Starch
Starch or amylum is a polymeric carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as energy storage, it is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, maize and cassava. Pure starch is a white and odorless powder, insoluble in cold water or alcohol, it consists of two types of molecules: the branched amylopectin. Depending on the plant, starch contains 20 to 25% amylose and 75 to 80% amylopectin by weight. Glycogen, the glucose store of animals, is a more branched version of amylopectin. In industry, starch is converted into sugars, for example by malting, fermented to produce ethanol in the manufacture of beer and biofuel, it is processed to produce many of the sugars used in processed foods. Mixing most starches in warm water produces a paste, such as wheatpaste, which can be used as a thickening, stiffening or gluing agent; the biggest industrial non-food use of starch is as an adhesive in the papermaking process.
Starch can be applied to parts of some garments before ironing. The word "starch" is from a Germanic root with the meanings "strong, strengthen, stiffen". Modern German Stärke is related; the Greek term for starch, "amylon", is related. It provides the root amyl, used as a prefix for several 5-carbon compounds related to or derived from starch. Starch grains from the rhizomes of Typha as flour have been identified from grinding stones in Europe dating back to 30,000 years ago. Starch grains from sorghum were found on grind stones in caves in Ngalue, Mozambique dating up to 100,000 years ago. Pure extracted wheat starch paste was used in Ancient Egypt to glue papyrus; the extraction of starch is first described in the Natural History of Pliny the Elder around AD 77–79. Romans used it in cosmetic creams, to powder the hair and to thicken sauces. Persians and Indians used it to make dishes similar to gothumai wheat halva. Rice starch as surface treatment of paper has been used in paper production in China since 700 CE.
In addition to starchy plants consumed directly, by 2008 66 million tonnes of starch were being produced per year worldwide. In 2011 production was increased to 73 million ton. In the EU the starch industry produced about 8.5 million tonnes in 2008, with around 40% being used for industrial applications and 60% for food uses, most of the latter as glucose syrups. In 2017 EU production was 11 million ton of which 9,4 million ton was consumed in the EU and of which 54% were starch sweeteners. US produced about 27,5 million ton starch in 2017 of which about 8,2 million ton high fructose syrup and 6,2 million ton glucose syrups and 2,5 million ton starch products, the rest of the starch was used for producing ethanol. Most green plants use starch as their energy store; the extra glucose is changed into starch, more complex than glucose. An exception is the family Asteraceae. Inulin-like fructans are present in grasses such as wheat, in onions and garlic and asparagus. In photosynthesis, plants use light energy to produce glucose from carbon dioxide.
The glucose is used to generate the chemical energy required for general metabolism, to make organic compounds such as nucleic acids, lipids and structural polysaccharides such as cellulose, or is stored in the form of starch granules, in amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds and tubers store starch to prepare for the next growing season. Glucose is soluble in water, binds with water and takes up much space and is osmotically active. Glucose molecules are bound in starch by the hydrolyzed alpha bonds; the same type of bond is found in the animal reserve polysaccharide glycogen. This is in contrast to many structural polysaccharides such as chitin and peptidoglycan, which are bound by beta bonds and are much more resistant to hydrolysis. Plants produce starch by first converting glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase; this step requires energy in the form of ATP. The enzyme starch synthase adds the ADP-glucose via a 1,4-alpha glycosidic bond to a growing chain of glucose residues, liberating ADP and creating amylose.
The ADP-glucose is certainly added to the non-reducing end of the amylose polymer, as the UDP-glucose is added to the non-reducing end of glycogen during glycogen synthesis. Starch branching enzyme introduces 1,6-alpha glycosidic bonds between the amylose chains, creating the branched amylopectin; the starch debranching enzyme isoamylase removes some of these branches. Several isoforms of these enzymes exist, leading to a complex synthesis process. Glycogen and amylopectin have similar structure, but the former has about one branch point per ten 1,4-alpha bonds, compared to about one branch point per thirty 1,4-alpha bonds in amylopectin. Amylopectin is synthesized from ADP-glucose while mammals and fungi synthesize glycogen from UDP-glucose. In addition to starch synthesis in plants, starch can be synthesized from non-food starch mediated by an enzyme cocktail. In this cell-free biosystem, beta-1,4-glycosidic bond-linked cellulose is hydrolyzed to cello
Symbiosis
Symbiosis is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. The organisms, each termed a symbiont, may be of different species. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms"; the term was subject to a century-long debate about whether it should denote mutualism, as in lichens. Symbiosis can be obligatory, which means that one or both of the symbionts depend on each other for survival, or facultative when they can live independently. Symbiosis is classified by physical attachment; when one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis. The definition of symbiosis was a matter of debate for 130 years. In 1877, Albert Bernhard Frank used the term symbiosis to describe the mutualistic relationship in lichens. In 1879, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms".
