Insectivorous Plants

1 January 2017

The rapid acid growth allows the sundew tentacles to bend, aiding in the retention and digestion of prey. [5] 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 marsh 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.

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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. Sarracenia purpurea subsp. urpurea (the northern pitcher plant) can be 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. Nepenthes bicalcarata possesses two sharp thorns that project from the base of the operculum over the entrance to the pitcher.

These likely serve to lure insects into a precarious position over the pitcher mouth, where they may lose their footing and fall into the fluid within. [6] 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. [7] 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. [citation needed] Flypaper traps The leaf of a Drosera capensis bending in response to the trapping of an insect 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. In the genus Pinguicula, the mucilage glands are quite short (sessile), and the leaf, while 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 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. Drosera capensis responding to captured prey. This scene is about 4 hours in real-time. 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 the sensitive hairs on the leaf lobes are triggered. 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” , “bear trap” or “man trap”, based on their shape and 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. [8] 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 muscipula, 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. [9] 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. [10] 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,[9] flipping rapidly from convex to concave[11] and interring the prey.

This whole process takes less than a second. In the Venus flytrap, 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 close even tighter (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, depending on the growing conditions. The tip of one stolon of Utricularia vulgaris, showing stolon, branching leaf-shoots, and transparent bladder traps Bladder traps 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. acrorhiza appears to regulate the number of bladders it bears in response to the prevailing nutrient content of its habitat. Lobster-pot traps Genlisea violacea traps and leaves 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.

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