A chemical substance is a form of matter having constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. Chemical substances can be chemical compounds, or alloys. Chemical elements may not be included in the definition, depending on expert viewpoint. Chemical substances are called'pure' to set them apart from mixtures. A common example of a chemical substance is pure water. Other chemical substances encountered in pure form are diamond, table salt and refined sugar. However, in practice, no substance is pure, chemical purity is specified according to the intended use of the chemical. Chemical substances exist as solids, gases, or plasma, may change between these phases of matter with changes in temperature or pressure. Chemical substances may be converted to others by means of chemical reactions. Forms of energy, such as light and heat, are not matter, are thus not "substances" in this regard.
A chemical substance may well be defined as "any material with a definite chemical composition" in an introductory general chemistry textbook. According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. But, there are exceptions to this definition; the chemical substance index published by CAS includes several alloys of uncertain composition. Non-stoichiometric compounds are a special case that violates the law of constant composition, for them, it is sometimes difficult to draw the line between a mixture and a compound, as in the case of palladium hydride. Broader definitions of chemicals or chemical substances can be found, for example: "the term'chemical substance' means any organic or inorganic substance of a particular molecular identity, including – any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature". In geology, substances of uniform composition are called minerals, while physical mixtures of several minerals are defined as rocks.
Many minerals, mutually dissolve into solid solutions, such that a single rock is a uniform substance despite being a mixture in stoichiometric terms. Feldspars are a common example: anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium. In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. For example, the EU regulation REACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition"; the latter two consist of multiple chemical substances. For example, charcoal is an complex polymeric mixture that can be defined by its manufacturing process. Therefore, although the exact chemical identity is unknown, identification can be made to a sufficient accuracy; the CAS index includes mixtures. Polymers always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance.
However, the polymer may be defined by a known precursor or reaction and the molar mass distribution. For example, polyethylene is a mixture of long chains of -CH2- repeating units, is sold in several molar mass distributions, LDPE, MDPE, HDPE and UHMWPE; the concept of a "chemical substance" became established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate. He deduced; this is now known as the law of constant composition. With the advancement of methods for chemical synthesis in the realm of organic chemistry. However, there are some controversies regarding this definition because the large number of chemical substances reported in chemistry literature need to be indexed. Isomerism caused much consternation to early researchers, since isomers have the same composition, but differ in configuration of the atoms. For example, there was much speculation for the chemical identity of benzene, until the correct structure was described by Friedrich August Kekulé.
The idea of stereoisomerism – that atoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example, tartaric acid has three distinct isomers, a pair of diastereomers with one diastereomer forming two enantiomers. An element is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through a nuclear reaction; this is so, beca
Rodenticides, colloquially rat poison, are non-specific pest control chemicals made and sold for the purpose of killing rodents. Some rodenticides are lethal after one exposure. Rodents are disinclined to gorge on an unknown food, preferring to sample and observe whether it makes them or other rats sick; this phenomenon of bait shyness or poison shyness is the rationale for poisons that kill only after multiple doses. Besides being directly toxic to the mammals that ingest them, including dogs and humans, many rodenticides present a secondary poisoning risk to animals that hunt or scavenge the dead corpses of rats. Anticoagulants are defined as chronic, single-dose or multiple-dose rodenticides, acting by effective blocking of the vitamin K cycle, resulting in inability to produce essential blood-clotting factors—mainly coagulation factors II and VII. In addition to this specific metabolic disruption, massive toxic doses of 4-hydroxycoumarin, 4-thiochromenone and indandione anticoagulants cause damage to tiny blood vessels, increasing their permeability, causing internal bleeding.
These effects are gradual. In the final phase of the intoxication, the exhausted rodent collapses due to hemorrhagic shock or severe anemia and dies calmly; the question of whether the use of these rodenticides can be considered humane has been raised. The main benefit of anticoagulants over other poisons is that the time taken for the poison to induce death means that the rats do not associate the damage with their feeding habits. First generation rodenticidal anticoagulants have shorter elimination half-lives, require higher concentrations and consecutive intake over days in order to accumulate the lethal dose, are less toxic than second generation agents. Second generation agents are far more toxic than first generation, they are applied in lower concentrations in baits—usually on the order of 0.001% to 0.005%—are lethal after a single ingestion of bait and are effective against strains of rodents that became resistant to first generation anticoagulants. Vitamin K1 has been suggested, used, as antidote for pets or humans accidentally or intentionally exposed to anticoagulant poisons.
