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Cannabis indica is an annual plant species in the family Cannabaceae indigenous to the Hindu Kush mountains of Southern Asia. The plant produces large amounts of tetrahydrocannabinol (THC) and tetrahydrocannabivarin (THCV), with total cannabinoid levels being as high as 53.7%. It is now widely grown in China, India, Nepal, Thailand, Afghanistan, and Pakistan, as well as southern and western Africa, and is cultivated for purposes including hashish in India. The high concentrations of THC or THCV provide euphoric effects making it popular for use for several purposes, not only simple pleasure but also clinical drug research, potential new drug research, and use in alternative medicine, among many others.
Taxonomy
In 1785, Jean-Baptiste Lamarck published a description of a second species of Cannabis, which he named Cannabis indica. Lamarck based his description of the newly named species on plant specimens collected in India. Richard Evans Schultes described C. indica as relatively short, conical, and densely branched, whereas C. sativa was described as tall and laxly branched. Loran C. Anderson described C. indica plants as having short, broad leaflets whereas those of C. sativa were characterized as relatively long and narrow. C. indica plants conforming to Schultes's and Anderson's descriptions originated from the Hindu Kush mountain range. Because of the often harsh and variable climate of those parts (extremely cold winters and warm summers), C. indica is well-suited for cultivation in temperate climates.
The specific epithet indica is Latin for "of India" and has come to be synonymous with the cannabis strain.
There was very little debate about the taxonomy of Cannabis until the 1970s, when botanists like Richard Evans Schultes began testifying in court on behalf of accused persons who sought to avoid criminal charges of possession of C. sativa by arguing that the plant material could instead be C. indica.
Cultivation | Cannabis indica | Wikipedia | 417 | 2154325 | https://en.wikipedia.org/wiki/Cannabis%20indica | Biology and health sciences | Rosales | Plants |
Broad-leafed C. indica plants in the Indian Subcontinent are traditionally cultivated for the production of charas, a form of hashish. Pharmacologically, C. indica landraces tend to have higher THC content than C. sativa strains. Some users report more of a "stoned" feeling and less of a "high" from C. indica when compared to C. sativa. (The terms sativa and indica, used in this sense, are more appropriately termed "narrow-leaflet" and "wide-leaflet" drug type, respectively.) The C. indica high is often referred to as a "body buzz" and has beneficial properties such as pain relief in addition to being an effective treatment for insomnia and an anxiolytic, as opposed to C. sativa's more common reports of a cerebral, creative and energetic high, and even (albeit rarely) including hallucinations. Differences in the terpenoid content of the essential oil may account for some of these differences in effect. Common C. indica strains for recreational or medicinal use include Kush and Northern Lights.
A recent genetic analysis included both the narrow-leaflet and wide-leaflet drug "biotypes" under C. indica, as well as southern and eastern Asian hemp (fiber/seed) landraces and wild Himalayan populations.
Genome
In 2011, a team of Canadian researchers led by Andrew Sud announced that they had sequenced a draft genome of the Purple Kush strain of C. indica.
Gallery | Cannabis indica | Wikipedia | 322 | 2154325 | https://en.wikipedia.org/wiki/Cannabis%20indica | Biology and health sciences | Rosales | Plants |
Cannabis ruderalis is a variety, subspecies, or species of Cannabis native to Central and Eastern Europe and Russia. It contains a relatively low quantity of psychoactive compound tetrahydrocannabinol (THC) and does not require photoperiod to blossom (unlike C. indica and C. sativa). Some scholars accept C. ruderalis as its own species due to its unique traits and phenotypes which distinguish it from C. indica and C. sativa; others debate whether ruderalis is a subspecies under C. sativa.
Description
This species is smaller than other species of the genus, rarely growing over in height. The plants have "thin, slightly fibrous stems" with little branching. The foliage is typically open with large leaves. C. ruderalis reaches maturity much quicker than other species of Cannabis, typically 5–7 weeks after being planted from seed.
Unlike other species of the genus, C. ruderalis enters the flowering stage based on the plant's maturity rather than its light cycle. With C. sativa and C. indica varieties, the plant stays in the vegetative state indefinitely as long as a long daylight cycle is maintained. Cannabis geneticists today refer to this feature as "autoflowering" when C. ruderalis is cross-bred.
Regarding its cannabinoid profile, it usually contains less tetrahydrocannabinol (THC) in its resin compared to other Cannabis species but is often high in cannabidiol (CBD).
Taxonomy
Species description
There is no consensus in the botany community that C. ruderalis is one separate species, rather than a subspecies from C. sativa. It was first described in 1924 by D. E. Janischewsky, noting the visible differences in the fruits' seed (an achene), shape and size from previously classified Cannabis sativa.
Genomic studies
Recently, genomic DNA studies utilizing molecular markers and different varieties of plants from diverse geographical origins have been employed to enrich the Cannabis taxonomy discussion. In 2005, Hillig reinforced the polytypic classification system based on allozyme variation at 17 genomic loci. Hillig's approach, proposed a more detailed taxonomy encompassing three species with seven subspecies or varieties: | Cannabis ruderalis | Wikipedia | 469 | 2154347 | https://en.wikipedia.org/wiki/Cannabis%20ruderalis | Biology and health sciences | Rosales | Plants |
C. sativa
C. sativa subsp. sativa var. sativa
C. sativa subsp. sativa var. spontanea
C. sativa subsp. indica var. kafiristanica
C. indica
C. indica
C. indica sensu
C. chinensis
C. ruderalis.
Clarke and Merlin carried out more studies in 2013 to analyze the genus mixing molecular markers, chemotypes and morphological characteristics. They proposed a refinement in Hillig's hypothesis and suggested that C. ruderalis could be the wild ancestor of C. sativa and C. indica. However, these affirmations were based on a limited sample size.
Etymology
The term ruderalis is derived from the Latin rūdera, which is the plural form of rūdus, meaning "rubble", "lump", or "rough piece of bronze". In botanical Latin, ruderalis means "weedy" or "growing among waste". A ruderal species refers to any plant that is the first to colonize land after a disturbance removing competition.
Distribution and habitat
C. ruderalis was first scientifically described in 1924 (from plants collected in southern Siberia), although it grows wild in other areas of Russia. The Russian botanist, Janischewski, was studying wild Cannabis in the Volga River system and realized he had come upon a third species. C. ruderalis is a hardier variety grown in the northern Himalayas and southern states of the former Soviet Union, characterized by a more sparse, "weedy" growth.
Similar C. ruderalis populations can be found in most of the areas where hemp cultivation was once prevalent. The most notable region in North America is the midwestern United States, though populations occur sporadically throughout the United States and Canada. Large wild C. ruderalis populations are found in central and eastern Europe, most of them in Ukraine, Lithuania, Belarus, Latvia, Estonia and adjacent countries. Without human selection, these plants have lost many of the traits they were originally selected for, and have acclimated to their environment.
Cultivation
Seeds of C. ruderalis were brought to Amsterdam in the early 1980s in order to enhance the breeding program of seed banks. | Cannabis ruderalis | Wikipedia | 458 | 2154347 | https://en.wikipedia.org/wiki/Cannabis%20ruderalis | Biology and health sciences | Rosales | Plants |
C. ruderalis has lower THC content than either C. sativa or C. indica, so it is rarely grown for recreational use. Also, the shorter stature of C. ruderalis limits its application for hemp production. C. ruderalis strains are high in the cannabіnoid cannabidiol (CBD), so they are grown by some medical marijuana users.
Because C. ruderalis transitions from the vegetative stage to the flowering stage with age, as opposed to the light cycle required with photoperiod strains, it is bred with other household sativa and indica strains of cannabis to create "auto-flowering cannabis strains". This trait offers breeders some agricultural possibilities and advantages over the photoperiodic flowering varieties, as well as resistance aspects to insect and disease pressures.
C. indica strains are frequently cross-bred with C. ruderalis to produce autoflowering plants with high THC content, improved hardiness and reduced height. Cannabis x intersita Sojak, a strain identified in 1960, is a cross between C. sativa and C. ruderalis. Attempts to produce a Cannabis strain with a shorter growing season are another application of cultivating C. ruderalis. C. ruderalis when crossed with sativa and indica strains will carry the recessive autoflowering trait. Further crosses will stabilise this trait and give a plant which flowers automatically and can be fully mature in as little as 10 weeks.
Cultivators also favor ruderalis plants due to their reduced production time, typically finishing in 3–4 months rather than 6–8 months . The auto-flowering trait is extremely beneficial because it allows for multiple harvests in one outdoor growing season without the use of light deprivation techniques necessary for multiple harvests of photoperiod-dependent strains.
Uses
C. ruderalis is traditionally used in Russian and Mongolian folk medicine, especially for uses in treating depression. Because C. ruderalis is among the lowest THC producing biotypes of Cannabis, C. ruderalis is rarely used for recreational purposes.
In modern use, C. ruderalis has been crossed with Bedrocan strains to produce the strain Bediol for patients with medical prescriptions. The typically higher concentration of CBD may make ruderalis plants viable for the treatment of anxiety or epilepsy.
Bibliography
Books
Articles | Cannabis ruderalis | Wikipedia | 492 | 2154347 | https://en.wikipedia.org/wiki/Cannabis%20ruderalis | Biology and health sciences | Rosales | Plants |
In ecology, resilience is the capacity of an ecosystem to respond to a perturbation or disturbance by resisting damage and subsequently recovering. Such perturbations and disturbances can include stochastic events such as fires, flooding, windstorms, insect population explosions, and human activities such as deforestation, fracking of the ground for oil extraction, pesticide sprayed in soil, and the introduction of exotic plant or animal species. Disturbances of sufficient magnitude or duration can profoundly affect an ecosystem and may force an ecosystem to reach a threshold beyond which a different regime of processes and structures predominates. When such thresholds are associated with a critical or bifurcation point, these regime shifts may also be referred to as critical transitions.
Human activities that adversely affect ecological resilience such as reduction of biodiversity, exploitation of natural resources, pollution, land use, and anthropogenic climate change are increasingly causing regime shifts in ecosystems, often to less desirable and degraded conditions. Interdisciplinary discourse on resilience now includes consideration of the interactions of humans and ecosystems via socio-ecological systems, and the need for shift from the maximum sustainable yield paradigm to environmental resource management and ecosystem management, which aim to build ecological resilience through "resilience analysis, adaptive resource management, and adaptive governance". Ecological resilience has inspired other fields and continues to challenge the way they interpret resilience, e.g. supply chain resilience.
Definitions
The IPCC Sixth Assessment Report defines resilience as, “not just the ability to maintain essential function, identity and structure, but also the capacity for transformation.” The IPCC considers resilience both in terms of ecosystem recovery as well as the recovery and adaptation of human societies to natural disasters. | Ecological resilience | Wikipedia | 354 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
The concept of resilience in ecological systems was first introduced by the Canadian ecologist C.S. Holling in order to describe the persistence of natural systems in the face of changes in ecosystem variables due to natural or anthropogenic causes. Resilience has been defined in two ways in ecological literature:
as the time required for an ecosystem to return to an equilibrium or steady-state following a perturbation (which is also defined as stability by some authors). This definition of resilience is used in other fields such as physics and engineering, and hence has been termed ‘engineering resilience’ by Holling.
as "the capacity of a system to absorb disturbance and reorganize while undergoing change so as to still retain essentially the same function, structure, identity, and feedbacks".
The second definition has been termed ‘ecological resilience’, and it presumes the existence of multiple stable states or regimes.
For example, some shallow temperate lakes can exist within either clear water regime, which provides many ecosystem services, or a turbid water regime, which provides reduced ecosystem services and can produce toxic algae blooms. The regime or state is dependent upon lake phosphorus cycles, and either regime can be resilient dependent upon the lake's ecology and management.
Likewise, Mulga woodlands of Australia can exist in a grass-rich regime that supports sheep herding, or a shrub-dominated regime of no value for sheep grazing. Regime shifts are driven by the interaction of fire, herbivory, and variable rainfall. Either state can be resilient dependent upon management.
Theory
Ecologists Brian Walker, C S Holling and others describe four critical aspects of resilience: latitude, resistance, precariousness, and panarchy.
The first three can apply both to a whole system or the sub-systems that make it up. | Ecological resilience | Wikipedia | 380 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
Latitude: the maximum amount a system can be changed before losing its ability to recover (before crossing a threshold which, if breached, makes recovery difficult or impossible).
Resistance: the ease or difficulty of changing the system; how “resistant” it is to being changed.
Precariousness: how close the current state of the system is to a limit or “threshold.”.
Panarchy: the degree to which a certain hierarchical level of an ecosystem is influenced by other levels. For example, organisms living in communities that are in isolation from one another may be organized differently from the same type of organism living in a large continuous population, thus the community-level structure is influenced by population-level interactions.
Closely linked to resilience is adaptive capacity, which is the property of an ecosystem that describes change in stability landscapes and resilience. Adaptive capacity in socio-ecological systems refers to the ability of humans to deal with change in their environment by observation, learning and altering their interactions.
Human impacts
Resilience refers to ecosystem's stability and capability of tolerating disturbance and restoring itself. If the disturbance is of sufficient magnitude or duration, a threshold may be reached where the ecosystem undergoes a regime shift, possibly permanently. Sustainable use of environmental goods and services requires understanding and consideration of the resilience of the ecosystem and its limits. However, the elements which influence ecosystem resilience are complicated. For example, various elements such as the water cycle, fertility, biodiversity, plant diversity and climate, interact fiercely and affect different systems.
There are many areas where human activity impacts upon and is also dependent upon the resilience of terrestrial, aquatic and marine ecosystems. These include agriculture, deforestation, pollution, mining, recreation, overfishing, dumping of waste into the sea and climate change.
Agriculture | Ecological resilience | Wikipedia | 365 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
Agriculture can be used as a significant case study in which the resilience of terrestrial ecosystems should be considered. The organic matter (elements carbon and nitrogen) in soil, which is supposed to be recharged by multiple plants, is the main source of nutrients for crop growth. In response to global food demand and shortages, however, intensive agriculture practices including the application of herbicides to control weeds, fertilisers to accelerate and increase crop growth and pesticides to control insects, reduce plant biodiversity while the supply of organic matter to replenish soil nutrients and prevent surface runoff is diminished. This leads to a reduction in soil fertility and productivity. More sustainable agricultural practices would take into account and estimate the resilience of the land and monitor and balance the input and output of organic matter.
Deforestation
The term deforestation has a meaning that covers crossing the threshold of forest's resilience and losing its ability to return to its originally stable state. To recover itself, a forest ecosystem needs suitable interactions among climate conditions and bio-actions, and enough area. In addition, generally, the resilience of a forest system allows recovery from a relatively small scale of damage (such as lightning or landslide) of up to 10 percent of its area. The larger the scale of damage, the more difficult it is for the forest ecosystem to restore and maintain its balance.
Deforestation also decreases biodiversity of both plant and animal life and can lead to an alteration of the climatic conditions of an entire area. According to the IPCC Sixth Assessment Report, carbon emissions due to land use and land use changes predominantly come from deforestation, thereby increasing the long-term exposure of forest ecosystems to drought and other climate change-induced damages. Deforestation can also lead to species extinction, which can have a domino effect particularly when keystone species are removed or when a significant number of species is removed and their ecological function is lost.
Climate change
Overfishing
It has been estimated by the United Nations Food and Agriculture Organisation that over 70% of the world's fish stocks are either fully exploited or depleted which means overfishing threatens marine ecosystem resilience and this is mostly by rapid growth of fishing technology. One of the negative effects on marine ecosystems is that over the last half-century the stocks of coastal fish have had a huge reduction as a result of overfishing for its economic benefits. Blue fin tuna is at particular risk of extinction. Depletion of fish stocks results in lowered biodiversity and consequently imbalance in the food chain, and increased vulnerability to disease. | Ecological resilience | Wikipedia | 512 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
In addition to overfishing, coastal communities are suffering the impacts of growing numbers of large commercial fishing vessels in causing reductions of small local fishing fleets. Many local lowland rivers which are sources of fresh water have become degraded because of the inflows of pollutants and sediments.
Dumping of waste into the sea
Dumping both depends upon ecosystem resilience whilst threatening it. Dumping of sewage and other contaminants into the ocean is often undertaken for the dispersive nature of the oceans and adaptive nature and ability for marine life to process the marine debris and contaminants. However, waste dumping threatens marine ecosystems by poisoning marine life and eutrophication.
Poisoning marine life
According to the International Maritime Organisation oil spills can have serious effects on marine life. The OILPOL Convention recognized that most oil pollution resulted from routine shipboard operations such as the cleaning of cargo tanks. In the 1950s, the normal practice was simply to wash the tanks out with water and then pump the resulting mixture of oil and water into the sea. OILPOL 54 prohibited the dumping of oily wastes within a certain distance from land and in 'special areas' where the danger to the environment was especially acute. In 1962 the limits were extended by means of an amendment adopted at a conference organized by IMO. Meanwhile, IMO in 1965 set up a Subcommittee on Oil Pollution, under the auspices of its Maritime Safety committee, to address oil pollution issues.
The threat of oil spills to marine life is recognised by those likely to be responsible for the pollution, such as the International Tanker Owners Pollution Federation:
The marine ecosystem is highly complex and natural fluctuations in species composition, abundance and distribution are a basic feature of its normal function. The extent of damage can therefore be difficult to detect against this background variability. Nevertheless, the key to understanding damage and its importance is whether spill effects result in a downturn in breeding success, productivity, diversity and the overall functioning of the system. Spills are not the only pressure on marine habitats; chronic urban and industrial contamination or the exploitation of the resources they provide are also serious threats. | Ecological resilience | Wikipedia | 422 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
Eutrophication and algal blooms
The Woods Hole Oceanographic Institution calls nutrient pollution the most widespread, chronic environmental problem in the coastal ocean. The discharges of nitrogen, phosphorus, and other nutrients come from agriculture, waste disposal, coastal development, and fossil fuel use. Once nutrient pollution reaches the coastal zone, it stimulates harmful overgrowths of algae, which can have direct toxic effects and ultimately result in low-oxygen conditions. Certain types of algae are toxic. Overgrowths of these algae result in harmful algal blooms, which are more colloquially referred to as "red tides" or "brown tides". Zooplankton eat the toxic algae and begin passing the toxins up the food chain, affecting edibles like clams, and ultimately working their way up to seabirds, marine mammals, and humans. The result can be illness and sometimes death.
Sustainable development
There is increasing awareness that a greater understanding and emphasis of ecosystem resilience is required to reach the goal of sustainable development. A similar conclusion is drawn by Perman et al. who use resilience to describe one of 6 concepts of sustainability; "A sustainable state is one which satisfies minimum conditions for ecosystem resilience through time". Resilience science has been evolving over the past decade, expanding beyond ecology to reflect systems of thinking in fields such as economics and political science. And, as more and more people move into densely populated cities, using massive amounts of water, energy, and other resources, the need to combine these disciplines to consider the resilience of urban ecosystems and cities is of paramount importance.
Academic perspectives
The interdependence of ecological and social systems has gained renewed recognition since the late 1990s by academics including Berkes and Folke and developed further in 2002 by Folke et al. As the concept of sustainable development has evolved beyond the 3 pillars of sustainable development to place greater political emphasis on economic development. This is a movement which causes wide concern in environmental and social forums and which Clive Hamilton describes as "the growth fetish". | Ecological resilience | Wikipedia | 417 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
The purpose of ecological resilience that is proposed is ultimately about averting our extinction as Walker cites Holling in his paper: "[..] "resilience is concerned with [measuring] the probabilities of extinction” (1973, p. 20)". Becoming more apparent in academic writing is the significance of the environment and resilience in sustainable development. Folke et al state that the likelihood of sustaining development is raised by "Managing for resilience" whilst Perman et al. propose that safeguarding the environment to "deliver a set of services" should be a "necessary condition for an economy to be sustainable". The growing application of resilience to sustainable development has produced a diversity of approaches and scholarly debates.
