The fundamental theorem of arithmetic states that every positive integer can be expressed as the product of prime factors in essentially a single way. A prime number is a number whose only factors are itself and 1 (the first few prime numbers are 1, 2, 3, 5, 7, 11, 13). Integers that are not prime are called composite. The number 99 is composite because it can be factored into the product 9 …
The number n! is the product 1 × 2 × 3 × 4 ×... × n, that is, the product of all the natural numbers from 1 up to n, including n itself where 1 is a natural number. It is called either "n factorial" or "factorial n." Thus 5! is the number 1 × 2 × 3 × 4 × 5, or 120. Older books sometimes used the symbol In fo…
Falcons are birds of prey in the family Falconidae. There are 39 species of true falcons, all in the genus Falco. Like other species in the order Falconiformes (which also includes hawks, eagles, osprey, and vultures), falcons have strong raptorial (or grasping) talons, a hooked beak, extremely acute vision, and a fierce demeanor. Falcons can be distinguished from other raptors by the small toothl…
The Faraday effect is manifest when a changing magnetic field induces an electric field. Hence the effect is also known as "induction." It is most simply exemplified by a loop of wire and a bar magnet. If one moves the magnet through the loop of wire, the changing magnetic field within the loop gives rise to an electrical current in the wire. The current is larger for stronger magnet…
A fat is a solid triester of glycerol. It is formed when a molecule of glycerol, an alcohol with three hydroxyl groups, reacts with three molecules of fatty acids. A fatty acid is a long-chain aliphatic carboxylic acid. The more correct name for a fat is a triglyceride. The three fatty acid fragments in a fat may be all the same (a simple triglyceride) or they may be different from each other (a m…
A fatty acid is a combination of a chain of carbon and hydrogen atoms, known as a hydrocarbon, and a particular acid group (-COOH). Three fatty-acid molecules combined with a glycerol form a triglyceride fat or oil. While several varieties of fatty acid occur in nature, all belong in one of two categories—saturated or unsaturated. In a saturated fatty-acid molecule, all the carbon atoms in …
A fault is a geologic term describing a fracture at which two bodies of rock have been displaced relative to each other. Bedrock faults are those in which bodies of rock meet; small, local movements may occur on bedrock faults. Much larger movements or displacements occur along Faults where plates of Earth's crust abut each other. Faults may be inches (centimeters) to hundreds of miles (kil…
Fauna is a generic term for the list of animal species occurring in a particular, large region. Fauna can refer to a prehistoric collection of animals, as might be inferred from the fossil record, or to a modern assemblage of species living in a region. The botanical analogue is known as flora. More locally, a faunation refers to the communities of individuals of the various animal species and occ…
The facsimile, or fax, machine is both a transmitting and receiving device that "reads" text, maps, photographs, fingerprints, and graphics and communicates via telephone line. Since 1980s, fax machines have undergone rapid development and refinement and are now indispensable communication aids for news services, businesses, government agencies, and individuals. The fax was invented …
Feather stars, or comatulids, are echinoderms that belong to the class Crinoidea (phylum Echinodermata) which they share with the sea lilies. Unlike the latter group, however, feather stars are not obliged to remain in one place; instead they can swim or even crawl over short distances before attaching themselves to some support. Swimming movements are achieved by waving the arms up and down in a …
In its broadest sense, fermentation refers to any process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms. The most familiar type of fermentation is the conversion of sugars and starches to alcohol by enzymes in yeast. To distinguish this reaction from other kinds of fermentation, the process is sometimes known as alcoholic or eth…
Ferns are plants in the Filicinophyta phylum, also called the Pteridophyta phylum. They are intermediate in complexity between the more primitive (i.e., evolutionarily ancient) bryophytes (mosses, liverworts, and hornworts) and the more advanced (or recent) seed plants. Like bryophytes, ferns reproduce sexually by making spores rather than seeds. Most ferns produce spores on the underside or margi…
Ferrets are small carnivores belonging to the weasel family (Mustelidae). The name is most commonly given to the fitch, or European polecat (Mustela putorius), which has been domesticated and used for hunting rodents and as a pet for hundreds of years. Like most weasels, ferrets are long and slender, and are determined hunters. Their color varies from yellowish to all black, and they are about 2 f…
In animals, fertilization is the fusion of a sperm cell with an egg cell. The penetration of the egg cell by the chromosome-containing part of the sperm cell causes a reaction, which prevents additional sperm cells from entering the egg. The egg and sperm each contribute half of the new organism's genetic material. A fertilized egg cell is known as a zygote. The zygote undergoes continuous …
A fertilizer is any substance applied to land to increase plant growth and produce higher crop yield. Fertilizers may be made from organic material, such as animal manure or compost, or it may be chemically manufactured. Manufactured fertilizers contain varying amounts of inorganic nitrogen, phosphorus, and potassium, all of which are nutrients that plants need to grow. Since the 1950s crop produc…
Fetal alcohol syndrome (FAS) represents a preventable pattern of clinical abnormalities that develop during embryogenesis (the developmental stages shortly after conception) due to exposure to alcohol during pregnancy. FAS is currently the leading cause of birth defects and developmental delay, with as many as 12,000 babies born affected in the United States each year. Although the prevalence of F…
American physicist Richard Feynman's (1918–1988), work and writings were fundamental to the development of quantum electrodynamic theory (QED theory). With regard to QED theory, Feynman is perhaps best remembered for his invention of what are now known as Feynman diagrams, to portray the complex interactions of atomic particles. Moreover, Feynman diagrams allow visual representation …
Optical fiber is a very thin strand of glass or plastic capable of transmitting light from one point to another. Optical fiber can also be called an optical waveguide, since it is a device that guides light. Optical fibers consist of a light-carrying core and a cladding surrounding the core. There are generally three types of construction: glass core/cladding, glass core with plastic cladding, or …
The Fibonacci sequence is a series of numbers in which each succeeding number (after the second) is the sum of the previous two. The most famous Fibonacci sequence is 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89.... This sequence expresses many naturally occurring relationships in the plant world. …
A field is the name given to a pair of numbers and a set of operations which together satisfy several specific laws. A familiar example of a field is the set of rational numbers and the operations addition and multiplication. An example of a set of numbers that is not a field is the set of integers. It is an "integral domain." It is not a field because it lacks multiplicative inverse…
Figurative numbers are numbers which can be represented by dots arranged in various geometric patterns. For example, triangular numbers are represented by the patterns shown in Figure 1. The numbers they represent are 1, 3, 6, 10, and so on. Figurative numbers also include the square numbers which can be represented by square arrays of dots, as shown in Figure 2. The first few square numbers are 1…
Filtration is the process by which solid materials are removed from a fluid mixture, either a gas or liquid mixture. One of the most familiar kinds of filtration is that which students of chemistry encounter in their early laboratory experiences. In these experiences, suspensions of a solid precipitate in water are passed through filter paper supported in a glass funnel. The filter paper traps sol…
Finches are species of arboreal, perching birds that make up the large, widespread family, the Fringillidae. There are three subfamilies in this group, the largest being the Carduelinae or cardueline finches, a geographically widespread group that contains about 122 species. The subfamily Fringillinae or fringillid finches consists of three species breeding in woodlands of Eurasia, while the Drepa…
The true firs are about 40 species of conifer trees in the genus Abies, occurring in cool-temperate, boreal, and montane forests of the northern hemisphere. Firs are members of the pine family (Pinaceae). Firs are characterized by flattened needles, usually having two white lines running the length of the leaf. Firs do not have a petiole joining the needles to the twigs, and after the foliage is s…
More than three quarters of Earth's surface is covered by salt water; in addition, large areas are inundated with freshwater in the form of lakes, rivers, canals, swamps, and marshes. It is therefore not surprising that animals and plants have undergone a wide radiation in such habitats. One of the most successful groups of animals that have evolved to fill all of these habitats is the fish…
Flagella are long, thread-like appendages which provide some live single cells with the ability to move, motility. Bacteria which have flagella are either rod or spiral-shaped and are known as bacilli and spirochetes, respectively. Cocci, or round bacteria, are almost all nonmotile. Animal sperm cells also have flagella. However, prokaryotic cells (such as bacteria) have flagella made up of the pr…
Allowing analysis of the light (photons) from excited atoms, flame analysis is a form of atomic emission spectroscopy (AES). German chemist Robert Bunsen's (1811–1999) invention of the Bunsen burner—a tool now commonly used in modern chemistry laboratories—also spurred the development of flame analysis. Working with Gustav Kirchhoff (1824–1887), Bunsen helped to …
Flamingos are five species of large, colorful, very unusual-looking wading birds that encompass the family Phoenicopteridae. The flamingo lineage is ancient, with fossils of these birds being known from the early Tertiary. These birds occur in tropical and temperate regions of Africa, Madagascar, India, southern Europe, Caribbean coasts, highlands of the Andes in South America, and on the Galapago…
Flatfish are a group of mostly saltwater, carnivorous, bottom-dwelling fish in which both eyes are located the same side of the head. The under side of a flatfish is white while the upper side with the two eyes may be brightly colored. Many of these fish can change color to match their surroundings, making them hard to detect. When flatfish hatch, the eyes are located normally on each side of the …
Flatworms are small, multicelled animals with elongated bodies that have clearly defined anterior (front) and posterior (rear) ends. These worms are bilaterally symmetrical, meaning that their two sides reflect each other. They usually have a recognizable head, which houses gravity and light-receptive organs, and eye spots. They lack circulatory and respiratory systems and have only one opening th…
The flax plant, genus Linum, family Linaceae, is the source of two important commodities. Linen is a historic, economically important cloth made from the fiber of flax. Linseed oil is obtained from the pressed seeds of the plant. There are about 200 species of Linum. The species that is cultivated most extensively is L. usitatissimum, an annual plant grown for its fiber and seed. Varieties of L. u…
Fleas are about one thousand species of small insects in the order Siphonaptera, including several hundred species in North America. Adult fleas are external parasites (that is, ectoparasites) of mammals or birds, living on skin or in fur or feathers, and feeding on the blood of their hosts. Some fleas are serious parasites of birds or mammals, and may cause important damage to domestic animals, a…
Flies belong to the order Diptera, a group that also includes mosquitoes, gnats, and midges. Flies make up the fourth largest order of insects, with about 100,000 species recognized. Dipterans are amongst the most advanced insects in terms of morphology and biological adaptations. Their versatility and extreme range of anatomical and behavioral adaptations have enabled them to thrive in almost eve…
Ratites are flightless birds that lack the keel (high ridge) on the breastbone to which the flight muscles of flying birds are attached. Instead, the entire breastbone looks rather like a turtle's shell. It has also been described as a raft, which gives this group of flightless birds its name, Ratitae (Ratis means raft in Latin). Ratites have heavy, solid bones, while flying birds have ligh…
Flooding, although it usually carries a negative connotation, is quite a natural process and is simply the response of a natural system (a river system) to the presence of too much water during an interval of time. Rivers and streams are governed by a simple equation, Q = A × V, where Q is discharge (amount of water), A is area of the river channel, and V is velocity. When excess discharge …
Flora is a word used to describe the assemblage of plant species that occurs in some particular area or large region. Flora can refer to a modern assemblage of plant species, or to a prehistoric group of species that is inferred from the fossil record. The zoological analogue is known as a fauna, although this word is usually used in reference to a large region. More locally, "vegetation&#x…
A flower is the reproductive structure of an Angiosperm plant. Flowers have ovaries with ovules that develop into fruits with seeds. There are over 300,000 species of Angiosperms, and their flowers and fruits vary significantly. Flowers and fruits are among the most useful features for the identification of plant species and determination of their evolutionary relationships. …
Fluid dynamics is the study of the flow of liquids and gases, usually in and around solid surfaces. The flow patterns depend on the characteristics of the fluid, the speed of flow, and the shape of the solid surface. Scientists try to understand the principles and mechanisms of fluid dynamics by studying flow patterns experimentally in laboratories and also mathematically, with the aid of powerful…
Fluid mechanics is the study of gases and liquids at rest and in motion. Fluid statics studies the behavior of stationary fluids and tells us, for instance, how much air to put in our tires and whether a boat in a lake will float or sink. Fluid dynamics studies the flow behavior of moving fluids. Both global weather patterns and the flow of water from a faucet are governed by the laws of fluid dyn…
Fluorescence is the process by which a substance absorbs electromagnetic radiation (visible or invisible light) from another source, then re-emits the radiation with a wavelength that is longer than the wavelength of the illuminating radiation. It can be observed in gases at low pressure and in certain liquids and solids, such as the ruby gemstone. Fluorescence is the principle that is the basis o…
Fluorescent in situ hybridization (FISH) is a powerful technique for detecting RNA or DNA sequences in cells, tissues, and tumors. FISH provides a unique link among the studies of cell biology, cytogenetics, and molecular genetics. Fluorescent in situ hybridization is a technique in which single-stranded nucleic acids (usually DNA, but RNA may also be used) are permitted to interact so that comple…
Fluorescent light is the most common type of electrical light found in the United States; it is used for practically all commercial lighting, i.e. offices, factories, stores and schools, and it is estimated that there are 1.5 billion fluorescent lamps in use nationwide. Fluorescent lighting is popular due to its high efficacy, i.e. it produces between three to five times more light than an incande…
Fluoridation consists of adding fluoride to a substance (often drinking water) to reduce tooth decay. Fluoridation was first introduced into the United States in the 1940s in an attempt to study its effect on the reduction of tooth decay. Since then many cities have added fluoride to their water supply systems. Proponents of fluoridation have claimed that it dramatically reduced tooth decay, which…
Flying fish belong to the family Exocoetidae in the bony fish order Atheriniformes. They are close relatives of the needlefish, halfbeaks, and sauries. Flying fish are characterized by a low lateral line, soft fins without spines, and a caudal fin with the lower lobe larger than the upper lobe. The lower jaw of the young flying fish has an extended filament longer than the body, which becomes deta…
Focused ion beams have been used since the 1960s to investigate the chemical and isotopic composition of In addition to precise imaging, FIB technology can be used in a variety of manufacturing environments requiring high levels of precision and accuracy. The image above, using time-lapse, shows a computer-controlled ion beam helping shape a mirror for the Keck telescope. © Roger Ressmey…
Fog is caused by the condensation of water at or near Earth's surface. The atmosphere is obscured—essentially by cloud formation—near the surface and fog conditions are generally characterized as existing when atmospheric visibility is reduced to about one-half mile (approximately 0.8 km). Precipitation fog is a type of evaporation fog that happens when relatively warm rain or…
A fold is a bend in a body of rock or sediment that forms due to a change in pressure. Wave-like folds are composed of layers of the earth's crust that bend and buckle under enormous pressure as the crust hardens, compresses, and shortens. Folds form much the same way as a hump arises in a sheet of paper pushed together from both ends. Folds may be softly rolling or severe and steep, depend…
A food chain is a series of organisms dependent on each other for food; a food web is an interconnected set of food chains in the same ecosystem. Organisms that eat similar foods are assigned to a particular trophic level, or feeding level, within a food web. Food web is a more accurate term because food chains only exist on paper. In nature feeding habits are complex because many organisms feed o…
Food irradiation refers to a process where food is exposed to a type of radiation called ionizing radiation. The high-energy of the radiation, which can come from a radioactive or a non-radioactive source, breaks apart the genetic material of microorganisms that are on the surface of the food. Microorganisms and other surface contaminants, including insects, are killed as a result. This scrutiny o…
Food poisoning refers to an illness that is caused by the presence of bacteria, poisonous chemicals, or another kind of harmful compound in a food. Bacterial growth in the food is usually required. Food poisoning is different from food intoxication, which is the presence of preformed bacterial toxin in food. There are over 250 different foodborne diseases. The majority of these are infections, and…
The term food preservation refers to any one of a number of techniques used to prevent food from spoiling. It includes methods such as canning, pickling, drying and freeze-drying, irradiation, pasteurization, smoking, and the addition of chemical additives. Food preservation has become an increasingly important component of the food industry as fewer people eat foods produced on their own lands, a…
The food pyramid was developed by the U.S. Department of Agriculture (USDA) as a nutrition guide for healthy persons over the age of two years. The guide stresses eating a wide variety of foods from the five major food groups while minimizing the intake of fats and sugars. The daily quantity of foods from each group is represented by the triangular shape. The pyramid is composed of four levels. Th…
Foot and mouth disease is caused by a particular type of virus. The disease affects cloven hooved animals; that is, animals with hooves that are split into two main segments. Examples of domestic cloven hooved animals include cattle, sheep, pigs, and goats. Wild cloven hooved animals that are susceptible to foot and mouth disease include elephants, hedgehogs, and rats. Foot and mouth disease occur…
Force is the term used for an outside influence exerted by one body on another which produces a change in state of motion or state of configuration. This limited meaning in science compared to our everyday usage is most important because of the specific results of this out-side influence. Force producing a change in state of motion gives a body acceleration. If forces acting on a body produces no …
Forensic science reflects multidisciplinary scientific approach to examining crime scenes and in examining evidence to be used in legal proceedings. Forensic science techniques are also used to verify compliance with international treaties and resolutions regarding weapons production and use. Forensic science techniques incorporate techniques and principles of biology, chemistry, medicine, physics…
Forestry is the science of harvesting, planting, and tending trees, within the broader context of the management of forested landscapes. Traditionally, forestry has focused on providing society with sustainable yields of economically important products, especially wood for the manufacturing of lumber or paper, or for the generation of energy. Increasingly, however, forestry must consider other, no…
A forest is any ecological community that is structurally dominated by tree-sized woody plants. Forests occur anywhere that the climate is suitable in terms of length of the growing season, air and soil temperature, and sufficiency of soil moisture. Forests can be classified into broad types on the basis of their geographic range and dominant types of trees. The most extensive of these types are b…
There are several different ways to represent compounds in structural formulas, depending on how much detail needs to be shown about the molecule under consideration. Illustration by Hans & Cassidy. Courtesy of Gale Group. We will look at complete structural formulas, condensed formulas, line formulas, and three-dimensional formulas. After you become familiar with the rules for wr…
Fossas are cat-like Madagascan carnivores in the family Viverridae, which also includes civets, linsangs, genets, and mongooses. Fossas are quite different from other viverrids and are the sole members of the subfamily Cryptoproctinae. They are the largest Madagascan carnivores, measuring 24-30 in (60-75 cm) long. With a number of cat-like features-including a rounded head, long whiskers, large fr…
Fossils are a significant window into Earth's history and organic evolution. The term fossil literally means something that has been 'dug up,' but its modern meaning has been restricted to evidence of past life. Such evidence may take the form of body fossils (both plant and animal), trace fossils or ichnofossils (physical features formed in rock due to animal-sediment interac…
Fossil fuels are buried deposits of petroleum, coal, peat, natural gas, and other carbon-rich organic compounds derived from the dead bodies of plants and animals that lived many millions of years ago. Over long periods of time, pressure and heat generated by overlying sediments concentrate and modified these materials into valuable energy sources for human purposes. Fossil fuels currently provide…
A fractal is a geometric figure, often characterized as being self-similar; that is, irregular, fractured, fragmented, or loosely connected in appearance. Benoit Mandelbrot coined the term fractal to describe such figures, deriving the word from the Latin "fractus" meaning broken, fragmented, or irregular. He also pointed out amazing similarities in appearance between some fractal se…
Fraction is the name for part of something as distinct from the whole of it. The word itself means a small amount as, for example, when we ask someone to "move over a fraction." We mean them to move over part of the way, not all the way. Fractional parts such as half, quarter, eighth, and so on form a part of daily language usage. When, for example, we refer to "half an hour,&…
Fraunhofer lines are dark absorption lines in the solar spectrum that can be seen when sunlight is passed through a prism to separate it into the colors of the rainbow. They occur because cooler gas, which is higher in the Sun's atmosphere, absorbs some colors of the light emitted by hotter gas lower in the Sun's atmosphere. Sir Isaac Newton (1642-1727) discovered that if white light…
Freeways, also called superhighways, are roads specifically designed to allow for the free flow of traffic. Freeways typically feature two or more traffic lanes in each direction, medians to divide the opposing directions, full access control, a system of ramps to prevent merging and diverging traffic from interrupting the traffic flow, and grading to separate intersecting traffic on other roads. …
Any process that is repetitive or periodic has an associated frequency. The frequency is the number of repetitions, or cycles, during a given time interval. The inverse of the frequency is called the period of the process. Pendulums, as in a grandfather clock, also have a frequency of a certain number of swings per minute. A complete oscillation for a pendulum requires the pendulum bob to start an…
Freshwater is chemically defined as containing a concentration of less than two parts per thousand (<0.2%) of dissolved salts. Although water is abundant on the surface of Earth, freshwater is a very limited resource. Freshwater, in all forms, makes up less than 2.8% of the world water supply. Freshwater on Earth exists in several forms. These include lakes, which represent 0.009% of the gl…
Friction is the force that resists motion when the surface of one object slides over the surface of another. Frictional forces are always parallel to the surfaces in contact, and they oppose any motion or attempted motion. No movement will occur unless a force equal to or greater than the frictional force is applied to the body or bodies that can move. While friction is often regarded as a nuisanc…
A frigate bird on its nest on Barbuda Island in the West Indies. Photograph by Ormond Gigli. Stock Market. Reproduced by permission. Frigate birds are five species of oceanic birds that make up the family Fregatidae. Frigate birds occur along the coasts of the tropical oceans, but also hundreds of miles out to sea. Frigate birds typically weigh about 3 lb (1.5 kg), but the spread of their l…
The frog's-bit or tape-grass family (Hydrocharitaceae) is a relatively small group of herbaceous, aquatic, monocotyledonous plants, occurring in fresh and marine waters. There are about 100 species in the frog's-bit family, distributed among 15 genera. The flowers of members of this family are water pollinated, shedding their pollen into the water, which disperses it to the stigmatic…
Frogs are tail-less amphibians (class Amphibia, order Anura). With some 3,500 living species, frogs are the most numerous and best known of amphibians. They are found on all continents except Antarctica and are common on many oceanic islands. The terms "frog" and "toad" are derived from early usage in England and northern Europe, where two families of the order Anura oc…
Frostbite is the freezing of tissues. It occurs when body parts, most commonly the fingers, toes, and the tips of ears and the nose, are exposed for long periods to the cold. Frostbite is a direct result of limited blood circulation. The blood is the body's internal heating system; it carries heat to the body tissues. But prolonged exposure to the cold can constrict blood vessels, causing b…
A fruit is an often edible part of a plant that is derived from a fertilized, ripened ovary. As a dietary staple, fruits are appreciated for their sweetness and as a rich source of nutrients, especially vitamins. Gardeners enjoy planting fruit-bearing plants for their usefulness as a food and also for the array of color and diversity they bring into the garden. In agricultural industry, fruits are…
Fuel cells are a clean and quiet way to convert chemical-energy of fuels directly into electricity. Specifically, they transform hydrogen and oxygen into electric power, emitting water as their only waste product. A fuel cell consists of two electrodes, an anode and a cathode, sandwiched around an electrolyte. (An electrolyte is a substance, usually liquid, capable of conducting electricity by mea…
A function represents a mathematical relationship between two sets of real numbers. These sets of numbers are related to each other by a rule which assigns each value from one set to exactly one value in the other set. The standard notation for a function y = f(x), developed in the 18th century, is read "y equals f of x." Other representations of functions include graphs and tables. …
A fundamental theorem is a statement or proposition so named because it has consequences for the subject matter that are difficult to overestimate. Put another way, a fundamental theorem lies at the very heart of the subject. Mathematicians have designated one theorem in each main branch as fundamental to that branch. The fundamental theorem of arithmetic states that every number can be written as…
Fungi are one of the five kingdoms of organisms. Like higher plants (of the kingdom Plantae), most fungi are attached to the substrate they grow on. Unlike plants, fungi do not have chlorophyll and are not photosynthetic. Another key difference from plants is that fungi have cell walls composed of chitin, a nitrogen containing carbohydrate. All fungi have nuclei and the nuclei of most species are …
A fungus is a tiny plant-like organism that obtains its nourishment from dead or living organic matter. Some examples of fungi include mushrooms, toadstools, smuts, molds, rusts and mildew. Fungi have long been recognized as a serious threat to plants and crops. They attack food both while it is growing and after it has been harvested and placed in storage. One of the great agricultural disasters …
Gaia, Earth, was believed by the ancient Greeks to be a living, fertile ancestor of many of their important gods. The Romans, who adopted many Greek gods and ideas as their own, also believed in this organismic entity, who they renamed Terra. The Gaian notion has been personified in more recent interpretations as "Mother Earth." The Gaia hypothesis is a recent and highly controversia…
A galaxy is a large collection of stars similar to the Milky Way galaxy in which our solar system is located. Astronomers classify galaxies according to their shape as either spiral, elliptical, or irregular. Spiral galaxies are further subdivided into normal and barred spirals. Elliptical galaxies can be either giant or dwarf ellipticals, depending on their size. Galaxies can contain anywhere fro…
Game theory is a branch of mathematics concerned with the analysis of conflict situations. It involves determining a strategy for a given situation and the costs or benefits realized by using the strategy. First developed in the early twentieth century, it was originally applied to parlor games such as bridge, chess, and poker. Now, game theory is applied to a wide range of subjects such as econom…
A gamete is a specialized reproductive cell. The cells usually have one half as many chromosomes in their nuclei as the majority of body cells, which are known as somatic cells. All sexually-reproducing plants, animals, and microbes produce gametes sometime during their life span. During the second and third quarters of the nineteenth century the scientists J. L. Prevost, J. B. Dumas, T. Schwann, …
Gametogenesis is the production of haploid sex cells (in humans, ovum and spermatozoa) that each carry one-half the genetic compliment of the parents from the germ cell line of each parent. The production of ovum is termed oogenesis and the production of spermatozoa is called spermatogenesis. Both oogenesis and spermatogenesis provide a mechanism through which genetic information may be passed to …