Some of these poisons act by inhibiting liver functions and in advanced stages of poisoning, several blood-clotting factors are absent, the volume of circulating blood is diminished, so that a blood transfusion can save a person, poisoned, an advantage over some older poisons. Metal phosphides have been used as a means of killing rodents and are considered single-dose fast acting rodenticides. A bait consisting of food and a phosphide is left; the acid in the digestive system of the rodent reacts with the phosphide to generate the toxic phosphine gas. This method of vermin control has possible use in places where rodents are resistant to some of the anticoagulants for control of house and field mice. Inversely, the individual rodents, that survived anticoagulant bait poisoning can be eradicated by pre-baiting them with nontoxic bait for a week or two and subsequently applying poisoned bait of the same sort as used for pre-baiting until all consumption of the bait ceases; these methods of alternating rodenticides with different modes of action gives actual or 100% eradications of the rodent population in the area, if the acceptance/palatability of baits are good.
Zinc phosphide is added to rodent baits in a concentration of 0.75% to 2.0%. The baits have pungent garlic-like odor due to the phosphine liberated by hydrolysis; the odor has an repulsive effect on other mammals. Birds, notably wild turkeys, are not sensitive to the smell, will feed on the bait, thus become collateral damage; the tablets or pellets may contain other chemicals which evolve ammonia, which helps reduce the potential for spontaneous combustion or explosion of the phosphine gas. Metal phosphides do not accumulate in the tissues of poisoned animals, so the risk of secondary poisoning is low. Before the advent of anticoagulants, phosphides were the favored kind of rat poison. During World War II, they came into use in United States because of shortage of strychnine due to the Japanese occupation of the territories where the strychnine tree is grown. Phosphides are rather fast-acting rat poisons, resulting in the rats dying in open areas, instead of in the affected buildings. Phosphides used as rodenticides include: aluminium phosphide calcium phosphide magnesium phosphide zinc phosphide
Environmental impact of pesticides
The impact of pesticides consists of the effects of pesticides on non-target species. Pesticides are chemical preparations used to kill fungal or animal pests. Over 98% of sprayed insecticides and 95% of herbicides reach a destination other than their target species, because they are sprayed or spread across entire agricultural fields. Runoff can carry pesticides into aquatic environments while wind can carry them to other fields, grazing areas, human settlements and undeveloped areas affecting other species. Other problems emerge from poor production and storage practices. Over time, repeated application increases pest resistance, while its effects on other species can facilitate the pest's resurgence; each pesticide or pesticide class comes with a specific set of environmental concerns. Such undesirable effects have led many pesticides to be banned, while regulations have limited and/or reduced the use of others. Over time, pesticides have become less persistent and more species-specific, reducing their environmental footprint.
In addition the amounts of pesticides applied per hectare have declined, in some cases by 99%. However, the global spread of pesticide use, including the use of older/obsolete pesticides that have been banned in some jurisdictions, has increased overall; the arrival of humans in an area, to live or to conduct agriculture has environmental impacts. These range from simple crowding out of wild plants in favor of more desirable cultivars to larger scale impacts such as reducing biodiversity by reducing food availability of native species, which can propagate across food chains; the use of agricultural chemicals such as fertilizer and des magnify those impacts. While advances in agrochemistry have reduced those impacts, for example by the replacement of long-lived chemicals with those that reliably degrade in the best case they remain substantial; these effects are magnified by the use of poor management practices. While concern for ecotoxicology began with acute poisoning events in the late 19th century.
Shortly thereafter, DDT used to combat malaria, its metabolites were shown to cause population-level effects in raptorial birds. Initial studies in industrialized countries focused on acute mortality effects involving birds or fish. Data on pesticide usage not publicly available; the common practice of incident registration is inadequate for understanding the entirety of effects. Since 1990, research interest has shifted from documenting incidents and quantifying chemical exposure to studies aimed at linking laboratory and field experiments; the proportion of effect-related publications has increased. Animal studies focus on fish, birds and arachnids. Since 1993, the United States and the European Union have updated pesticide risk assessments, ending the use of acutely toxic organophosphate and carbamate insecticides. Newer pesticides aim at efficiency in target and minimum side effects in nontarget organisms; the phylogenetic proximity of beneficial and pest species complicates the project. One of the major challenges is to link the results from cellular studies through many levels of increasing complexity to ecosystems.