The flaw of the free market
The challenge of applying the concept of ecological resilience to the context of sustainable development is that it sits at odds with conventional economic ideology and policy making. Resilience questions the free market model within which global markets operate. Inherent to the successful operation of a free market is specialisation which is required to achieve efficiency and increase productivity. This very act of specialisation weakens resilience by permitting systems to become accustomed to and dependent upon their prevailing conditions. In the event of unanticipated shocks; this dependency reduces the ability of the system to adapt to these changes. Correspondingly; Perman et al. note that; "Some economic activities appear to reduce resilience, so that the level of disturbance to which the ecosystem can be subjected to without parametric change taking place is reduced".
Moving beyond sustainable development
Berkes and Folke table a set of principles to assist with "building resilience and sustainability" which consolidate approaches of adaptive management, local knowledge-based management practices and conditions for institutional learning and self-organisation.
More recently, it has been suggested by Andrea Ross that the concept of sustainable development is no longer adequate in assisting policy development fit for today's global challenges and objectives. This is because the concept of sustainable development is "based on weak sustainability" which doesn't take account of the reality of "limits to earth's resilience". Ross draws on the impact of climate change on the global agenda as a fundamental factor in the "shift towards ecological sustainability" as an alternative approach to that of sustainable development. | Ecological resilience | Wikipedia | 474 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
Because climate change is a major and growing driver of biodiversity loss, and that biodiversity and ecosystem functions and services, significantly contribute to climate change adaptation, mitigation and disaster risk reduction, proponents of ecosystem-based adaptation suggest that the resilience of vulnerable human populations and the ecosystem services upon which they depend are critical factors for sustainable development in a changing climate.
In environmental policy
Scientific research associated with resilience is beginning to play a role in influencing policy-making and subsequent environmental decision making.
This occurs in a number of ways:
Observed resilience within specific ecosystems drives management practice. When resilience is observed to be low, or impact seems to be reaching the threshold, management response can be to alter human behavior to result in less adverse impact to the ecosystem.
Ecosystem resilience impacts upon the way that development is permitted/environmental decision making is undertaken, similar to the way that existing ecosystem health impacts upon what development is permitted. For instance, remnant vegetation in the states of Queensland and New South Wales are classified in terms of ecosystem health and abundance. Any impact that development has upon threatened ecosystems must consider the health and resilience of these ecosystems. This is governed by the Threatened Species Conservation Act 1995 in New South Wales and the Vegetation Management Act 1999 in Queensland.
International level initiatives aim at improving socio-ecological resilience worldwide through the cooperation and contributions of scientific and other experts. An example of such an initiative is the Millennium Ecosystem Assessment whose objective is "to assess the consequences of ecosystem change for human well-being and the scientific basis for action needed to enhance the conservation and sustainable use of those systems and their contribution to human well-being". Similarly, the United Nations Environment Programme aim is "to provide leadership and encourage partnership in caring for the environment by inspiring, informing, and enabling nations and peoples to improve their quality of life without compromising that of future generations.
Environmental management in legislation
Ecological resilience and the thresholds by which resilience is defined are closely interrelated in the way that they influence environmental policy-making, legislation and subsequently environmental management. The ability of ecosystems to recover from certain levels of environmental impact is not explicitly noted in legislation, however, because of ecosystem resilience, some levels of environmental impact associated with development are made permissible by environmental policy-making and ensuing legislation. | Ecological resilience | Wikipedia | 469 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
Some examples of the consideration of ecosystem resilience within legislation include:
Environmental Planning and Assessment Act 1979 (NSW) – A key goal of the Environmental Assessment procedure is to determine whether proposed development will have a significant impact upon ecosystems.
Protection of the Environment (Operations) Act 1997 (NSW) – Pollution control is dependent upon keeping levels of pollutants emitted by industrial and other human activities below levels which would be harmful to the environment and its ecosystems. Environmental protection licenses are administered to maintain the environmental objectives of the POEO Act and breaches of license conditions can attract heavy penalties and in some cases criminal convictions.
Threatened Species Conservation Act 1995 (NSW) – This Act seeks to protect threatened species while balancing it with development.
History
The theoretical basis for many of the ideas central to climate resilience have actually existed since the 1960s. Originally an idea defined for strictly ecological systems, resilience in ecology was initially outlined by C.S. Holling as the capacity for ecological systems and relationships within those systems to persist and absorb changes to "state variables, driving variables, and parameters." This definition helped form the foundation for the notion of ecological equilibrium: the idea that the behavior of natural ecosystems is dictated by a homeostatic drive towards some stable set point. Under this school of thought (which maintained quite a dominant status during this time period), ecosystems were perceived to respond to disturbances largely through negative feedback systems – if there is a change, the ecosystem would act to mitigate that change as much as possible and attempt to return to its prior state.
As greater amounts of scientific research in ecological adaptation and natural resource management was conducted, it became clear that oftentimes, natural systems were subjected to dynamic, transient behaviors that changed how they reacted to significant changes in state variables: rather than work back towards a predetermined equilibrium, the absorbed change was harnessed to establish a new baseline to operate under. Rather than minimize imposed changes, ecosystems could integrate and manage those changes, and use them to fuel the evolution of novel characteristics. This new perspective of resilience as a concept that inherently works synergistically with elements of uncertainty and entropy first began to facilitate changes in the field of adaptive management and environmental resources, through work whose basis was built by Holling and colleagues yet again. | Ecological resilience | Wikipedia | 461 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
By the mid 1970s, resilience began gaining momentum as an idea in anthropology, culture theory, and other social sciences. There was significant work in these relatively non-traditional fields that helped facilitate the evolution of the resilience perspective as a whole. Part of the reason resilience began moving away from an equilibrium-centric view and towards a more flexible, malleable description of social-ecological systems was due to work such as that of Andrew Vayda and Bonnie McCay in the field of social anthropology, where more modern versions of resilience were deployed to challenge traditional ideals of cultural dynamics. | Ecological resilience | Wikipedia | 124 | 13224331 | https://en.wikipedia.org/wiki/Ecological%20resilience | Biology and health sciences | Ecology | Biology |
Clostridium tetani is a common soil bacterium and the causative agent of tetanus. Vegetative cells of Clostridium tetani are usually rod-shaped and up to 2.5 μm long, but they become enlarged and tennis racket- or drumstick-shaped when forming spores. C. tetani spores are extremely hardy and can be found globally in soil or in the gastrointestinal tract of animals. If inoculated into a wound, C. tetani can grow and produce a potent toxin, tetanospasmin, which interferes with motor neurons, causing tetanus. The toxin's action can be prevented with tetanus toxoid vaccines, which are often administered to children worldwide.
Characteristics
Clostridium tetani is a rod-shaped, Gram-positive bacterium, typically up to 0.5 μm wide and 2.5 μm long. It is motile by way of various flagella that surround its body. C. tetani cannot grow in the presence of oxygen. It grows best at temperatures ranging from 33 to 37 °C.
Upon exposure to various conditions, C. tetani can shed its flagellums and form a spore. Each cell can form a single spore, generally at one end of the cell, giving the cell a distinctive drumstick shape. C. tetani spores are extremely hardy and are resistant to heat, various antiseptics, and boiling for several minutes. The spores are long-lived and are distributed worldwide in soils as well as in the intestines of various livestock and companion animals.
Evolution
Clostridium tetani is classified within the genus Clostridium, a broad group of over 150 species of Gram-positive bacteria. C. tetani falls within a cluster of nearly 100 species that are more closely related to each other than they are to any other genus. This cluster includes other pathogenic Clostridium species such as C. botulinum and C. perfringens. The closest relative to C. tetani is C. cochlearium. Other Clostridium species can be divided into a number of genetically related groups, many of which are more closely related to members of other genera than they are to C. tetani. Examples of this include the human pathogen C. difficile, which is more closely related to members of genus Peptostreptococcus than to C. tetani.
Role in disease | Clostridium tetani | Wikipedia | 507 | 13233321 | https://en.wikipedia.org/wiki/Clostridium%20tetani | Biology and health sciences | Gram-positive bacteria | Plants |
While C. tetani is frequently benign in the soil or in the intestinal tracts of animals, it can sometimes cause the severe disease tetanus. Disease generally begins with spores entering the body through a wound. In deep wounds, such as those from a puncture or contaminated needle injection the combination of tissue death and limited exposure to surface air can result in a very low-oxygen environment, allowing C. tetani spores to germinate and grow. As C. tetani grows at the wound site, it releases the toxins tetanolysin and tetanospasmin as cells lyse. The function of tetanolysin is unclear, although it may help C. tetani to establish infection within a wound. Tetanospasmin ("tetanus toxin") is a potent toxin with an estimated lethal dose less than 2.5 nanograms per kilogram of body weight, and is responsible for the symptoms of tetanus. Tetanospasmin spreads via the lymphatic system and bloodstream throughout the body, where it is taken up into various parts of the nervous system. In the nervous system, tetanospasmin acts by blocking the release of the inhibitory neurotransmitters glycine and gamma-aminobutyric acid at motor nerve endings. This blockade leads to the widespread activation of motor neurons and spasming of muscles throughout the body. These muscle spasms generally begin at the top of the body and move down, beginning about 8 days after infection with lockjaw, followed by spasms of the abdominal muscles and the limbs. Muscle spasms continue for several weeks.
The gene encoding tetanospasmin is found on a plasmid carried by many strains of C. tetani; strains of bacteria lacking the plasmid are unable to produce toxin. The function of tetanospasmin in bacterial physiology is unknown.
Treatment and prevention
Clostridium tetani is susceptible to a number of antibiotics, including chloramphenicol, clindamycin, erythromycin, penicillin G, and tetracycline. However, the usefulness of treating C. tetani infections with antibiotics remains unclear. Instead, tetanus is often treated with tetanus immune globulin to bind up circulating tetanospasmin. Additionally, benzodiazepines or muscle relaxants may be given to reduce the effects of the muscle spasms. | Clostridium tetani | Wikipedia | 511 | 13233321 | https://en.wikipedia.org/wiki/Clostridium%20tetani | Biology and health sciences | Gram-positive bacteria | Plants |
Damage from C. tetani infection is generally prevented by administration of a tetanus vaccine consisting of tetanospasmin inactivated by formaldehyde, called tetanus toxoid. This is made commercially by growing large quantities of C. tetani in fermenters, then purifying the toxin and inactivating in 40% formaldehyde for 4–6 weeks. The toxoid is generally coadministered with diphtheria toxoid and some form of pertussis vaccine as DPT vaccine or DTaP. This is given in several doses spaced out over months or years to elicit an immune response that protects the host from the effects of the toxin.
Research
Clostridium tetani can be grown on various anaerobic growth media such as thioglycolate media, casein hydrolysate media, and blood agar. Cultures grow particularly well on media at a neutral to alkaline pH, supplemented with reducing agents. The genome of a C. tetani strain has been sequenced, containing 2.80 million base pairs with 2,373 protein coding genes.
History
Clinical descriptions of tetanus associated with wounds are found at least as far back as the 4th century BCE, in Hippocrates' Aphorisms. The first clear connection to the soil was in 1884, when Arthur Nicolaier showed that animals injected with soil samples would develop tetanus. In 1889, C. tetani was isolated from a human victim by Kitasato Shibasaburō, who later showed that the organism could produce disease when injected into animals, and that the toxin could be neutralized by specific antibodies. In 1897, Edmond Nocard showed that tetanus antitoxin induced passive immunity in humans, and could be used for prophylaxis and treatment. In World War I, injection of tetanus antiserum from horses was widely used as a prophylaxis against tetanus in wounded soldiers, leading to a dramatic decrease in tetanus cases over the course of the war. The modern method of inactivating tetanus toxin with formaldehyde was developed by Gaston Ramon in the 1920s; this led to the development of the tetanus toxoid vaccine by P. Descombey in 1924, which was widely used to prevent tetanus induced by battle wounds during World War II. | Clostridium tetani | Wikipedia | 487 | 13233321 | https://en.wikipedia.org/wiki/Clostridium%20tetani | Biology and health sciences | Gram-positive bacteria | Plants |
In chemistry, the ball-and-stick model is a molecular model of a chemical substance which displays both the three-dimensional position of the atoms and the bonds between them. The atoms are typically represented by spheres, connected by rods which represent the bonds. Double and triple bonds are usually represented by two or three curved rods, respectively, or alternately by correctly positioned sticks for the sigma and pi bonds. In a good model, the angles between the rods should be the same as the angles between the bonds, and the distances between the centers of the spheres should be proportional to the distances between the corresponding atomic nuclei. The chemical element of each atom is often indicated by the sphere's color.
In a ball-and-stick model, the radius of the spheres is usually much smaller than the rod lengths, in order to provide a clearer view of the atoms and bonds throughout the model. As a consequence, the model does not provide a clear insight about the space occupied by the model. In this aspect, ball-and-stick models are distinct from space-filling (calotte) models, where the sphere radii are proportional to the Van der Waals atomic radii in the same scale as the atom distances, and therefore show the occupied space but not the bonds.
Ball-and-stick models can be physical artifacts or virtual computer models. The former are usually built from molecular modeling kits, consisting of a number of coil springs or plastic or wood sticks, and a number of plastic balls with pre-drilled holes. The sphere colors commonly follow the CPK coloring. Some university courses on chemistry require students to buy such models as learning material.
History
In 1865, German chemist August Wilhelm von Hofmann was the first to make ball-and-stick molecular models. He used such models in lecture at the Royal Institution of Great Britain.
Specialist companies manufacture kits and models to order. One of the earlier companies was Woosters at Bottisham, Cambridgeshire, UK. Besides tetrahedral, trigonal and octahedral holes, there were all-purpose balls with 24 holes. These models allowed rotation about the single rod bonds, which could be both an advantage (showing molecular flexibility) and a disadvantage (models are floppy). The approximate scale was 5 cm per ångström (0.5 m/nm or 500,000,000:1), but was not consistent over all elements. | Ball-and-stick model | Wikipedia | 485 | 7115718 | https://en.wikipedia.org/wiki/Ball-and-stick%20model | Physical sciences | Substance | Chemistry |
The Beeverses Miniature Models company in Edinburgh (now operating as Miramodus) produced small models beginning in 1961 using PMMA balls and stainless steel rods. In these models, the use of individually drilled balls with precise bond angles and bond lengths enabled large crystal structures to be accurately created in a light and rigid form. | Ball-and-stick model | Wikipedia | 64 | 7115718 | https://en.wikipedia.org/wiki/Ball-and-stick%20model | Physical sciences | Substance | Chemistry |
The element sulfur exists as many allotropes. In number of allotropes, sulfur is second only to carbon. In addition to the allotropes, each allotrope often exists in polymorphs (different crystal structures of the same covalently bonded Sn molecules) delineated by Greek prefixes (α, β, etc.).
Furthermore, because elemental sulfur has been an item of commerce for centuries, its various forms are given traditional names. Early workers identified some forms that have later proved to be single or mixtures of allotropes. Some forms have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them, e.g. "Muthmann's sulfur I" or "Engel's sulfur".
The most commonly encountered form of sulfur is the orthorhombic polymorph of , which adopts a puckered ring – or "crown" – structure. Two other polymorphs are known, also with nearly identical molecular structures. In addition to , sulfur rings of 6, 7, 9–15, 18, and 20 atoms are known. At least five allotropes are uniquely formed at high pressures, two of which are metallic.
The number of sulfur allotropes reflects the relatively strong S−S bond of 265 kJ/mol. Furthermore, unlike most elements, the allotropes of sulfur can be manipulated in solutions of organic solvents and are analysed by HPLC.
Phase diagram
The pressure-temperature (P-T) phase diagram for sulfur is complex (see image). The region labeled I (a solid region), is α-sulfur.
High-pressure solid allotropes
In a high-pressure study at ambient temperatures, four new solid forms, termed II, III, IV, V have been characterized, where α-sulfur is form I. Solid forms II and III are polymeric, while IV and V are metallic (and are superconductive below 10 K and 17 K, respectively). Laser irradiation of solid samples produces three sulfur forms below 200–300 kbar (20–30 GPa). | Allotropes of sulfur | Wikipedia | 462 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
Solid cyclo allotrope preparation
Two methods exist for the preparation of the cyclo-sulfur allotropes. One of the methods, which is most famous for preparing hexasulfur, is to treat hydrogen polysulfides with polysulfur dichloride:
A second strategy uses titanocene pentasulfide as a source of the unit. This complex is easily made from polysulfide solutions:
Titanocene pentasulfide reacts with polysulfur chloride:
Solid cyclo-sulfur allotropes
Cyclo-hexasulfur, cyclo-
This allotrope was first prepared by M. R. Engel in 1891 by treating thiosulfate with HCl. Cyclo- is orange-red and forms a rhombohedral crystal. It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. Another method of preparation involves the reaction of a polysulfane with sulfur monochloride:
(dilute solution in diethyl ether)
The sulfur ring in cyclo- has a "chair" conformation, reminiscent of the chair form of cyclohexane. All of the sulfur atoms are equivalent.
Cyclo-heptasulfur, cyclo-
It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are known. Two forms (γ-, δ-) have been characterized. The cyclo- ring has an unusual range of bond lengths of 199.3–218.1 pm. It is said to be the least stable of all of the sulfur allotropes.
Cyclo-octasulfur, cyclo-
Octasulfur contains puckered rings, and is known in three forms that differ only in the way the rings are packed in the crystal. | Allotropes of sulfur | Wikipedia | 399 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
α-Sulfur
α-Sulfur is the form most commonly found in nature. When pure it has a greenish-yellow colour (traces of cyclo- in commercially available samples make it appear yellower). It is practically insoluble in water and is a good electrical insulator with poor thermal conductivity. It is quite soluble in carbon disulfide: 35.5 g/100 g solvent at 25 °C. It has an orthorhombic crystal structure. α-Sulfur is the predominant form found in "flowers of sulfur", "roll sulfur" and "milk of sulfur". It contains puckered rings, alternatively called a crown shape. The S–S bond lengths are all 203.7 pm and the S-S-S angles are 107.8° with a dihedral angle of 98°. At 95.3 °C, α-sulfur converts to β-sulfur.
β-Sulfur
β-Sulfur is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. It is unusual because it is only stable above 95.3 °C; below this temperature it converts to α-sulfur. β-Sulfur can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur. It has a melting point variously quoted as 119.6 °C and 119.8 °C but as it decomposes to other forms at around this temperature the observed melting point can vary. The 119 °C melting point has been termed the "ideal melting point" and the typical lower value (114.5 °C) when decomposition occurs, the "natural melting point".
γ-Sulfur
γ-Sulfur was first prepared by F.W. Muthmann in 1890. It is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. It crystallises in pale yellow monoclinic needles. It is the densest form of the three. It can be prepared by slowly cooling molten sulfur that has been heated above 150 °C or by chilling solutions of sulfur in carbon disulfide, ethyl alcohol or hydrocarbons. It is found in nature as the mineral rosickyite. It has been tested in carbon fiber-stabilized form as a cathode in lithium-sulfur (Li-S) batteries and was observed to stop the formation of polysulfides that compromise battery life.
Cyclo- (n = 9–15, 18, 20) | Allotropes of sulfur | Wikipedia | 503 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
These allotropes have been synthesised by various methods for example, treating titanocene pentasulfide and a dichlorosulfane of suitable sulfur chain length, :
or alternatively treating a dichlorosulfane, and a polysulfane, :
, , and can also be prepared from . With the exception of cyclo-, the rings contain S–S bond lengths and S-S-S bond angle that differ one from another.
Cyclo- is the most stable cyclo-allotrope. Its structure can be visualised as having sulfur atoms in three parallel planes, 3 in the top, 6 in the middle and three in the bottom.
Two forms (α-, β-) of cyclo- are known, one of which has been characterized.
Two forms of cyclo- are known where the conformation of the ring is different. To differentiate these structures, rather than using the normal crystallographic convention of α-, β-, etc., which in other cyclo- compounds refer to different packings of essentially the same conformer, these two conformers have been termed endo- and exo-.
Cyclo-·cyclo- adduct
This adduct is produced from a solution of cyclo- and cyclo- in . It has a density midway between cyclo- and cyclo-. The crystal consists of alternate layers of cyclo- and cyclo-. This material is a rare example of an allotrope that contains molecules of different sizes.