The definition has varied among scientists, with some advocating that it should only refer to persistent mutualisms, while others thought it should apply to all persistent biological interactions, in other words mutualisms, commensalism, or parasitism, but excluding brief interactions such as predation. Current biology and ecology textbooks use the latter "de Bary" definition, or an broader one where symbiosis means all interspecific interactions. In 1949, Edward Haskell proposed an integrative approach, proposing a classification of "co-actions" adopted by biologists as "interactions". Biological interactions can involve individuals of the same species or individuals of different species; these can be further classified by either the mechanism of the interaction or the strength and direction of their effects. Relationships can be obligate, meaning that one or both of the symbionts depend on each other for survival. For example, in lichens, which consist of fungal and photosynthetic symbionts, the fungal partners cannot live on their own.
The algal or cyanobacterial symbionts in lichens, such as Trentepohlia, can live independently, their symbiosis is, facultative. Endosymbiosis is any symbiotic relationship in which one symbiont lives within the tissues of the other, either within the cells or extracellularly. Examples include diverse microbiomes, nitrogen-fixing bacteria that live in root nodules on legume roots. Ectosymbiosis is any symbiotic relationship in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. Examples of this include ectoparasites such as lice. Competition can be defined as an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both facilitates this type of interaction, although the competition may exist over other'amenities', such as females for reproduction. Mutualism or interspecies reciprocal altruism is a long-term relationship between individuals of different species where both individuals benefit.
Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both. A large percentage of herbivores have mutualistic gut flora to help them digest plant matter, more difficult to digest than animal prey; this gut flora is made up of cellulose-digesting protozoans or bacteria living in the herbivores' intestines. Coral reefs are the result of mutualisms between coral organisms and various types of algae which live inside them. Most land plants and land ecosystems rely on mutualisms between the plants, which fix carbon from the air, mycorrhyzal fungi, which help in extracting water and minerals from the ground. An example of mutualism is the relationship between the ocellaris clownfish that dwell among the tentacles of Ritteri sea anemones; the territorial fish protects the anemone from anemone-eating fish, in turn the stinging tentacles of the anemone protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.
A further example is a fish which sometimes lives together with a shrimp. The shrimp cleans up a burrow in the sand in which both the shrimp and the goby fish live; the shrimp is blind, leaving it vulnerable to predators when outside its burrow. In case of danger, the goby touches the shrimp with its tail to warn it; when that happens both the shrimp and goby retreat into the burrow. Different species of gobies clean up ectoparasites in other fish another kind of mutualism. A non-obligate symbiosis is seen in encru
Hydrothermal vent
A hydrothermal vent is a fissure on the seafloor from which geothermally heated water issues. Hydrothermal vents are found near volcanically active places, areas where tectonic plates are moving apart at spreading centers, ocean basins, hotspots. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents. Hydrothermal vents exist because the earth is both geologically active and has large amounts of water on its surface and within its crust. Under the sea, hydrothermal vents may form features called white smokers. Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive hosting complex communities fueled by the chemicals dissolved in the vent fluids. Chemosynthetic bacteria and archaea form the base of the food chain, supporting diverse organisms, including giant tube worms, clams and shrimp. Active hydrothermal vents are believed to exist on Jupiter's moon Europa, Saturn's moon Enceladus, it is speculated that ancient hydrothermal vents once existed on Mars.
