Lacewings are insects in the order Neuroptera, sub-order Planipennia. Lacewings are named after the fine, complex, cross-branched venation of their four wings, which presents a beautiful, lacelike pattern. Lacewings are rather poor, fluttery fliers. When at rest, they hold their wings tentlike over their back. Lacewings have a complete metamorphosis, with four life history stages: egg, larva, pupa…
The "hydroxy" portion of the name tells chemists that there is an alcohol (OH) group in the molecule, and the "alpha" part of the name means that the alcohol is attached to the carbon atom adjacent to the acid (COOH) group. The "prop" portion of the name indicates that there are three carbon atoms. Lactic acid can also be called 2-hydroxypropanoic aci…
Lagomorphs are herbivorous mammals such as rabbits, hares, and pikas, in the order Lagomorpha. Because they exploit similar ecological niches, lagomorphs and rodents (order Rodentia) are rather similar in many aspects of their morphology and behavior. However, these orders are also different in important respects, and each represents an ancient evolutionary lineage. One distinguishing feature of t…
Lakes are inland bodies of water—millions of which are scattered over the earth's surface. Lakes are classified on the basis of origin, age, salinity, fertility, and water circulation. Lakes can be formed by glaciers, tectonic plate movements, river and wind currents, and volcanic or meteorite activity. Lakes can also be a phase of evolution in the aging process of a bay or estuary. …
Chevalier de Lamarck was a French naturalist and invertebrate zoologist who lived from 1844-1829. He is best known for a theory of evolution developed in his book, Philosophie zoologique, published in 1809. This theory, known today as Lamarckism, is based on the socalled "inheritance of acquired traits," meaning that characteristics that an organism may develop during its lifetime ar…
Lampreys and hagfishes are unusual, jaw-less fish that comprise the order Cyclostomata, so named because of the circular shape of the mouth. The 41 species of lampreys are in the superfamily Petromyzontoidea, while the approximately 35 species of hagfishes and slime hags are in the superfamily Myxinoidea. Lampreys and hagfishes lack the scales typical of most fish, and are covered with a slimy muc…
Land and sea breezes are wind and weather phenomena associated with coastal areas. A land breeze is a breeze blowing from land out toward a body of water. A sea breeze is a wind blowing from the water onto the land. Land breezes and sea breezes arise because of differential heating between land and water surfaces. Land and sea breezes can extend inland up to 100 mi (161 km), or manifest as local p…
Land use is a geographical concept that refers to the ways in which parcels of land are utilized by people and society. Land-use planning is an activity that examines the factors that influence the nature and dynamics of land usage and develops ways to optimize those variables to achieve larger social, economic, and ecological benefits. …
The term "sanitary landfill" was first used in the 1930s to refer to the compacting of solid waste materials. Initially adopted by New York City and Fresno, California, the sanitary landfill used heavy earth-moving equipment to compress waste materials and then cover them with soil. The practice of covering solid waste was evident in Greek civilization over 2,000 years ago, but the G…
A landform is a natural sculpture of the surface of the earth. Most landforms are produced by the actions of weathering and erosion, carving away material from higher elevations and depositing it down lower. Different kinds of rock erode at a variety of rates under particular climatic conditions. As softer rock is worn away the more resistant rock is exposed, producing another series of landforms.…
Langurs belong to the primate family Cercopitecidae, of which 13 species are represented in the genus Semnopithecus. This represents one of the largest and most diverse groups of colobine monkeys in Asia, with most species restricted to the south and southeast. Many species are distinguished by their vocalizations and the color of their fur, which ranges from a silver-grey in the common or Hanuman…
Light plays a vital role in the life of all oceans. At the simplest level, it provides one of the basic requirements for photosynthesis and promotes development of a food chain. Some species of fish that live in the darker reaches of the oceans also rely on light for survival. Some of these species, such as lantern fish, have even developed their own artificial means of generating light. Lantern f…
The lanthanides are a series of 14 metallic elements that appear at the bottom of the periodic table. Lanthanum, the element preceding the lanthanides in the periodic table, is usually also included in a discussion of the lanthanides since all 15 elements have very similar properties. When first discovered and isolated, the lanthanides were called the rare earth elements. Many uses have been found…
Larks are 75 species of small, terrestrial songbirds that make up the family Alaudidae. Larks breed on all of the continents except Antarctica. Their usual habitats are all open areas and typically include prairies, savannas, alpine and arctic tundras, heathlands, and some types of agricultural fields. Larks have long, pointed wings, a notched tail, and rather long legs and toes, with the hind, ba…
Laryngitis is an inflammation of the larynx. Located at the upper end of the trachea, or windpipe, the larynx contains the vocal chords that are used to form sounds. Because the larynx plays such an important role in speech, it is sometimes called the "voice box." When the larynx becomes inflamed in laryngitis, it swells and reddens. The major symptom of laryngitis is hoarseness. Oth…
The laser is a device that uses the principle of stimulated emission to produce light. The qualities of the light generated by a laser are significantly different from that generated by a conventional source such as an incandescent light bulb or fluorescent light tube. These major differences include: divergence: the laser generally emits a pencil thin beam of light whose divergent angle is closel…
Laser (light amplification by stimulated emission of radiation) surgery uses an intensely hot, precisely focused beam of light to remove or vaporize tissue and control bleeding in a wide variety of non-invasive and minimally invasive procedures. Laser surgery is used to: The first working laser was introduced in 1960. The device was initially used to treat diseases and disorders of the eye, whose …
The concepts of latitude and longitude create a grid system for the unique expression of any location on Earth's surface. Latitudes—also known as parallels—mark and measure distance north or south from the equator. Earth's equator (the great circle or middle circumference) is designated 0° latitude. The north and south geographic poles, respectively, measure 90&#…
The laurels are a family of flowering plants known to botanists as the Lauraceae. Lauraceae contains about 45 genera and 2,000 species, and is the most diverse family in the order Laurales. Most species grow in the tropical forests of Southeast Asia and Central and South America. The best known species is Laurus nobilis, a Mediterranean shrub used by the ancient Greeks to decorate the head of vict…
What makes a bird fly? A person run? A judo expert flip a heavier opponent? Earth orbit the Sun? These and any other motions are governed by three deceptively simple laws first stated by Isaac Newton in the seventeenth century. These three laws of motion when coupled with Newton's law of gravity form the basis for explaining both the motions we see on theearth and the motions of the heavenl…
LCD is short for liquid crystal display. LCDs are devices that use liquid crystals to create images. Liquid crystal images are being used in watch faces, laptop computer screens, camcorder viewers, virtual reality helmet displays, and television screens. The two main LCDs are passive and active. In passive LCDs, once the image has been made, it cannot be changed. In active LCDs, the image can be m…