Catena sulfur forms
The term "Catena sulfur forms" refers to mixtures of sulfur allotropes that are high in catena (polymer chain) sulfur. The naming of the different forms is very confusing and care has to be taken to determine what is being described because some names are used interchangeably. | Allotropes of sulfur | Wikipedia | 393 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
Amorphous sulfur
Amorphous sulfur is the quenched product from molten sulfur hotter than the λ-transition at 160 °C, where polymerization yields catena sulfur molecules. (Above this temperature, the properties of the liquid melt change remarkably. For example, the viscosity increases more than 10000-fold as the temperature increases through the transition). As it anneals, solid amorphous sulfur changes from its initial glassy form, to a plastic form, hence its other names of plastic, and glassy or vitreous sulfur. The plastic form is also called χ-sulfur. Amorphous sulfur contains a complex mixture of catena-sulfur forms mixed with cyclo-forms.
Insoluble sulfur
Insoluble sulfur is obtained by washing quenched liquid sulfur with . It is sometimes called polymeric sulfur, μ-S or ω-S.
Fibrous (φ-) sulfur
Fibrous (φ-) sulfur is a mixture of the allotropic ψ- form and γ-cyclo-.
ω-Sulfur
ω-Sulfur is a commercially available product prepared from amorphous sulfur that has not been stretched prior to extraction of soluble forms with . It sometimes called "white sulfur of Das" or supersublimated sulfur. It is a mixture of ψ-sulfur and lamina sulfur. The composition depends on the exact method of production and the sample's history. One well known commercial form is "Crystex". ω-sulfur is used in the vulcanization of rubber.
λ-Sulfur
λ-Sulfur is molten sulfur just above the melting temperature. It is a mixture containing mostly cyclo-. Cooling λ-sulfur slowly gives predominantly β-sulfur.
μ-Sulfur
μ-Sulfur is the name applied to solid insoluble sulfur and the melt prior to quenching.
π-Sulfur
π-Sulfur is a dark-coloured liquid formed when λ-sulfur is left to stay molten. It contains mixture of rings.
Biradical catena () chains
This term is applied to biradical catena-chains in sulfur melts or the chains in the solid.
Solid catena allotropes
The production of pure forms of catena-sulfur has proved to be extremely difficult. Complicating factors include the purity of the starting material and the thermal history of the sample. | Allotropes of sulfur | Wikipedia | 481 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
ψ-Sulfur
This form, also called fibrous sulfur or ω1-sulfur, has been well characterized. It has a density of 2.01 g·cm−3 (α-sulfur 2.069 g·cm−3) and decomposes around its melting point of 104 °C. It consists of parallel helical sulfur chains. These chains have both left and right-handed "twists" and a radius of 95 pm. The S–S bond length is 206.6 pm, the S-S-S bond angle is 106° and the dihedral angle is 85.3°, (comparable figures for α-sulfur are 203.7 pm, 107.8° and 98.3°).
Lamina sulfur
Lamina sulfur has not been well characterized but is believed to consist of criss-crossed helices. It is also called χ-sulfur or ω2-sulfur.
High-temperature gaseous allotropes
Monatomic sulfur can be produced from photolysis of carbonyl sulfide.
Disulfur,
Disulfur, , is the predominant species in sulfur vapour above 720 °C (a temperature above that shown in the phase diagram); at low pressure (1 mmHg) at 530 °C, it comprises 99% of the vapor. It is a triplet diradical (like dioxygen and sulfur monoxide), with an S−S bond length of 188.7 pm. The blue colour of burning sulfur is due to the emission of light by the molecule produced in the flame.
The molecule has been trapped in the compound (E = As, Sb) for crystallographic measurements, produced by treating elemental sulfur with excess iodine in liquid sulfur dioxide. The cation has an "open-book" structure, in which each ion donates the unpaired electron in the π* molecular orbital to a vacant orbital of the molecule.
Trisulfur,
is found in sulfur vapour, comprising 10% of vapour species at 440 °C and 10 mmHg. It is cherry red in colour, with a bent structure, similar to ozone, .
Tetrasulfur,
has been detected in the vapour phase, but it has not been well characterized. Diverse structures (e.g. chains, branched chains and rings) have been proposed.
Theoretical calculations suggest a cyclic structure.
Pentasulfur,
Pentasulfur has been detected in sulfur vapours but has not been isolated in pure form. | Allotropes of sulfur | Wikipedia | 505 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
List of allotropes and forms
Allotropes are in Bold. | Allotropes of sulfur | Wikipedia | 16 | 14383139 | https://en.wikipedia.org/wiki/Allotropes%20of%20sulfur | Physical sciences | Group 16 | Chemistry |
The Oriental rat flea (Xenopsylla cheopis), also known as the tropical rat flea or the rat flea, is a parasite of rodents, primarily of the genus Rattus, and is a primary vector for bubonic plague and murine typhus. This occurs when a flea that has fed on an infected rodent bites a human, although this flea can live on any warm blooded mammal.
Body structure
The Oriental rat flea has no genal or pronotal combs. This characteristic can be used to differentiate the Oriental rat flea from the cat flea, dog flea, and other fleas. The flea's body is about one tenth of an inch long (about ). Its body is constructed to make it easier to jump long distances. The flea's body consists of three regions: head, thorax, and abdomen. The head and the thorax have rows of bristles (called combs), and the abdomen consists of eight visible segments. A flea's mouth has two functions: one for squirting saliva or partly digested blood into the bite, and one for sucking up blood from the host. This process mechanically transmits pathogens that may cause diseases it might carry. Fleas smell exhaled carbon dioxide from humans and animals and jump rapidly to the source to feed on the newly found host. The flea is wingless so it can not fly, but it can jump long distances with the help of small, powerful legs. A flea's leg consists of four parts: the part that is closest to the body is the coxa; next are the femur, tibia, and tarsus. A flea can use its legs to jump up to 200 times its own body length (about ).
Life cycle | Oriental rat flea | Wikipedia | 355 | 5444531 | https://en.wikipedia.org/wiki/Oriental%20rat%20flea | Biology and health sciences | Insects: General | Animals |
There are four stages in a flea's life. The first stage is the egg stage. Microscopic white eggs fall easily from the female to the ground or from the animal she lays on. If they are laid on an animal, they soon fall off in the dust or in the animal's bedding. If the eggs do fall immediately on the ground, then they fall into crevices on the floor where they will be safe until they hatch one to ten days later (depending on the environment that they live in, it may take longer to hatch). They hatch into a larva that looks very similar to a worm and is about two millimeters long. It only has a small body and a mouth part. At this stage, the flea does not drink blood; instead it eats dead skin cells, flea droppings, and other smaller parasites lying around them in the dust. When the larva is mature it makes a silken cocoon around itself and pupates. The flea remains a pupa from one week to six months changing in a process called metamorphosis. When the flea emerges, it begins the final cycle, called the adult stage. A flea can now suck blood from hosts and mate with other fleas. A single female flea can mate once and lay eggs every day with up to 50 eggs per day.
Experimentally, it has been shown that the fleas flourish in dry climatic conditions with temperatures of , they can live up to a year and can stay in the cocoon stage for up to a year if the conditions are not favourable.
History
The Oriental rat flea was collected in Shendi, Sudan by Charles Rothschild along with Karl Jordan and described in 1903. He named it cheopis after the Cheops pyramids.
Disease transmission
This species can act as a vector for plague, Yersinia pestis, Rickettsia typhi and also act as a host for the tapeworms Hymenolepis diminuta and Hymenolepis nana. Diseases can be transmitted from one generation of fleas to the next through the eggs.
Gallery | Oriental rat flea | Wikipedia | 422 | 5444531 | https://en.wikipedia.org/wiki/Oriental%20rat%20flea | Biology and health sciences | Insects: General | Animals |
Plain weave (also called tabby weave, linen weave or taffeta weave) is the most basic of three fundamental types of textile weaves (along with satin weave and twill). It is strong and hard-wearing, and is used for fashion and furnishing fabrics. Fabrics with a plain weave are generally strong, durable, and have a smooth surface. They are often used for a variety of applications, including clothing, home textiles, and industrial fabrics.
In plain weave cloth, the warp and weft threads cross at right angles, aligned so they form a simple criss-cross pattern. Each weft thread crosses the warp threads by going over one, then under the next, and so on. The next weft thread goes under the warp threads that its neighbor went over, and vice versa.
Balanced plain weaves are fabrics in which the warp and weft are made of threads of the same weight (size) and the same number of ends per inch as picks per inch.
Basketweave is a variation of plain weave in which two or more threads are bundled and then woven as one in the warp or weft, or both.
A balanced plain weave can be identified by its checkerboard-like appearance. It is also known as one-up-one-down weave or over and under pattern.
Examples of fabric with plain weave are chiffon, organza, percale and taffeta.
Etymology
According to the 12th-century geographer al-Idrīsī, in Andalusī-era Almería, imitations of Iraqī and Persian silks called «عَتَّابِيِّ» —‘attābī— were manufactured, which David Jacoby identifies as "a taffeta fabric made of silk and cotton (natural fibers) originally produced in Attabiya, a district of Baghdad." The word was adopted into Medieval Latin as attabi, then French as tabis and English as tabby, as in "tabby weave".
End uses
Its uses range from heavy and coarse canvas and blankets made of thick yarns to the lightest and finest cambries and muslins made in extremely fine yarns. Chiffon, organza, percale and taffeta are also plain weave fabrics. | Plain weave | Wikipedia | 466 | 5446107 | https://en.wikipedia.org/wiki/Plain%20weave | Technology | Weaving | null |
In graph theory, Vizing's theorem states that every simple undirected graph may be edge colored using a number of colors that is at most one larger than the maximum degree of the graph. At least colors are always necessary, so the undirected graphs may be partitioned into two classes: "class one" graphs for which colors suffice, and "class two" graphs for which colors are necessary. A more general version of Vizing's theorem states that every undirected multigraph without loops can be colored with at most colors, where is the multiplicity of the multigraph. The theorem is named for Vadim G. Vizing who published it in 1964.
Discovery
The theorem discovered by Soviet mathematician Vadim G. Vizing was published in 1964 when Vizing was working in Novosibirsk and became known as Vizing's theorem. Indian mathematician R. P. Gupta independently discovered the theorem, while undertaking his doctorate (1965-1967).
Examples
When , the graph must itself be a matching, with no two edges adjacent, and its edge chromatic number is one. That is, all graphs with are of class one.
When , the graph must be a disjoint union of paths and cycles. If all cycles are even, they can be 2-edge-colored by alternating the two colors around each cycle. However, if there exists at least one odd cycle, then no 2-edge-coloring is possible. That is, a graph with is of class one if and only if it is bipartite.
Proof
This proof is inspired by .
Let be a simple undirected graph. We proceed by induction on , the number of edges. If the graph is empty, the theorem trivially holds. Let and suppose a proper -edge-coloring exists for all where .
We say that color } is missing in with respect to proper -edge-coloring if for all . Also, let -path from denote the unique maximal path starting in with -colored edge and alternating the colors of edges (the second edge has color , the third edge has color and so on), its length can be . Note that if is a proper -edge-coloring of then every vertex has a missing color with respect to .
Suppose that no proper -edge-coloring of exists. This is equivalent to this statement:
(1) Let and be arbitrary proper -edge-coloring of and be missing from and be missing from with respect to . Then the -path from ends in . | Vizing's theorem | Wikipedia | 508 | 5449464 | https://en.wikipedia.org/wiki/Vizing%27s%20theorem | Mathematics | Graph theory | null |
This is equivalent, because if (1) doesn't hold, then we can interchange the colors and on the -path and set the color of to be , thus creating a proper -edge-coloring of from . The other way around, if a proper -edge-coloring exists, then we can delete , restrict the coloring and (1) won't hold either.
Now, let and be a proper -edge-coloring of and be missing in with respect to . We define to be a maximal sequence of neighbours of such that is missing in with respect to for all .
We define colorings as
for all ,
not defined,
otherwise.
Then is a proper -edge-coloring of due to definition of . Also, note that the missing colors in are the same with respect to for all .
Let be the color missing in with respect to , then is also missing in with respect to for all . Note that cannot be missing in , otherwise we could easily extend , therefore an edge with color is incident to for all . From the maximality of , there exists such that . From the definition of this holds:
Let be the -path from with respect to . From (1), has to end in . But is missing in , so it has to end with an edge of color . Therefore, the last edge of is . Now, let be the -path from with respect to . Since is uniquely determined and the inner edges of are not changed in , the path uses the same edges as in reverse order and visits . The edge leading to clearly has color . But is missing in , so ends in . Which is a contradiction with (1) above.
Classification of graphs
Several authors have provided additional conditions that classify some graphs as being of class one or class two, but do not provide a complete classification. For instance, if the vertices of the maximum degree in a graph form an independent set, or more generally if the induced subgraph for this set of vertices is a forest, then must be of class one.
showed that almost all graphs are of class one. That is, in the Erdős–Rényi model of random graphs, in which all -vertex graphs are equally likely, let be the probability that an -vertex graph drawn from this distribution is of class one; then approaches one in the limit as goes to infinity. For more precise bounds on the rate at which converges to one, see . | Vizing's theorem | Wikipedia | 487 | 5449464 | https://en.wikipedia.org/wiki/Vizing%27s%20theorem | Mathematics | Graph theory | null |
Planar graphs
showed that a planar graph is of class one if its maximum degree is at least eight.
In contrast, he observed that for any maximum degree in the range from two to five, there exist
planar graphs of class two. For degree two, any odd cycle is such a graph, and for degree three, four, and five, these graphs can be constructed from platonic solids by replacing a single edge by a path of two adjacent edges.
In Vizing's planar graph conjecture, states that all simple, planar graphs with maximum degree six or seven are of class one, closing the remaining possible cases.
Independently, and partially proved Vizing's planar graph conjecture by showing that all planar graphs with maximum degree seven are of class one.
Thus, the only case of the conjecture that remains unsolved is that of maximum degree six. This conjecture has implications for the total coloring conjecture.
The planar graphs of class two constructed by subdivision of the platonic solids are not regular: they have vertices of degree two as well as vertices of higher degree. The four color theorem (proved by ) on vertex coloring of planar graphs, is equivalent to the statement that every bridgeless 3-regular planar graph is of class one .
Graphs on nonplanar surfaces
In 1969, Branko Grünbaum conjectured that every 3-regular graph with a polyhedral embedding on any two-dimensional oriented manifold such as a torus must be of class one. In this context, a polyhedral embedding is a graph embedding such that every face of the embedding is topologically a disk and such that the dual graph of the embedding is simple, with no self-loops or multiple adjacencies. If true, this would be a generalization of the four color theorem, which was shown by Tait to be equivalent to the statement that 3-regular graphs with a polyhedral embedding on a sphere are of class one. However, showed the conjecture to be false by finding snarks that have polyhedral embeddings on high-genus orientable surfaces. Based on this construction, he also showed that it is NP-complete to tell whether a polyhedrally embedded graph is of class one.
Algorithms | Vizing's theorem | Wikipedia | 465 | 5449464 | https://en.wikipedia.org/wiki/Vizing%27s%20theorem | Mathematics | Graph theory | null |
describe a polynomial time algorithm for coloring the edges of any graph with colors, where is the maximum degree of the graph. That is, the algorithm uses the optimal number of colors for graphs of class two, and uses at most one more color than necessary for all graphs. Their algorithm follows the same strategy as Vizing's original proof of his theorem: it starts with an uncolored graph, and then repeatedly finds a way of recoloring the graph in order to increase the number of colored edges by one.
More specifically, suppose that is an uncolored edge in a partially colored graph. The algorithm of Misra and Gries may be interpreted as constructing a directed pseudoforest (a graph in which each vertex has at most one outgoing edge) on the neighbors of : for each neighbor of , the algorithm finds a color that is not used by any of the edges incident to , finds the vertex (if it exists) for which edge has color , and adds as an edge to . There are two cases:
If the pseudoforest constructed in this way contains a path from to a vertex that has no outgoing edges in , then there is a color that is available both at and . Recoloring edge with color allows the remaining edge colors to be shifted one step along this path: for each vertex in the path, edge takes the color that was previously used by the successor of in the path. This leads to a new coloring that includes edge .
If, on the other hand, the path starting from in the pseudoforest leads to a cycle, let be the neighbor of at which the path joins the cycle, let be the color of edge , and let be a color that is not used by any of the edges at vertex . Then swapping colors and on a Kempe chain either breaks the cycle or the edge on which the path joins the cycle, leading to the previous case.
With some simple data structures to keep track of the colors that are used and available at each vertex, the construction of and the recoloring steps of the algorithm can all be implemented in time , where is the number of vertices in the input graph. Since these steps need to be repeated times, with each repetition increasing the number of colored edges by one, the total time is .
In an unpublished technical report, claimed a faster time bound for the same problem of coloring with colors. | Vizing's theorem | Wikipedia | 479 | 5449464 | https://en.wikipedia.org/wiki/Vizing%27s%20theorem | Mathematics | Graph theory | null |
History
In both and , Vizing mentions that his work was motivated by a theorem of showing that multigraphs could be colored with at most colors. Although Vizing's theorem is now standard material in many graph theory textbooks, Vizing had trouble publishing the result initially, and his paper on it appears in an obscure journal, Diskret. Analiz. | Vizing's theorem | Wikipedia | 71 | 5449464 | https://en.wikipedia.org/wiki/Vizing%27s%20theorem | Mathematics | Graph theory | null |
Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths. There are numerous species of these parasites, which are broadly classified into tapeworms, flukes, and roundworms. They often live in the gastrointestinal tract of their hosts, but they may also burrow into other organs, where they induce physiological damage.
Soil-transmitted helminthiasis and schistosomiasis are the most important helminthiases, and are among the neglected tropical diseases. These group of helminthiases have been targeted under the joint action of the world's leading pharmaceutical companies and non-governmental organizations through a project launched in 2012 called the London Declaration on Neglected Tropical Diseases, which aimed to control or eradicate certain neglected tropical diseases by 2020.
Helminthiasis has been found to result in poor birth outcome, poor cognitive development, poor school and work performance, poor socioeconomic development, and poverty. Chronic illness, malnutrition, and anemia are further examples of secondary effects.
Soil-transmitted helminthiases are responsible for parasitic infections in as much as a quarter of the human population worldwide. One well-known example of soil-transmitted helminthiases is ascariasis.
Types of parasitic helminths
Of all the known helminth species, the most important helminths with respect to understanding their transmission pathways, their control, inactivation and enumeration in samples of human excreta from dried feces, faecal sludge, wastewater, and sewage sludge are:
soil-transmitted helminths, including Ascaris lumbricoides (the most common worldwide), Trichuris trichiura, Necator americanus, Strongyloides stercoralis and Ancylostoma duodenale
Hymenolepis nana
Taenia saginata
Enterobius
Fasciola hepatica
Schistosoma mansoni
Toxocara canis
Toxocara cati
Helminthiases are classified as follows (the disease names end with "-sis" and the causative worms are in brackets): | Helminthiasis | Wikipedia | 459 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
Roundworm infection (nematodiasis)
Filariasis (Wuchereria bancrofti, Brugia malayi infection)
Onchocerciasis (Onchocerca volvulus infection)
Soil-transmitted helminthiasis – this includes ascariasis (Ascaris lumbricoides infection), trichuriasis (Trichuris infection), and hookworm infection (includes necatoriasis and Ancylostoma duodenale infection)
Trichostrongyliasis (Trichostrongylus spp. infection)
Dracunculiasis (guinea worm infection)
Baylisascaris (raccoon roundworm, may be transmitted to pets, livestock, and humans)
Tapeworm infection (cestodiasis)
Echinococcosis (Echinococcus infection)
Hymenolepiasis (Hymenolepis infection)
Taeniasis/cysticercosis (Taenia infection)
Coenurosis (T. multiceps, T. serialis, T. glomerata, and T. brauni infection)
Trematode infection (trematodiasis)
Amphistomiasis (amphistomes infection)
Clonorchiasis (Clonorchis sinensis infection)
Fascioliasis (Fasciola infection)
Fasciolopsiasis (Fasciolopsis buski infection)
Opisthorchiasis (Opisthorchis infection)
Paragonimiasis (Paragonimus infection)
Schistosomiasis/bilharziasis (Schistosoma infection)
Acanthocephala infection
Moniliformis infection
Signs and symptoms
The signs and symptoms of helminthiasis depend on a number of factors including: the site of the infestation within the body; the type of worm involved; the number of worms and their volume; the type of damage the infesting worms cause; and, the immunological response of the body. Where the burden of parasites in the body is light, there may be no symptoms.