Hydrothermal vents in the deep ocean form along the mid-ocean ridges, such as the East Pacific Rise and the Mid-Atlantic Ridge. These are locations where two tectonic plates are diverging and new crust is being formed; the water that issues from seafloor hydrothermal vents consists of sea water drawn into the hydrothermal system close to the volcanic edifice through faults and porous sediments or volcanic strata, plus some magmatic water released by the upwelling magma. In terrestrial hydrothermal systems, the majority of water circulated within the fumarole and geyser systems is meteoric water plus ground water that has percolated down into the thermal system from the surface, but it commonly contains some portion of metamorphic water, magmatic water, sedimentary formational brine, released by the magma; the proportion of each varies from location to location. In contrast to the 2 °C ambient water temperature at these depths, water emerges from these vents at temperatures ranging from 60 °C up to as high as 464 °C.
Due to the high hydrostatic pressure at these depths, water may exist in either its liquid form or as a supercritical fluid at such temperatures. The critical point of water is 375 °C at a pressure of 218 atmospheres. However, introducing salinity into the fluid raises the critical point to higher temperatures and pressures; the critical point of seawater is 407 °C and 298.5 bars, corresponding to a depth of ~2,960 m below sea level. Accordingly, if a hydrothermal fluid with a salinity of 3.2 wt. % NaCl vents above 407 °C and 298.5 bars, it is supercritical. Furthermore, the salinity of vent fluids have been shown to vary due to phase separation in the crust; the critical point for lower salinity fluids is at lower temperature and pressure conditions than that for seawater, but higher than that for pure water. For example, a vent fluid with a 2.24 wt. % NaCl salinity has the critical point at 280.5 bars. Thus, water emerging from the hottest parts of some hydrothermal vents can be a supercritical fluid, possessing physical properties between those of a gas and those of a liquid.
Examples of supercritical venting are found at several sites. Sister Peak vents low salinity phase-separated, vapor-type fluids. Sustained venting was not found to be supercritical but a brief injection of 464 °C was well above supercritical conditions. A nearby site, Turtle Pits, was found to vent low salinity fluid at 407 °C, above the critical point of the fluid at that salinity. A vent site in the Cayman Trough named Beebe, the world's deepest known hydrothermal site at ~5,000 m below sea level, has shown sustained supercritical venting at 401 °C and 2.3 wt% NaCl. Although supercritical conditions have been observed at several sites, it is not yet known what significance, if any, supercritical venting has in terms of hydrothermal circulation, mineral deposit formation, geochemical fluxes or biological activity; the initial stages of a vent chimney begin with the deposition of the mineral anhydrite. Sulfides of copper and zinc precipitate in the chimney gaps, making it less porous over the course of time.
Vent growths on the order of 30 cm per day have been recorded. An April 2007 exploration of the deep-sea vents off the coast of Fiji found those vents to be a significant source of dissolved iron; some hydrothermal vents form cylindrical chimney structures. These form from minerals; when the superheated water contacts the near-freezing sea water, the minerals precipitate out to form particles which add to the height of the stacks. Some of these chimney structures can reach heights of 60 m. An example of such a towering vent was "Godzilla", a structure on the Pacific Ocean deep seafloor near Oregon that rose to 40 m before it fell over in 1996. A black smoker or deep sea vent is a type of hydrothermal vent found on the seabed in the bathyal zone, but in lesser depths as well as deeper in abyssal zone, they appear as chimney-like structures that emit a cloud of black material. Black smokers emit particles with high levels of sulfur-bearing minerals, or sulfides. Black smokers are formed in fields hundreds of meters wide when superheated water from below Earth's crust comes through the ocean floor.
This water is most notably sulfides. When it
Paraphyly
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—monophyletic subgroups. The group is said to be paraphyletic with respect to the excluded subgroups; the arrangement of the members of a paraphyletic group is called a paraphyly. The term is used in phylogenetics and in linguistics; the term was coined to apply to well-known taxa like Reptilia which, as named and traditionally defined, is paraphyletic with respect to mammals and birds. Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor—including all extant reptiles as well as the extinct synapsids—except for mammals and birds. Other recognized paraphyletic groups include fish and lizards. If many subgroups are missing from the named group, it is said to be polyparaphyletic. A paraphyletic group cannot be a clade, or monophyletic group, any group of species that includes only a common ancestor and all of its descendants.