Leaching usually refers to the movement of dissolved substances with water percolating through soil. Sometimes, leaching may also refer to the movement of soluble chemicals out of biological tissues, as when rainfall causes potassium and other ions to be lost by foliage. Leaching occurs naturally in all soils, as long as the rate of water input through precipitation is greater than water losses by…
A metallic element with atomic number 82. Symbol Pb, atomic weight 207.19, specific gravity 11.35, melting point 621.32°F (327.4°C), boiling point 3,191°F (1,755°C). Lead is in column IVA of the periodic table. It has four naturally occurring stable isotopes, lead-204, lead-206, lead-207, and lead-208. The last three of these are all end products of one or another radio…
A leaf is a plant organ which is an outgrowth of the stem, and has three main parts: the blade, a flattened terminal portion; the petiole, a basal stalk which connects the blade to the stem; and the stipules, small appendages at the base of the petiole. However, the leaves of many species lack one or more of these three parts. Leaves function in photosynthesis, or the biological conversion of ligh…
Leafhoppers are a species of insects in the family Cicadellidae, order Homoptera, a group that also includes the cicadas, whiteflies, aphids, and scale insects. There are about 20,000 species of leafhoppers, including about 2,500 species in North America. Leafhoppers are leaf-feeding herbivores that use their sucking mouthparts to pierce the tissues of plants and feed on their juices, in some case…
Learning is the alteration of behavior as a result of experience. When an organism is observed to change its behavior, it is said to learn. Many theories have been formulated by psychologists to explain the process of learning. Early in the twentieth century, learning was primarily described through behaviorist principles that included associative, or conditioned response. Associative learning is …
A common denominator for a set of fractions is simply the same (common) lower symbol (denominator). In practice the common denominator is chosen to be a number that is divisible by all of the denominators in an addition or subtraction problem. Thus for the fractions 2/3, 1/10, and 7/15, a common denominator is 30. Other common denominators are 60, 90, etc. The smallest of the common denominators i…
Lecithin is a phospholipid which consists of glycerol, two fatty acids, a phosphate group and choline. Lecithin was first found in eggs in 1846, so its name was coined from the Greek word for egg yolk, lekithos. Though lecithin is its common name, chemists refer to it as phosphatidylcholine. It is a yellow-brown fatty substance. In contrast to fats, which function as fuel molecules, lecithin serve…
A LED (light-emitting diode) converts electrical energy to light by means of a semiconductor, made of a solid material, such as silicon, whose electrical conductivity when hot is as great as that of metals and very low when cold. LEDs were commonly referred to early in their history as solid-state lamps. The light produced by LEDs is known as electroluminescence, distinguishing it from incandescen…
Legionnaires' disease is a type of pneumonia caused by Legionella bacteria. The bacterial species re sponsible for Legionnaires' disease is L. pneumophila. Major symptoms include fever, chills, muscle aches, and a cough that is initially nonproductive. Definitive diagnosis relies on specific laboratory tests for the bacteria, bacterial antigens, or antibodies produced by the body�…
Legumes or beans are species of plants in the family Fabaceae (also known as Leguminoseae). The legume family is very large, containing about 12,000 species and 440 genera with species occurring on all of the habitable continents. The most species-rich groups in the legume family are the milk-vetches (Astragalus spp.) with 2,000 species, indigos (Indigoifera spp.; 500 species), clovers (Trifolium …
Lemmings are small mouselike rodents in the family Muridae, which also includes the voles, gerbils, hamsters, rats, and mice. Lemmings occur in open, northern habitats, especially in alpine and arctic tundra of North America and Eurasia. Lemmings are herbivores, feeding on sedges, grasses, berries, roots, and lichens. Lemmings are ecologically important in their habitat, in part because they are t…
Lemurs are primitive primates, or prosimians, found only on the island of Madagascar and nearby small islands off the eastern coast of Africa. Although lemurs, lorises, and tarsiers are all prosimians, or "premonkeys," only the lemurs and lorises have the typical prosimian snout that, like a dog's, remains moist. This wet snout, called a rhinarium, suggests that scent is a par…
In the field of optics, a lens is a device used for focusing or defocusing a beam of light. It is commonly formed from a disk-shaped blank of transparent material, such as glass, plastic, or fused quartz; both sides are ground and polished, with at least one surface being polished with a curve. The word lens is derived from the Latin word for lentil, since the shape of a lens resembles the curved …
Leprosy, also called Hansen's disease, affects 10-12 million people worldwide. Caused by an unusual bacterium called Mycobacterium leprae, leprosy primarily affects Leprosy. CNRI/Phototake NYC. Reproduced by permission. humans. Leprosy is found in tropical areas, such as Africa, South and Southeast Asia, and Central and South America. In the United States, cases of leprosy have bee…
From the Greek words for white (leukos) and blood (hemia), leukemia refers to abnormally shaped and functioning leukocytes (white blood cells). Because the leukocytes multiply at an uncontrolled and rapid rate, leukemia is considered a cancer of the blood. Leukemia is neither contagious nor infectious, nor acquired from the mother prior to or during birth, but some researchers have suggested genet…
Lewis structures (also called electron-dot structures) are formed when Lewis symbols (also called electron-dot symbols) are combined. Lewis symbols are a simple way of visualizing the valence electrons in an atom. In a Lewis symbol, the symbol for the element is used to represent the atom and its core electrons. Dots placed around the atom are used to indicate the valence electrons. When combined …
Lice are small, wingless, biting or sucking insects, many of which are ecto-parasites. There are about 3,000 species of lice in the orders Mallophaga and Anopleura. The Anopleura are sucking lice, which are parasites of mammals, and which feed only on blood. The Mallophaga are chewing or biting lice, and are primarily pests of birds, feeding on skin and feathers. Most species of lice are specific …
Lichens are an intimate symbiosis, in which two species live together as a type of composite organism. Lichens are an obligate mutualism between a fungus mycobiont and an alga or blue-green bacterium phycobiont. Each lichen mutualism is highly distinctive, and can be identified on the basis of its size, shape, color, and biochemistry. Even though lichens are not true "species" in the…
Life history is an ecological term that refers to the significant features of the life cycle of organisms and their relationships with environmental conditions. Life cycle refers to the sequence of discrete developmental stages of an organism from their origin as gametes to their eventual death. Life cycle also refers to the stages through which generations of organisms pass from their own origin …
In inorganic chemistry, ligands are molecules or electrically charged atoms (ions) which are bonded to metal atoms or ions. The ligand changes the metal's ability to dissolve in or react with its surroundings. In biochemistry, ligands are defined as molecules, usually Figure 1. Two ligands (ammonia) each donate a pair of electrons to bond with a silver ion. (N = nitrogen, H = hydrogen, A…
Light can be narrowly defined as the visible portion of the electromagnetic spectrum. A broader definition would include infrared, ultraviolet, and x-ray wavelengths, which are not visible to the eye. The nature of light has been the subject of controversy for thousands of years. Even today, while scientists know how light behaves, they do not always know why light behaves as it does. The next con…
