Carnivorous plant: Encyclopedia II - Carnivorous plant - Trapping mechanisms

Carnivorous plant - Trapping mechanisms

There are five basic trapping mechanisms that have evolved in carnivorous plants. These are:

• Pitfall traps (pitcher plants), which trap prey in a rolled leaf that contains a pool of digestive enzymes and/or bacteria;
• 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;
• Lobster-pot traps, 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, such as Triphyophyllum, which secrete mucilage, but whose leaves do not grow or move in response to prey capture; and there are also active flypapers, such as sundews, whose leaves undergo rapid growth, aiding in the retention and digestion of prey.

Carnivorous plant - 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 traps are quite clearly evolutionarily derived from a simple rolled leaf, whose margins have been 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: 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. S. purpurea (the huntman's cup) has a more cosmopolitan distribution. Sarracenia is the pitcher plant genus most commonly encountered in cultivation, and its members are most commonly known as trumpet pitchers.

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, 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. These 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, which are chlorophyll-free patches through which light can penetrate. Insects (mostly ants) enter the chamber via the opening, which is underneath the balloon, and once inside, tire themselves trying to escape from these false exits, until they eventually fall into the tube. Prey access is increased by the 'fangs' (outgrowths of the operculum), which give the plant its name. Seedling Sarracenia species also have long, overhanging opercular outgrowths: Darlingtonia may therefore represent an example of neoteny.

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, 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 covered in a loose coating of waxy, flakes which provides a very uncertain 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 also found in hemlock, which probably increases the efficiency of the traps by intoxicating the prey items.

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.

Carnivorous plant - 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 superficially appear particularly carnivorous. However, this belies the fact that the leaf is an extremely effective trap of small flying insects (such as fungus gnats), and whose 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 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 are capable of bending 180° in only a minute or so. Sundews are extremely cosmopolitan, and are found on all the continents except the Antarctic mainland. They are probably at their most diverse in Australia, the home of the large subgroup of pygmy sundews, such as D. pygmaea, and 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: the leaves are incapable of rapid movement or growth. Unrelated, but similar in habit, are the Australian rainbow plants (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.

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, however, in its juvenile phase, the plant is carnivorous: this may be related to a requirement for specific nutrients for flowering.

Carnivorous plant - 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 vesiculosa). Aldrovanda is aquatic, and specialised in catching small invertebrates; Dionaea is terrestrial and catches mostly flies. The traps are very similar: they have leaves whose terminal section is divided into two lobes, hinged along the midrib. Trigger hairs (three on each lobe in the case of Dionaea, many more in the case of Aldrovanda) inside the trap lobes are sensitive to touch. When the trigger hairs are bent, 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's having a simple memory: for the lobes to shut, two stimuli are required, between 0.5 and 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.

Carnivorous plant - Bladder traps

Bladder traps are exclusive to the genus Utricularia, or bladderworts. These possess bladders (vesicula), 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 (such as U. sandersonii) are terrestrial, growing on waterlogged soil, and their trapping mechanism is triggered in a slightly different manner. Bladderworts lack roots, although terrestrial species have anchoring stems that resemble them. Temperate aquatic bladderworts genearlly die back to a resting turion during the winter months, and U. macrorhiza appears to regulate the number of bladders its bears in response to the prevailing nutrient content of its habitat.

Carnivorous plant - 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 to prey, but not exit. This is achieved by inward-pointing hairs, which force the prey to move in a particular direction. Prey items 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.

Carnivorous plant - 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 is a matter of definition, although to many horticulturalists, it is a matter of taste. So there is a spectrum of carnivory: from 'non-carnivores' like cabbages, through borderline carnivores, to the true carnivores, including both unspecialised and simple traps, like Heliamphora, to extremely specialised and complex traps, like that of the Venus flytrap.

The borderline carnivores of most interest are Roridula and Catopsis berteroniana. Catopsis is a borderline carnivorous bromeliad, like Brocchinia; however, Roridula has a more intricate relationship with its 'prey'. 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 species of assassin bug (Pameridea), which eat the trapped insects: the plant benefits by absorbing nutrients from the bugs' faeces.

A number of species in the Martyniaceae (previously the Pedaliaceae), such as Ibicella lutea have sticky leaves that trap insects; however, these plants have not been shown conclusively to be carnivorous. Likewise, the seeds of Shepherd's Purse, urns of Paepalanthus bromelioides and bracts of Passiflora foetida 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 diagnostic criterion for carnivory. However, this would probably discount Byblis, Heliamphora and Darlingtonia, all of which appear to rely on the enzymes of symbiotic bacteria to break down their prey, but are generally considered to be acceptable as carnivores. However, discouting the enzyme-based definition leaves open the question of Roridula: there is no clear reason why a plant's possession of symbiotic bacteria that allow it to benfit from trapped prey should allow the plant to be considered carnivorous, whilst possession of symbiotic bugs should not.

Adapted from the Wikipedia article "Trapping mechanisms", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki