non-carnivorous leaves (phyllodes) in winter. Light levels are lower than in summer, so light is more limiting than nutrients, and carnivory does not pay. The lack of insects in winter exacerbates the problem. Damage to growing pitcher leaves prevent them from forming proper pitchers, and again, the plant produces a phyllode instead.

Part-time carnivory in Triphyophyllum peltatum may be due to an unusually high need for potassium at a certain point in the life cycle, just before flowering.

Many other carnivores shut down in some season. Tuberous sundews die back to tubers in the dry season, bladderworts to turions in winter, and non-carnivorous leaves are made by most butterworts and Cephalotus in the less favourable seasons. Utricularia macrorhiza varies the number of bladders its produces based on the expected density of prey.^[28] Part-time carnivory in Triphyophyllum peltatum may be due to an unusually high need for potassium at a certain point in the life cycle, just before flowering.

The more carnivorous a plant is, the more conventional its habitat is likely to be. Venus flytraps live in a very stereotypical and very specialised habitat, whereas less carnivorous plants (Byblis, Pinguicula) are found in more unusual habitats (i.e., those typical for non-carnivores). Byblis and Drosophyllum both come from relatively arid regions and are both passive flypapers, arguably the lowest maintenance form of trap. Venus flytraps filter their prey using the teeth around the trap's edge, so as not to waste energy on hard-to-digest prey. In evolution, laziness pays, because energy can be used for reproduction, and short-term benefits in reproduction will outweigh long-term benefits in anything else.

Carnivory rarely pays, so even carnivorous plants avoid it when there is too little light or an easier source of nutrients, and they use as few carnivorous features as are required at a given time or for a given prey item. There are very few habitats stressful enough to make investing biomass and energy in trigger hairs and enzymes worthwhile. Many plants occasionally benefit from animal protein rotting on their leaves, but carnivory that is obvious enough for the casual observer to notice is rare.

Bromeliads seem very well preadapted to carnivory, but only one or two species can be classified as truly carnivorous. By their very shape, bromeliads will benefit from increased prey-derived nutrient input. In this sense, bromeliads are probably carnivorous, but their habitats are too dark for more extreme, recognisable carnivory to evolve. Most bromeliads are epiphytes, and most epiphytes grow in partial shade on tree branches. Brocchinia reducta, on the other hand, is a ground dweller.

Classification

See also: List of carnivorous plants

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 (now subsumed into the Lamiales^[29]).

In more modern classification, such as that of the Angiosperm Phylogeny Group, the families have been retained, but they have been redistributed amongst several disparate orders. It is also recommended that Drosophyllum be considered in a monotypic family outside the rest of the Droseraceae, probably more closely allied to the Dioncophyllaceae. The current recommendations are shown below (only carnivorous genera are listed):

Dicots

Stylidium turbinatum

Aldrovanda vesiculosa

Byblis liniflora

Cephalotus follicularis

• Asterales (sunflower and daisy order)
  • Stylidiaceae
    • Stylidium (trigger plants, a borderline carnivore)
• Caryophyllales, (carnation order)
  • Dioncophyllaceae
    • Triphyophyllum (a tropical liana)
  • Drosophyllaceae
    • Drosophyllum (Portuguese dewy pine)
  • Droseraceae (sundew family)
    • Aldrovanda (waterwheel plant)
    • Dionaea (Venus flytrap)
    • Drosera (sundews)
    • †Droserapollis
    • †Droserapites
    • †Droseridites
    • †Fischeripollis
    • †Palaeoaldrovanda
    • †Saxonipollis
  • 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)
    • †Archaeamphora
    • Sarracenia (North American trumpet pitchers)
    • Darlingtonia (cobra plant/lily)
    • 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 and Biovularia an obsolete genus)
  • Martyniaceae (all borderline carnivores, related to the sesame plant)
    • Ibicella
• Oxalidales (wood sorrel order)
  • Cephalotus (Albany pitcher plant)

Monocots

• Poales (grass order)
  • Bromeliaceae (bromeliad or pineapple family)
    • Brocchinia (a terrestrial bromeliad)
    • Catopsis (a borderline carnivore)
  • Eriocaulaceae (pipewort family)
    • Paepalanthus (a borderline carnivore)

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.

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 rarely die, although its growth may be impaired. In general, these plants are best left to their own devices: after underwatering with tap-water, the most common cause of Venus flytrap death is prodding the traps to watch them close and feeding them cheese and other inappropriate items.

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 native to cold temperate regions and can be grown outside in a bog garden year-round. Most Sarracenia can tolerate temperatures well below freezing, despite most species being native to the southeastern United States. Species of Drosera and Pinguicula also tolerate subfreezing temperatures. Nepenthes species, which are tropical, require temperatures from 20 to 30 °C to thrive.

Many Sarracenia hybrids are easy to grow.

Carnivorous plants require appropriate nutrient-poor soil. Most appreciate a 3:1 mixture of Sphagnum peat to sharp horticultural 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, particularly on scale insects. 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 and well ventilated 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.
• Pinguicula grandiflora, the common butterwort: purple flowers in spring, hibernates as a bud (hibernaculum) in winter. Fully hardy.
• Pinguicula moranensis, the Mexican butterwort: pink flowers, non-carnivorous leaves in winter.

Venus flytraps will do well under these conditions but are actually rather difficult to grow: even if treated well, they 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.

Cultural depictions

Audrey Junior, the man-eating plant from the cult film The Little Shop of Horrors

Carnivorous plants have long been the subject of popular interest and exposition, much of it highly inaccurate. Fictional plants have been featured in a number of books, movies, television series, and video games. Typically, these fictional depictions include exaggerated characteristics, such as enormous size or possession of abilities beyond the realm of reality, and can be viewed as a kind of artistic license. The most famous examples of fictional carnivorous plants in popular culture include the 1960s black comedy The Little Shop of Horrors, the triffids of John Wyndham's The Day of the Triffids, and others. Other movies and television series utilize accurate depictions of carnivorous plants for cinematic purposes.

Depiction of a native being consumed by a Ya-te-veo ("I can see you") carnivorous tree of Central America, from Land and Sea by J.W. Buel, 1887

The earliest known depiction of carnivorous plants in popular culture was a case where a large man-eating tree was reported to have consumed a young woman in Madagascar in 1878, as witnessed by Dr. Carl Liche. Liche reported the events in the South Australian Register in 1881. The woman, pictured in an accompanying artwork, was supposed to have been a member of the Mkodos, a "little known but cruel tribe". The account has been debunked as pure myth as it appears Dr. Liche, the Mkodos, and the tree were all fabrications.^[30]

See also

• Aggressive mimicry
• Carnivorous fungus
• Predatory dinoflagellate

References

Further reading

• Slack A (1986). Insect-eating plants and how to grow them. Sherborne UK: Alphabooks. ISBN ISBN 0-906670-42-X. 
• Juniper BE, Robins RJ, Joel DM (1989). The carnivorous plants. Academic Press, San Diego. 
• Carnivorous Plant Database provides an up-to-date, searchable database of all the published species of carnivorous plants.
• Carnivorous Plant FAQ at Sarracenia.com
• List of films and TV shows that feature carnivorous plants-most of them fictional
• Botanical Society of America - Carnivorous Plants Online
• Ellison, A.M. 2006. Nutrient Limitation and Stoichiometry of Carnivorous Plants.PDF (334 KiB) Plant Biol. 8: 740–747.