Certain worms may cause particular constellations of symptoms. For instance, taeniasis can lead to seizures due to neurocysticercosis. | Helminthiasis | Wikipedia | 462 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
Mass and volume
In extreme cases of intestinal infestation, the mass and volume of the worms may cause the outer layers of the intestinal wall, such as the muscular layer, to tear. This may lead to peritonitis, volvulus, and gangrene of the intestine.
Immunological response
As pathogens in the body, helminths induce an immune response. Immune-mediated inflammatory changes occur in the skin, lung, liver, intestine, central nervous system, and eyes. Signs of the body's immune response may include eosinophilia, edema, and arthritis. An example of the immune response is the hypersensitivity reaction that may lead to anaphylaxis. Another example is the migration of Ascaris larvae through the bronchi of the lungs causing asthma.
Secondary effects
Immune changes
In humans, T helper cells and eosinophils respond to helminth infestation. It is well established that T helper 2 cells are the central players of protective immunity to helminths, while the roles for B cells and antibodies are context-dependent. Inflammation leads to encapsulation of egg deposits throughout the body. Helminths excrete into the intestine toxic substances after they feed. These substances then enter the circulatory and lymphatic systems of the host body.
Chronic immune responses to helminthiasis may lead to increased susceptibility to other infections such as tuberculosis, HIV, and malaria. There is conflicting information about whether deworming reduces HIV progression and viral load and increases CD4 counts in antiretroviral naive and experienced individuals, although the most recent Cochrane review found some evidence that this approach might have favorable effects. Helminth infection also lowers the immune responses to vaccination for other diseases such as BCG, measles, and Hepatitis B.
Chronic illness
Chronic helminthiasis may cause severe morbidity. Helminthiasis has been found to result in poor birth outcome, poor cognitive development, poor school and work performance, decreased productivity, poor socioeconomic development, and poverty. | Helminthiasis | Wikipedia | 439 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
Malnutrition
Helminthiasis may cause chronic illness through malnutrition including vitamin deficiencies, stunted growth, anemia, and protein-energy malnutrition. Worms compete directly with their hosts for nutrients, but the magnitude of this effect is likely minimal as the nutritional requirements of worms is relatively small. In pigs and humans, Ascaris has been linked to lactose intolerance and vitamin A, amino acid, and fat malabsorption. Impaired nutrient uptake may result from direct damage to the intestinal mucosal wall or from more subtle changes such as chemical imbalances and changes in gut flora. Alternatively, the worms’ release of protease inhibitors to defend against the body's digestive processes may impair the breakdown of other nutrients. In addition, worm induced diarrhoea may shorten gut transit time, thus reducing absorption of nutrients.
Malnutrition due to worms can give rise to anorexia. A study of 459 children in Zanzibar revealed spontaneous increases in appetite after deworming. Anorexia might be a result of the body's immune response and the stress of combating infection. Specifically, some of the cytokines released in the immune response to worm infestation have been linked to anorexia in animals.
Anemia
Helminths may cause iron-deficiency anemia. This is most severe in heavy hookworm infections, as Necator americanus and Ancylostoma duodenale feed directly on the blood of their hosts. Although the daily consumption of an individual worm (0.02–0.07 ml and 0.14–0.26 ml respectively) is small, the collective consumption under heavy infection can be clinically significant. Intestinal whipworm may also cause anemia. Anemia has also been associated with reduced stamina for physical labor, a decline in the ability to learn new information, and apathy, irritability, and fatigue. A study of the effect of deworming and iron supplementation in 47 students from the Democratic Republic of the Congo found that the intervention improved cognitive function. Another study found that in 159 Jamaican schoolchildren, deworming led to better auditory short-term memory and scanning and retrieval of long-term memory over a period of nine-weeks. | Helminthiasis | Wikipedia | 472 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
Cognitive changes
Malnutrition due to helminths may affect cognitive function leading to low educational performance, decreased concentration and difficulty with abstract cognitive tasks. Iron deficiency in infants and preschoolers is associated with "lower scores ... on tests of mental and motor development ... [as well as] increased fearfulness, inattentiveness, and decreased social responsiveness". Studies in the Philippines and Indonesia found a significant correlation between helminthiasis and decreased memory and fluency. Large parasite burdens, particularly severe hookworm infections, are also associated with absenteeism, under-enrollment, and attrition in school children.
Transmission
Helminths are transmitted to the final host in several ways. The most common infection is through ingestion of contaminated vegetables, drinking water, and raw or undercooked meat. Contaminated food may contain eggs of nematodes such as Ascaris, Enterobius, and Trichuris; cestodes such as Taenia, Hymenolepis, and Echinococcus; and trematodes such as Fasciola. Raw or undercooked meats are the major sources of Taenia (pork, beef and venison), Trichinella (pork and bear), Diphyllobothrium (fish), Clonorchis (fish), and Paragonimus (crustaceans). Schistosomes and nematodes such as hookworms (Ancylostoma and Necator) and Strongyloides can penetrate the skin directly.
The roundworm, Dracunculus has a complex mode of transmission: it is acquired from drinking infested water or eating frogs and fish that contain (had eaten) infected crustaceans (copepods); and can also be transmitted from infected pets (cats and dogs). Roundworms such as Brugia, Wuchereria and Onchocerca are directly transmitted by mosquitoes. In the developing world, the use of contaminated water is a major risk factor for infection. Infection can also take place through the practice of geophagy, which is not uncommon in parts of sub-Saharan Africa. Soil is eaten, for example, by children or pregnant women to counteract a real or perceived deficiency of minerals in their diet.
Diagnosis | Helminthiasis | Wikipedia | 470 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
Specific helminths can be identified through microscopic examination of their eggs (ova) found in faecal samples. The number of eggs is measured in units of eggs per gram. However, it does not quantify mixed infections, and in practice, is inaccurate for quantifying the eggs of schistosomes and soil-transmitted helminths. Sophisticated tests such as serological assays, antigen tests, and molecular diagnosis are also available; however, they are time-consuming, expensive and not always reliable.
Prevention
Disrupting the cycle of the worm will prevent infestation and re-infestation. Prevention of infection can largely be achieved by addressing the issues of WASH—water, sanitation and hygiene. The reduction of open defecation is particularly called for, as is stopping the use of human waste as fertilizer.
Further preventive measures include adherence to appropriate food hygiene, wearing of shoes, regular deworming of pets, and the proper disposal of their feces.
Scientists are also searching for a vaccine against helminths, such as a hookworm vaccine.
Treatment
Medications
Broad-spectrum benzimidazoles (such as albendazole and mebendazole) are the first line treatment of intestinal roundworm and tapeworm infections. Macrocyclic lactones (such as ivermectin) are effective against adult and migrating larval stages of nematodes. Praziquantel is the drug of choice for schistosomiasis, taeniasis, and most types of food-borne trematodiases. Oxamniquine is also widely used in mass deworming programmes. Pyrantel is commonly used for veterinary nematodiasis. Artemisinins and derivatives are proving to be candidates as drugs of choice for trematodiasis.
Mass deworming
In regions where helminthiasis is common, mass deworming treatments may be performed, particularly among school-age children, who are a high-risk group. Most of these initiatives are undertaken by the World Health Organization (WHO) with positive outcomes in many regions. Deworming programs can improve school attendance by 25 percent. Although deworming improves the health of an individual, outcomes from mass deworming campaigns, such as reduced deaths or increases in cognitive ability, nutritional benefits, physical growth, and performance, are uncertain or not apparent.
Surgery | Helminthiasis | Wikipedia | 492 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
If complications of helminthiasis, such as intestinal obstruction occur, emergency surgery may be required. Patients who require non-emergency surgery, for instance for removal of worms from the biliary tree, can be pre-treated with the anthelmintic drug albendazole.
Epidemiology
Areas with the highest prevalence of helminthiasis are tropical and subtropical areas including sub-Saharan Africa, central and east Asia, and the Americas.
Neglected tropical diseases
Some types of helminthiases are classified as neglected tropical diseases. They include:
Soil-transmitted helminthiases
Roundworm infections such as lymphatic filariasis, dracunculiasis, and onchocerciasis
Trematode infections, such as schistosomiasis, and food-borne trematodiases, including fascioliasis, clonorchiasis, opisthorchiasis, and paragonimiasis
Tapeworm infections such as cysticercosis, taeniasis, and echinococcosis
Prevalence
The soil-transmitted helminths (A. lumbricoides, T. trichiura, N. americanus, A. duodenale), schistosomes, and filarial worms collectively infect more than a quarter of the human population worldwide at any one time, far surpassing HIV and malaria together. Schistosomiasis is the second most prevalent parasitic disease of humans after malaria.
In 2014–15, the WHO estimated that approximately 2 billion people were infected with soil-transmitted helminthiases, 249 million with schistosomiasis, 56 million people with food-borne trematodiasis, 120 million with lymphatic filariasis, 37 million people with onchocerciasis, and 1 million people with echinococcosis. Another source estimated a much higher figure of 3.5 billion infected with one or more soil-transmitted helminths.
In 2014, only 148 people were reported to have dracunculiasis because of a successful eradication campaign for that particular helminth, which is easier to eradicate than other helminths as it is transmitted only by drinking contaminated water.
Because of their high mobility and lower standards of hygiene, school-age children are particularly vulnerable to helminthiasis. Most children from developing nations will have at least one infestation. Multi-species infections are very common. | Helminthiasis | Wikipedia | 506 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
The most common intestinal parasites in the United States are Enterobius vermicularis, Giardia lamblia, Ancylostoma duodenale, Necator americanus, and Entamoeba histolytica.
In a developing country like Bangladesh, the most common species are round worm (Ascaris lumbricoides), whipworm (Tricurias tricuras) and hookworm
(Ancylostoma duodenalis).
Variations within communities
Even in areas of high prevalence, the frequency and severity of infection is not uniform within communities or families. A small proportion of community members harbour the majority of worms, and this depends on age. The maximum worm burden is at five to ten years of age, declining rapidly thereafter. Individual predisposition to helminthiasis for people with the same sanitation infrastructure and hygiene behavior is thought to result from differing immunocompetence, nutritional status, and genetic factors. Because individuals are predisposed to a high or a low worm burden, the burden reacquired after successful treatment is proportional to that before treatment.
Disability-adjusted life years
It is estimated that intestinal nematode infections cause 5 million disability-adjusted life years (DALYS) to be lost, of which hookworm infections account for more than 3 million DALYS and ascaris infections more than 1 million. There are also signs of progress: The Global Burden of Disease Study published in 2015 estimates a 46 percent (59 percent when age standardised) reduction in years lived with disability (YLD) for the 13-year time period from 1990 to 2013 for all intestinal/nematode infections, and even a 74 percent (80 percent when age standardised) reduction in YLD from ascariasis.
Deaths
As many as 135,000 die annually from soil transmitted helminthiasis.
The 1990–2013 Global Burden of Disease Study estimated 5,500 direct deaths from schistosomiasis, while more than 200,000 people were estimated in 2013 to die annually from causes related to schistosomiasis. Another 20 million have severe consequences from the disease. It is the most deadly of the neglected tropical diseases. | Helminthiasis | Wikipedia | 449 | 971658 | https://en.wikipedia.org/wiki/Helminthiasis | Biology and health sciences | Concepts | Health |
Plant reproductive morphology is the study of the physical form and structure (the morphology) of those parts of plants directly or indirectly concerned with sexual reproduction.
Among all living organisms, flowers, which are the reproductive structures of angiosperms, are the most varied physically and show a correspondingly great diversity in methods of reproduction. Plants that are not flowering plants (green algae, mosses, liverworts, hornworts, ferns and gymnosperms such as conifers) also have complex interplays between morphological adaptation and environmental factors in their sexual reproduction. The breeding system, or how the sperm from one plant fertilizes the ovum of another, depends on the reproductive morphology, and is the single most important determinant of the genetic structure of nonclonal plant populations. Christian Konrad Sprengel (1793) studied the reproduction of flowering plants and for the first time it was understood that the pollination process involved both biotic and abiotic interactions. Charles Darwin's theories of natural selection utilized this work to build his theory of evolution, which includes analysis of the coevolution of flowers and their insect pollinators.
Use of sexual terminology
Plants have complex lifecycles involving alternation of generations. One generation, the sporophyte, gives rise to the next generation, the gametophyte asexually via spores. Spores may be identical isospores or come in different sizes (microspores and megaspores), but strictly speaking, spores and sporophytes are neither male nor female because they do not produce gametes. The alternate generation, the gametophyte, produces gametes, eggs and/or sperm. A gametophyte can be monoicous (bisexual), producing both eggs and sperm, or dioicous (unisexual), either female (producing eggs) or male (producing sperm). | Plant reproductive morphology | Wikipedia | 387 | 971961 | https://en.wikipedia.org/wiki/Plant%20reproductive%20morphology | Biology and health sciences | Plant reproduction | null |
In the bryophytes (liverworts, mosses, and hornworts), the sexual gametophyte is the dominant generation. In ferns and seed plants (including cycads, conifers, flowering plants, etc.) the sporophyte is the dominant generation; the obvious visible plant, whether a small herb or a large tree, is the sporophyte, and the gametophyte is very small. In bryophytes and ferns, the gametophytes are independent, free-living plants, while in seed plants, each female megagametophyte, and the megaspore that gives rise to it, is hidden within the sporophyte and is entirely dependent on it for nutrition. Each male gametophyte typically consists of two to four cells enclosed within the protective wall of a pollen grain.
The sporophyte of a flowering plant is often described using sexual terms (e.g. "female" or "male") . For example, a sporophyte that produces spores that give rise only to male gametophytes may be described as "male", even though the sporophyte itself is asexual, producing only spores. Similarly, flowers produced by the sporophyte may be described as "unisexual" or "bisexual", meaning that they give rise to either one sex of gametophyte or both sexes of the gametophyte.
Flowering plants
Basic flower morphology
The flower is the characteristic structure concerned with sexual reproduction in flowering plants (angiosperms). Flowers vary enormously in their structure (morphology). A perfect flower, like that of Ranunculus glaberrimus shown in the figure, has a calyx of outer sepals and a corolla of inner petals and both male and female sex organs. The sepals and petals together form the perianth. Next inwards there are numerous stamens, which produce pollen grains, each containing a microscopic male gametophyte. Stamens may be called the "male" parts of a flower and collectively form the androecium. Finally in the middle there are carpels, which at maturity contain one or more ovules, and within each ovule is a tiny female gametophyte. Carpels may be called the "female" parts of a flower and collectively form the gynoecium. | Plant reproductive morphology | Wikipedia | 500 | 971961 | https://en.wikipedia.org/wiki/Plant%20reproductive%20morphology | Biology and health sciences | Plant reproduction | null |
Each carpel in Ranunculus species is an achene that produces one ovule, which when fertilized becomes a seed. If the carpel contains more than one seed, as in Eranthis hyemalis, it is called a follicle. Two or more carpels may be fused together to varying degrees and the entire structure, including the fused styles and stigmas may be called a pistil. The lower part of the pistil, where the ovules are produced, is called the ovary. It may be divided into chambers (locules) corresponding to the separate carpels.
Variations
A perfect flower has both stamens and carpels, and is described as "bisexual" or "hermaphroditic". A unisexual flower is one in which either the stamens or the carpels are missing, vestigial or otherwise non-functional. Each flower is either staminate (having only functional stamens and thus male), or carpellate or pistillate (having only functional carpels and thus female). If separate staminate and carpellate flowers are always found on the same plant, the species is described as monoecious. If separate staminate and carpellate flowers are always found on different plants, the species is described as dioecious. A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species.
Members of the birch family (Betulaceae) are examples of monoecious plants with unisexual flowers. A mature alder tree (Alnus species) produces long catkins containing only male flowers, each with four stamens and a minute perianth, and separate stalked groups of female flowers, each without a perianth. (See the illustration of Alnus serrulata.) | Plant reproductive morphology | Wikipedia | 397 | 971961 | https://en.wikipedia.org/wiki/Plant%20reproductive%20morphology | Biology and health sciences | Plant reproduction | null |
Most hollies (members of the genus Ilex) are dioecious. Each plant produces either functionally male flowers or functionally female flowers. In Ilex aquifolium (see the illustration), the common European holly, both kinds of flower have four sepals and four white petals; male flowers have four stamens, female flowers usually have four non-functional reduced stamens and a four-celled ovary. Since only female plants are able to set fruit and produce berries, this has consequences for gardeners. Amborella represents the first known group of flowering plants to separate from their common ancestor. It too is dioecious; at any one time, each plant produces either flowers with functional stamens but no carpels, or flowers with a few non-functional stamens and a number of fully functional carpels. However, Amborella plants may change their "sex" over time. In one study, five cuttings from a male plant produced only male flowers when they first flowered, but at their second flowering three switched to producing female flowers.
In extreme cases, almost all of the parts present in a complete flower may be missing, so long as at least one carpel or one stamen is present. This situation is reached in the female flowers of duckweeds (Lemna), which consist of a single carpel, and in the male flowers of spurges (Euphorbia) which consist of a single stamen. | Plant reproductive morphology | Wikipedia | 305 | 971961 | https://en.wikipedia.org/wiki/Plant%20reproductive%20morphology | Biology and health sciences | Plant reproduction | null |
A species such as Fraxinus excelsior, the common ash of Europe, demonstrates one possible kind of variation. Ash flowers are wind-pollinated and lack petals and sepals. Structurally, the flowers may be bisexual, consisting of two stamens and an ovary, or may be male (staminate), lacking a functional ovary, or female (carpellate), lacking functional stamens. Different forms may occur on the same tree, or on different trees. The Asteraceae (sunflower family), with close to 22,000 species worldwide, have highly modified inflorescences made up of flowers (florets) collected together into tightly packed heads. Heads may have florets of one sexual morphology – all bisexual, all carpellate or all staminate (when they are called homogamous), or may have mixtures of two or more sexual forms (heterogamous). Thus goatsbeards (Tragopogon species) have heads of bisexual florets, like other members of the tribe Cichorieae, whereas marigolds (Calendula species) generally have heads with the outer florets bisexual and the inner florets staminate (male).
Like Amborella, some plants undergo sex-switching. For example, Arisaema triphyllum (Jack-in-the-pulpit) expresses sexual differences at different stages of growth: smaller plants produce all or mostly male flowers; as plants grow larger over the years the male flowers are replaced by more female flowers on the same plant. Arisaema triphyllum thus covers a multitude of sexual conditions in its lifetime: nonsexual juvenile plants, young plants that are all male, larger plants with a mix of both male and female flowers, and large plants that have mostly female flowers. Other plant populations have plants that produce more male flowers early in the year and as plants bloom later in the growing season they produce more female flowers.
Terminology | Plant reproductive morphology | Wikipedia | 406 | 971961 | https://en.wikipedia.org/wiki/Plant%20reproductive%20morphology | Biology and health sciences | Plant reproduction | null |
The complexity of the morphology of flowers and its variation within populations has led to a rich terminology.
Androdioecious: having male flowers on some plants, bisexual ones on others.
Androecious: having only male flowers (the male of a dioecious population); producing pollen but no seed.
Androgynous: see bisexual.
Androgynomonoecious: having male, female, and bisexual flowers on the same plant, also called trimonoecious.
Andromonoecious: having both bisexual and male flowers on the same plant.
Bisexual: each flower of each individual has both male and female structures, i.e. it combines both sexes in one structure. Flowers of this kind are called perfect, having both stamens and carpels. Other terms used for this condition are androgynous, hermaphroditic, monoclinous and synoecious.
Dichogamous: having sexes developing at different times; producing pollen when the stigmas are not receptive, either protandrous or protogynous. This promotes outcrossing by limiting self-pollination. Some dichogamous plants have bisexual flowers, others have unisexual flowers.
Diclinous: see Unisexual.