Gamma rays are a highly energetic form of electromagnetic radiation. The wavelength of a gamma ray is very short—less than the radius of an atom—the energy they carry can be millions of electron volts. Gamma rays originate in the nucleus of an atom, and are created when cosmic rays collide with atoms in molecules of gas. In the collision, the nucleus of the atom is destroyed, and gam…
Gamma ray bursts are brief, seconds-long, blasts of radiation of mysterious origin that, in nature, seem to come from the depths of interstellar space. Bursts of gamma radiation also have been measured coming from severe thunderstorms and are a component of nuclear bomb detonation. The accidental discovery of cosmic gamma ray bursts was confirmed in 1973. Ten years earlier, the United States Air F…
Gangrene involves the death of human tissue, usually due to ischemia, which is an interruption in the blood supply to a particular area. Loss of blood supply means loss of oxygen delivery to that tissue, as well as loss of other nutritive factors usually carried in the blood circulation. Tissue deprived in this manner will die, and often becomes infected with bacteria during this process. …
Garpike (gar) are bony fish classified in the family Lepisosteidae. These fish are differentiated from garfish which belong to the family Belonidae. Garpike were once Longnose gar (Lepisosteus osseus). Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission. abundant and widely distributed, but are now rare. Some species of garpike are found in Mexico, Central Ame…
Liquefaction of gases is the process by which substances in their gaseous state are converted to the liquid state. When pressure on a gas is increased, its molecules closer together, and its temperature is reduced, which removes enough energy to make it change from the gaseous to the liquid state. …
The fundamental physical properties of a gas are related to its temperature, pressure and volume. These properties can be described and predicted by a set of equations, known as the gas laws. While these laws were originally based on mathematical interpretations for an ideal or perfect gas, modern atomic and kinetic theory of gases has led to a modified expression that more accuratel…
Gazelles are medium-sized fawn-colored antelopes found in arid parts of the world, mainly in Ethiopia, Somalia, northern Africa and around the Sahara Desert, parts of the Middle East, India, and Central Asia. Gazelles are horned animals with a four-chambered stomach and cloven hooves. Gazelles are cud chewers (ruminants), and they lack upper canine and incisor teeth. Gazelles tear grass, foliage, …
A gear is a toothed disk attached to a rotating rod or shaft that transmits and modifies rotary motion by working in conjunction with another gear. Usually circular in shape, the protrusions of one gear mesh into the profile of its mate to obtain a predetermined mechanical advantage. For example, if one gear wheel has ten times as many teeth as the wheel that drives it, it will make one tenth of a…
Geckos are small night-lizards found in the tropics and subtropics, and number more than 650 species in the A parachute gecko (Ptychozoon kuhli). Photograph by Tom McHugh. The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. family Gekkonidae, divided into four subfamilies (the Diplodactylinae, the Gekkoninae, the Sphaerodactylinae, and the Eublepharinae)…
Geese are large birds in the subfamily Anserinae of the waterfowl family Anatidae, consisting of ducks, geese, and swans. Geese occur in many types of aquatic habitats, on all continents but Antarctica. Most geese breed in freshwater marshes, salt marshes, or marsh-fringed, open-water wetlands. Geese typically winter in those sorts of natural habitats and in estuaries, although in some regions the…
Gelatin is an edible protein made from the skin, bones and ligaments of animals. It is clear, usually colorless or pale yellow, odorless and tasteless, and dissolves in water. The hot solution is liquid, but as it cools, it "gels," forming a semi-solid, which is soft and flexible, yet firm enough to hold any shape into which it may be molded or cut. A familiar example of gelatin is t…
A gene is the basic structural unit of inheritance in biological organisms. It is made up of a short segment of DNA and contains the necessary information to produce a specific protein. Each gene is separated from each other by non-coding sequences that serve other functions. Genes are strung together and tightly packed into structures called chromosomes. All the genes in an organism are located o…
The Human Genome Project began in 1990, with the goal of sequencing the complete human genome. Although estimates to complete the daunting project initially ranged up to forty years, with advances in technolo gy—including gene chip and microarray technology— Gloved hand holding a device for rapidly analyzing samples of DNA at the scene of a crime. Photograph by Sam Ogden. Photo …
The term mutation was originally coined by Dutch botanist Hugo De Vries (1848–1935) to describe a new approach to explain evolution, although it is quite different than the current definition. De Vries discovered new forms of the Evening Primrose (Oenothera lamarcklana) that were growing in a meadow. He attributed these new varieties and the method for which new species arise to what he cal…
Genes are DNA sequences that code for protein. Gene splicing is a form of genetic engineering where specific genes or gene sequences are inserted into the genome of a different organism. Gene splicing can also specifically refer to a step during the processing of deoxyribonucleic acid (DNA) to prepare it to be translated into protein. Gene splicing can also be applied to molecular biology techniqu…
Gene therapy is a rapidly growing field of medicine in which genes are introduced into the body to treat diseases. Gene therapy is the name applied to the treatment of inherited diseases by corrective genetic engineering of the dysfunctional genes. It is part of a broader field called genetic medicine, which involves the screening, diagnosis, prevention and treatment of hereditary conditions in hu…
A generator is a machine by which mechanical energy is transformed into electrical energy. Generators can be sub-divided into two major categories depending on whether the electric current produced is alternating current (AC) or direct current (DC). The basic principle on which both types of generator works is the same, although the details of construction of the two may differ somewhat. Generator…
Genetic disorders refer to medical conditions that develop as the result of abnormalities in an individuals genetic material, usually that is inherited. Inheriting or developing a genetic disorder leads to a collection of clinical manifestations known as a syndrome. These clinical manifestations can vary from person to person with the same genetic defect or have similar presentations. …
Genetic engineering is the alteration of genetic material with a view to producing new substances or creating new functions. The technique became possible in the 1950s, when scientists discovered the structure of DNA molecules and learned how these molecules store and transmit genetic information. Largely as a result of the pioneering work of James Watson and Francis Crick, scientists were able to…
The advent of molecular technologies and the application of genetic identification in clinical and forensic microbiology have greatly improved the capability of laboratories to detect and specifically identify an organism quickly and accurately. In the wake of the 2001 anthrax attacks utilizing the United States mail, a great deal of investigative attention turned to identification of the source o…
The use of genetic information to predict future onset of disease in an asymptomatic (presymptomatic) person is called predictive genetic testing. Every aspect of our being is influenced by both genes and environment. In the future, a strategy for influencing development may be to alter genes. At present, the environment in which genes act can sometimes be changed, and thereby moderate their impac…
While the term genetically modified organisms has arisen within the past decade, humans have for centuries been using microorganisms to make products like beer and cheese, and plants and animals have been carefully bred to improve the quality and quantity of the food supply. The elucidation of the structure of DNA and the development of the discipline of molecular biology has made possible the acc…
Genetics is the branch of biology concerned with the science of heredity, or the transfer of specific characteristics from one generation to the next. Genetics focuses primarily on genes, coded units found along the DNA molecules of the chromosomes, housed by the cell nucleus. Together, genes make up the blueprints that determine the entire development of the species of organisms down to specific …
Genets are mongoose-like mammals in the family Viverridae in the order Carnivora. Other members of this family include civets, linsangs, mongooses and the fossa. The genet genus Genetta has three subgenera and nine species. Genets are found in Africa south of the Sahara desert, in the southwestern Arabian Peninsula, and in southern Europe. Genets have a long body, short legs, a pointed snout, prom…
The genome (sometimes spelled geneome) is, in the broadest use of the term, the full set of genes or genetic material carried by a particular organism representing a particular species or population. The size of a genome is usually measured in numbers of genes or base pairs. With the success of the Human Genome Project and other international genome projects and programs, by 2003, scientists have,…
The study of an organism's total complement of genetic material, called its genome, has become indispensable for shedding light on its biochemistry, physiology, and patterns of inheritance. Even more can be gained by comparing the genomes of multiple organisms to discern how their DNA sequences have changed over evolutionary time. This technique has become increasingly valuable with the exp…
A genotype describes the actual set (complement) of genes carried by an organism. In contrast, phenotype refers to the observable expression of characters and traits coded for by those genes. Although phenotypes are based upon the content of the underlying genes comprising the genotype, the expression of those genes in observable traits (phenotypic expression) is also, to varying degrees, influenc…
Rejected by modern science, the geocentric theory (in Greek, ge means earth), which maintained that Earth was the center of the universe, dominated ancient and medieval science. It seemed evident to early astronomers that the rest of the universe moved about a stable, motionless Earth. The Sun, Moon, planets, and stars could be seen moving about Earth along circular paths day after day. It appeare…
Geochemical analysis is the process through which scientists discover and unravel the chemical compounds that make up the earth, its atmosphere, and its seas. The process requires a thorough grounding in chemistry and the earth sciences, and an understanding of the different ways elements can interact in a given geologic situation. Geochemical analysis can predict where petroleum, metals, water, a…
Geochemistry is the science or study of the chemistry of the earth. Geochemists who practice this science are interested in the origin of chemical elements, their evolution, the classes and many divisions of minerals and rocks and how they are created and changed by earth processes, and the circulation of chemical elements through all parts of the earth including the atmosphere and biological form…
Geodes are hollow rock masses that are lined with crystals that have grown toward the center of the cavity. Geodes are usually roughly spherical in shape, up to 12 A blue geode. © Royalty-Free/Corbis. Reproduced by permission. in (30 cm) or more in diameter. Most frequently, the crystals growing within a geode are quartz, calcite, or fluorite, though occurrences of other minerals are …
A geodesic is the shortest path between two points along a surface. On a plane, it is the straight line segment joining the two points. On a sphere, it is the shorter arc of a great circle joining the two points. …
A geodesic dome is a spherical building in which the supporting structure is a lattice of interconnecting tetrahedrons (a pyramid with three sides and a base) and octahedrons (an eight sided figure—two pyramids with four sides and a base, placed base to base). The first contemporary geodesic dome on record is Walter Bauersfeld's, who realized the utility of projecting the constellati…
Earth's geographic poles are fixed by the axis of Earth's rotation. On maps, the north and south geographic poles are located at the congruence of lines of longitude. Earth's geographic poles and magnetic poles are not located in the same place – in fact they are hundreds of miles apart. As are all points on Earth, the northern magnetic pole is south of the northern geo…
Geologic maps are graphical representations of rocks, sediments, and other geologic features observed or inferred to exist at or beneath Earth's surface. They can be based on observations of outcrops in the field, interpretation of aerial photographs or satellite images, and information obtained during the drilling of exploratory boreholes. Outcrops can be obscured, particularly in areas co…
Although historical time covers centuries, and archeological times covers millennia, geologic time describes the immense span of time—billions of years—revealed in the fossil and rock record of Earth. Geochronology is the science of finding out how old rocks and minerals are. Absolute time and relative time are terms used to describe the age of rocks and events used by geologists. Ra…
Geology is the study of Earth. Modern geology includes studies in seismology (earthquake studies), volcanology, energy resources exploration and development, tectonics (structural and mountain building studies), hydrology and hydrogeology (water-resources studies), geologic mapping, economic geology (e.g., mining), paleontology (ancient life studies), soil science, historical geology and stratigra…
Geometry, the study of points, lines, and other figures in space, is a very old branch of mathematics. Its ideas were undoubtedly used, intuitively if not formally, from earliest times. Walking along a straight line toward a particular destination is the shortest way to get there; lining an arrow up with the target is the way to hit it; sitting in a circle around a fire is the most equitable way t…
Geomicrobiology refers to the activities of microorganisms (usually bacteria) that live beneath the surface of the Earth. The field of study is also referred to as biogeochemistry and subsurface microbiology. Habitats of the organisms include the ocean and deep within the rock that makes up Earth's crust. The study of the identities and activities of such organisms is important from a basic…
Geophysics is the study of Earth's physical character, including the solid planet, the atmosphere, and bodies of water. Geophysical investigations, therefore, often draw upon information and techniques developed in scientific disciplines such as physics, geology, and astronomy. Major areas of modern geophysical research include seismology, volcanology and geothermal studies, tectonics, geom…
Plants can sense the Earth's gravitational field. Geotropism is the term applied to the consequent orientation response of growing plant parts. Roots are positively geotropic, that is, they will bend and grow downwards, towards the center of the Earth. In contrast, shoots are negatively geotropic, that is, they will bend and grow upwards, or away, from the surface. These geotropisms can be …
Gerbils are rat-like rodents in the mammalian family Muridae, which also includes rats, mice, voles and lemmings. Some authorities place the gerbils in a separate family Gerbilidae, together with the pigmy gerbils. Wild gerbils are rat-sized, long-tailed rodents with rather long hind feet. Nearly all live in self-dug burrows and forage at night feeding mostly on seeds. Gerbils are probably derived…
Germ cells are one of two fundamental cell types in the human body. Germ cells are responsible for the production of sex cells or gametes (in humans, ovum and spermatozoa). Germ cells also constitute a cell line through which genes are passed from generation to generation. The vast majority of cells in the body are somatic cells. Indeed, the term somatic cell encompasses all of the differentiated …
The germ theory is a fundamental tenet of medicine that states that microorganisms, which are too small to be seen without the aid of a microscope, can invade the body and cause certain diseases. Until the acceptance of the germ theory, many people believed that disease was punishment for a person's evil behavior. When entire populations fell ill, the disease was often blamed on swamp vapor…
Germination is the process by which a seed begins its development into a mature plant. Germination begins with an increase of metabolic activity within the seed. The first visible sign of germination in angiosperms (flowering plants) is generally an enlargement of the seed, due to intake of water from the environment. The seed's covering may wrinkle and crack at this time. Soon afterward, t…
Gerontology is a branch of sociology that studies aging among populations internationally, and monitors efforts to deal with problems arising in old age. It differs from geriatrics the same way that psychology is separate from psychiatry. A psychologist's inquiries apply to general questions about how the human brain and mind work. A psychiatrist is more concerned with involving patients in…
Members of the Gesneria family, the Gesneriaceae, are herbs, shrubs, sometimes trees or woody vines. The Gesneriaceae is a large family composed of approximately 120 genera and 1,800 species. With the exception of two genera (Haberlea and Ramonda), which are native to temperate Europe, they are found only in the tropical and subtropical regions of the world. Although none are native to the United …
A geyser is an intermittent or semi-regularly periodic spout of geothermally heated groundwater and steam. Any subsurface encounter between water and heat produces a hydrothermal process. The heat is usually supplied by upwellings of magma from the mantle, the water by precipitation that percolates downward through surface rocks. Some oceanic water enters the mantle at subduction zones and becomes…
Gibbons are species of tropical forest apes in the family Pongidae. This family contains all of the anthropoid apes, which are the closest living relatives of humans (Homo sapiens), in terms of their anatomy, physiology, and behavior. Like other anthropoid primates, gibbons lack a tail, they have a more-or-less upright posture, and they have a well-developed brain. However, gibbons are generally r…
The Gila monster (Heloderma suspectum) is a large, strikingly-colored venomous lizard. The gila monster and the Mexican beaded lizard (Heloderma horridum) are the only members of the beaded lizard family, Helodermatidae. The Gila monster occurs in rocky, semi-arid habitats from the Colorado River basin in the southwestern United States to the western regions of Mexico and Guatemala. The Gila monst…
The ginger family, Zingiberaceae, includes about 50 genera and 1,300 species of plants, a few of which have culinary or medicinal uses. The common ginger (Zingiber officinale) is one of the oldest and most commonly used spices. Ginger for these uses is obtained from the tuberous rhizome, or underground stem of the plant. The common ginger is native to Southeast Asia, where it has been cultivated f…
The ginkgo, or maidenhair tree (Ginkgo biloba) is an unusual species of gymnosperm, having broad leaves, and seasonally deciduous foliage that turns yellow and is dropped in autumn. The ginkgo is a dioecious plant, which means that male and female functions are performed by separate trees. The ginkgo is famous as a socalled "living fossil," because it is the only surviving member of …
Ginseng refers to several species of plants in the genus Panax, family Araliaceae. Ginseng is a perennial, herbaceous plant, with compound leaves that grow from a starchy root. The natural habitat of ginseng is the understory of mature angiosperm forest in the temperate zones of east Asia and eastern North America. The root of ginseng is highly valued as having many therapeutic properties by pract…
Giraffes are a species of large, long-legged, long-necked ungulates in the family Giraffidae, order Artiodactyla. Giraffes are the tallest living animals on Earth. Okapis are a close relative, but these animals do not have such long legs or neck. The giraffe is a widespread animal of grasslands and savannas of sub-Saharan Africa. The okapi is a much rarer animal and occurs in tropical forest. Both…
GIS is the common abbreviation for geographic information systems, a powerful and widely used computer database and software program that allows scientists to link geographically referenced information related to any number of variables to a map of a geographical area. GIS allows its users to analyze and display data using digitized maps. In addition, GIS can generate maps and tables useful to a w…
Glaciers are flowing masses of ice, created by years of snowfall and cold local temperatures. Approximately one tenth of the Earth is covered by glaciers. Glaciers are most numerous near the poles, covering most of Antarctica and Greenland and parts of Iceland, Canada, Russia, and Alaska; they also exist in mountainous regions on every continent except Australia. From the air, a glacier looks dece…