A light-year is the distance that light (or any other form of electromagnetic radiation, such as radio waves) travels in a vacuum in one year. Since light travels at a velocity of 186,171.1 mi/s (299,792.5 km/s), one light-year equals 5,878,489,000,000 miles (9,460,530,000,000 km). The light-year is a convenient unit of measurement to use when discussing distances to the stars in the Milky Way gal…
Lightning is an electrical discharge usually, but not always, produced by well-developed thunderstorms. Although there is a clear-air lightening phenomena,lightning most frequently occurs within a cloud (intra-cloud), between two clouds (inter-cloud), or from the cloud to the ground. A lightning discharge can heat the air as much as five times hotter than the surface temperature of the Sun (about …
Lilacs (Syringa spp.) are about 10 species of shrubs and small trees in the olive family (Oleaceae). Lilacs are native to Eurasia but have been widely planted elsewhere as ornamental shrubs. The common lilac (Syringa vulgaris) is the most familiar species to most people. The common lilac has shiny green wedge-shaped leaves without teeth on the margins which are arranged alternately on the twigs. …
Lilies are the classic representatives of the monocotyledons—those plants with only one seed leaf. Lilies are mostly perennial, erect herbs arising from a bulb. Some climb, a few are woody, but most arise from underground stems or other structures. The leaves vary in number from one to many, and are arranged on the stem alternately or in whorls. The leaves are flat, linear to lance-shaped, …
In mathematics the concept of limit formally expresses the notion of arbitrary closeness. That is, a limit is a value that a variable quantity approaches as closely as one desires. The operations of differentiation and integration from calculus are both based on the theory of limits. The theory of limits is based on a particular property of the real numbers; namely that between any two real number…
Limiting factors are environmental influences that constrain the productivity of organisms, populations, or communities and thereby prevent them from achieving their full biological potential which could be realized under optimal conditions. Limiting factors can be single elements or a group of related factors. The environment of organisms must be suitable in many respects. Environmental factors m…
Limpets are a common mollusk of the class Gastropoda. Its shell is generally low, flat, oval, and more bilaterally symmetrical than coiled (like a snail's), and it covers the entire soft body, so that the living animal inside is rarely visible. Limpets adhere strongly to rocks by means of a broad muscular foot. It is important for their survival that they are not dislodged easily, since the…
There are many different ways of writing the equation of a line in a coordinate plane. They all stem from the form ax + by + c = 0. Thus 2x + 3y - 5 = 0 is an equation of a line, with a = 2, b = 3, and c = -5. When the equation is written in the form y = mx + b we have slope-intercept form: m is the slope of the line and b is the y-intercept. The equation 2x + 3y - 5 = 0 becomes So the line h…
Two important concepts emerge in linear algebra to help facilitate the expression and solution of systems of simultaneous linear equations. They are the vector and the matrix. Vectors correspond to directed line segments. They have both magnitude (length) and direction. Matrices are rectangular arrays of numbers. They are used in dealing with the coefficients of simultaneous equations. Using vecto…
Lipids are a class of natural, organic compounds in plants and animals, defined by a specific way they behave: they are soluble in non-polar solvents. That is, lipids are not soluble in water but dissolve in solvents like gasoline, ether, carbon tetrachloride, or oil. The vast majority of lipids are colorless and mostly fats and oils. Lipids comprise one of the three broad classifications into whi…
Liquid crystals are pure substances in a state of matter that shows properties of both liquids and solids over a specific temperature range. At temperatures lower than this range, the liquid crystals are only like solids. They do not flow and their molecules maintain a regular arrangement. At temperatures above this range, the liquid crystals behave only like liquids. They can flow and the molecul…
Lithium has been the treatment of choice for manic-depressive illness for several decades. Lithium is a trace element found in plants, mineral rocks, and in the human body. Today, the major source of medical lithium is mines in North Carolina. Lithium is classified as an antimonic medication because of its ability to reverse mania, a mood disorder characterized by extreme excitement and activity. …
Lithography is a method of printing an image by applying patterned layers of color to paper with a series of etched metal or stone plates. This is the process used to print many newspapers and multi-colored lithographs. It is also the general name for the techniques used to fabricate integrated circuits (ICs). In the manufacture of integrated circuits, the silicon wafer that acts as the base and a…
The word lithosphere is derived from the word "sphere," combined with the Greek word "lithos" which means rock. The lithosphere is the solid outer section of Earth which includes Earth's crust (the "skin" of rock on the outer layer of planet Earth), as well as the underlying cool, dense, and fairly rigid upper part of the upper mantle. The lithosphe…
Lithotripsy, extracorporeal shock wave (ESWL), is the first non-invasive (not requiring surgical opening of the body) treatment for eliminating kidney stones by breaking them into sand-like particles, usually by means of high pressure waves generated in water. The particles are then eliminated from the body during urination. The ESWL machine, called a lithotripter, generates shock waves in a reser…
The liverworts are one of three classes in the plant phylum Bryophyta. The other two classes are mosses and hornworts. Liverworts are small, green, terrestrial plants. They do not have true roots, stems, or leaves. Instead, they have an above ground leaf-like structure, known as a thallus, and an underground structure, known as a rhizoid. Most liverworts are found in moist environments and they te…
Livestock is a collective term for domesticated animals that are kept, mostly for the production of meat, milk, wool, or other products. The most common species are cattle, pigs, sheep, goats, horses, and chickens. The term is not used in reference to animals that are kept as pets or companions. Livestock are domesticated species, which have been genetically modified over time through the artifici…
Lobsters are large crustaceans in the order Decapoda, which also includes about 10,000 species of crayfish, crabs, and shrimps. Decapods are characterized by having their carapace fused with their thoracic segments to form a gill chamber above where the legs join the body, and the first three of their eight pairs of thoracic legs are modified into grasping, clawlike structures known as maxillipeds…
A lock or water lock is an enclosed, rectangular chamber with gates at each end, within which water is raised or lowered to allow boats or ships to overcome differences in water level. Locks have a history of over 2,000 years, and although they are most often used by boats on canals, they also are used to transport massive ships between seas. All locks operate on the simple buoyancy principle that…
A lock is a mechanical device for securing a door, chest, or other receptacle so that it can only be opened by an authorized person. Most locks are opened by a key which is placed in the lock and turned. Combination locks do not use a key but rather have a cylinder that is turned to certain stops. Today, many hotels use special plastic cards with magnetic strips as keys which cause a door to open …