Dioecious: having either only male or only female flowers. No individual plant of the population produces both pollen and ovules. (From the Greek for "two households". | Plant reproductive morphology | Wikipedia | 297 | 971961 | https://en.wikipedia.org/wiki/Plant%20reproductive%20morphology | Biology and health sciences | Plant reproduction | null |
Mass wasting, also known as mass movement, is a general term for the movement of rock or soil down slopes under the force of gravity. It differs from other processes of erosion in that the debris transported by mass wasting is not entrained in a moving medium, such as water, wind, or ice. Types of mass wasting include creep, solifluction, rockfalls, debris flows, and landslides, each with its own characteristic features, and taking place over timescales from seconds to hundreds of years. Mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, Mars, Venus, Jupiter's moon Io, and on many other bodies in the Solar System.
Subsidence is sometimes regarded as a form of mass wasting. A distinction is then made between mass wasting by subsidence, which involves little horizontal movement, and mass wasting by slope movement.
Rapid mass wasting events, such as landslides, can be deadly and destructive. More gradual mass wasting, such as soil creep, poses challenges to civil engineering, as creep can deform roadways and structures and break pipelines. Mitigation methods include slope stabilization, construction of walls, catchment dams, or other structures to contain rockfall or debris flows, afforestation, or improved drainage of source areas.
Types
Mass wasting is a general term for any process of erosion that is driven by gravity and in which the transported soil and rock is not entrained in a moving medium, such as water, wind, or ice. The presence of water usually aids mass wasting, but the water is not abundant enough to be regarded as a transporting medium. Thus, the distinction between mass wasting and stream erosion lies between a mudflow (mass wasting) and a very muddy stream (stream erosion), without a sharp dividing line. Many forms of mass wasting are recognized, each with its own characteristic features, and taking place over timescales from seconds to hundreds of years.
Based on how the soil, regolith or rock moves downslope as a whole, mass movements can be broadly classified as either creeps or landslides. Subsidence is sometimes also regarded as a form of mass wasting. A distinction is then made between mass wasting by subsidence, which involves little horizontal movement, and mass wasting by slope movement.
Creep | Mass wasting | Wikipedia | 472 | 972457 | https://en.wikipedia.org/wiki/Mass%20wasting | Physical sciences | Geomorphology: General | Earth science |
Soil creep is a slow and long term mass movement. The combination of small movements of soil or rock in different directions over time is directed by gravity gradually downslope. The steeper the slope, the faster the creep. The creep makes trees and shrubs curve to maintain their perpendicularity, and they can trigger landslides if they lose their root footing. The surface soil can migrate under the influence of cycles of freezing and thawing, or hot and cold temperatures, inching its way towards the bottom of the slope forming terracettes. Landslides are often preceded by soil creep accompanied with soil sloughing—loose soil that falls and accumulates at the base of the steepest creep sections.
Solifluction
Solifluction is a form of creep characteristics of arctic or alpine climates. It takes place in soil saturated with moisture that thaws during the summer months to creep downhill. It takes place on moderate slopes, relatively free of vegetation, that are underlain by permafrost and receive a constant supply of new debris by weathering. Solifluction affects the entire slope rather than being confined to channels and can produce terrace-like landforms or stone rivers.
Landslide
A landslide, also called a landslip, is a relatively rapid movement of a large mass of earth and rocks down a hill or a mountainside. Landslides can be further classified by the importance of water in the mass wasting process. In a narrow sense, landslides are rapid movement of large amounts of relatively dry debris down moderate to steep slopes. With increasing water content, the mass wasting takes the form of debris avalanches, then earthflows, then mudflows. Further increase in water content produces a sheetflood, which is a form of sheet erosion rather than mass wasting.
Occurrences
On Earth, mass wasting occurs on both terrestrial and submarine slopes. Submarine mass wasting is particularly common along glaciated coastlines where glaciers are retreating and great quantities of sediments are being released. Submarine slides can transport huge volumes of sediments for hundreds of kilometers in a few hours.
Mass wasting is a common phenomenon throughout the Solar System, occurring where volatile materials are lost from a regolith. Such mass wasting has been observed on Mars, Io, Triton, and possibly Europa and Ganymede. Mass wasting also occurs in the equatorial regions of Mars, where stopes of soft sulfate-rich sediments are steepened by wind erosion. Mass wasting on Venus is associated with the rugged terrain of tesserae. Io shows extensive mass wasting of its volcanic mountains. | Mass wasting | Wikipedia | 511 | 972457 | https://en.wikipedia.org/wiki/Mass%20wasting | Physical sciences | Geomorphology: General | Earth science |
Deposits and landforms
Mass wasting affects geomorphology, most often in subtle, small-scale ways, but occasionally more spectacularly.
Soil creep is rarely apparent but can produce such subtle effects as curved forest growth and tilted fences and telephone poles. It occasionally produces low scarps and shallow depressions. Solifluction produced lobed or sheetlike deposits, with fairly definite edges, in which clasts (rock fragments) are oriented perpendicular to the contours of the deposit.
Rockfall can produce talus slopes at the feet of cliffs. A more dramatic manifestation of rockfall is rock glaciers, which form from rockfall from cliffs oversteepened by glaciers.
Landslides can produce scarps and step-like small terraces. Landslide deposits are poorly sorted. Those rich in clay may show stretched clay lumps (a phenomenon called boudinage) and zones of concentrated shear.
Debris flow deposits take the form of long, narrow tracks of very poorly sorted material. These may have natural levees at the sides of the tracks, and sometimes consist of lenses of rock fragments alternating with lenses of fine-grained earthy material. Debris flows often form much of the upper slopes of alluvial fans.
Causes
Triggers for mass wasting can be divided into passive and activating (initiating) causes. Passive causes include:
Rock and soil lithology. Unconsolidated or weak debris are more susceptible to mass wasting, as are materials that lose cohesion when wetted.
Stratigraphy, such as thinly bedded rock or alternating beds of weak and strong or impermeable or permiable rock lithologies.
Faults or other geologic structures that weaken the rock.
Topography, such as steep slopes or cliffs.
Climate, with large temperature swings, frequent freezing and thawing, or abundant rainfall
Lack of vegetation
Activating causes include:
Undercutting of the slope by excavation or erosion
Increased overburden from structures
Increased soil moisture
Earthquakes
Hazards and mitigation
Mass wasting causes problems for civil engineering, particularly highway construction. It can displace roads, buildings, and other construction and can break pipelines. Historically, mitigation of landslide hazards on the Gaillard Cut of the Panama Canal accounted for of the of material removed while excavating the cut. | Mass wasting | Wikipedia | 463 | 972457 | https://en.wikipedia.org/wiki/Mass%20wasting | Physical sciences | Geomorphology: General | Earth science |
Rockslides or landslides can have disastrous consequences, both immediate and delayed. The Oso disaster of March 2014 was a landslide that caused 43 fatalities in Oso, Washington, US. Delayed consequences of landslides can arise from the formation of landslide dams, as at Thistle, Utah, in April 1983.
Volcano flanks can become over-steep resulting in instability and mass wasting. This is now a recognised part of the growth of all active volcanoes. It is seen on submarine volcanoes as well as surface volcanoes: Kamaʻehuakanaloa (formerly Loihi) in the Hawaiian–Emperor seamount chain and Kick 'em Jenny in the Lesser Antilles Volcanic Arc are two submarine volcanoes that are known to undergo mass wasting. The failure of the northern flank of Mount St. Helens in 1980 showed how rapidly volcanic flanks can deform and fail.
Methods of mitigation of mass wasting hazards include:
Afforestation
Construction of fences, walls, or ditches to contain rockfall
Construction of catchment dams to contain debris flows
Improved drainage of source areas
Slope stabilization | Mass wasting | Wikipedia | 215 | 972457 | https://en.wikipedia.org/wiki/Mass%20wasting | Physical sciences | Geomorphology: General | Earth science |
In model theory, a branch of mathematical logic, two structures M and N of the same signature σ are called elementarily equivalent if they satisfy the same first-order σ-sentences.
If N is a substructure of M, one often needs a stronger condition. In this case N is called an elementary substructure of M if every first-order σ-formula φ(a1, …, an) with parameters a1, …, an from N is true in N if and only if it is true in M.
If N is an elementary substructure of M, then M is called an elementary extension of N. An embedding h: N → M is called an elementary embedding of N into M if h(N) is an elementary substructure of M.
A substructure N of M is elementary if and only if it passes the Tarski–Vaught test: every first-order formula φ(x, b1, …, bn) with parameters in N that has a solution in M also has a solution in N when evaluated in M. One can prove that two structures are elementarily equivalent with the Ehrenfeucht–Fraïssé games.
Elementary embeddings are used in the study of large cardinals, including rank-into-rank.
Elementarily equivalent structures
Two structures M and N of the same signature σ are elementarily equivalent if every first-order sentence (formula without free variables) over σ is true in M if and only if it is true in N, i.e. if M and N have the same complete first-order theory.
If M and N are elementarily equivalent, one writes M ≡ N.
A first-order theory is complete if and only if any two of its models are elementarily equivalent.
For example, consider the language with one binary relation symbol '<'. The model R of real numbers with its usual order and the model Q of rational numbers with its usual order are elementarily equivalent, since they both interpret '<' as an unbounded dense linear ordering. This is sufficient to ensure elementary equivalence, because the theory of unbounded dense linear orderings is complete, as can be shown by the Łoś–Vaught test. | Elementary equivalence | Wikipedia | 466 | 972601 | https://en.wikipedia.org/wiki/Elementary%20equivalence | Mathematics | Model theory | null |
More generally, any first-order theory with an infinite model has non-isomorphic, elementarily equivalent models, which can be obtained via the Löwenheim–Skolem theorem. Thus, for example, there are non-standard models of Peano arithmetic, which contain other objects than just the numbers 0, 1, 2, etc., and yet are elementarily equivalent to the standard model.
Elementary substructures and elementary extensions
N is an elementary substructure or elementary submodel of M if N and M are structures of the same signature σ such that for all first-order σ-formulas φ(x1, …, xn) with free variables x1, …, xn, and all elements a1, …, an of N, φ(a1, …, an) holds in N if and only if it holds in M:
This definition first appears in Tarski, Vaught (1957). It follows that N is a substructure of M.
If N is a substructure of M, then both N and M can be interpreted as structures in the signature σN consisting of σ together with a new constant symbol for every element of N. Then N is an elementary substructure of M if and only if N is a substructure of M and N and M are elementarily equivalent as σN-structures.
If N is an elementary substructure of M, one writes N M and says that M is an elementary extension of N: M N.
The downward Löwenheim–Skolem theorem gives a countable elementary substructure for any infinite first-order structure in at most countable signature; the upward Löwenheim–Skolem theorem gives elementary extensions of any infinite first-order structure of arbitrarily large cardinality.
Tarski–Vaught test
The Tarski–Vaught test (or Tarski–Vaught criterion) is a necessary and sufficient condition for a substructure N of a structure M to be an elementary substructure. It can be useful for constructing an elementary substructure of a large structure. | Elementary equivalence | Wikipedia | 445 | 972601 | https://en.wikipedia.org/wiki/Elementary%20equivalence | Mathematics | Model theory | null |
Let M be a structure of signature σ and N a substructure of M. Then N is an elementary substructure of M if and only if for every first-order formula φ(x, y1, …, yn) over σ and all elements b1, …, bn from N, if M x φ(x, b1, …, bn), then there is an element a in N such that M φ(a, b1, …, bn).
Elementary embeddings
An elementary embedding of a structure N into a structure M of the same signature σ is a map h: N → M such that for every first-order σ-formula φ(x1, …, xn) and all elements a1, …, an of N,
N φ(a1, …, an) if and only if M φ(h(a1), …, h(an)).
Every elementary embedding is a strong homomorphism, and its image is an elementary substructure.
Elementary embeddings are the most important maps in model theory. In set theory, elementary embeddings whose domain is V (the universe of set theory) play an important role in the theory of large cardinals (see also Critical point). | Elementary equivalence | Wikipedia | 264 | 972601 | https://en.wikipedia.org/wiki/Elementary%20equivalence | Mathematics | Model theory | null |
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. They are among the flattest, smoothest, and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills).
The creation of the abyssal plain is the result of the spreading of the seafloor (plate tectonics) and the melting of the lower oceanic crust. Magma rises from above the asthenosphere (a layer of the upper mantle), and as this basaltic material reaches the surface at mid-ocean ridges, it forms new oceanic crust, which is constantly pulled sideways by spreading of the seafloor. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons into deeper water. The rest is composed chiefly of pelagic sediments. Metallic nodules are common in some areas of the plains, with varying concentrations of metals, including manganese, iron, nickel, cobalt, and copper. There are also amounts of carbon, nitrogen, phosphorus and silicon, due to material that comes down and decomposes.
Owing in part to their vast size, abyssal plains are believed to be major reservoirs of biodiversity. They also exert significant influence upon ocean carbon cycling, dissolution of calcium carbonate, and atmospheric CO2 concentrations over time scales of a hundred to a thousand years. The structure of abyssal ecosystems is strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles. Factors such as climate change, fishing practices, and ocean fertilization have a substantial effect on patterns of primary production in the euphotic zone. Animals absorb dissolved oxygen from the oxygen-poor waters. Much dissolved oxygen in abyssal plains came from polar regions that had melted long ago. Due to scarcity of oxygen, abyssal plains are inhospitable for organisms that would flourish in the oxygen-enriched waters above. Deep sea coral reefs are mainly found in depths of 3,000 meters and deeper in the abyssal and hadal zones. | Abyssal plain | Wikipedia | 499 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Abyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until recently, none had been studied on a systematic basis. They are poorly preserved in the sedimentary record, because they tend to be consumed by the subduction process. Due to darkness and a water pressure that can reach about 750 times atmospheric pressure (76 megapascal), abyssal plains are not well explored.
Oceanic zones
The ocean can be conceptualized as zones, depending on depth, and presence or absence of sunlight. Nearly all life forms in the ocean depend on the photosynthetic activities of phytoplankton and other marine plants to convert carbon dioxide into organic carbon, which is the basic building block of organic matter. Photosynthesis in turn requires energy from sunlight to drive the chemical reactions that produce organic carbon.
The stratum of the water column nearest the surface of the ocean (sea level) is referred to as the photic zone. The photic zone can be subdivided into two different vertical regions. The uppermost portion of the photic zone, where there is adequate light to support photosynthesis by phytoplankton and plants, is referred to as the euphotic zone (also referred to as the epipelagic zone, or surface zone). The lower portion of the photic zone, where the light intensity is insufficient for photosynthesis, is called the dysphotic zone (dysphotic means "poorly lit" in Greek). The dysphotic zone is also referred to as the mesopelagic zone, or the twilight zone. Its lowermost boundary is at a thermocline of , which, in the tropics generally lies between 200 and 1,000 metres. | Abyssal plain | Wikipedia | 362 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
The euphotic zone is somewhat arbitrarily defined as extending from the surface to the depth where the light intensity is approximately 0.1–1% of surface sunlight irradiance, depending on season, latitude and degree of water turbidity. In the clearest ocean water, the euphotic zone may extend to a depth of about 150 metres, or rarely, up to 200 metres. Dissolved substances and solid particles absorb and scatter light, and in coastal regions the high concentration of these substances causes light to be attenuated rapidly with depth. In such areas the euphotic zone may be only a few tens of metres deep or less. The dysphotic zone, where light intensity is considerably less than 1% of surface irradiance, extends from the base of the euphotic zone to about 1,000 metres. Extending from the bottom of the photic zone down to the seabed is the aphotic zone, a region of perpetual darkness.
Since the average depth of the ocean is about 4,300 metres, the photic zone represents only a tiny fraction of the ocean's total volume. However, due to its capacity for photosynthesis, the photic zone has the greatest biodiversity and biomass of all oceanic zones. Nearly all primary production in the ocean occurs here. Life forms which inhabit the aphotic zone are often capable of movement upwards through the water column into the photic zone for feeding. Otherwise, they must rely on material sinking from above, or find another source of energy and nutrition, such as occurs in chemosynthetic archaea found near hydrothermal vents and cold seeps.
The aphotic zone can be subdivided into three different vertical regions, based on depth and temperature. First is the bathyal zone, extending from a depth of 1,000 metres down to 3,000 metres, with water temperature decreasing from to as depth increases. Next is the abyssal zone, extending from a depth of 3,000 metres down to 6,000 metres. The final zone includes the deep oceanic trenches, and is known as the hadal zone. This, the deepest oceanic zone, extends from a depth of 6,000 metres down to approximately 11,034 meters, at the very bottom of the Mariana Trench, the deepest point on planet Earth. Abyssal plains are typically in the abyssal zone, at depths from 3,000 to 6,000 metres.
The table below illustrates the classification of oceanic zones:
Formation | Abyssal plain | Wikipedia | 505 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Oceanic crust, which forms the bedrock of abyssal plains, is continuously being created at mid-ocean ridges (a type of divergent boundary) by a process known as decompression melting. Plume-related decompression melting of solid mantle is responsible for creating ocean islands like the Hawaiian islands, as well as the ocean crust at mid-ocean ridges. This phenomenon is also the most common explanation for flood basalts and oceanic plateaus (two types of large igneous provinces). Decompression melting occurs when the upper mantle is partially melted into magma as it moves upwards under mid-ocean ridges. This upwelling magma then cools and solidifies by conduction and convection of heat to form new oceanic crust. Accretion occurs as mantle is added to the growing edges of a tectonic plate, usually associated with seafloor spreading. The age of oceanic crust is therefore a function of distance from the mid-ocean ridge. The youngest oceanic crust is at the mid-ocean ridges, and it becomes progressively older, cooler and denser as it migrates outwards from the mid-ocean ridges as part of the process called mantle convection.
The lithosphere, which rides atop the asthenosphere, is divided into a number of tectonic plates that are continuously being created and consumed at their opposite plate boundaries. Oceanic crust and tectonic plates are formed and move apart at mid-ocean ridges. Abyssal hills are formed by stretching of the oceanic lithosphere. Consumption or destruction of the oceanic lithosphere occurs at oceanic trenches (a type of convergent boundary, also known as a destructive plate boundary) by a process known as subduction. Oceanic trenches are found at places where the oceanic lithospheric slabs of two different plates meet, and the denser (older) slab begins to descend back into the mantle. At the consumption edge of the plate (the oceanic trench), the oceanic lithosphere has thermally contracted to become quite dense, and it sinks under its own weight in the process of subduction. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old. The overall process of repeated cycles of creation and destruction of oceanic crust is known as the Supercontinent cycle, first proposed by Canadian geophysicist and geologist John Tuzo Wilson. | Abyssal plain | Wikipedia | 488 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
New oceanic crust, closest to the mid-oceanic ridges, is mostly basalt at shallow levels and has a rugged topography. The roughness of this topography is a function of the rate at which the mid-ocean ridge is spreading (the spreading rate). Magnitudes of spreading rates vary quite significantly. Typical values for fast-spreading ridges are greater than 100 mm/yr, while slow-spreading ridges are typically less than 20 mm/yr. Studies have shown that the slower the spreading rate, the rougher the new oceanic crust will be, and vice versa. It is thought this phenomenon is due to faulting at the mid-ocean ridge when the new oceanic crust was formed. These faults pervading the oceanic crust, along with their bounding abyssal hills, are the most common tectonic and topographic features on the surface of the Earth. The process of seafloor spreading helps to explain the concept of continental drift in the theory of plate tectonics.
The flat appearance of mature abyssal plains results from the blanketing of this originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited from turbidity currents that have been channeled from the continental margins along submarine canyons down into deeper water. The remainder of the sediment comprises chiefly dust (clay particles) blown out to sea from land, and the remains of small marine plants and animals which sink from the upper layer of the ocean, known as pelagic sediments. The total sediment deposition rate in remote areas is estimated at two to three centimeters per thousand years. Sediment-covered abyssal plains are less common in the Pacific Ocean than in other major ocean basins because sediments from turbidity currents are trapped in oceanic trenches that border the Pacific Ocean.
Abyssal plains are typically covered by deep sea, but during parts of the Messinian salinity crisis much of the Mediterranean Sea's abyssal plain was exposed to air as an empty deep hot dry salt-floored sink.