Glands are aggregates of specialized cells that secrete or excrete chemical substances which are used elsewhere in the body. Glands carry out regulatory, digestive, reproductive, and other functions in the body. A gland may be an independent structure or may be incorporated into another, larger, structure that has still other functions. In addition, a gland can be endocrine, secreting its hormones…
Early peoples were likely to have discovered natural glass, which is created when lightning strikes sand, and were certain to have used obsidian-a dark volcanic glass-for weapons, ornaments, and money. The first manufactured glass probably took the form either of glass beads or ceramic glaze and appeared around 4000-5000 B.C. Surviving examples of Egyptian and Mesopotamian glass objects date to ar…
The long-term distribution of heat and precipitation on Earth's surface is called global climate. Heat from the sun keeps the Earth's average temperature at about 60°F(16°C), within a range that allows for biological life and maintains the planet's life-sustaining reservoirs of liquid water. Astronomical variations and atmospheric shielding cause incoming solar r…
Long before the space age, people used the heavens for navigation. Besides relying on the Sun, Moon, and Stars, the early travelers invented the magnetic compass, the sextant, and the seagoing chronometer. Eventually, radio navigation in which a position could be determined by receiving radio signals broadcast from multiple transmitters, came into existence. Improved high frequency signals gave gr…
Global warming refers to a long-term increase in the Earth's surface temperature that results in large-scale changes in global climate, namely redistribution of climatic zones defined by temperature, precipitation, and associated adapted ecosystems. Global climate changes, and episodes of global warming, have occurred throughout geologic history as a result of natural variations in incoming…
One important property of glycerol or glycerin is that is not poisonous to humans. Therefore it is used in foods, syrups, ointments, medicines, and cosmetics. Glycerol also has special chemical properties that allow it to be used where oil would fail. Glycerol is a thick syrup that is used as the "body" to many syrups, for example, cough medicines and lotions used to treat ear infect…
A glycol is an aliphatic organic compound in which two hydroxyl (OH) groups are present. The most important glycols are those in which the hydroxyl groups are attached to adjacent carbon atoms, and the term glycol is often interpreted as applying only to such compounds. The latter are also called vicinal diols, or 1,2-diols. Compounds in which two hydroxyl groups are attached to the same carbon at…
Glycolysis, a series of enzymatic steps in which the six-carbon glucose molecule is degraded to yield two three-carbon pyruvate molecules, is a central catabolic pathway in plants, animals and many microorganisms. In a sequence of 10 enzymatic steps, energy released from glucose is conserved by glycolysis in the form of adenosine triphosphate (ATP). So central is glycolysis to life that its sequen…
Goats belong to the order Artiodactyla (genus Capra), which is made up of a number of hoofed mammals having an even number of toes. Goats have existed on Earth for at least 35 million years and, during the course of evolution, have undergone an incredibly wide radiation, both in distribution and ecology. Although the taxonomy of this group is still unclear, eight species are generally recognized a…
The goatsuckers, nightjars, and nighthawks number 70 species of birds in the family Caprimulgidae. These birds have a relatively large head, with a wide beak, and a large mouth with a seemingly enormous gape. The mouth is fringed by long, stiff bristles, and is an adaptation for catching insects in flight. The unusually large mouth of goatsuckers was once believed to be useful for suckling milk at…
Gobies, belonging to the suborder Gobidioidei, are small fish that usually live off the coast in tropical and warm temperate regions. They spend the majority of their time resting on the bottom near protective cracks in coral reefs or burrows in the sand. Most species of this fish have fused pelvic fins which form a suction cup on their undersides. A goby uses this suction cup to cling to rocks so…
Goldenseal (Hydrastis canadensis L.) is a woodland plant belonging to the family Ranunculaceae. The plant is also known as eyebalm, eyeroot, hydrastis, orangeroot, tumeric root, and yellowroot. Mainly found in the wild, goldenseal grows to a height of about 1 ft (30 cm). It has an erect, hairy stem, and produces small, greenish-white flowers that bloom in early spring, and later turn into clusters…
Gophers are small rodents. Although the name is often used popularly to refer to a variety of animals, including snakes, in the United States gophers are the pocket gophers that live in the grasslands of western Canada, eastward to the Great Lakes, and down into northern South America. Pocket gophers (family Geomyidae) have fur-lined cheek pouches that let them carry food in large quantities. Thes…
Gorillas inhabit forests of Central Africa, and are the largest and most powerful of all primates. Adult males stand 6 ft (1.8 m) upright (although this is an unnatural position for a gorilla) and weigh up to 450 lb (200 kg), while females are much smaller. Gorillas live up to about 44 years. Mature males (older than 13 years), or silverbacks, are marked by a band of silver-gray hair on their back…
Gourds and their relatives are various species of plants in the family Cucurbitaceae. There are about 750 species in this family divided among 90 genera. Some members of the gourd family include the cucumber, squash, melon, and pumpkin. Most species of gourds are tropical or subtropical, but a few occur in temperate climates. A few species in the gourd family produce large, edible fruits, and some…
A graft is a horticultural term for a bud or shoot of one variety or species of plant that is positioned on the stem of another, compatible plant, in such a way that integrated growth results. The recipient plant is called the stock or rootstock, and the grafted part is referred to as the scion. A simple method for stem grafting involves both stems being cut with a sharp blade at the same acute an…
One of the major theoretical hurdles to a reachable synthesis of current theories of particles and force interactions into a grand unification theory (also known as Grand Unified Field Theory, Grand Unified Theory, or GUT) is the need to reconcile the evolving principles of quantum theory with the principles of general relativity advanced by German-American physicist Albert Einstein (1879-1955) ne…
Grapes are various species of woody vines in the genus Vitis, family Vitaceae. This family contains about 700 species most of which occur in tropical and subtropical climates, although some occur in temperate habitats. The genus Vitis has about 50 species. Grapes are ecologically important as food for wildlife. They are also cultivated by humans in large quantities, mostly for the production of ta…
In mathematics, a graph is a geometric representation, a picture, of a relation or function. A relation is a subset of the set of all ordered pairs (x,y) for which each x is a member of some set X and each y is a member of another set Y. A specific relationship between each x and y determines which ordered pairs are in the subset. A function is a similar set of ordered pairs, with the added restri…
Grasses are monocotyledonous plants in the family Poaceae (also known as Gramineae). There are as many as 10,000 species of grasses distributed among more than 600 genera. The richest genera of grasses are the panic-grasses (Panicum spp.) with 400 species, the bluegrasses (Poa spp.) and love-grasses (Eragrostis spp.) with 300 species each, and the needle-grasses (Stipa spp.) with 200 species. Spec…
Grasshoppers are plant-eating insects characterized by long hind legs designed for locomotion by jumping. Like all insects, the body of grasshoppers is divided into three main parts: head, thorax, and abdomen. On the head are two antennae for feeling and detecting scent, and two compound eyes comprised of many optical units called facets, each of which is like a miniature eye. The chewing mouthpar…
Grasslands are environments in which herbaceous species, especially grasses, make up the dominant vegetation. Natural grasslands, commonly called prairie, pampas, shrub steppe, palouse, and many other regional names, occur in regions where rainfall is sufficient for grasses and forbs but too sparse or too seasonal to support tree growth. Such conditions occur at both temperate and tropical latitud…
Gravitational lenses are accidental natural arrangements of gravity, light, and distant astronomical objects that create altered images of the those objects. Commonly, a lens is a piece of glass shaped so as to bend light passing through it. In the process, it alters the image of the light source as observed through the lens. A gravitational lens bends light using gravity rather than glass. Gravit…
Gravity is a force of attraction that exists between every pair of objects in the Universe. This force is proportional to the mass of each object in each pair, and inversely proportional to the square of the distance between the two; thus, …
The Great Barrier Reef lies off the northeastern coast of Australia and is both a scientific wonder and an increasingly popular tourist attraction. It has been described as "the most complex and perhaps the most productive biological system in the world." The Great Barrier Reef is the largest structure ever made by living organisms including human beings, consisting of the skeletons …
The greatest common factor (or greatest common divisor) of a set of natural numbers is the largest natural number that divides each member of the set evenly (with no remainder). For example, 6 is the greatest common factor of the set because 1246 = 2, 1846 = 3, and 3046 = 5. …
Grebes are aquatic birds that make up the family Podicipedidae. This is the only family in the order Podicipitiformes, a rather unique group of birds that is not Western grebes (Aechmophorus occidentalis) displaying across the water. Photograph by Phil Dotson/The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. closely related to other living orders, an…
To understand the greenhouse effect, Earth's energy budget must be known. An energy budget is an account of all of the energy coming into and leaving a system and of any energy that is stored in (or produced by) the system itself. Almost all of the energy coming to Earth from space has been radiated by the closest star, the Sun. The Sun emits electromagnetic energy at a rate and spectral qu…