The concept of a half-life has been utilized for pesticides in plants, certain authors maintain that pesticide risk and impact assessment models rely on and are sensitive to information describing dissipation from plants. Half-life for pesticides is explained in two NPIC fact sheets. Known degradation pathways are through: photolysis, chemical dissociation, sorption and plant or animal metabolism. A USDA fact sheet published in 1994 lists the soil adsorption coefficient and soil half-life for then-commonly used pesticides. Pesticides can contribute to air pollution. Pesticide drift occurs when pesticides suspended in the air as particles are carried by wind to other areas contaminating them. Pesticides that are applied to crops can volatilize and may be blown by winds into nearby areas posing a threat to wildlife. Weather conditions at the time of application as well as temperature and relative humidity change the spread of the pesticide in the air; as wind velocity increases so does exposure.
Low relative humidity and high temperature result in more spray evaporating. The amount of inhalable pesticides in the outdoor environment is therefore dependent on the season. Droplets of sprayed pesticides or particles from pesticides applied as dusts may travel on the wind to other areas, or pesticides may adhere to particles that blow in the wind, such as dust particles. Ground spraying produces less pesticide drift. Farmers can employ a buffer zone around their crop, consisting of empty land or non-crop plants such as evergreen trees to serve as windbreaks and absorb the pesticides, preventing drift into other areas; such windbreaks are required in the Netherlands. Pesticides that are sprayed on to fields and used to fumigate soil can give off chemicals called volatile organic compounds, which can react with other chemicals and form a pollutant called tropospheric ozone. Pesticide use accounts for about 6 percent of total tropospheric ozone levels. In the United States, pesticides were found to pollute every stream and over 90% of wells sampled in a study by the US Geological Survey.
Pesticide residues have been found in rain and groundwater. Studies by the UK government showed that pesticide concentrations exceeded those allowable for drinking water in some samples of river water and groundwate
Pesticides are substances that are meant to control pests, including weeds. The term pesticide includes all of the following: herbicide, insecticides nematicide, piscicide, rodenticide, insect repellent, animal repellent and fungicide; the most common of these are herbicides which account for 80% of all pesticide use. Most pesticides are intended to serve as plant protection products, which in general, protect plants from weeds, fungi, or insects. In general, a pesticide is a chemical or biological agent that deters, kills, or otherwise discourages pests. Target pests can include insects, plant pathogens, molluscs, mammals, fish and microbes that destroy property, cause nuisance, or spread disease, or are disease vectors. Along with these benefits, pesticides have drawbacks, such as potential toxicity to humans and other species; the Food and Agriculture Organization has defined pesticide as: any substance or mixture of substances intended for preventing, destroying, or controlling any pest, including vectors of human or animal disease, unwanted species of plants or animals, causing harm during or otherwise interfering with the production, storage, transport, or marketing of food, agricultural commodities and wood products or animal feedstuffs, or substances that may be administered to animals for the control of insects, arachnids, or other pests in or on their bodies.
The term includes substances intended for use as a plant growth regulator, desiccant, or agent for thinning fruit or preventing the premature fall of fruit. Used as substances applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport. Pesticides can be classified by target organism, chemical structure, physical state. Biopesticides include biochemical pesticides. Plant-derived pesticides, or "botanicals", have been developing quickly; these include the pyrethroids, nicotinoids, a fourth group that includes strychnine and scilliroside. Many pesticides can be grouped into chemical families. Prominent insecticide families include organochlorines and carbamates. Organochlorine hydrocarbons could be separated into dichlorodiphenylethanes, cyclodiene compounds, other related compounds, they operate by disrupting the sodium/potassium balance of the nerve fiber, forcing the nerve to transmit continuously. Their toxicities vary but they have been phased out because of their persistence and potential to bioaccumulate.
Organophosphate and carbamates replaced organochlorines. Both operate through inhibiting the enzyme acetylcholinesterase, allowing acetylcholine to transfer nerve impulses indefinitely and causing a variety of symptoms such as weakness or paralysis. Organophosphates are quite toxic to vertebrates and have in some cases been replaced by less toxic carbamates. Thiocarbamate and dithiocarbamates are subclasses of carbamates. Prominent families of herbicides include phenoxy and benzoic acid herbicides, triazines and Chloroacetanilides. Phenoxy compounds tend to selectively kill broad-leaf weeds rather than grasses; the phenoxy and benzoic acid herbicides function similar to plant growth hormones, grow cells without normal cell division, crushing the plant's nutrient transport system. Triazines interfere with photosynthesis. Many used pesticides are not included in these families, including glyphosate; the application of pest control agents is carried out by dispersing the chemical in a solvent-surfactant system to give a homogeneous preparation.