A locus is a set of points that contains all the points, and only the points, that satisfy the condition, or conditions, required to describe a geometric figure. The word locus is Latin for place or location. A locus may also be defined as the path traced out by a point in motion, as it moves according to a stated set of conditions, since all the points on the path satisfy the stated conditions. T…
Because logarithms are exponents, they have an intimate connection with exponential functions and with the laws of exponents. The major laws of logarithms and the exponential laws from which they are derived are shown in Table 1. In all these rules, the bases a and b and the arguments x and y are limited to positive numbers. The exponents m, n, p, and r and the logarithms can be positive, negative…
Loons are the only surviving members of an ancient order of birds, the Gaviiformes, which has a fossil record extending back to the Lower Cretaceous, more than 100 million years ago. Loons comprise their own family, the Gaviidae, which consists of 12 extinct and five surviving (extant) species. All of the extant species of loons live in the Northern Hemisphere, where they breed on lakes and ponds,…
LORAN (Long Range Navigation) is a radio-based navigational aid first used during World War II to locate ships and planes with greater accuracy than could be achieved with conventional techniques. LORAN determines location by comparing accurately-synchronized powerful radio pulses originating from different reference transmitter sites. Pulses from nearby transmitters arrive earlier than pulses fro…
The loris or bushbaby family, Lorisidae, includes 14 species of Asian and African primates. Loris is a Dutch word for clown, given to these amusing creatures by European seaman who saw them. With the lemurs, these attractive little primates make up the group called prosimians, or "pre-monkeys." All lemurs are found on the big island of Madagascar; other members of the loris family ca…
Light generation by a process other than by heating is luminescence. For example, an incandescent light bulb, in which the filament is heated until it is literally white-hot, is not luminescent; a fluorescent light tube (which is cool to the touch) is luminescent. Luminescence is generated as part of a process in which atoms or molecules with electrons excited into higher energy states shed energy…
Bony fish are divided into two major groups: rayfinned and fleshy-finned fish. The fleshy-finned fish are further subdivided into two orders: the lungfish, or Dipnoi, and the lobe-finned fish, or Crossopterygii. Although crossopterygian fish are the group that is thought to be close to the ancestors of the land vertebrates, lungfish also display many of these characteristics. In the early stages o…
Lycophytes are vascular plants in the class Lycopodiopsida, a division of vascular plants known as Pteridophytes (the ferns and their allies). The class Lycopodiopsida is divided into three subclasses: the Lycopodiidae, Selaginellidae, and Isoetidae. Like other pteridophytes, the lycophytes have an alternation of generations, consisting of two generations of morphologically different plants. The l…
Lyme disease is an infection transmitted by the bite of ticks carrying the spiral-shaped bacterium Borrelia burgdorferi (Bb). The disease was named for Lyme, Connecticut, the town where it was first diagnosed in 1975, after a puzzling outbreak of arthritis. The organism was named for its discoverer, Willy Burgdorfer. The effects of this disease can be long-term and disabling unless it is recognize…
The lymphatic system is the body's network of organs, ducts, and tissues that filter harmful substances out of the fluid that surrounds body tissues. Lymphatic organs include the bone marrow, thymus, spleen, appendix, tonsils, adenoids, lymph nodes, and Peyer's patches (in the small intestine). The thymus and bone marrow are called primary lymphatic organs, because lymphocytes are pr…
Lyrebirds are named for the male's magnificent tail, which spreads in a fan-like display, resembling a lyre, an ancient Greek stringed instrument. The male's body is little longer than 12 in (30.5 cm), but the tail may be longer than 16 in (40.5 cm). The only two species of lyrebird in the world are indigenous to a strip of rugged, hilly bushland along the east coast of the Australia…
Macaques are medium to large-sized monkeys native to Asia and Africa belonging to the genus Macaca, family Cercopithecidae, order Primates. Macaques are usually various shades of brown, gray, or black in fur color, although golden and white color phases occur rarely. Approximately 20 species are known. Locomotion is mainly quadrupedal, and most species are terrestrial in habit, although they take …
The Mach number is used in fluid mechanics and is especially useful in studies involving supersonic aerodynamics. It is named after Ernst Mach (1838-1916), the Austrian physicist and philosopher who pioneered the study of supersonic projectiles. The Mach number is the ratio of the velocity of a fluid to the velocity of sound in that same fluid. In the case of a body moving through a fluid, the Mac…
A machine tool is an electrically powered tool which is used to remove material, usually metal, at a controlled rate to achieve a desired shape or finish. A machine tool typically holds the workpiece and a cutting tool, and moves either the workpiece, tool or both to provide a means of machining the material to the desired shape. Machining, another term for metalcutting, is performed by shaving aw…
Machine vision, also referred to as computer or robot vision, is a term that describes the many techniques by which machines visually sense the physical world. These techniques, used primarily for monitoring industrial manufacturing, are becoming increasingly popular as today's manufacturing environments become more automated and quality control standards increase. Whether the task is to so…
A simple machine is a device for doing work that has only one part. Most authorities list six kinds of simple machines: levers, pulleys, wheels and axles, inclined planes, wedges, and screws. One can argue, however, that these six machines are not entirely different from each other. Pulleys and wheels and axles, for example, are really special kinds of levers, and wedges and screws are special kin…
The Atlantic mackerel, Somber scombrus, supports one of the most important commercial fisheries and supports a significant sport fishing interest. The fish is a close relative of the tuna. The attraction of mackerel as sport fish is due primarily to the streamlined body, forked tail, pointed head, and high-speed swimming. An unusual characteristic of the mackerel is that it does not possess a swim…
Magic square is an unusual numerical configuration containing consecutive integers in arrangements so that the sum of numbers in any row, column, or diagonal are identical. Such squares were known approximately 4,000 years ago in China. The basic magic square is a square containing consecutive integers starting with number 1. Three of the basic magic squares are shown in Table 1. Other magic …
In geology, magma refers to molten rock deep within Earth that consists of liquids, gases, and particles of rocks and crystals. Magma has been observed in the form of hot lava and the various rocks made from the solidification of magma. Geologists have created magmas (artificial melts) in the laboratory to learn more about the physical conditions in which magma originated and its composition. Magm…
Magnesium (Mg) is one of the most abundant structural metals (those metals used to build things), ranking third behind iron and aluminum. Magnesium compounds are found in mineral rocks such as dolomite and magnesite, while the Earth's oceans contain a practically unlimited supply of the metal in the form of dissolved magnesium chloride. One cubic mile of seawater holds some six million tons…
Magnetic levitation is the phenomenon in which two magnetic objects are repelled from each other in a vertical direction. The phenomenon, also known as MAGLEV, has long been recognized as having some important commercial applications. The most significant of these is the construction of MAGLEV trains which are propelled a few inches above a track at very high rates of speed. …