The groundhog or woodchuck (Marmota monax) is a husky, waddling rodent in the squirrel family Sciuridae, order Rodentia. The groundhog is a type of marmot (genus Marmota), and is also closely related to the ground squirrels and gophers. The natural habitat of the groundhog is forest edges and grasslands, ranging from the eastern United States and Canada through much of the Midwest, to parts of the…
Groundwater occupies the void space in a geological strata. It is one element in the continuous process of moisture circulation on Earth, termed the hydrologic cycle. Almost all groundwater originates as surface water. Some portion of rain hitting the earth runs off into streams and lakes, and another portion soaks into the soil, where it is available for use by plants and subject to evaporation b…
A group is a simple mathematical system, so basic that groups appear wherever one looks in mathematics. Despite the primitive nature of a group, mathematicians have developed a rich theory about them. Specifically, a group is a mathematical system consisting of a set G and a binary operation * which has the following properties: Note that commutativity is not required. That is, it need not be true…
Grouse (and ptarmigan) are medium-sized birds in the family Tetraonidae, order Galliformes. Grouse and ptarmigan are often hunted for food and sport, and are sometimes broadly referred to as upland gamebirds because they are not hunted in wetlands, as are ducks and geese. Grouse are ground-dwelling birds with a short, turned-down bill. They have long, heavy feet with a short elevated fourth toe be…
Growth and decay refers to a class of problems in mathematics that can be modeled or explained using increasing or decreasing sequences (also called series). A sequence is a series of numbers, or terms, in which each successive term is related to the one before it by precisely the same formula. There are many practical applications of sequences. One example is predicting the growth of human popula…
Several hormones play important roles in human growth. The major human growth hormone (hGH), or somatotropin, is a protein made up of 191 amino acids secreted by the anterior pituitary and coordinates normal growth and development. Human growth is characterized by two spurts, one at birth and the other at puberty. HGH plays an important role at both of these times. Normal individuals have measurab…
Guenons are small to medium-sized monkeys widespread throughout sub-Saharan Africa. These primates are classified in the infraorder of Old World simian primates (Cataffhina) and the family Cercopithecidae. Their genus, Cercopithecus, is large, very diverse, and successful. The Cercopithecidae family consists of two subfamilies: the omnivorous Cercopithecinae (including guenons, talapoin, and baboo…
Guillain-Barre syndrome (GBS) is a cause of progressive muscle weakness and paralysis which evolves over days or weeks, and resolves over the next several weeks or months. About 85% of patients recover completely, with no residual problems. Diagnosis of GBS is made by virtue of the cluster of symptoms (ascending muscle weakness and then paralysis) and by examining the fluid which bathes the brain …
Guinea fowl are seven species of medium-sized terrestrial birds in the family Phasianidae, order Galliformes, which also includes other fowl-like birds, such as the grouse, ptarmigan, turkey, quail, peafowl, and pheasants. The natural range of guinea fowl is sub-Saharan Africa, the Arabian Peninsula, and Madagascar. However, these birds have been introduced to some other places, and are commonly k…
Guinea pigs, or cavies, are about 20 species of rodents in the family Caviidae. Guinea pigs are native to South America, occurring from Colombia and Venezuela in the north, to Brazil and northern Argentina. These animals occur in rocky habitats, savannas, forest edges, and swamps, and can be rather common within their preferred habitat. Guinea pigs have a stout body, with a relatively large head, …
Gulls are 43 species of seabirds, in the subfamily Larinae of the family Laridae, which also includes the terns. Gulls occur in a wide range of coastal habitats, ranging from inland lakes, rivers, and wetlands, to marine shores and estuaries. Their distribution is virtually world-wide, but most species occur in the Northern Hemisphere. Species of gulls range in body length from 8-32 in (20-81 cm).…
One of the most popular species of freshwater topical fish is the guppy. The first specimens were brought to the British Museum in London for description in 1859 by R. J. L. Guppy, a biologist from Trinidad (West Indies) after whom the fish is named. The species originally possessed the scientific name Lebistes reticulatus, but in 1963, the Latin name was changed to Poecilia reticulata and remains…
The Gutenberg discontinuity occurs within Earth's interior at a depth of about 1,800 mi (2,900 km) below the surface, where there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, …
Gutta percha is a rubberlike gum obtained from the milky sap of trees of the Sapotaceae family, found in Indonesia and Malaysia. Once of great economic value, gutta percha is now being replaced by plastics in many items, although it is still used in some electrical insulation and dental work. The English natural historian John Tradescant (c. 1570-1638), introduced gutta percha to Europe in the 162…
Gymnosperms are one of the two major groups of plants that produce seeds; the other is the angiosperms. Gymnosperm literally means "naked seed," which refers to the development of seeds exposed on a flat structure, that is, not within an ovary as in the angiosperms. Gymnosperms became common about 290 million years ago and although many of the earlier types are now extinct, four kind…
Gynecology, from the Greek meaning "the study of women," is a medical specialty dealing with the health of a woman's genital tract. The genital tract is made up of the reproductive organs including the vagina, cervix, uterus, ovaries, fallopian tubes, and their supporting structures. Marked changes occur in a woman's reproductive organs upon her reaching menarche (the a…
A gyroscope is heavy disk placed on a spindle that is mounted within a system of circles such that it can turn freely. When the disk, called a flywheel, is made to spin, the gyroscope becomes extremely resistant to any change in its orientation in space. If it is mounted in gimbals, a set of pivot and frame mountings that allow it freedom of rotation about all three axes, a fast-spinning gyroscope…
The term habitat refers to the type of environment in which an organism or species occurs. For plants, habitat is mostly defined by its physical attributes (e.g., rainfall, temperature, topographic position, soil texture and moisture) and its chemical properties (e.g., soil acidity, concentrations of nutrients and toxins, oxidation reduction status). For terrestrial animals, the habitat is defined…
A primitive group of fish, hagfish (order Hyperotreti, family Myxinidae) resemble eels in their external appearance. These fish lack a backbone, jaws, true fins, and scales. Their body is tubelike and often covered in a slimy substance that is secreted from abundant glands in the skin. The body is often a pale fleshy pink, but is occasionally brown-gray above and pink below. They may reach up to 2…
The half-life of a process is an indication of how fast that process proceeds—a measure of the rate or rapidity of the process. Specifically, the half-life is the length of time that it takes for a substance involved in that process to diminish to one-half of its initial amount. The faster the process, the less time it will take to use up one-half of the substance, so the shorter the half-l…
Organic halides are organic compounds containing a halogen atom bonded to a carbon (C) atom. Fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) are all types of halogen atoms. A compound that contains a carbon atom bonded to a fluorine atom (C-F) is called an organofluoride. If the carbon atom is part of a chain of carbon atoms, the organofluoride compound is referred to as an alkyl fluorid…
A current-carrying body placed in a magnetic field with the current direction unaligned with the field experiences a force leading to a transient sidewise drift of the charge carriers of the current. This drift continues until the force is balanced by an electric field produced by the charge accumulating at points on the body's surface in the direction of the drift. At points on the body&#x…
Hallucinogens are substances that alter the user's thought processes or mood to the extent that he perceives objects or experiences sensations that in fact have no reality. Many natural and some manmade substances have the ability to bring about hallucinations. In fact, because of the ready market for such chemicals, they are manufactured in illegal chemical laboratories for sale as halluci…
Halogenated hydrocarbons are derivatives of hydrocarbons (that is, organic compounds that only contain carbon and hydrogen atoms) which include some halogen atoms within their chemical structure. The most commonly encountered halogens in halogenated hydrocarbons are fluorine and chlorine, but sometimes bromine or iodine occur, or combinations of any of these. Some halogenated hydrocarbons occur na…
The halogens are a group of chemical elements that includes fluorine, chlorine, bromine, iodine, and astatine. Halogen comes from Greek terms meaning "produce sea salt." None of the halogens occur naturally in the form of elements, but, except for astatine, they are very widespread and abundant in chemical compounds where they are combined with other elements. Sodium chloride, common…
A halosaur is a thin, elongated fish resembling an eel. The largest of halosaurs grows to about 20 in (51 cm) long. Unlike the eel, the halosaur has a backbone composed of many vertebrae. It has somewhat large scales, numbering fewer than 30 horizontal rows on each of its sides. This fish lives close to or on the bottom of the sea and is thus referred to as a benthic fish. It feeds on the ocean fl…
Hamsters are small rodents with dense fur, a short tail, and large cheek pouches. They belong to the mammalian family Muridae, which also includes rats, mice, gerbils, voles, and lemmings. During foraging trips, hamsters use their cheek pouches to carry seeds and grains back to underground food stores that are sometimes quite large. Hamsters mostly eat plant matter, especially seeds, nuts, soft fr…
Hand tools can be as easily found as made, and the earliest tools used by people included sticks and rocks picked up and used as projectiles, or to pound or dig. The earliest fashioned hand tools date back to the Stone Age. Currently new technologies make hand tools that are battery-powered, so they are still portable, yet easier to use than their precursors. Tools are an extension of human limbs …