Discovery | Abyssal plain | Wikipedia | 412 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
The landmark scientific expedition (December 1872 – May 1876) of the British Royal Navy survey ship HMS Challenger yielded a tremendous amount of bathymetric data, much of which has been confirmed by subsequent researchers. Bathymetric data obtained during the course of the Challenger expedition enabled scientists to draw maps, which provided a rough outline of certain major submarine terrain features, such as the edge of the continental shelves and the Mid-Atlantic Ridge. This discontinuous set of data points was obtained by the simple technique of taking soundings by lowering long lines from the ship to the seabed.
The Challenger expedition was followed by the 1879–1881 expedition of the Jeannette, led by United States Navy Lieutenant George Washington DeLong. The team sailed across the Chukchi Sea and recorded meteorological and astronomical data in addition to taking soundings of the seabed. The ship became trapped in the ice pack near Wrangel Island in September 1879, and was ultimately crushed and sunk in June 1881.
The Jeannette expedition was followed by the 1893–1896 Arctic expedition of Norwegian explorer Fridtjof Nansen aboard the Fram, which proved that the Arctic Ocean was a deep oceanic basin, uninterrupted by any significant land masses north of the Eurasian continent.
Beginning in 1916, Canadian physicist Robert William Boyle and other scientists of the Anti-Submarine Detection Investigation Committee (ASDIC) undertook research which ultimately led to the development of sonar technology. Acoustic sounding equipment was developed which could be operated much more rapidly than the sounding lines, thus enabling the German Meteor expedition aboard the German research vessel Meteor (1925–27) to take frequent soundings on east-west Atlantic transects. Maps produced from these techniques show the major Atlantic basins, but the depth precision of these early instruments was not sufficient to reveal the flat featureless abyssal plains.
As technology improved, measurement of depth, latitude and longitude became more precise and it became possible to collect more or less continuous sets of data points. This allowed researchers to draw accurate and detailed maps of large areas of the ocean floor. Use of a continuously recording fathometer enabled Tolstoy & Ewing in the summer of 1947 to identify and describe the first abyssal plain. This plain, south of Newfoundland, is now known as the Sohm Abyssal Plain. Following this discovery many other examples were found in all the oceans. | Abyssal plain | Wikipedia | 474 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
The Challenger Deep is the deepest surveyed point of all of Earth's oceans; it is at the south end of the Mariana Trench near the Mariana Islands group. The depression is named after HMS Challenger, whose researchers made the first recordings of its depth on 23 March 1875 at station 225. The reported depth was 4,475 fathoms (8184 meters) based on two separate soundings. On 1 June 2009, sonar mapping of the Challenger Deep by the Simrad EM120 multibeam sonar bathymetry system aboard the R/V Kilo Moana indicated a maximum depth of 10971 meters (6.82 miles). The sonar system uses phase and amplitude bottom detection, with an accuracy of better than 0.2% of water depth (this is an error of about 22 meters at this depth).
Terrain features
Hydrothermal vents
A rare but important terrain feature found in the bathyal, abyssal and hadal zones is the hydrothermal vent. In contrast to the approximately 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 barometric pressure at these depths, water may exist in either its liquid form or as a supercritical fluid at such temperatures.
At a barometric pressure of 218 atmospheres, the critical point of water is 375 °C. At a depth of 3,000 meters, the barometric pressure of sea water is more than 300 atmospheres (as salt water is denser than fresh water). At this depth and pressure, seawater becomes supercritical at a temperature of 407 °C (see image). However the increase in salinity at this depth pushes the water closer to its critical point. Thus, water emerging from the hottest parts of some hydrothermal vents, black smokers and submarine volcanoes can be a supercritical fluid, possessing physical properties between those of a gas and those of a liquid. | Abyssal plain | Wikipedia | 397 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Sister Peak (Comfortless Cove Hydrothermal Field, , elevation −2996 m), Shrimp Farm and Mephisto (Red Lion Hydrothermal Field, , elevation −3047 m), are three hydrothermal vents of the black smoker category, on the Mid-Atlantic Ridge near Ascension Island. They are presumed to have been active since an earthquake shook the region in 2002. These vents have been observed to vent phase-separated, vapor-type fluids. In 2008, sustained exit temperatures of up to 407 °C were recorded at one of these vents, with a peak recorded temperature of up to 464 °C. These thermodynamic conditions exceed the critical point of seawater, and are the highest temperatures recorded to date from the seafloor. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge. The initial stages of a vent chimney begin with the deposition of the mineral anhydrite. Sulfides of copper, iron, and zinc then precipitate in the chimney gaps, making it less porous over the course of time. Vent growths on the order of 30 cm (1 ft) per day have been recorded.[11] 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 (see iron cycle).
Hydrothermal vents in the deep ocean typically 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.
Cold seeps
Another unusual feature found in the abyssal and hadal zones is the cold seep, sometimes called a cold vent. This is an area of the seabed where seepage of hydrogen sulfide, methane and other hydrocarbon-rich fluid occurs, often in the form of a deep-sea brine pool. The first cold seeps were discovered in 1983, at a depth of 3200 meters in the Gulf of Mexico. Since then, cold seeps have been discovered in many other areas of the World Ocean, including the Monterey Submarine Canyon just off Monterey Bay, California, the Sea of Japan, off the Pacific coast of Costa Rica, off the Atlantic coast of Africa, off the coast of Alaska, and under an ice shelf in Antarctica.
Biodiversity | Abyssal plain | Wikipedia | 484 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Though the plains were once assumed to be vast, desert-like habitats, research over the past decade or so shows that they teem with a wide variety of microbial life. However, ecosystem structure and function at the deep seafloor have historically been poorly studied because of the size and remoteness of the abyss. Recent oceanographic expeditions conducted by an international group of scientists from the Census of Diversity of Abyssal Marine Life (CeDAMar) have found an extremely high level of biodiversity on abyssal plains, with up to 2000 species of bacteria, 250 species of protozoans, and 500 species of invertebrates (worms, crustaceans and molluscs), typically found at single abyssal sites. New species make up more than 80% of the thousands of seafloor invertebrate species collected at any abyssal station, highlighting our heretofore poor understanding of abyssal diversity and evolution. Richer biodiversity is associated with areas of known phytodetritus input and higher organic carbon flux.
Abyssobrotula galatheae, a species of cusk eel in the family Ophidiidae, is among the deepest-living species of fish. In 1970, one specimen was trawled from a depth of 8370 meters in the Puerto Rico Trench. The animal was dead, however, upon arrival at the surface. In 2008, the hadal snailfish (Pseudoliparis amblystomopsis) was observed and recorded at a depth of 7700 meters in the Japan Trench. In December 2014 a type of snailfish was filmed at a depth of 8145 meters, followed in May 2017 by another sailfish filmed at 8178 meters. These are, to date, the deepest living fish ever recorded. Other fish of the abyssal zone include the fishes of the family Ipnopidae, which includes the abyssal spiderfish (Bathypterois longipes), tripodfish (Bathypterois grallator), feeler fish (Bathypterois longifilis), and the black lizardfish (Bathysauropsis gracilis). Some members of this family have been recorded from depths of more than 6000 meters. | Abyssal plain | Wikipedia | 442 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
CeDAMar scientists have demonstrated that some abyssal and hadal species have a cosmopolitan distribution. One example of this would be protozoan foraminiferans, certain species of which are distributed from the Arctic to the Antarctic. Other faunal groups, such as the polychaete worms and isopod crustaceans, appear to be endemic to certain specific plains and basins. Many apparently unique taxa of nematode worms have also been recently discovered on abyssal plains. This suggests that the deep ocean has fostered adaptive radiations. The taxonomic composition of the nematode fauna in the abyssal Pacific is similar, but not identical to, that of the North Atlantic. A list of some of the species that have been discovered or redescribed by CeDAMar can be found here.
Eleven of the 31 described species of Monoplacophora (a class of mollusks) live below 2000 meters. Of these 11 species, two live exclusively in the hadal zone. The greatest number of monoplacophorans are from the eastern Pacific Ocean along the oceanic trenches. However, no abyssal monoplacophorans have yet been found in the Western Pacific and only one abyssal species has been identified in the Indian Ocean. Of the 922 known species of chitons (from the Polyplacophora class of mollusks), 22 species (2.4%) are reported to live below 2000 meters and two of them are restricted to the abyssal plain. Although genetic studies are lacking, at least six of these species are thought to be eurybathic (capable of living in a wide range of depths), having been reported as occurring from the sublittoral to abyssal depths. A large number of the polyplacophorans from great depths are herbivorous or xylophagous, which could explain the difference between the distribution of monoplacophorans and polyplacophorans in the world's oceans. | Abyssal plain | Wikipedia | 409 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Peracarid crustaceans, including isopods, are known to form a significant part of the macrobenthic community that is responsible for scavenging on large food falls onto the sea floor. In 2000, scientists of the Diversity of the deep Atlantic benthos (DIVA 1) expedition (cruise M48/1 of the German research vessel RV Meteor III) discovered and collected three new species of the Asellota suborder of benthic isopods from the abyssal plains of the Angola Basin in the South Atlantic Ocean. In 2003, De Broyer et al. collected some 68,000 peracarid crustaceans from 62 species from baited traps deployed in the Weddell Sea, Scotia Sea, and off the South Shetland Islands. They found that about 98% of the specimens belonged to the amphipod superfamily Lysianassoidea, and 2% to the isopod family Cirolanidae. Half of these species were collected from depths of greater than 1000 meters.
In 2005, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) remotely operated vehicle, KAIKO, collected sediment core from the Challenger Deep. 432 living specimens of soft-walled foraminifera were identified in the sediment samples. Foraminifera are single-celled protists that construct shells. There are an estimated 4,000 species of living foraminifera. Out of the 432 organisms collected, the overwhelming majority of the sample consisted of simple, soft-shelled foraminifera, with others representing species of the complex, multi-chambered genera Leptohalysis and Reophax. Overall, 85% of the specimens consisted of soft-shelled allogromiids. This is unusual compared to samples of sediment-dwelling organisms from other deep-sea environments, where the percentage of organic-walled foraminifera ranges from 5% to 20% of the total. Small organisms with hard calciferous shells have trouble growing at extreme depths because the water at that depth is severely lacking in calcium carbonate. The giant (5–20 cm) foraminifera known as xenophyophores are only found at depths of 500–10,000 metres, where they can occur in great numbers and greatly increase animal diversity due to their bioturbation and provision of living habitat for small animals. | Abyssal plain | Wikipedia | 482 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
While similar lifeforms have been known to exist in shallower oceanic trenches (>7,000 m) and on the abyssal plain, the lifeforms discovered in the Challenger Deep may represent independent taxa from those shallower ecosystems. This preponderance of soft-shelled organisms at the Challenger Deep may be a result of selection pressure. Millions of years ago, the Challenger Deep was shallower than it is now. Over the past six to nine million years, as the Challenger Deep grew to its present depth, many of the species present in the sediment of that ancient biosphere were unable to adapt to the increasing water pressure and changing environment. Those species that were able to adapt may have been the ancestors of the organisms currently endemic to the Challenger Deep.
Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to the deepest oceanic trenches. The robot ocean probe Nereus observed a 2–3 cm specimen (still unclassified) of polychaete at the bottom of the Challenger Deep on 31 May 2009. There are more than 10,000 described species of polychaetes; they can be found in nearly every marine environment. Some species live in the coldest ocean temperatures of the hadal zone, while others can be found in the extremely hot waters adjacent to hydrothermal vents. | Abyssal plain | Wikipedia | 275 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Within the abyssal and hadal zones, the areas around submarine hydrothermal vents and cold seeps have by far the greatest biomass and biodiversity per unit area. Fueled by the chemicals dissolved in the vent fluids, these areas are often home to large and diverse communities of thermophilic, halophilic and other extremophilic prokaryotic microorganisms (such as those of the sulfide-oxidizing genus Beggiatoa), often arranged in large bacterial mats near cold seeps. In these locations, chemosynthetic archaea and bacteria typically form the base of the food chain. Although the process of chemosynthesis is entirely microbial, these chemosynthetic microorganisms often support vast ecosystems consisting of complex multicellular organisms through symbiosis. These communities are characterized by species such as vesicomyid clams, mytilid mussels, limpets, isopods, giant tube worms, soft corals, eelpouts, galatheid crabs, and alvinocarid shrimp. The deepest seep community discovered thus far is in the Japan Trench, at a depth of 7700 meters.
Probably the most important ecological characteristic of abyssal ecosystems is energy limitation. Abyssal seafloor communities are considered to be food limited because benthic production depends on the input of detrital organic material produced in the euphotic zone, thousands of meters above. Most of the organic flux arrives as an attenuated rain of small particles (typically, only 0.5–2% of net primary production in the euphotic zone), which decreases inversely with water depth. The small particle flux can be augmented by the fall of larger carcasses and downslope transport of organic material near continental margins.
Exploitation of resources
In addition to their high biodiversity, abyssal plains are of great current and future commercial and strategic interest. For example, they may be used for the legal and illegal disposal of large structures such as ships and oil rigs, radioactive waste and other hazardous waste, such as munitions. They may also be attractive sites for deep-sea fishing, and extraction of oil and gas and other minerals. Future deep-sea waste disposal activities that could be significant by 2025 include emplacement of sewage and sludge, carbon sequestration, and disposal of dredge spoils. | Abyssal plain | Wikipedia | 485 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
As fish stocks dwindle in the upper ocean, deep-sea fisheries are increasingly being targeted for exploitation. Because deep sea fish are long-lived and slow growing, these deep-sea fisheries are not thought to be sustainable in the long term given current management practices. Changes in primary production in the photic zone are expected to alter the standing stocks in the food-limited aphotic zone.
Hydrocarbon exploration in deep water occasionally results in significant environmental degradation resulting mainly from accumulation of contaminated drill cuttings, but also from oil spills. While the oil blowout involved in the Deepwater Horizon oil spill in the Gulf of Mexico originates from a wellhead only 1500 meters below the ocean surface, it nevertheless illustrates the kind of environmental disaster that can result from mishaps related to offshore drilling for oil and gas.
Sediments of certain abyssal plains contain abundant mineral resources, notably polymetallic nodules. These potato-sized concretions of manganese, iron, nickel, cobalt, and copper, distributed on the seafloor at depths of greater than 4000 meters, are of significant commercial interest. The area of maximum commercial interest for polymetallic nodule mining (called the Pacific nodule province) lies in international waters of the Pacific Ocean, stretching from 118°–157°, and from 9°–16°N, an area of more than 3 million km2. The abyssal Clarion-Clipperton fracture zone (CCFZ) is an area within the Pacific nodule province that is currently under exploration for its mineral potential.
Eight commercial contractors are currently licensed by the International Seabed Authority (an intergovernmental organization established to organize and control all mineral-related activities in the international seabed area beyond the limits of national jurisdiction) to explore nodule resources and to test mining techniques in eight claim areas, each covering 150,000 km2. When mining ultimately begins, each mining operation is projected to directly disrupt 300–800 km2 of seafloor per year and disturb the benthic fauna over an area 5–10 times that size due to redeposition of suspended sediments. Thus, over the 15-year projected duration of a single mining operation, nodule mining might severely damage abyssal seafloor communities over areas of 20,000 to 45,000 km2 (a zone at least the size of Massachusetts). | Abyssal plain | Wikipedia | 479 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
Limited knowledge of the taxonomy, biogeography and natural history of deep sea communities prevents accurate assessment of the risk of species extinctions from large-scale mining. Data acquired from the abyssal North Pacific and North Atlantic suggest that deep-sea ecosystems may be adversely affected by mining operations on decadal time scales. In 1978, a dredge aboard the Hughes Glomar Explorer, operated by the American mining consortium Ocean Minerals Company (OMCO), made a mining track at a depth of 5000 meters in the nodule fields of the CCFZ. In 2004, the French Research Institute for Exploitation of the Sea (IFREMER) conducted the Nodinaut expedition to this mining track (which is still visible on the seabed) to study the long-term effects of this physical disturbance on the sediment and its benthic fauna. Samples taken of the superficial sediment revealed that its physical and chemical properties had not shown any recovery since the disturbance made 26 years earlier. On the other hand, the biological activity measured in the track by instruments aboard the crewed submersible bathyscaphe Nautile did not differ from a nearby unperturbed site. This data suggests that the benthic fauna and nutrient fluxes at the water–sediment interface has fully recovered.
List of abyssal plains | Abyssal plain | Wikipedia | 263 | 972800 | https://en.wikipedia.org/wiki/Abyssal%20plain | Physical sciences | Oceanic and coastal landforms | null |
The Gerridae are a family of insects in the order Hemiptera, commonly known as water striders, water skeeters, water scooters, water bugs, pond skaters, water skippers, water gliders, water skimmers or puddle flies. Consistent with the classification of the Gerridae as true bugs (i.e., suborder Heteroptera), gerrids have mouthparts evolved for piercing and sucking, and distinguish themselves by having the unusual ability to walk on water, making them pleuston (surface-living) animals. They are anatomically built to transfer their weight to be able to run on top of the water's surface. As a result, one could likely find water striders present in any pond, river, or lake. Over 1,700 species of gerrids have been described, 10% of them being marine.
While 90% of the Gerridae are freshwater bugs, the oceanic Halobates makes the family quite exceptional among insects. The genus Halobates was first heavily studied between 1822 and 1883 when Francis Buchanan White collected several different species during the Challenger Expedition. Around this time, Eschscholtz discovered three species of the Gerridae, bringing attention to the species, though little of their biology was known. Since then, the Gerridae have been continuously studied due to their ability to walk on water and unique social characteristics.
Description
The family Gerridae is physically characterized by having hydrofuge hairpiles, retractable preapical claws, and elongated legs and body.
Hydrofuge hairpiles are small, hydrophobic microhairs. These are tiny hairs with more than one thousand microhairs per mm. The entire body is covered by these hairpiles, providing the water strider resistance to splashes or drops of water. These hairs repel the water, preventing drops from weighing down the body. | Gerridae | Wikipedia | 392 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Size
They are generally small, long-legged insects and the body length of most species is between . A few are between . Among widespread genera, the North Hemisphere Aquarius includes the largest species, generally exceeding , at least among females, and the largest species averaging about . Females typically average larger than males of their own species, but it appears to be reversed in the largest species, the relatively poorly known Gigantometra gigas of streams in northern Vietnam and adjacent southern China. It typically reaches a body length of about in wingless males and in winged females (winged males, however, only average marginally larger than females). In this species each middle and hind leg can surpass .
Antennae
Water striders have two antennae with four segments on each. Antennal segments are numbered from closest to the head to farthest. The antennae have short, stiff bristles in segment III. Relative lengths of the antennae segments can help identify unique species within the family Gerridae, but in general, segment I is longer and stockier than the remaining three. The four segments combined are usually no longer than the length of the water strider head.
Thorax
The thorax of water striders is generally long, narrow, and small in size. It generally ranges from 1.6 mm to 3.6 mm long across the species, with some bodies more cylindrical or rounder than others. The pronotum, or outer layer of the thorax, of the water strider can be either shiny or dull depending on the species, and covered with microhairs to help repel water. The abdomen of a water strider can have several segments and contains both the metasternum and omphalium.
Appendages
Gerridae have front, middle, and back legs. The front legs are shortest and have preapical claws adapted to puncture prey. Preapical claws are claws that are not at the end of the leg, but rather halfway through, like mantises. The middle legs are longer than the first pair and shorter than the last pair and are adapted for propulsion through the water. The hind pair is the longest and is used for spreading weight over a large surface area, as well as steering the bug across the surface of the water. The front legs are attached just posterior to the eyes, while the middle legs are attached closer to the back legs which attach midthorax but extend beyond the terminal end of the body. | Gerridae | Wikipedia | 493 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Wings
Some water striders have wings present on the dorsal side of their thorax, while other species of Gerridae do not, particularly Halobates. Water striders experience wing length polymorphism that has affected their flight ability and evolved in a phylogenetic manner where populations are either long-winged, wing-dimorphic, or short-winged. Wing dimorphism consists of summer gerrid populations evolving different length wings than winter populations within the same species. Habitats with rougher waters are likely to hold gerrids with shorter wings, while habitats with calm waters are likely to hold long-winged gerrids. This is due to potential for damage of the wings and ability for dispersal.