Formally, a paraphyletic group is the relative complement of one or more subclades within a clade: removing one or more subclades leaves a paraphyletic group. The term paraphyly, or paraphyletic, derives from the two Ancient Greek words παρά, meaning "beside, near", φῦλον, meaning "genus, species", refers to the situation in which one or several monophyletic subgroups of organisms are left apart from all other descendants of a unique common ancestor. Conversely, the term monophyly, or monophyletic, builds on the Ancient Greek prefix μόνος, meaning "alone, unique", refers to the fact that a monophyletic group includes organisms consisting of all the descendants of a unique common ancestor. By comparison, the term polyphyly, or polyphyletic, uses the Ancient Greek prefix πολύς, meaning "many, a lot of", refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. Groups that include all the descendants of a common ancestor are said to be monophyletic.
A paraphyletic group is a monophyletic group from which one or more subsidiary clades are excluded to form a separate group. Ereshefsky has argued that paraphyletic taxa are the result of anagenesis in the excluded group or groups. A group whose identifying features evolved convergently in two or more lineages is polyphyletic. More broadly, any taxon, not paraphyletic or monophyletic can be called polyphyletic; these terms were developed during the debates of the 1960s and 1970s accompanying the rise of cladistics. Paraphyletic groupings are considered problematic by many taxonomists, as it is not possible to talk about their phylogenetic relationships, their characteristic traits and literal extinction. Related terminology that may be encountered are stem group, budding cladogenesis, anagenesis, or'grade' groupings. Paraphyletic groups are a relic from previous erroneous assessments about phylogenic relationships, or from before the rise of cladistics; the prokaryotes, because they exclude the eukaryotes, a descendant group.
Bacteria and Archaea are prokaryotes, but archaea and eukaryotes share a common ancestor, not ancestral to the bacteria. The prokaryote/eukaryote distinction was proposed by Edouard Chatton in 1937 and was accepted after being adopted by Roger Stanier and C. B. van Niel in 1962. The botanical code abandoned consideration of bacterial nomenclature in 1975. Among plants, dicotyledons are paraphyletic. "Dicotyledon" has not been used as an ICBN classification for decades, but is allowed as a synonym of Magnoliopsida. Phylogenetic analysis indicates. Excluding monocots from the dicots makes the latter a paraphyletic group. Among animals, several familiar groups are not, in fact, clades; the order Artiodactyla is paraphyletic. In the ICZN Code, the two taxa are orders of equal rank. Molecular studies, have shown that the Cetacea descend from artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetacean descendants the Artiodactyls must be paraphyletic; the class Reptilia as traditionally defined is paraphyletic because it excludes mammals.
In the ICZN Code, the three taxa are classes of equal rank. However, mammals hail from the synapsids and birds are descended from the dinosaurs, both of which are reptiles. Alternatively, reptiles are paraphyletic. Birds and reptiles together make Sauropsids. Osteichthyes, bony fish, are paraphyletic when they include only Actinopterygii and Sarcopterygii, excluding tetrapods; the wasps are paraphyletic, consisting of the narrow-waisted Apocrita without the bees. The sawflies are paraphyletic, forming all of the Hymenoptera except for the Apocrita, a clade deep within the sawfly tree. Crustaceans are not a clade; the modern clade that spans all of them is the Tetraconata. Species have a special status in systematics as being an observable feature of nature itself and a
Marine biology
Marine biology is the scientific study of marine life, organisms in the sea. Given that in biology many phyla and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy. A large proportion of all life on Earth lives in the ocean; the exact size of this large proportion is unknown, since many ocean species are still to be discovered. The ocean is a complex three-dimensional world covering 71% of the Earth's surface; the habitats studied in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the oceanic trenches, sometimes 10,000 meters or more beneath the surface of the ocean. Specific habitats include coral reefs, kelp forests, seagrass meadows, the surrounds of seamounts and thermal vents, muddy and rocky bottoms, the open ocean zone, where solid objects are rare and the surface of the water is the only visible boundary.