A virus lethality study performed in 1977 demonstrated that a particular pesticide did not increase the lethality of the virus, however combinations which included some surfactants and the solvent showed that pretreatment with them markedly increased the viral lethality in the test mice. Pesticides can be classified based upon their biological mechanism application method. Most pesticides work by poisoning pests. A systemic pesticide moves inside a plant following absorption by the plant. With insecticides and most fungicides, this movement is upward and outward. Increased efficiency may be a result. Systemic insecticides, which poison pollen and nectar in the flowers, may kill bees and other needed pollinators. In 2010, the development of a new class of fungicides called; these work by taking advantage of natural defense chemicals released by plants called phytoalexins, which fungi detoxify using enzymes. The paldoxins inhibit the fungi's detoxification enzymes, they are believed to be greener.
Since before 2000 BC, humans have utilized pesticides to protect their crops. The first known pesticide was elemental sulfur dusting used in ancient Sumer about 4,500 years ago in ancient Mesopotamia; the Rig Veda, about 4,000 years old, mentions the use of poisonous plants for pest control. By the 15th century, toxic chemicals such as arsenic and lead were being applied to crops to kill pests. In the 17th century, nicotine sulfate was extracted from tobacco leaves for use as an insecticide; the 19th century saw the introduction of two more natural pesticides, derived fr
Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: organic polysulfides. Anions have the general formula S2−n; these anions are the conjugate bases of the hydrogen polysulfides H2Sn. Organic polysulfides have the formulae RSnR, where R = alkyl or aryl; the alkali metal polysulfides arise by treatment of a solution of sulfide, e.g. sodium sulfide, with elemental sulfur: S2− + n S → S2−n+1In some cases, these anions have been obtained as organic salts, which are soluble in organic solvents. The energy released in the reaction of sodium and elemental sulfur is the basis of battery technology; the sodium–sulfur battery and the lithium–sulfur battery require high temperatures to maintain liquid polysulfide and Na+-conductive membranes that are unreactive toward sodium and sodium sulfide. Polysulfides are ligands in coordination chemistry. Examples of transition metal polysulfido complexes include 2TiS5, 2−, 2−. Main group elements form polysulfides.
In commerce, the term "polysulfide" refers to a class of polymers with alternating chains of several sulfur atoms and hydrocarbons. They have the formula R2Sx. In this formula x indicates the number of sulfur atoms. Polysulfide polymers can be synthesized by condensation polymerization reactions between organic dihalides and alkali metal salts of polysulfide anions: n Na2S5 + n ClCH2CH2Cl → n + 2n NaClDihalides used in this condensation polymerization are dichloroalkanes; the polymers are called thiokols. In some cases, polysulfide polymers can be formed by ring-opening polymerization reactions. Polysulfide polymers are prepared by the addition of polysulfanes to alkenes. An idealized equation is: 2 RCH=CH2 + H2Sx → 2SxIn reality, homogeneous samples of H2Sx are difficult to prepare. Polysulfide polymers are insoluble in water and many other organic solvents; because of their solvent resistance, these materials find use as sealants to fill the joints in pavement, automotive window glass, aircraft structures.
Polymers containing one or two sulfur atoms separated by hydrocarbon sequences are not classified polysulfides, e.g. poly sulfide n. Many commercial elastomers contain polysulfides as crosslinks; these crosslinks interconnect neighboring polymer chains. The degree of rigidity is related to the number of crosslinks. Elastomers therefore have a characteristic ability to "snap back" to their original shape after being stretched or compressed; because of this memory for their original cured shape, elastomers are referred to as rubbers. The process of crosslinking the polymer chains in these polymers with sulfur is called vulcanization; the sulfur chains attach themselves to the "allylic" carbon atoms, which are adjacent to C=C linkages. Vulcanization is a step in the processing of several classes of rubbers, including polychloroprene, styrene-butadiene, polyisoprene, chemically similar to natural rubber. Charles Goodyear's discovery of vulcanization, involving the heating of polyisoprene with sulfur, was revolutionary because it converted a sticky and useless material into an elastomer which could be fabricated into useful products.