Evolution
Cretogerris, from the Cretaceous (Albian) Charentese amber of France, was initially suggested as a gerrid. However, it was later interpreted as an indeterminate member of Gerroidea. They are morphologically similar to the unrelated Chresmoda, an enigmatic genus of insect known from the Late Jurassic to the Mid Cretaceous with a presumably similar lifestyle.
Molecular analysis suggest an origin of the family Gerridae about 128 Million years ago (Mya) in the Cretaceous, splitting from the sister group Veliidae, with whom they share a single origin of rowing as a locomotive mechanism. According on the transcriptome-based phylogeny, Gerridae is a monophyletic group. | Gerridae | Wikipedia | 293 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Wing polymorphism
Wing polymorphism (i.e., the presence of multiple wing morphs in a given species) has independently evolved multiple times in Gerridae, as well as complete wing loss, something that has been important for the evolution of the variety in species we see today, and dispersal of Gerridae. The existence of wing polymorphism in a given species can be explained as a particular case oogenesis-flight syndrome. Following this rationale, which is commonly applied in insects, developing short wings provides the individual with the capacity to dedicate the energy stores that would usually be used for wing and wing muscle development to increasing egg production and reproducing early, ultimately enhancing the individual's fitness. The ability for one brood to have young with wings and the next not allows water striders to adapt to changing environments. Long, medium, short, and nonexistent wing forms are all necessary depending on the environment and season. Long wings allow for flight to a neighboring water body when one gets too crowded, but they can get wet and weigh a water strider down. Short wings may allow for short travel, but limit how far a gerrid can disperse. Nonexistent wings prevent a gerrid from being weighed down, but prevent dispersal. | Gerridae | Wikipedia | 260 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Wing polymorphism is common in the Gerridae despite most univoltine populations being completely apterous (wingless) or macropterous (with wings). Apterous populations of gerrids would be restricted to stable aquatic habitats that experience little change in environment, while macropterous populations can inhabit more changing, variable water supplies. Stable waters are usually large lakes and rivers, while unstable waters are generally small and seasonal. Gerrids produce winged forms for dispersal purposes and macropterous individuals are maintained due to their ability to survive in changing conditions. Wings are necessary if the body of water is likely to dry since the gerrid must fly to a new source of water. However, wingless forms are favored due to competition for ovarian development and wings and reproductive success is the main goal due to the selfish gene theory. Overwintering gerrids usually are macropterous, or with wings, so they can fly back to their aquatic habitat after winter. An environmental switch mechanism controls seasonal dimorphism observed in bivoltine species, or species having two broods per year. This switch mechanism is what helps determine whether or not a brood with wings will evolve. Temperature also plays an important role in photoperiodic switch. Temperatures signify the seasons and thus when wings are needed since they hibernate during winter. Ultimately, these switching mechanisms alter genetic alleles for wing characteristics, helping to maintain biological dispersal.
Ability to walk on water
Water striders are able to walk on top of water due to a combination of several factors. Water striders use the high surface tension of water and long, hydrophobic legs to help them stay above water.
Gerridae species use this surface tension to their advantage through their highly adapted legs and distributed weight. | Gerridae | Wikipedia | 366 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
The legs of a water strider are long and slender, allowing the weight of the water strider body to be distributed over a large surface area. The legs are strong, but have flexibility that allows the water striders to keep their weight evenly distributed and flow with the water movement. Hydrofuge hairs line the body surface of the water strider. There are several thousand hairs per square millimeter, providing the water strider with a hydrofuge body that prevents wetting from waves, rain, or spray, which could inhibit their ability to keep their entire body above the water surface if the water stuck and weighed down the body. This position of keeping the majority of the body above the water surface, called epipleustonic, is a defining characteristic of water striders. If the body of the water strider were to accidentally become submerged, for instance by a large wave, the tiny hairs would trap air. Tiny air bubbles throughout the body act as buoyancy to bring the water strider to the surface again, while also providing air bubbles to breathe from underwater. Despite their success in overcoming submergence in water, however, water striders are not as competent in oil, and experimental oil spills have suggested that oil spilled in freshwater systems can drive water strider immobility and death.
The tiny hairs on the legs provide both a hydrophobic surface as well as a larger surface area to spread their weight over the water. The middle legs used for rowing have particularly well developed fringe hairs on the tibia and tarsus to help increase movement through the ability to thrust. The hind pair of legs are used for steering When the rowing stroke begins, the middle tarsi of gerrids are quickly pressed down and backwards to create a circular surface wave in which the crest can be used to propel a forward thrust. The semicircular wave created is essential to the ability of the water strider to move rapidly since it acts as a counteracting force to push against. As a result, water striders often move at 1 meter per second or faster.
Life cycle
Gerrids generally lay their eggs on submerged rocks or vegetation using a gelatinous substance as a glue. Gravid females carry between two and twenty eggs. The eggs are creamy white or translucent, but become bright orange. | Gerridae | Wikipedia | 468 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Gerrids go through the egg stage, five instar stages of nymphal forms, and then the adult stage. Instar durations of water striders are highly correlated throughout the larval period. This means that individuals tend to develop at the same rate through each instar stage. Each nymphal stage lasts 7–10 days and the water strider molts, shedding its old cuticle through a Y-shaped suture dorsal to the head and thorax. Nymphs are very similar to adults in behavior and diet, but are smaller (1 mm long), paler, and lack differentiation in tarsal and genital segments. It takes approximately 60 to 70 days for a water strider to reach adulthood, though this development rate has been found highly correlated to the water temperature the eggs are in.
Ecology
Habitat
Gerridae generally inhabit surfaces of calm waters. The majority of water striders inhabit freshwater areas, with the exception of Asclepios, Halobates, Stenobates and a few other genera, which inhabit marine waters. The marine species are generally coastal, but a few Halobates live offshore (oceanic) and are the only insects of this habitat. Gerridae prefer an environment abundant with insects or zooplankton and one that contains several rocks or plants to oviposit eggs on. It has been studied by prevalence of water striders in varying environments, that water striders most prefer waters around . Any water temperature lower than is unfavorable. This is likely due to the fact that development rates of young are temperature dependent [5]. The cooler the surrounding waters, the slower the development of the young is. Prominent genera Gerridae are present in Europe, the former USSR, Canada, US, South Africa, South America, Australia, China and Malaysia [5]. None have been yet identified in New Zealand waters.
Diet | Gerridae | Wikipedia | 386 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Gerrids are aquatic predators and feed on invertebrates, mainly spiders and insects, that fall onto the water surface. Water striders are attracted to this food source by ripples produced by the struggling prey. The water strider uses its front legs as sensors for the vibrations produced by the ripples in the water. The water strider punctures the prey item's body with its proboscis, injects salivary enzymes that break down the prey's internal structures, and then sucks out the resulting fluid. Gerrids prefer living prey, though they are indiscriminate feeders when it comes to terrestrial insect type. Halobates, which are found on open sea, feed off floating insects, zooplankton, and occasionally resort to cannibalism of their own nymphs. Cannibalism is frequent and helps control population sizes and restrict conflicting territories. During the non-mating season when gerrids live in cooperative groups, and cannibalism rates are lower, water striders will openly share large kills with others around them. Some gerrids are collectors, feeding off sediment or deposit surface.
Predators
Gerrids, or water striders, are preyed upon largely by birds and some fish. Petrels, terns, and some marine fish prey on Halobates. Fish do not appear to be the main predators of water striders, but will eat them in cases of starvation. Scent gland secretions from the thorax are responsible for repelling fish from eating them. Gerrids are largely hunted by birds of a wide range of species dependent on habitat. Some water striders are hunted by frogs, but they are not their main food source. Water striders are also sometimes hunted by each other. Water strider cannibalism involves mainly hunting nymphs for mating territory and sometimes for food.
Parasites
Several endoparasites have been found in gerrids. Trypanosamatid flagellates, nematodes, and parasitic Hymenoptera all act as endoparasites. Water mite larvae act as ectoparasites of water striders. | Gerridae | Wikipedia | 440 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Dispersal
Sudden increases in salt concentration in the water of gerrid habitats can trigger migration of water striders. Water striders will move to areas of lower salt concentration, resulting in the mix of genes within brackish and freshwater bodies. Nymphal population density also affects the dispersal of water striders. The higher density of water striders in the nymphal stage results in a higher percentage of brachypterous adults developing flight muscles. These flight muscles allow for the water striders to fly to neighboring bodies of water and mate, resulting in the spread of genes. This spread and mixing of genes can be beneficial due to a heterozygotic advantage. Generally, water striders will try to disperse in such a way to lower the density of gerrids in one area or pool of water. Most do this by flight, but those that lack wings or wing muscles will rely on the current of their water body or flooding. Eggs in Halobates are often laid on floating ocean debris and thus spread across the ocean by this drifting matter.
Mating behavior | Gerridae | Wikipedia | 216 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Sex discrimination in some Gerridae species is determined through communication of ripple frequency produced on the water surface. Males predominantly produce these ripples in the water. There are three main frequencies found in ripple communication: 25 Hz as a repel signal, 10 Hz as a threat signal, and 3 Hz as a courtship signal. An approaching gerrid will first give out a repel signal to let the other water strider know they are in its area. If the other gerrid does not return the repel signal, then the bug knows it is a female and will switch to the courtship signal. A receptive female will lower her abdomen and allow the male to mount her and mate. A non-receptive female will raise her abdomen and emit a repel signal. Males that are allowed to mate stay attached to the same female for the entire reproductive season. This is to ensure that the female's young belong to the mounting male and thus guarantee the spread of his genes. Females oviposit, or lay their eggs, by submerging and attaching the eggs to stable surfaces such as plants or stones. Some water strider species will lay the eggs at the water edge if the body of water is calm enough. The amount of eggs laid depends on the amount of food available to the mother during the reproductive season. The availability of food and dominance among other gerrids in the area both play crucial roles in the amount of food obtained and thus, resulting fecundity. Water striders will reproduce all year long in tropical regions where it remains warm, but only during the warm months in seasonal habitats. Gerrids that live in environments with winters will overwinter in the adult stage. This is due to the large energy cost which would need to be spent to maintain their body temperature at functional levels. These water striders have been found in leaf litter or under stationary shelters such as logs and rocks during the winter in seasonal areas. This reproductive diapause is a result of shortening day lengths during larval development and seasonal variation in lipid levels. Shorter day length signals the water strider of the coming temperature drops, also acting as a physical signal the body uses to store lipids throughout the body as food sources. Water striders use these lipids to metabolize during their hibernation. The length of the hibernation depends when the environment warms and the days become longer again. | Gerridae | Wikipedia | 492 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
Social behavior
Kin discrimination is rare in Gerridae, only really being seen in Halobates. Without hunger playing a role, several studies have shown that neither Aquarius remigis nor Limnoporus dissortis parents preferentially cannibalize on non-kin. Those two species are highly prevalent in American waters. These species do not show familial tendencies, leaving their young to forage on their own. Females cannibalize more on young than males do and, in particular, on first-instar nymphs. Young must disperse as soon as their wings are fully developed to avoid cannibalism and other territorial conflicts since neither parents nor siblings can identify members genetically related to themselves.
Gerridae are territorial insects and make this known by their vibration patterns. Both female and male adult Gerridae hold separate territories, though usually the male territories are larger than the female. During the mating season, gerrids will emit warning vibrations through the water and defend both their territory and the female in it. Even though gerridae are very conspicuous, making their presence known through repel signals, they often live in large groups. These large groups usually form during the non-mating season since there is less need to compete. Instead of competing to reproduce, water striders can work together to obtain nutrition and shelter outside of the mating season. Water striders will attempt to disperse when these groups become too dense. They do so by flying away or cannibalizing.
In popular culture
In the video game Super Mario 64, in the level Wet-Dry World, there are enemies named Skeeter that are based on water striders and their movement. The name comes from "water skeeter", an alternative name for water striders.
In the 2002 film The Tuxedo, water striders are genetically modified by bioterrorists to have bacteria that can spread from person to person, causing severe dehydration and instant death. | Gerridae | Wikipedia | 404 | 972966 | https://en.wikipedia.org/wiki/Gerridae | Biology and health sciences | Hemiptera (true bugs) | Animals |
In computing, formatted text, styled text, or rich text, as opposed to plain text, is digital text which has styling information beyond the minimum of semantic elements: colours, styles (boldface, italic), sizes, and special features in HTML (such as hyperlinks).
Terminology
Formatted text cannot rightly be identified with binary files or be distinct from ASCII text. This is because formatted text is not necessarily binary, it may be text-only, such as HTML, RTF or enriched text files, and it may be ASCII-only. Conversely, a plain text file may be non-ASCII (in an encoding such as Unicode UTF-8). Text-only formatted text is achieved by markup which too is textual, while some editors of formatted text like Microsoft Word save in a binary format.
Beginnings of formatted text
Formatted text has its genesis in the pre-computer use of underscoring to embolden passages in typewritten manuscripts. In the first interactive systems of early computer technology, underlining was not possible, and users made up for this lack (and the lack of formatting in ASCII) by using certain symbols as substitutes. Emphasis, for example, could be achieved in ASCII in a number of ways:
Capitalization:
Surrounding with underscores:
Surrounding with asterisks:
Spacing:
Surrounding by underscores was also used for book titles:
Markup languages
Formatting can be marked by tags distinguished from the body text by special characters, such as angle brackets in HTML. For example, this text:
The dog is classified as Canis familiaris in taxonomy.
is marked up in HTML thus:
<p>The dog is classified as <i>Canis familiaris</i> in taxonomy.</p>
The italicised text is enclosed by an opening and a closing italics tag. In LaTeX, the text would be marked up like this:
The dog is classified as \textit{Canis familiaris} in taxonomy.
Most markup languages can be edited with any text editor, needing no special software. Many markup languages can also be edited with specialized software designed to automate some functions or present the output as WYSIWYG. | Formatted text | Wikipedia | 468 | 973737 | https://en.wikipedia.org/wiki/Formatted%20text | Technology | Data storage and memory | null |
Formatted document files
Since the invention of MacWrite, the first WYSIWYG word processor, in which the typist codes the formatting visually rather than by inserting textual markup, word processors have tended to save to binary files. Opening such files with a text editor reveals them embedded with various binary characters, either around the formatted text (e.g. in WordPerfect) or separate from it, at the beginning or end of the file (e.g. in Microsoft Word).
Formatted text documents in binary files have, however, the disadvantages of formatting scope and secrecy. Whereas the extent of formatting is accurately marked in markup languages, WYSIWYG formatting is based on memory, that is, keeping for example your pressing of the boldface button until cancelled. This can lead to formatting mistakes and maintenance troubles. As for secrecy, formatted text document file formats tend to be proprietary and undocumented, leading to difficulty in coding compatibility by third parties, and also to unnecessary upgrades because of version changes.
WordStar was a popular word processor that did not use binary files with hidden characters.
OpenOffice.org Writer saves files in an XML format. However, the resultant file is a binary since it is compressed (a tarball equivalent).
PDF is another formatted text file format that is usually binary (using compression for the text, and storing graphics and fonts in binary). It is generally an end-user format, written from an application such as Microsoft Word or OpenOffice.org Writer, and not editable by the user once done. | Formatted text | Wikipedia | 331 | 973737 | https://en.wikipedia.org/wiki/Formatted%20text | Technology | Data storage and memory | null |
Erinaceidae is a family in the order Eulipotyphla, consisting of the hedgehogs and moonrats. Until recently, it was assigned to the order Erinaceomorpha, which has been subsumed with the paraphyletic Soricomorpha into Eulipotyphla. Eulipotyphla has been shown to be monophyletic; Soricomorpha is paraphyletic because both Soricidae and Talpidae share a more recent common ancestor with Erinaceidae than with solenodons.
Erinaceidae contains the well-known hedgehogs (subfamily Erinaceinae) of Eurasia and Africa and the gymnures or moonrats (subfamily Galericinae) of Southeast Asia. This family was once considered part of the order Insectivora, but that polyphyletic order is now considered defunct.
Characteristics
Erinaceids are generally shrew-like in form, with long snouts and short tails. They are, however, much larger than shrews, ranging from in body length and in weight, in the case of the short-tailed gymnure, up to and in the moonrat. All but one species have five toes in each foot, in some cases with strong claws for digging, and they have large eyes and ears. Hedgehogs possess hair modified into sharp spines to form a protective covering over the upper body and flanks, while gymnures have only normal hair. Most species have anal scent glands, but these are far better developed in gymnures, which can have a powerful odor.
Erinaceids are omnivorous, with the major part of their diet consisting of insects, earthworms, and other small invertebrates. They also eat seeds and fruit, and occasionally birds' eggs, along with any carrion they come across. Their teeth are sharp and suited for impaling invertebrate prey. The dental formula for erinaceids is:
Hedgehogs are nocturnal, but gymnures are less so, and may be active during the day. Many species live in simple burrows, while others construct temporary nests on the surface from leaves and grass, or shelter in hollow logs or similar hiding places. Erinaceids are solitary animals outside the breeding season, and the father plays no role in raising the young. | Erinaceidae | Wikipedia | 477 | 973884 | https://en.wikipedia.org/wiki/Erinaceidae | Biology and health sciences | Eulipotyphla | Animals |
Female erinaceids give birth after a gestation period of around six to seven weeks. The young are born blind and hairless, although hedgehogs begin to sprout their spines within 36 hours of birth.
Evolution
Erinaceids are a group of placental mammals that have retained many of their ancestral traits, having changed little since their origin in the Eocene. The so-called 'giant hedgehog' (actually a gymnure) Deinogalerix, from the Miocene of Gargano Island (part of modern Italy), was the size of a large rabbit, and may have eaten vertebrate prey or carrion, rather than insects.
Classification | Erinaceidae | Wikipedia | 137 | 973884 | https://en.wikipedia.org/wiki/Erinaceidae | Biology and health sciences | Eulipotyphla | Animals |
Order Eulipotyphla
†Family Amphilemuridae
†Genus Alsaticopithecus
†Genus Amphilemur
†Genus Gesneropithex
†Genus Macrocranion
†Macrocranion germonpreae
†Macrocranion junnei
†Macrocranion nitens
†Macrocranion robinsoni
†Macrocranion tenerum
†Macrocranion vandebroeki
†Genus Pholidocercus
†Pholidocercus hassiacus
Family Erinaceidae
†Genus Silvacola
†Silvacola acares
†Genus Oligoechinus
Subfamily Erinaceinae
†Genus Amphechinus
†Amphechinus akespensis
†Amphechinus arverniensis
†Amphechinus baudelotae
†Amphechinus edwardsi
†Amphechinus ginsburgi
†Amphechinus golpeae
†Amphechinus horncloudi
†Amphechinus intermedius
†Amphechinus kreuzae
†Amphechinus major
†Amphechinus microdus
†Amphechinus minutissimus
†Amphechinus robinsoni
†Amphechinus taatsiingolensis
Genus †Ladakhechinus
†Ladakhechinus iugummontis
Genus Atelerix
Four-toed hedgehog, Atelerix albiventris
North African hedgehog, Atelerix algirus
Southern African hedgehog, Atelerix frontalis
Somali hedgehog, Atelerix sclateri
Genus Erinaceus
Amur hedgehog, Erinaceus amurensis
Southern white-breasted hedgehog, Erinaceus concolor
European hedgehog, Erinaceus europaeus
Northern white-breasted hedgehog, Erinaceus roumanicus
Genus Hemiechinus
Long-eared hedgehog, Hemiechinus auritus
Indian long-eared hedgehog, Hemiechinus collaris
Genus Mesechinus
Daurian hedgehog, Mesechinus dauuricus
Hugh's hedgehog, Mesechinus hughi
Gaoligong forest hedgehog, Mesechinus wangi
Small-toothed forest hedgehog, Mesechinus miodon
Genus Paraechinus
Desert hedgehog, Paraechinus aethiopicus
Brandt's hedgehog, Paraechinus hypomelas
Indian hedgehog, Paraechinus micropus
Bare-bellied hedgehog, Paraechinus nudiventris
Subfamily Galericinae | Erinaceidae | Wikipedia | 512 | 973884 | https://en.wikipedia.org/wiki/Erinaceidae | Biology and health sciences | Eulipotyphla | Animals |
†Genus Deinogalerix
†Deinogalerix brevirostris
†Deinogalerix freudenthali
†Deinogalerix intermedius
†Deinogalerix koenigswaldi
†Deinogalerix minor
Genus Echinosorex
Moonrat, Echinosorex gymnura
†Genus Galerix
†Galerix aurelianensis
†Galerix exilis
†Galerix kostakii
†Galerix remmerti
†Galerix rutlandae
†Galerix saratji
†Galerix stehlini
†Galerix symeonidisi
†Galerix uenayae
Genus Hylomys
Long-eared gymnure, Hylomys megalotis
Dwarf gymnure, Hylomys parvus
Javan short-tailed gymnure or Lesser Moonrat, Hylomys suillus
Genus Neohylomys
Hainan gymnure, Neonylomys hainanensis
Genus Neotetracus
Shrew gymnure, Neotetracus sinensis
Genus Podogymnura
Dinagat gymnure, Podogymnura aureospinula
Eastern Mindanao gymnure, Podogymnura intermedia
Mindanao gymnure, Podogymnura truei | Erinaceidae | Wikipedia | 264 | 973884 | https://en.wikipedia.org/wiki/Erinaceidae | Biology and health sciences | Eulipotyphla | Animals |
The backstaff is a navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back (hence the name) and observed the shadow cast by the upper vane on a horizon vane. It was invented by the English navigator John Davis, who described it in his book Seaman's Secrets in 1594.