The organisms studied range from microscopic phytoplankton and zooplankton to huge cetaceans 25–32 meters in length. Marine ecology is the study of how marine organisms interact with the environment. Marine life is a vast resource, providing food and raw materials, in addition to helping to support recreation and tourism all over the world. At a fundamental level, marine life helps determine the nature of our planet. Marine organisms contribute to the oxygen cycle, are involved in the regulation of the Earth's climate. Shorelines are in part shaped and protected by marine life, some marine organisms help create new land. Many species are economically important to humans, including shellfish, it is becoming understood that the well-being of marine organisms and other organisms are linked in fundamental ways. The human body of knowledge regarding the relationship between life in the sea and important cycles is growing, with new discoveries being made nearly every day; these cycles include those of matter and of air.
Large areas beneath the ocean surface still remain unexplored. The study of marine biology dates back to Aristotle, who made many observations of life in the sea around Lesbos, laying the foundation for many future discoveries. In 1768, Samuel Gottlieb Gmelin published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the new binomial nomenclature of Linnaeus, it included elaborate illustrations of seaweed and marine algae on folded leaves. The British naturalist Edward Forbes is regarded as the founder of the science of marine biology; the pace of oceanographic and marine biology studies accelerated during the course of the 19th century. The observations made in the first studies of marine biology fueled the age of discovery and exploration that followed. During this time, a vast amount of knowledge was gained about the life that exists in the oceans of the world. Many voyages contributed to this pool of knowledge. Among the most significant were the voyages of HMS Beagle where Charles Darwin came up with his theories of evolution and on the formation of coral reefs.
Another important expedition was undertaken by HMS Challenger, where findings were made of unexpectedly high species diversity among fauna stimulating much theorizing by population ecologists on how such varieties of life could be maintained in what was thought to be such a hostile environment. This era was important for the history of marine biology but naturalists were still limited in their studies because they lacked technology that would allow them to adequately examine species that lived in deep parts of the oceans; the creation of marine laboratories was important because it allowed marine biologists to conduct research and process their specimens from expeditions. The oldest marine laboratory in the world, Station biologique de Roscoff, was established in France in 1872. In the United States, the Scripps Institution of Oceanography dates back to 1903, while the prominent Woods Hole Oceanographic Institute was founded in 1930; the development of technology such as sound navigation ranging, scuba diving gear and remotely operated vehicles allowed marine biologists to discover and explore life in deep oceans, once thought to not exist.
As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for all the photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen and other nutrients and trace elements. Microscopic life undersea is diverse and still poorly understood. For example, the role of viruses in marine ecosystems is being explored in the beginning of the 21st century; the role of phytoplankton is better understood due to their critical position as the most numerous primary producers on Earth. Phytoplankton are categorized into cyanobacteria, various types of algae, dinoflagellates, coccolithophorids, chrysophytes, chlorophytes and silicoflagellates. Zooplankton tend to be somewhat larger, not all are microscopic. Many Protozoa are zooplankton, including dinoflagellates, zooflagellates and radiolarians; some of these are phytoplankton.
Species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more they represent problematic species concepts. For example, the boundaries between related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, in a ring species. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, each clone is a microspecies. All species are given a two-part name, a "binomial"; the first part of a binomial is the genus.
The second part is called the specific epithet. For example, Boa constrictor is one of four species of the genus Boa. None of these is satisfactory definitions, but scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, to grade into one another. Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being. In the 19th century, biologists grasped. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection; that understanding was extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures.
Genes can sometimes be exchanged between species by horizontal gene transfer. Viruses are a special case, driven by a balance of mutation and selection, can be treated as quasispecies. Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. Early taxonomists such as Linnaeus had no option but to describe what they saw: this was formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or impossible to test. Biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: agamospecies for asexual organisms biospecies for reproductively isolated sexual organisms ecospecies based on ecological niches evolutionary species based on lineage genetic species based on gene pool morphospecies based on form or phenotype and taxonomic species, a species as determined by a taxonomist.