Hantavirus infections are infections of the lungs caused by hantaviruses. There are five known types of hantaviruses, which differ only slightly from one another. These types are: Hantaan, Seoul, Puumala, Prospect Hill, and Sin Nombre. The Sin Nombre virus was the cause of the 1993 outbreak in the Southwestern United States, which led to a greater understanding of the virus and its transmission to…
Hard water is water that contains large amounts of calcium, magnesium, or iron ions. Hard water is undesirable since it often has an unpleasant taste, interferes with the ability of soaps to dissolve (although some synthetic detergents dissolve well in hard water), and can cause scaling (the building up of insoluble precipitates) in pipes and hot water systems. Water hardness is most commonly the …
What makes a note from a musical instrument sound rich? The volume of the sound is determined by the amplitude of the oscillations in a sound wave, the distance individual molecules oscillate. A larger amplitude produces a louder sound and transmits more energy. The pitch of a note is the frequency or number of oscillations per second. A higher frequency produces a higher pitched note. The richnes…
Hartebeests are even-toed hoofed antelopes in the family Bovidae, which are found throughout Africa south of the Sahara. Included among the grazing antelopes are the reedbuck, waterbuck, rhebok, addax, oryx, bluebuck, gemsbok, and roan and sable antelopes. More closely related to hartebeests are gnus, impala, topi, wildebeest, and bontebok. These are medium to large antelopes that forage for food …
Hawks (family Accipitridae) are one of the major groups of predatory birds that are active during the day. They are members of the order Falconiformes, which also includes the falcons, vultures, and osprey, and like the other Falconiformes, they have the characteristic sharp, strong claws and hooked beak suited for catching and tearing up prey. Found on all continents but Antarctica, hawks are a d…
Hazardous wastes are by-products of human activities that could cause substantial harm to human health or the environment if improperly managed. The United States Environmental Protection Agency (EPA) classifies liquid, solid, and gaseous discarded materials and emissions as hazardous if they are poisonous (toxic), flammable, corrosive, or chemically reactive at levels above specified safety thres…
Hazels or filberts are shrub-sized woody plants in the birch family (Betulaceae) found in temperate forests of North America and Eurasia. Hazels have simple, coarse-toothed, hairy leaves that are deciduous in the autumn. Hazel species native to North America include the American hazel (Corylus americana) of the east and beaked hazel (C. cornuta) of a wider distribution. The giant filbert (C. maxim…
Hearing is the ability to collect, process and interpret sound. Sound vibrations travel through air, water, or solids in the form of pressure waves. When a sound wave hits a flexible object such as the eardrum it causes it to vibrate, which begins the process of hearing. The process of hearing involves the conversion of acoustical energy (sound waves) to mechanical, hydraulic, chemical, and finall…
A heart is a means to circulate blood through the body of an animal. Among the lower species such as insects, arachnids, and others, the heart may simply be an expanded area in a blood vessel and may occur a number of times. The earthworm, for example, has 10 such "hearts." These areas contract rhythmically to force the blood through the aorta, or blood vessel. Not until the evolutio…
Heart diseases (cardiovascular disease) is any abnormal organic condition of the heart or the heart and circulation. A number of conditions can lead to the development of heart disease, including angina, atherosclerosis, cardiac arrhythmia, cardiomyopathy, chronic venous insufficiency, diabetes, heart attack, high cholesterol, high homocysteine, high triglycerides, hypertension, insulin resistance…
The developing fetal heart accounts for a large percentage of the volume of the early thorax. About 20 days after fertilization, the heart develops from the fusion of paired endothelial tubes into a single tube. Heart growth subsequently involves the growth, expansion, and partitioning of this tube into four chambers separated by thickened septa of cardiac muscle and valves. Atrial development is …
The heart-lung machine is a device used to provide blood circulation and oxygenation while the heart is stopped. It is a means of keeping a patient alive while his heart is stopped or even removed from his body. Usually called the heart-lung machine, the device also is referred to as cardiopulmonary bypass, indicating its function as a means to substitute for the normal functions of the heart (car…
Heat exchange reflects and drives changes in energy state between two objects—or more generally systems—in thermal contact due to a difference in temperature. Heat flows from a system at higher temperature to one at lower temperature until both systems are at the same temperature. Systems at the same temperature are said to be in thermal equilibrium. The term "heat" is …
Heat capacity (often abbreviated Cp) is defined as the amount of heat required to raise the temperature of a given mass of a substance by one degree Celsius. Heat capacity may also be defined as the energy required to raise the temperature of one mole of a substance by one degree Celsius (the molar heat capacity) or to raise one gram of a substance by one degree Celsius (the specific heat capacity…
The heat index is a measure of how warm an average person feels as a consequence of moisture in the air compared to the actual temperature measured by a thermometer at the same time and location. Generally speaking, the higher the relative humidity, the warmer the temperature will seem to be to a person. The reason for this relationship is that the human body normally loses heat through the proces…
Heat transfer is the net change in energy as a result of temperature differences. This energy is transferred in the direction of decreasing temperature until thermal equilibrium (equality of temperatures) is achieved. The basic mechanisms involved in this process include radiation (the transfer of energy in the form of electromagnetic waves) and conduction (the transfer of kinetic energy). Heat tr…
The heath family, or Ericaceae, contains about 100-125 genera of vascular plants comprising 3,000-3,500 species. These plants are widespread in North and South America, Eurasia, and Africa, but are rare in Australasia. Species of heaths are most diverse and ecologically prominent in temperate and subtropical regions. The most species-rich genus in the heath family are the rhododendrons (Rhododendr…
Hedgehogs are small, often spine-covered members of the insectivore family Erinaceidae. The spiny hedgehogs are 13 species in subfamily Erinaceidae. Most famed is the European hedgehog, Erinaceus europaeus, which is also a resident of New Zealand, where it was introduced. Not all members of the hedgehog family have tough spines. The moonrats, or gymnures, of Southeast Asia have coarse hair instead…
The Heisenberg uncertainty principle first formulated by German physicist Werner Heisenberg (1901–1976), has broad implications for quantum theory. The principle asserts that it is physically impossible to measure both the exact position and the exact momentum of a particle (like an electron) at the same time. The more precisely one quantity is measured, the less precisely the other is know…
The heliocentric theory argues that the Sun is the central body of the solar system and perhaps of the universe. Everything else (planets and their satellites, asteroids, comets, etc.) revolves around it. The prevailing theory of the universe at that time was a geocentric (Earth-centered) one, in which all celestial bodies were believed to revolve around Earth. This was seen as a more plausible th…
Hematology is the study of blood and its basic biological components, including red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (erythrocytes). Hematologists study and help treat a variety of hematological malfunctions and diseases, one of the primary being the various anemias. Anemias, like sickle cell anemia, result in a loss of erythrocytes, which reduces the…
Hemophilia is an inheritable disorder of the mechanism of blood clotting. Depending on the degree of the disorder present in an individual, excess bleeding may occur only after specific, predictable events (such as surgery, dental procedures, or trauma), or may occur spontaneously, with no initiating event. Various tests are available to measure, under very carefully controlled conditions, the len…
Hemorrhagic diseases are caused by infection with certain viruses or bacteria. Viruses cause virtually all the hemorrhagic diseases of microbiological origin that arise with any frequency. The various viral diseases are also known as viral hemorrhagic fevers. Bacterial hemorrhagic disease does occur, but rarely. One example of a bacterial hemorrhagic disease is scrub typhus. Copious bleeding is th…
Hemp, or Cannabis sativa, is a tall, annual plant that thrives in temperate and subtropical climates. It is native to central and western Asia, and is one of the oldest cultivated plants. The word "hemp" is derived from the old English word "hanf," and refers to both the plant and the long fibers that are processed from its stems. The most common use of hemp has been as…
Henna, species Lawsonia intermis of the family Lythracea, is a perennial shrub that grows wild in northern Africa and southern Asia. The name henna, which comes from the Arabic word al kenna, refers to both the plant and the dye that comes from the leaves. The henna plant has narrow, grayish green leaves and small, sweet smelling clustered flowers that are white, yellow, or rose in color. One of t…
Hepatitis is inflammation of the liver, a potentially life-threatening disease most frequently caused by viral infections but which may also result from liver damage caused by toxic substances such as alcohol and certain drugs. Hepatitis viruses identified to date occur in five types: hepatitis A (HAV), hepatitis B (HBV), hepatitis C (HCV), hepatitis D (HDV), and hepatitis E (HEV). All types are p…
An herb is an aromatic plant that is used by people most commonly in cooking, but sometimes for medicinal purposes, as an insect repellant, as a source of dye, and sometimes for their attractive aesthetics. Herbs are not necessarily plants that are taxonomically related to each other—what these plants share is a usefulness to humans, not an evolutionary lineage. In general, herbs are non-wo…
Modern medicine has provided many breakthrough treatments for serious diseases. Some conditions, however, have eluded the healing grasp of contemporary western medicine,