Types of backstaffs
Backstaff is the name given to any instrument that measures the altitude of the sun by the projection of a shadow. It appears that the idea for measuring the sun's altitude using back observations originated with Thomas Harriot. Many types of instruments evolved from the cross-staff that can be classified as backstaffs. Only the Davis quadrant remains dominant in the history of navigation instruments. Indeed, the Davis quadrant is essentially synonymous with backstaff. However, Davis was neither the first nor the last to design such an instrument and others are considered here as well.
Davis quadrant
Captain John Davis invented a version of the backstaff in 1594. Davis was a navigator who was quite familiar with the instruments of the day such as the mariner's astrolabe, the quadrant and the cross-staff. He recognized the inherent drawbacks of each and endeavoured to create a new instrument that could reduce those problems and increase the ease and accuracy of obtaining solar elevations.
One early version of the quadrant staff is shown in Figure 1. It had an arc affixed to a staff so that it could slide along the staff (the shape is not critical, though the curved shape was chosen). The arc (A) was placed so that it would cast its shadow on the horizon vane (B). The navigator would look along the staff and observe the horizon through a slit in the horizon vane. By sliding the arc so that the shadow aligned with the horizon, the angle of the sun could be read on the graduated staff. This was a simple quadrant, but it was not as accurate as one might like. The accuracy in the instrument is dependent on the length of the staff, but a long staff made the instrument more unwieldy. The maximum altitude that could be measured with this instrument was 45°. | Backstaff | Wikipedia | 450 | 974163 | https://en.wikipedia.org/wiki/Backstaff | Technology | Navigation | null |
The next version of his quadrant is shown in Figure 2. The arc on the top of the instrument in the previous version was replaced with a shadow vane placed on a transom. This transom could be moved along a graduated scale to indicate the angle of the shadow above the staff. Below the staff, a 30° arc was added. The horizon, seen through the horizon vane on the left, is aligned with the shadow. The sighting vane on the arc is moved until it aligns with the view of the horizon. The angle measured is the sum of the angle indicated by the position of the transom and the angle measured on the scale on the arc.
The instrument that is now identified with Davis is shown in Figure 3. This form evolved by the mid-17th century. The quadrant arc has been split into two parts. The smaller radius arc, with a span of 60°, was mounted above the staff. The longer radius arc, with a span of 30° was mounted below. Both arcs have a common centre. At the common centre, a slotted horizon vane was mounted (B). A moveable shadow vane was placed on the upper arc so that its shadow was cast on the horizon vane. A moveable sight vane was mounted on the lower arc (C).
It is easier for a person to place a vane at a specific location than to read the arc at an arbitrary position. This is due to Vernier acuity, the ability of a person to align two line segments accurately. Thus an arc with a small radius, marked with relatively few graduations, can be used to place the shadow vane accurately at a specific angle. On the other hand, moving the sight vane to the location where the line to the horizon meets the shadow requires a large arc. This is because the position may be at a fraction of a degree and a large arc allows one to read smaller graduations with greater accuracy. The large arc of the instrument, in later years, was marked with transversals to allow the arc to be read to greater accuracy than the main graduations allow.
Thus Davis was able to optimize the construction of the quadrant to have both a small and a large arc, allowing the effective accuracy of a single arc quadrant of large radius without making the entire instrument so large. This form of the instrument became synonymous with the backstaff. It was one of the most widely used forms of the backstaff. Continental European navigators called it the English Quadrant. | Backstaff | Wikipedia | 501 | 974163 | https://en.wikipedia.org/wiki/Backstaff | Technology | Navigation | null |
A later modification of the Davis quadrant was to use a Flamsteed glass in place of the shadow vane; this was suggested by John Flamsteed. This placed a lens on the vane that projected an image of the sun on the horizon vane instead of a shadow. It was useful under conditions where the sky was hazy or lightly overcast; the dim image of the sun was shown more brightly on the horizon vane where a shadow could not be seen.
Usage
In order to use the instrument, the navigator would place the shadow vane at a location anticipating the altitude of the sun. Holding the instrument in front of him, with the sun at his back, he holds the instrument so that the shadow cast by the shadow vane falls on the horizon vane at the side of the slit. He then moves the sight vane so that he observes the horizon in a line from the sight vane through the horizon vane's slit while simultaneously maintaining the position of the shadow. This permits him to measure the angle between the horizon and the sun as the sum of the angle read from the two arcs.
Since the shadow's edge represents the limb of the sun, he must correct the value for the semidiameter of the sun.
Instruments that derived from the Davis quadrant
The Elton's quadrant derived from the Davis quadrant. It added an index arm with spirit levels to provide an artificial horizon.
Demi-cross
The demi-cross was an instrument that was contemporary with the Davis quadrant. It was popular outside England.
The vertical transom was like a half-transom on a cross-staff, hence the name demi-cross. It supported a shadow vane (A in Figure 4) that could be set to one of several heights (three according to May, four according to de Hilster). By setting the shadow vane height, the range of angles that could be measured was set. The transom could be slid along the staff and the angle read from one of the graduated scales on the staff.
The sight vane (C) and horizon vane (B) were aligned visually with the horizon. With the shadow vane's shadow cast on the horizon vane and aligned with the horizon, the angle was determined. In practice, the instrument was accurate but more unwieldy than the Davis quadrant.
Plough | Backstaff | Wikipedia | 462 | 974163 | https://en.wikipedia.org/wiki/Backstaff | Technology | Navigation | null |
The plough was the name given to an unusual instrument that existed for a short time. It was part cross-staff and part backstaff. In Figure 5, A is the transom that casts its shadow on the horizon vane at B. It functions in the same manner as the staff in Figure 1. C is the sighting vane. The navigator uses the sighting vane and the horizon vane to align the instrument horizontally. The sighting vane can be moved left to right along the staff. D is a transom just as one finds on a cross-staff. This transom has two vanes on it that can be moved closer or farther from the staff to emulate different-length transoms. The transom can be moved on the staff and used to measure angles.
Almucantar staff
The Almucantar staff is a device specifically used for measuring the altitude of the sun at low altitudes.
Cross-staff
The cross-staff was normally a direct observation instrument. However, in later years it was modified for use with back observations.
Quadrant
There was a variation of the quadrant – the Back observation quadrant – that was used for measuring the sun's altitude by observing the shadow cast on a horizon vane.
Thomas Hood cross-staff
Thomas Hood invented this cross-staff in 1590. It could be used for surveying, astronomy or other geometric problems.
It consists of two components, a transom and a yard. The transom is the vertical component and is graduated from 0° at the top to 45° at the bottom. At the top of the transom, a vane is mounted to cast a shadow. The yard is horizontal and is graduated from 45° to 90°. The transom and yard are joined by a special fitting (the double socket in Figure 6) that permits independent adjustments of the transom vertically and the yard horizontally.
It was possible to construct the instrument with the yard at the top of the transom rather than at the bottom. | Backstaff | Wikipedia | 396 | 974163 | https://en.wikipedia.org/wiki/Backstaff | Technology | Navigation | null |
Initially, the transom and yard are set so that the two are joined at their respective 45° settings. The instrument is held so that the yard is horizontal (the navigator can view the horizon along the yard to assist in this). The socket is loosened so that the transom is moved vertically until the shadow of the vane is cast at the yard's 90° setting. If the movement of just the transom can accomplish this, the altitude is given by the transom's graduations. If the sun is too high for this, the yard horizontal opening in the socket is loosened and the yard is moved to allow the shadow to land on the 90° mark. The yard then yields the altitude.
It was a fairly accurate instrument, as the graduations were well spaced compared to a conventional cross-staff. However, it was a bit unwieldy and difficult to handle in wind.
Benjamin Cole quadrant
A late addition to the collection of backstaves in the navigation world, this device was invented by Benjamin Cole in 1748.
The instrument consists of a staff with a pivoting quadrant on one end. The quadrant has a shadow vane, which can optionally take a lens like the Davis quadrant's Flamsteed glass, at the upper end of the graduated scale (A in Figure 7). This casts a shadow or projects an image of the sun on the horizon vane (B). The observer views the horizon through a hole in the sight vane (D) and a slit in the horizon vane to ensure the instrument is level. The quadrant component is rotated until the horizon and the sun's image or shadow are aligned. The altitude can then be read from the quadrant's scale. In order to refine the reading, a circular vernier is mounted on the staff (C).
The fact that such an instrument was introduced in the middle of the 18th century shows that the quadrant was still a viable instrument even in the presence of the octant.
English scientist George Adams created a very similar backstaff at the same time. Adam's version ensured that the distance between the Flamsteed glass and horizon vane was the same as the distance from the vane to the sight vane.
Cross bow quadrant
Edmund Gunter invented the cross bow quadrant, also called the mariner's bow, around 1623. It gets its name from the similarity to the archer's crossbow. | Backstaff | Wikipedia | 486 | 974163 | https://en.wikipedia.org/wiki/Backstaff | Technology | Navigation | null |
This instrument is interesting in that the arc is 120° but is only graduated as a 90° arc. As such, the angular spacing of a degree on the arc is slightly greater than one degree. Examples of the instrument can be found with a 0° to 90° graduation or with two mirrored 0° to 45° segments centred on the midpoint of the arc.
The instrument has three vanes, a horizon vane (A in Figure 8) which has an opening in it to observe the horizon, a shadow vane (B) to cast a shadow on the horizon vane and a sighting vane (C) that the navigator uses to view the horizon and shadow at the horizon vane. This serves to ensure the instrument is level while simultaneously measuring the altitude of the sun. The altitude is the difference in the angular positions of the shadow and sighting vanes.
With some versions of this instrument, the sun's declination for each day of the year was marked on the arc. This permitted the navigator to set the shadow vane to the date and the instrument would read the altitude directly. | Backstaff | Wikipedia | 219 | 974163 | https://en.wikipedia.org/wiki/Backstaff | Technology | Navigation | null |
In mathematics, an algebraic function is a function that can be defined
as the root of an irreducible polynomial equation. Algebraic functions are often algebraic expressions using a finite number of terms, involving only the algebraic operations addition, subtraction, multiplication, division, and raising to a fractional power. Examples of such functions are:
Some algebraic functions, however, cannot be expressed by such finite expressions (this is the Abel–Ruffini theorem). This is the case, for example, for the Bring radical, which is the function implicitly defined by
.
In more precise terms, an algebraic function of degree in one variable is a function that is continuous in its domain and satisfies a polynomial equation of positive degree
where the coefficients are polynomial functions of , with integer coefficients. It can be shown that the same class of functions is obtained if algebraic numbers are accepted for the coefficients of the 's. If transcendental numbers occur in the coefficients the function is, in general, not algebraic, but it is algebraic over the field generated by these coefficients.
The value of an algebraic function at a rational number, and more generally, at an algebraic number is always an algebraic number.
Sometimes, coefficients that are polynomial over a ring are considered, and one then talks about "functions algebraic over ".
A function which is not algebraic is called a transcendental function, as it is for example the case of . A composition of transcendental functions can give an algebraic function: .
As a polynomial equation of degree n has up to n roots (and exactly n roots over an algebraically closed field, such as the complex numbers), a polynomial equation does not implicitly define a single function, but up to n
functions, sometimes also called branches. Consider for example the equation of the unit circle:
This determines y, except only up to an overall sign; accordingly, it has two branches:
An algebraic function in m variables is similarly defined as a function which solves a polynomial equation in m + 1 variables:
It is normally assumed that p should be an irreducible polynomial. The existence of an algebraic function is then guaranteed by the implicit function theorem.
Formally, an algebraic function in m variables over the field K is an element of the algebraic closure of the field of rational functions K(x1, ..., xm).
Algebraic functions in one variable | Algebraic function | Wikipedia | 477 | 974169 | https://en.wikipedia.org/wiki/Algebraic%20function | Mathematics | Functions: General | null |
Introduction and overview
The informal definition of an algebraic function provides a number of clues about their properties. To gain an intuitive understanding, it may be helpful to regard algebraic functions as functions which can be formed by the usual algebraic operations: addition, multiplication, division, and taking an nth root. This is something of an oversimplification; because of the fundamental theorem of Galois theory, algebraic functions need not be expressible by radicals.
First, note that any polynomial function is an algebraic function, since it is simply the solution y to the equation
More generally, any rational function is algebraic, being the solution to
Moreover, the nth root of any polynomial is an algebraic function, solving the equation
Surprisingly, the inverse function of an algebraic function is an algebraic function. For supposing that y is a solution to
for each value of x, then x is also a solution of this equation for each value of y. Indeed, interchanging the roles of x and y and gathering terms,
Writing x as a function of y gives the inverse function, also an algebraic function.
However, not every function has an inverse. For example, y = x2 fails the horizontal line test: it fails to be one-to-one. The inverse is the algebraic "function" .
Another way to understand this, is that the set of branches of the polynomial equation defining our algebraic function is the graph of an algebraic curve.
The role of complex numbers
From an algebraic perspective, complex numbers enter quite naturally into the study of algebraic functions. First of all, by the fundamental theorem of algebra, the complex numbers are an algebraically closed field. Hence any polynomial relation p(y, x) = 0 is guaranteed to have at least one solution (and in general a number of solutions not exceeding the degree of p in y) for y at each point x, provided we allow y to assume complex as well as real values. Thus, problems to do with the domain of an algebraic function can safely be minimized.
Furthermore, even if one is ultimately interested in real algebraic functions, there may be no means to express the function in terms of addition, multiplication, division and taking nth roots without resorting to complex numbers (see casus irreducibilis). For example, consider the algebraic function determined by the equation
Using the cubic formula, we get | Algebraic function | Wikipedia | 474 | 974169 | https://en.wikipedia.org/wiki/Algebraic%20function | Mathematics | Functions: General | null |
For the square root is real and the cubic root is thus well defined, providing the unique real root. On the other hand, for the square root is not real, and one has to choose, for the square root, either non-real square root. Thus the cubic root has to be chosen among three non-real numbers. If the same choices are done in the two terms of the formula, the three choices for the cubic root provide the three branches shown, in the accompanying image.
It may be proven that there is no way to express this function in terms of nth roots using real numbers only, even though the resulting function is real-valued on the domain of the graph shown.
On a more significant theoretical level, using complex numbers allows one to use the powerful techniques of complex analysis to discuss algebraic functions. In particular, the argument principle can be used to show that any algebraic function is in fact an analytic function, at least in the multiple-valued sense.
Formally, let p(x, y) be a complex polynomial in the complex variables x and y. Suppose that
x0 ∈ C is such that the polynomial p(x0, y) of y has n distinct zeros. We shall show that the algebraic function is analytic in a neighborhood of x0. Choose a system of n non-overlapping discs Δi containing each of these zeros. Then by the argument principle
By continuity, this also holds for all x in a neighborhood of x0. In particular, p(x, y) has only one root in Δi, given by the residue theorem:
which is an analytic function.
Monodromy
Note that the foregoing proof of analyticity derived an expression for a system of n different function elements fi(x), provided that x is not a critical point of p(x, y). A critical point is a point where the number of distinct zeros is smaller than the degree of p, and this occurs only where the highest degree term of p or the discriminant vanish. Hence there are only finitely many such points c1, ..., cm.
A close analysis of the properties of the function elements fi near the critical points can be used to show that the monodromy cover is ramified over the critical points (and possibly the point at infinity). Thus the holomorphic extension of the fi has at worst algebraic poles and ordinary algebraic branchings over the critical points.
Note that, away from the critical points, we have | Algebraic function | Wikipedia | 506 | 974169 | https://en.wikipedia.org/wiki/Algebraic%20function | Mathematics | Functions: General | null |
since the fi are by definition the distinct zeros of p. The monodromy group acts by permuting the factors, and thus forms the monodromy representation of the Galois group of p. (The monodromy action on the universal covering space is related but different notion in the theory of Riemann surfaces.)
History
The ideas surrounding algebraic functions go back at least as far as René Descartes. The first discussion of algebraic functions appears to have been in Edward Waring's 1794 An Essay on the Principles of Human Knowledge in which he writes:
let a quantity denoting the ordinate, be an algebraic function of the abscissa x, by the common methods of division and extraction of roots, reduce it into an infinite series ascending or descending according to the dimensions of x, and then find the integral of each of the resulting terms. | Algebraic function | Wikipedia | 175 | 974169 | https://en.wikipedia.org/wiki/Algebraic%20function | Mathematics | Functions: General | null |
The Rialto Bridge (; ) is the oldest of the four bridges spanning the Grand Canal in Venice, Italy. Connecting the (districts) of San Marco and San Polo, it has been rebuilt several times since its first construction as a pontoon bridge in 1173, and is now a significant tourist attraction in the city.
The present stone bridge is a single span designed by Antonio da Ponte. Construction began in 1588 and was completed in 1591. It is similar to the wooden bridge it succeeded. Two ramps lead up to a central portico. On either side of the portico, the covered ramps carry rows of shops. The engineering of the bridge was considered so audacious that architect Vincenzo Scamozzi predicted future ruin. The bridge has defied its critics to become one of the architectural icons, and top tourist attractions, in Venice.
History
The first dry crossing of the Grand Canal was a pontoon bridge built in 1181 by Nicolò Barattieri. It was called the Ponte della Moneta, presumably because of the mint that stood near its eastern entrance.
The development and importance of the Rialto market on the eastern bank increased traffic on the floating bridge, so it was replaced in 1255 by a wooden bridge. This structure had two ramps meeting at a movable central section, that could be raised to allow the passage of tall ships. The connection with the market eventually led to a change of name for the bridge. During the first half of the 15th century, two rows of shops were built along the sides of the bridge. The rents brought an income to the State Treasury, which helped maintain the bridge.
Maintenance was vital for the timber bridge. It was partly burnt in the revolt led by Bajamonte Tiepolo in 1310. In 1444, it collapsed under the weight of a crowd rushing to see the marriage of the Marquis of Ferrara and it collapsed again in 1524.
The idea of rebuilding the bridge in stone was first proposed in 1503. Several projects were considered over the following decades. In 1551, the authorities requested proposals for the renewal of the Rialto Bridge, among other things. Plans were offered by famous architects, such as Jacopo Sansovino, Palladio and Vignola, but all involved a Classical approach with several arches, which was judged inappropriate to the situation. Michelangelo also was considered as designer of the bridge. | Rialto Bridge | Wikipedia | 485 | 974321 | https://en.wikipedia.org/wiki/Rialto%20Bridge | Technology | Bridges | null |
Other names
It was called Shylock's bridge in Robert Browning's poem "A Toccata of Galuppi's". | Rialto Bridge | Wikipedia | 29 | 974321 | https://en.wikipedia.org/wiki/Rialto%20Bridge | Technology | Bridges | null |
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