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that pre-literate people recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens would differentiate the species; this method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not different species. Species named in this manner are called morphospecies. In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms, it differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology, genes can move even between distantly related bacteria extending to the whole bacterial domain; as a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA hybridisation to decide if they belong to the same species or not. This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybri
Lichen
A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. The combined lichen has properties different from those of its component organisms. Lichens come in many colors and forms; the properties are sometimes plant-like, but lichens are not plants. Lichens may have tiny, leafless branches, flat leaf-like structures, flakes that lie on the surface like peeling paint, a powder-like appearance, or other growth forms. A macrolichen is a lichen, either bush-like or leafy. Here, "macro" and "micro" do not refer to size, but to the growth form. Common names for lichens may contain the word moss, lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant. Lichens do not have roots that absorb water and nutrients as plants do, but like plants, they produce their own nutrition by photosynthesis; when they grow on plants, they do not live as parasites, but instead use the plants as a substrate.
Lichens occur from sea level to high alpine elevations, in many environmental conditions, can grow on any surface. Lichens are abundant growing on bark, mosses, on other lichens, hanging from branches "living on thin air" in rain forests and in temperate woodland, they grow on rock, gravestones, exposed soil surfaces, in the soil as part of a biological soil crust. Different kinds of lichens have adapted to survive in some of the most extreme environments on Earth: arctic tundra, hot dry deserts, rocky coasts, toxic slag heaps, they can live inside solid rock, growing between the grains. It is estimated that 6% of Earth's land surface is covered by lichens. There are about 20,000 known species of lichens; some lichens have lost the ability to reproduce sexually, yet continue to speciate. Lichens can be seen as being self-contained miniature ecosystems, where the fungi, algae, or cyanobacteria have the potential to engage with other microorganisms in a functioning system that may evolve as an more complex composite organism.
Lichens may be long-lived, with some considered to be among the oldest living things. They are among the first living things to grow on fresh rock exposed after an event such as a landslide; the long life-span and slow and regular growth rate of some lichens can be used to date events. In American English, "lichen" is pronounced the same as the verb "liken". In British English, both this pronunciation and one rhyming with "kitchen" ) are used. English lichen derives from Greek λειχήν leichēn via Latin lichen; the Greek noun, which means "licker", derives from the verb λείχειν leichein, "to lick". Lichens grow in a wide range of shapes and forms; the shape of a lichen is determined by the organization of the fungal filaments. The nonreproductive tissues, or vegetative body parts, are called the thallus. Lichens are grouped by thallus type, since the thallus is the most visually prominent part of the lichen. Thallus growth forms correspond to a few basic internal structure types. Common names for lichens come from a growth form or color, typical of a lichen genus.
Common groupings of lichen thallus growth forms are: fruticose – growing like a tuft or multiple-branched leafless mini-shrub, upright or hanging down, 3-dimensional branches with nearly round cross section or flattened foliose – growing in 2-dimensional, leaf-like lobes crustose – crust-like, adhering to a surface like a thick coat of paint squamulose – formed of small leaf-like scales crustose below but free at the tips leprose – powdery gelatinous – jelly like filamentous – stringy or like matted hair byssoid – wispy, like teased wool structurelessThere are variations in growth types in a single lichen species, grey areas between the growth type descriptions, overlapping between growth types, so some authors might describe lichens using different growth type descriptions. When a crustose lichen gets old, the center may start to crack up like old-dried paint, old-broken asphalt paving, or like the polygonal "islands" of cracked-up mud in a dried lakebed; this is called being rimose or areolate, the "island" pieces separated by the cracks are called areolas.
The areolas appear separated, but are connected by an underlying "prothallus" or "hypothallus". When a crustose lichen grows from a center and appears to radiate out, it is called crustose placodioid; when the edges of the areolas lift up from the substrate, it is called squamulose. These growth form groups are not defined. Foliose lichens may sometimes branch and appear to be fruticose. Fruticose lichens may have flattened branching parts and appear leafy. Squamulose lichens may appear where the edges lift up. Gelatinous lichens may appear leafy when dry. Means of telling them apart in these cases are in the sections below. Structures involved in reproduction appear as discs, bumps, or squiggly lines on the surface of the thallus; the thallus is not always the part of the lichen, most visually noticeable. Some lichens can grow inside solid rock between the grains, with only the sexual fruiting part visible growing outside the rock; these may be dramatic in color or appearance. Forms of these sexual parts are not in the above growth form categories.
The most visually noticeable reproductive parts are circular, plate-like or disc-like outgrowths, with crinkly edges, are described in sections below. Lichens come in many colors. Coloratio