Audiocassette tape recorders are widely used to record and play back music or speech. Information is stored on a narrow ribbon of plastic tape that has one side coated with a magnetic material, such as iron oxide. An electromagnet aligns individual magnetic particles in a pattern that corresponds to the loudness and frequency of incoming sounds. In order to play back the recorded information, the …
Magnetic resonance imaging (MRI) is a medical technique which utilizes a magnetic field and the natural An example of a magnetic resonance imaging system (MRI). Photograph by Mason Morfit. FPG International Corp. Reproduced by permission. resonance of atoms to provide an image of human tissue. While the foundation for its development first took place in the late 1930s, it was not until…
Magnetism is a force generated in matter by the motion of electrons within its atoms. Magnetism and electricity represent different aspects of the force of electromagnetism, which is one part of Nature's fundamental electroweak force. The region in space that is penetrated by the imaginary lines of magnetic force describes a magnetic field. The strength of the magnetic field is determined b…
The magnetosphere is a comet-shaped region of Earth's outer atmosphere in which the behavior of Earth's magnetosphere. Illustration by Hans & Cassidy. Courtesy of Gale Group. charged particles is strongly influenced by magnetic and ionic phenomena. The term was first introduced by the British astronomer Thomas Gold in 1959 although speculation about the existence of suc…
Magnolias are species of trees and woody shrubs that comprise the family Magnoliaceae. This is an ancient and relatively primitive group of dicotyledonous plants with fossil materials known from as early as the Upper Cretaceous. The magnolia family contains about 220 species in 12 genera, including the true magnolias (Magnolia spp.), with about 100 species. Magnolias have seasonally deciduous or e…
Mahogany (Swietenia mahogani) is a member of the family Meliaceae, which contains about 500 other species of trees and shrubs native to tropical forests in the Americas, Africa, and Asia. Other common names for this species are the Spanish or West Indies mahogany. Various other species of trees have also been given the name mahogany, but the true mahogany is Swietenia mahogani. Mahogany is one of …
These are a group of ferns found in tropical and warm temperate regions. They are characterized by having delicate, fan-shaped fronds, arising from a thin black midrib, with small green leaflets. Maidenhair fern belong to the genus Adiantum, and some species are popular as house plants. In North America there are three common species, the northern maidenhair fern (A. pedatum), the southern maidenh…
Malaria has been described as the world's greatest public health problem. It is caused by one of several strains of the Plasmodium protozoan, a one-celled parasite that is transmitted by the bite of the Anopheles mosquito. Hundreds of millions of persons are struck by malaria each year, resulting in 1–3 million deaths annually, primarily in children under five-years-old in sub-Sahara…
Malnutrition is the condition that develops when the body does not get the right amount of the vitamins, minerals, and other nutrients it needs to maintain healthy tissues and organ function. Malnutrition occurs in people who are either under-nourished or over-nourished. Undernutrition is a consequence of consuming too few essential nutrients or using or excreting them more rapidly than they can b…
The more than 4,000 species of living mammals belong to the vertebrate class Mammalia. This diverse group of animals has certain common features: all have four legs, bodies covered by hair, a high and constant body temperature, a muscular diaphragm used in respiration, a lower jaw consisting of a single bone, a left systemic aortic arch leaving the left ventricle of the heart, and three bones in t…
Manakins are 53 species of small, tropical birds that comprise the family Pipridae, occurring from southern Mexico to Paraguay. Manakins are species that dwell in mature, tropical forests. Manakins are squat, compact birds, with short, rounded wings and a short tail. However, in some species the tail of the male is greatly lengthened by the occurrence of long, thin extensions, sometimes longer tha…
Mangroves are trees in the family Rhizophoraceae, occurring in tropical and subtropical environments as swampy forests fringing muddy, tidal, estuarine, and oceanic shores. Mangrove forests are generally the first type of woody ecosystem that is encountered when a low-lying tropical shore is approached from the ocean. Mangrove forests comprise a biome, that is, a distinctive ecosystem that occurs …
Mania is a mood disturbance marked by an abnormal degree of elation or irritability along with a number of other symptoms including restlessness, inflated self confidence, a marked decrease in the need for sleep, rapid and loud speech that is difficult to interrupt, racing thoughts, high distractibility, and a marked increase in certain goal-directed activities. Over time, manic episodes are usual…
Manic-depressive illness, clinically called bipolar disorder, is a major mental illness belonging to the category of illnesses designated as mood disorders. It is estimated that as many as two million Americans suffer from this illness and many more may go undiagnosed or underdiagnosed. Approximately one in five families will be confronted with a family member who may experience a manic episode or…
A map, or mapping, is a rule, often expressed as an equation, that specifies a particular element of one set for each element of another set. To help understand the notion of map, it is useful to picture the two sets schematically, and map one onto the other, by drawing connecting arrows from members of the first set to the appropriate members of the second set. For instance, let the set mapped fr…
The maples are about 150 species of angiosperm trees and shrubs in the genus Acer, family Aceraceae. Most maples occur in temperate forests of the Northern Hemisphere. Maples are characterized by the shape of their leaves, which in most species are broadly palmate with a three- or five-lobed outline, and are arranged in an opposite fashion on their branches. Maples have seasonally deciduous foliag…
Marfan syndrome is genetic disease that involves defects in the connective tissues of the body with the cardinal collection of abnormalities affecting the skeletal, ocular, and cardiovascular systems. It is inherited as an autosomal dominant disorder, meaning that inheriting only one defective gene from either the paternal or maternal side will lead to the disorder. Although the majority of mutati…
Marijuana is the common name for the hemp plant Cannabis sativa. Hemp grows in tropical as well as temperate climates. The dried ground leaves, flowers, and stems of the plant have a long history for their use as drugs. It has been cultivated in different regions of the world throughout centuries for its fiber to produce linen, rope, canvas, and oil. It has also been used as a medicine to relieve …
Marlins are large fish with elongated, bill-like snouts, fairly high dorsal fins, and a streamlined body. They belong to the order Perciformes and the suborder Scomproidei. Marlins are among the fastest of all fish and are highly valued by sporting fishers. …
Marmosets and tamarins are South and Central American primates of the Amazon Basin. Their family, Callitrichidae, includes 18 species that have been described as "near"-monkeys. All species are considered endangered or threatened by extinction. This plight is mostly caused by deforestation to develop new agricultural land, as well as disturbance of their forest habitat by logging, ro…
Marmots are species of medium-sized robust, short-legged burrowing herbivorous rodents in the genus Marmota, family Sciuridae, order Rodentia. Marmots are closely related to the ground squirrels and gophers. Marmots live in burrows that they dig themselves, or sometimes in the deep crevices of rock piles and talus slopes beneath cliffs. Most species of marmots occur in alpine or arctic tundra or i…
