Carnivorous plant

Carnivorous plants are plants that derive some or most of their nutrients from trapping and consuming animals or protozoans, typically insects and other arthropods. Carnivorous plants still generate some of their energy from photosynthesis. Carnivorous plants have adapted to grow in places where the soil is thin or poor in nutrients, especially nitrogen, such as acidic bogs.
They can be found on all continents except Antarctica, as well as many Pacific islands. In 1875 Charles Darwin published '' Insectivorous Plants'', the first treatise to recognize the significance of carnivory in plants, describing years of painstaking research.

True carnivory is believed to have evolved independently at least 12 times in five different orders of flowering plants, and is represented by more than a dozen genera. This classification includes at least 583 species that attract, trap, and kill prey, absorbing the resulting available nutrients. Venus flytrap (''Dionaea muscipula''), pitcher plant (''Cephalotus follicularis''), and bladderwort (''Utricularia gibba'') can be seen as exemplars of key traits genetically associated with carnivory: trap leaf development, prey digestion, and nutrient absorption.

The number of known species has increased by approximately 3 species per year since the year 2000. Additionally, over 300 protocarnivorous plant species in several genera show some but not all of these characteristics. A 2020 assessment has found that roughly one quarter are threatened with extinction from human actions.

Definition

Plants are considered carnivorous if they have these five traits:

# capture prey in traps
# kill the captured prey
# digest the captured prey
# absorb nutrients from the killed and digested prey
# use those nutrients to grow and develop.

Other traits may include the attraction and retention of prey.

Trapping mechanisms

Five basic trapping mechanisms are found in carnivorous 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 utilise rapid leaf movements.
# Bladder traps suck in prey with a bladder that generates an internal vacuum.
# Lobster-pot traps, also known as eel traps, use inward-pointing hairs to force prey to move towards a digestive organ.

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 flypaper traps whose leaves undergo rapid acid growth, which is an expansion of individual cells as opposed to cell division. The rapid acid growth allows the sundews' tentacles to bend, aiding in the retention and digestion of prey.

Pitfall traps

Characterised by an internal chamber, pitfall traps are thought to have evolved independently at least six times. This particular adaptation is found within the families Sarraceniaceae '' (Darlingtonia, Heliamphora, Sarracenia''), Nepenthaceae (''Nepenthes''), and Cephalotaceae (''Cephalotus''). Within the family Bromeliaceae'','' pitcher morphology and carnivory evolved twice ''(Brocchinia'' and ''Catopsis).'' Because these families do not share a common ancestor who also had pitfall trap morphology, carnivorous pitchers are an example of convergent evolution.

A passive trap, pitfall traps attract prey with nectar bribes secreted by the peristome and bright flower-like anthocyanin patterning within the pitcher. The linings of most pitcher plants are covered in a loose coating of waxy flakes which are slippery for insects, causing them to fall into the pitcher. Once within the pitcher structure, digestive enzymes or mutualistic species break down the prey into an absorbable form for the plant. Water can become trapped within the pitcher, making a habitat for other flora and fauna. This type of 'water body' is called a Phytotelma.

The simplest pitcher plants are probably those of ''Heliamphora'', the marsh pitcher plant. In this genus, the traps are clearly derived 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.

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; Most ''Heliamphora'' rely on bacterial digestion alone with the exception of a single species, Heliamphora tatei, which does produce digestive enzymes. The enzymes digest the proteins and nucleic acids in the prey, releasing amino acids and phosphate ions, which the plant absorbs. 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.

'' 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.

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, such as ''Nepenthes rajah'', also occasionally take small mammals and reptiles. ''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.

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 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 utilises sticky mucilage or glue. The leaf of flypaper traps is studded with mucilage-secreting glands, which may be short (like those of the butterworts), or long and mobile (like those of many sundews). Flypapers have evolved independently at least five times. There is evidence that some clades of flypaper traps have evolved from morphologically more complex traps such as pitchers.

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.

Similar 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