In addition to water and ammonia, the clouds in the atmospheres of the gas giant planets contain ammonium sulfides. The reddish-brownish clouds are attributed to polysulfides, arising from the exposure of the ammonium sulfides to light. Polysulfides, as sulfides, can induce stress corrosion cracking in carbon steel and stainless steel
Pest control is the regulation or management of a species defined as a pest, a member of the animal kingdom that impacts adversely on human activities. The human response depends on the importance of the damage done, will range from tolerance, through deterrence and management, to attempts to eradicate the pest. Pest control measures may be performed as part of an integrated pest management strategy. In agriculture, pests are kept at bay by cultural and biological means. Ploughing and cultivation of the soil before sowing reduces the pest burden and there is a modern trend to limit the use of pesticides as far as possible; this can be achieved by monitoring the crop, only applying insecticides when necessary, by growing varieties and crops which are resistant to pests. Where possible, biological means are used, encouraging the natural enemies of the pests and introducing suitable predators or parasites. In homes and urban environments, the pests are the rodents, birds and other organisms that share the habitat with humans, that feed on and spoil possessions.
Control of these pests is attempted through exclusion, physical removal or chemical means. Alternatively, various methods of biological control can be used including sterilisation programmes. Pest control is at least as old as agriculture, as there has always been a need to keep crops free from pests; as long ago as 3000 BC in Egypt, cats were used to control pests of grain stores such as rodents. Ferrets were domesticated by 500 AD in Europe for use as mousers. Mongooses were introduced into homes to control rodents and snakes by the ancient Egyptians; the conventional approach was the first to be employed, since it is comparatively easy to destroy weeds by burning them or ploughing them under, to kill larger competing herbivores. Techniques such as crop rotation, companion planting, the selective breeding of pest-resistant cultivars have a long history. Chemical pesticides were first used around 2500 BC, when the Sumerians used sulphur compounds as insecticides. Modern pest control was stimulated by the spread across the United States of the Colorado potato beetle.
After much discussion, arsenical compounds were used to control the beetle and the predicted poisoning of the human population did not occur. This led the way to a widespread acceptance of insecticides across the continent. With the industrialisation and mechanization of agriculture in the 18th and 19th centuries, the introduction of the insecticides pyrethrum and derris, chemical pest control became widespread. In the 20th century, the discovery of several synthetic insecticides, such as DDT, herbicides boosted this development. Biological control is first recorded around 300 AD in China, when colonies of weaver ants, Oecophylla smaragdina, were intentionally placed in citrus plantations to control beetles and caterpillars. In China, ducks were used in paddy fields to consume pests, as illustrated in ancient cave art. In 1762, an Indian mynah was brought to Mauritius to control locusts, about the same time, citrus trees in Burma were connected by bamboos to allow ants to pass between them and help control caterpillars.
In the 1880s, ladybirds were used in citrus plantations in California to control scale insects, other biological control experiments followed. The introduction of DDT, a cheap and effective compound, put an effective stop to biological control experiments. By the 1960s, problems of resistance to chemicals and damage to the environment began to emerge, biological control had a renaissance. Chemical pest control is still the predominant type of pest control today, although a renewed interest in traditional and biological pest control developed towards the end of the 20th century and continues to this day. Biological pest control is a method of controlling pests such as insects and mites by using other organisms, it relies on predation, herbivory or other natural mechanisms, but also involves an active human management role. Classical biological control involves the introduction of natural enemies of the pest that are bred in the laboratory and released into the environment. An alternative approach is to augment the natural enemies that occur in a particular area by releasing more, either in small, repeated batches, or in a single large-scale release.
Ideally, the released organism will breed and survive, provide long-term control. Biological control can be an important component of an integrated pest management programme. For example: mosquitoes are controlled by putting Bt Bacillus thuringiensis ssp. israelensis, a bacterium that infects and kills mosquito larvae, in local water sources. Mechanical pest control is the use of hands-on techniques as well as simple equipment and devices, that provides a protective barrier between plants and insects; this is referred to as tillage and is one of the oldest methods of weed control as well as being useful for pest control. Crop rotation can help to control pests by depriving them of their host plants, it is a major tactic in the control of corn rootworm, has reduced early season incidence of Colorado potato beetle by as much as 95%. A trap crop is a crop of a plant. Pests aggregated on the trap crop can be more controlled using pesticides or other methods. However, trap-cropping, on its own, has failed to cost reduce pest densities on large commercial scales, without the use of pesticides due to the pests' ability to disperse back into the main field
The potato is a starchy, tuberous crop from the perennial nightshade Solanum tuberosum. In many contexts, potato refers to the edible tuber, but it can refer to the plant itself. Common or slang terms include tater and spud. Potatoes were introduced to Europe in the second half of the 16th century by the Spanish. Today they are a staple food in many parts of the world and an integral part of much of the world's food supply; as of 2014, potatoes were the world's fourth-largest food crop after maize and rice. Wild potato species can be found from the United States to southern Chile; the potato was believed to have been domesticated independently in multiple locations, but genetic testing of the wide variety of cultivars and wild species traced a single origin for potatoes. In the area of present-day southern Peru and extreme northwestern Bolivia, from a species in the Solanum brevicaule complex, potatoes were domesticated 7,000–10,000 years ago. In the Andes region of South America, where the species is indigenous, some close relatives of the potato are cultivated.