Mars is the fourth planet from the center of the solar system, orbiting the Sun once every 687 (Earth) days at a mean distance of 141 million mi (227 million km). Called the "red planet" for its distinct orange-red color, Mars has been the object of intense interest for over a century. Popularly regarded as a possible source of life, Mars was thought to be barren after the Viking spa…
The Mars Pathfinder probe landed on Mars on July 4, 1997. Pathfinder was second in the Discovery series of robotic spacecraft, which the United States National Aeronautics and Space Administration (NASA) began to develop in the mid 1990s. Costing an average of $150 million per project, the Discovery shift to faster, cheaper, less-ambitious probes was prompted by the catastrophic failure in 1993 of…
Marsupial cats are native carnivores of Australia, in the family Dasyuridae. Like all marsupials, the young of marsupial cats are born when they are still in an embryonic state, and they migrate to a belly pouch (or marsupium) on the female, where they fix onto a nipple and suckle until they are almost fully grown and independent. Marsupial cats fill the ecological roles played by weasels, cats, f…
Marsupial rats and mice are a diverse group of about 40 species of small, native carnivores of Australia, Tasmania, and New Guinea, in the family Dasyuridae. The young of marsupial rats and mice, as with those of all marsupials, are born while still in a tiny, embryonic stage of development. The almost helpless babies migrate to the belly of their mother, where they fix on a nipple and suckle unti…
Marsupials belong to the order Marsupalia, one of three subclasses of mammals (Metatheria). Marsupials are named for the marsupium, which means pouch in Latin; most female marsupials carry their young in pouches. The order Marsupalia includes eight families, 75 genera, and 250 species. Marsupials are divided into two groups based on the number and shape of the incisor teeth. One group has numerous…
Marten, sable, and fisher are species of medium-sized carnivores in the family Mustelidae, which also includes the weasels, otters, badgers, minks, skunks, and wolverine. Marten, sable, and fisher are generally solitary animals, living in forests of the Northern Hemisphere. All of these species have highly valuable fur, and are trapped intensively. The American pine marten (Martes americana) range…
Maser is an acronym for microwave amplification by stimulated emission of radiation. Microwaves correspond to that portion of the electromagnetic spectrum where the radiation has wavelengths of 0.039-12 in (1 mm-30 cm), i.e., between the far infrared and radio frequencies. Crystals can be used as amplifiers of microwave radiation and as sources of radiation having a single wavelength and frequency…
Newton defined the mass of an object as the quantity of matter it possessed. A small rock, for example, has a mass—a fixed, unchanging quantity of matter. If you were to take that rock along with you on a trip to the moon, it would have the same quantity of matter (the same mass) that it had on Earth. Its weight, however, would be less on the moon. The rock's weight on earth was the …
Extinction, the death of all members of a species, is a natural process that has been occurring since the beginning of life on Earth. Nearly all species that have ever existed are now extinct, and extinction is an important process in the evolution of new species. Mass extinction, the death of large numbers of species over a relatively short span of geologic time, is also a natural process, but on…
The mass number of an atom is the total number of protons plus neutrons in its nucleus. To understand this situation, think of the binding energy as the strength of the "glue" that holds the protons and neutrons together as a nucleus. It is, therefore, the amount of energy that would be required to break the "glue" and pull the nucleus apart into its individual neutrons…
Mass production is an entire system of manufacturing products that uses specialized labor, machinery, the smooth and logical flow of materials, and an assembly line to turn out large volumes of the same product at the lowest possible cost. The fullest expression of mass production was probably found at the Ford Motor Company in the early years of the twentieth century, when hundreds of thousands o…
Mass spectrometry is an instrumental method of obtaining structure and mass information about either molecules or atoms by generating ionized particles and then accelerating them in a curved path through a magnetic field. Heavier particles are more difficult for the magnetic field to deflect around the curve, and thus travel in a straighter path than lighter particles. Consequently, by the time th…
Mass transportation is any kind of transportation system in which large numbers of people are carried within a single vehicle or combination of vehicles. Airplanes, railways, buses, trolleys, light rail systems, and subways are examples of mass transportation systems. The term mass transit is commonly used as a synonym for mass transportation. In many parts of the world, mass transit systems are a…
Mass wasting, or mass movement, is the process that moves Earth materials down a slope, under the influence of gravity. Mass wasting processes range from violent landslides to imperceptibly slow creep. Mass wasting decreases the steepness of slopes, leaving them more stable. While ice formation or water infiltration in sediments or rocks may aid mass wasting, the driving force is gravity. All mass…
Mathematics, in the very broadest sense, is the systematic study of relationships in the physical world and relationships between symbols which need not pertain to the real world. In relation to the world, mathematics is the language of science. It operates within the laws and constraints of science as it examines physical phenomena. Unlike science, however, mathematics has no constraints. So in r…
A matrix is a rectangular array of numbers or number-like elements: These rules for adding and subtracting matrices give matrix addition the same properties as ordinary addition and subtraction. It is closed (among matrices of the same size), commutative, and associative. There is an additive identity (the matrix consisting entirely of zeros) and an additive inverse: This latter def…
In 1804, the English scientist John Dalton formulated the atomic theory, which set out some fundamental characteristics of matter, and which is still used today. According to this theory, matter is composed of extremely small particles called atoms, which can be neither created nor destroyed. Atoms can, however, attach themselves (bond) to each other in various arrangements to form molecules. A ma…
The Maunder minimum is the name given to a period of extreme solar inactivity that occurred between 1645 and 1710. Of particular interest is that this period of inactivity corresponds closely to one of the coldest periods of the so-called "Little Ice Age" in Europe, a time of long, cold winters that caused severe hardships in the pre-industrial revolution world. This has led scientis…
Finding the maxima and minima, both absolute and relative, of various functions represents an important class of problems solvable by use of differential calculus. The theory behind finding maximum and minimum values of a function is based on the fact that the derivative of a function is equal to the slope of the tangent. When the values of a function increase as the value of the independent varia…
Mayflies, or shadflies, are aquatic insects in the order Ephemeroptera. There are several thousand species of mayflies, distributed among 20 families. Mayflies have a relatively protracted nymphal stage, which occurs in freshwater habitats. The adults are a short-lived stage during which dense aggregations of animals engage in a frenzied procreation, needed to perennate the species in its local ha…
The mean is also known as the average. It is one of the measures of central tendency, the others being the mode and median. …
The median is a measure of central tendency, like an average. It is a way of describing a group of items or characteristics instead of mentioning all of them. If the items are arranged in ascending order of magnitude, the median is the value of the middle item. If there is an odd number of items in the group, the median can be found precisely. For example, assume that 27 test scores are arranged f…
Medical genetics is a multi-dimensional study of medically significant human genetic variation. Molecular diagnostics is the newest specialty field in genetics and allows a better understanding of heredity and disease at the molecular level. Disorders due to mutations as small as a single base change in the DNA can now be explained. Once the mutation is known, clinical laboratory testing can usual…
Meiosis, also known as reduction division, consists of two successive cell divisions in diploid cells. The two cell divisions are similar to mitosis, but differ in that the chromosomes are duplicated only once, not twice. The end result of meiosis is four daughter cells, each of them haploid. Since meiosis only occurs in the sex organs (gonads), the daughter cells are the gametes (spermatozoa or o…
Cell membranes or plasma membranes surround cells, separating the cytoplasm and organelles on the inside from the extracellular fluid on the outside. Several cell organelles (mitochondria, endoplasmic reticulum, and Golgi bodies) are also bounded by membranes. The membrane allows a cell or organelle to maintain a constant internal environment, usually one that is quite different from the medium su…
Memory refers to the mental systems and processes involved in storing and recalling information about stimuli that are no longer present, as well as to all of the information that is stored. Memory is essential to healthy human functioning, and it can be said that every mental process involves some aspect of memory. Indeed, the ancient Greek philosopher Cicero once described memory as "the …
Mendelian genetics is the study of simple patterns of inheritance. Gregor Mendel (1822-1884), an Austrian monk was the first to publish an extensive study of how various traits are passed from parent to progeny. He studied simple garden pea plants, plotting in detail their various physical traits, and studying how combinations of various parental traits produced particular traits in the progeny. F…
Meningitis is a potentially fatal inflammation of the meninges, the thin, membranous covering of the brain and the spinal cord. Meningitis is most commonly caused by infection (by bacteria, viruses, or fungi), although it can also be caused by bleeding into the meninges, cancer, or diseases of the immune system. …
Menopause is the stage in the female life cycle during which menstrual cycles stop. On average, menopause occurs at age 51, and generally takes from five to seven years from start to finish. For years, the menopausal stage was rarely talked about in public. Beginning in the 1960s, physicians began treating menopause aggressively as a medical problem, using estrogen hormones. Contemporary debate fo…
The menstrual cycle technically refers to the cyclic changes that take place in the lining of the human uterus over the course of approximately 28 days in adult females. These cycle changes are associated with cyclic changes in the ovaries and in the brain and ovarian hormones. The term "menstrual" comes from the Latin word menses, meaning month. The purpose of the cyclic changes is …
Mercurous chloride has been most often used as a treatment for intestinal worms. In the past, large doses were often used to stimulate the intestines and remove blockages, although it is rarely used in medicine today due to the toxicity of mercury. When it is used as a laxative, if the treatment fails to work, large doses of other laxatives and water must be used to insure that no mercury is allow…
Mercury is a metallic chemical element identified by the symbol Hg on the periodic table. It is silver in color and, unlike other metals, is liquid at room temperature. The ancient name for mercury was quicksilver, meaning "living" silver. This name reflected mercury's lustrous silver color and its unusually lively behavior: when it is poured onto a smooth surface, it forms be…
Mercury is the closest planet to the Sun. It is a small world only slightly larger than Earth's Moon. Next to the planet Pluto, Mercury is one of the least explored planets within our solar system. Visited only once in all the years of solar system exploration (three brief fly-bys of Mariner 10 during 1974–75), only about 45% of Mercury's surface has been imaged from nearby. A…
The prefix "meso-" means "in between" or "intermediate." Mesoscopic systems are those that are larger than atoms and yet very much smaller than the largescale everyday objects that we can see and touch. They are a thousand to a hundred thousand times smaller than the diameter of a human hair and they range in size from several hundred nanometers or billion…
The phylum Mesozoa comprises a small group of parasitic animals that are related to flatworms—a widely dispersed group of free-living and parasitic organisms. Some 50 species have been identified in this phylum to date, all of which are exclusively marine in their lifestyle. Relatively few detailed studies have been conducted on the behavior and life cycles of these species and their taxono…
Metabolic disorders are diseases caused by errors in metabolism. The term "metabolism" refers to the sum of the chemical reactions in the body. Metabolic problems can be traced to numerous metabolic pathways found in cells throughout the body. Metabolic reactions are categorized into two types, anabolic and catabolic. Anabolic reactions construct complex molecules from simple molecul…
Metabolism refers to the highly integrated network of chemical reactions by which living cells grow and sustain themselves. This network is composed of two major types of pathways: anabolism and catabolism. Anabolism uses energy stored in the form of adenosine triphosphate (ATP) to build larger molecules from smaller molecules. Catabolic reactions degrade larger molecules in order to produce ATP a…
A material is called a metal based on the way it reacts to other elements. Metallic elements characteristically form positive ions when their compounds are in solution. Their oxides form hydroxides rather than acids with water. Nearly three-fourths of the elements in each group of the periodic table are metals except for the Group 17 (halogen) and Group 18 (noble gas) elements. Most metals form cr…
Metal fatigue is the process by which a material is slowly damaged by stresses and strains that are less than those needed to actually break the material apart. For example, a steel wire might be used to suspend weights that are less than the amount needed to cause the wire to break apart (its tensile strength). Over time, however, those weights might slowly cause defects to develop in the steel. …
The term metal production refers to all of the processes involved in the conversion of a raw material, such as a metallic ore, to a final form in which the metal can be used for some commercial or industrial purpose. In some instances, metal production involves relatively few steps since the metal already occurs in an elemental form in nature. Such is the case with gold, silver, platinum, and othe…
Metallurgy is the science and technology of metals. As indicated in Table 1, the recorded history of metal working goes back over 6,000 years. Chemical or extractive metallurgy is concerned with the extraction of metals from ores and with the refining of metals. Physical metallurgy is concerned with the physical and mechanical properties of metals as affected by composition, mechanical working, an…
Metamorphic grade is a scale denoting the level of pressure and temperature involved in forming a particular metamorphic rock. The scale is based on the first appearance of particular minerals, known as index minerals. Because each mineral crystallizes within a limited pressure and temperature range, the presence of particular index minerals indicates the relatively specific set of conditions that…