Following millennia of selective breeding, there are now over 1,000 different types of potatoes. Over 99% of presently cultivated potatoes worldwide descended from varieties that originated in the lowlands of south-central Chile, which have displaced popular varieties from the Andes; the importance of the potato as a food source and culinary ingredient varies by region and is still changing. It remains an essential crop in Europe eastern and central Europe, where per capita production is still the highest in the world, while the most rapid expansion in production over the past few decades has occurred in southern and eastern Asia, with China and India leading the world in overall production as of 2014. Being a nightshade similar to tomatoes, the vegetative and fruiting parts of the potato contain the toxin solanine and are not fit for human consumption. Normal potato tubers that have been grown and stored properly produce glycoalkaloids in amounts small enough to be negligible to human health, but if green sections of the plant are exposed to light, the tuber can accumulate a high enough concentration of glycoalkaloids to affect human health.
The English word potato comes from Spanish patata. The Spanish Royal Academy says the Spanish word is a hybrid of the Taíno batata and the Quechua papa; the name referred to the sweet potato although the two plants are not related. The 16th-century English herbalist John Gerard referred to sweet potatoes as "common potatoes", used the terms "bastard potatoes" and "Virginia potatoes" for the species we now call "potato". In many of the chronicles detailing agriculture and plants, no distinction is made between the two. Potatoes are referred to as "Irish potatoes" or "white potatoes" in the United States, to distinguish them from sweet potatoes; the name spud for a small potato comes from the digging of soil prior to the planting of potatoes. The word has an unknown origin and was used as a term for a short knife or dagger related to the Latin "spad-" a word root meaning "sword", it subsequently transferred over to a variety of digging tools. Around 1845, the name transferred to the tuber itself, the first record of this usage being in New Zealand English.
The origin of the word "spud" has erroneously been attributed to an 18th-century activist group dedicated to keeping the potato out of Britain, calling itself The Society for the Prevention of Unwholesome Diet. It was Mario Pei's 1949 The Story of Language. Pei writes, "the potato, for its part, was in disrepute some centuries ago; some Englishmen who did not fancy potatoes formed a Society for the Prevention of Unwholesome Diet. The initials of the main words in this title gave rise to spud." Like most other pre-20th century acronymic origins, this is false, there is no evidence that a Society for the Prevention of Unwholesome Diet existed. Potato plants are herbaceous perennials that grow about 60 cm high, depending on variety, with the leaves dying back after flowering and tuber formation, they bear white, red, blue, or purple flowers with yellow stamens. In general, the tubers of varieties with white flowers have white skins, while those of varieties with colored flowers tend to have pinkish skins.
Potatoes are cross-pollinated by insects such as bumblebees, which carry pollen from other potato plants, though a substantial amount of self-fertilizing occurs as well. Tubers form in response to decreasing day length, although this tendency has been minimized in commercial varieties. After flowering, potato plants produce small green fruits that resemble green cherry tomatoes, each containing about 300 seeds. Like all parts of the plant except the tubers, the fruit contain the toxic alkaloid solanine and are therefore unsuitable for consumption. All new potato varieties are grown from seeds called "true potato seed", "TPS" or "botanical seed" to distinguish it from seed tubers. New varieties grown from seed can be propagated vegetatively by planting tubers, pieces of tubers cut to include at least one or two eyes, or cuttings, a practice used in greenhouses for the production of healthy seed tubers. Plants propagated from tubers are clones of the parent, whereas those propagated from seed produce a range of different varieties.
There are about 5,000 potato varieties worldwide. Three thousand of them are found in the Andes alone in Peru, Ecuador and Colombia, they belong to eight or nine species, dependin