Metamorphic rock is rock that has changed from one type of rock into another. The word metamorphic (from Greek) means "of changing form." Metamorphic rock is produced from either igneous rock (rock formed from the cooling and hardening of magma) or sedimentary rock (rock formed from compressed and solidified layers of organic or inorganic matter). Most of Earth's crust is made…
Metamorphism is the process by which the structure and mineral content of rocks transform in response to changes in temperature, pressure, fluid content (gas or water), or a combination of these. Because the minerals that make up rocks are stable only within certain ranges of temperature and pressure, large changes in these conditions cause minerals to change chemically or to change shape, or both…
Metamorphosis is the transition in overall body pattern that occurs during the life history of some animals following birth or hatching. Two well-known examples are the development of caterpillars into butterflies and tadpoles into frogs. Metamorphosis is considered an indirect form of development, in that a metamorphic animal passes through morphologically distinct stages before reaching the adul…
Meteorology is a science that studies the processes and phenomena of the atmosphere. Meteorology is subdivided into many specialty areas including—but not limited to—physical meteorology (dealing with physical aspects of the atmosphere such as rain or cloud formation, or rainbows and mirages), synoptic meteorology (the analysis and forecast of large-scale weather systems), dynamic me…
The word meteor is derived from the Greek meteron, meaning something high up. Today, however, the term is used to describe the light phenomena produced by the entry of objects into Earth's atmosphere. A meteoroid is defined to be any solid object moving in interplanetary space smaller than a few meters in diameter. A visual meteor, or shooting star, is produced whenever a sandgrain-sized me…
The metric system of measurement is an internationally agreed-upon set of units for expressing the amounts of various quantities such as length, mass, time, temperature, and so on. Whenever we measure something, from the weight of a sack of potatoes to the distance to the moon, we must express the result as a number of specific units: for example, pounds and miles in the English system of measurem…
Mice are small fury mammals, usually living on the ground, with bright beady eyes, rounded ears, and long tails. Mice live all around the world, in almost every habitat, and are a very important part of nature. They are typically vegetarians, often eating seeds and grain, but some species have developed much more comprehensive diets. Known for their high rates of reproduction, females are normally…
In 1887 two American scientists, physicist Albert Michelson and physical chemist Edward Morley, performed an experiment that was designed to detect the motion of Earth through a hypothetical medium known as the luminiferous ether, which was thought to be present throughout space. They made their measurements with a very sensitive optical instrument now called a Michelson interferometer. Their obse…
Microorganisms include prokaryotes like bacteria, unicellular or mycelial eukaryotes e.g., yeasts and other fungi, and viruses, notably bacterial viruses (bacteriophages). Microbial genetics is concerned with the transmission of hereditary characters in microorganisms. Microbial genetics has played a unique role in developing the fields of molecular and cell biology and also has found applications…
Climate is the set of characteristic temperatures, humidities, sunshine, winds, and other weather conditions that prevail over large areas of space for long periods of time. Microclimate refers to a climate that holds over a very small area. Microclimates usually are slight modifications of the main background climate altered by features in the landscape. A forest creates a microclimate within the…
Microorganisms are minute organisms of microscopic dimensions, too small to be seen by the eye alone. To be viewed, microorganisms must be magnified by an optical or electron microscope. The most common types of microorganisms are viruses, bacteria, blue-green bacteria, some algae, some fungi, yeasts, and protozoans. Viruses, bacteria, and blue-green bacteria are all prokaryotes, meaning that they…
A microscope magnifies and resolves the image of an object that otherwise would be invisible to the naked eye, or whose detail could not be resolved using the unaided eye. These objects include such items as human skin, the eye of a fly, cells of a living organism, microorganisms such as bacteria, protozoa and viruses, individual molecules, and atoms. Some of the above objects are large enough to …
Microscopy is the science of producing and observing images of objects that cannot be seen by the unaided eye. A microscope is an instrument which produces the image. The primary function of a microscope is to resolve, that is distinguish, two closely spaced objects as separate. The secondary function of a microscope is to magnify. Microscopy has developed into an exciting field with numerous appl…
Microtechnology is the use of compact, or very small, technical devices. Microtechnology embraces microcomputer parts, space microdevices, microsurgery, and microelectronics. Both microfilm and microfiche, which store information on film, are also examples of microtechnology; microfiche generally stores more than microfilm. The term "micro," derived from the Greek word mikros, meanin…
Microwaves are radio signals with a very short wavelength. Microwave signals can be focused by antennas just as a searchlight concentrates light into a narrow beam. Signals are transmitted directly from a source to a receiver site. Reliable microwave signal range does not extend very far beyond the visible horizon. If microwave signals were visible to the eye, cities would be seen to be crisscross…
Migraine is a type of headache marked by severe head pain lasting several hours or more. Migraine is an intense, often debilitating type of headache. Migraines affect as many as 24 million people in the United States, and are responsible for billions of dollars in lost work, poor job performance, and direct medical costs. Approximately 18% of women and 6% of men experience at least one migraine at…
Migration is the act of moving from one place to another, and is often associated with seasonal movements of animals between their breeding territory and a wintering range. This activity is most readily observed in birds, but has been documented in many other animals as well, including insects, fish, whales, and other mammals. Migration is a complex behavior that involves timing, navigation and ot…
Mildews are whitish fungi that grow on moist surfaces. Some mildews are parasites growing on the surface of plant foliage or fruits. Other mildews grow on the moistened surfaces of materials made from plant or animal tissues, such as wood, paper, clothing, or leather. The downy mildews are in the fungal family Peronosporaceae. These fungi can only exist as parasites, and under conditions favorable…
Milkweeds are various species of perennial plants in the family Asclepiadaceae, a mostly tropical group that contains more than 1,800 species. Most species in this Milkweed in seed. JLM Visuals. Reproduced by permission. family are herbaceous, but others are woody climbers, shrubs, or small trees. The foliage and stems of milkweeds are often succulent, and when they are broken they weep a whi…
On a clear, moonless night, away from the bright lights of the city, the Milky Way is visible—a fuzzy, milky band stretching across the sky. The Milky Way is the plane of our galaxy, a vast spinning carousel of a few hundred billion stars. Our solar system is located about half way between the center and the edge of this 120,000-light-year diameter pancake shaped galactic disk. The "…
A classic experiment in molecular biology and genetics, the Miller-Urey experiment, established that the conditions that existed in Earth's primitive atmosphere were sufficient to produce amino acids, the subunits of proteins comprising and required by living organisms. In essence, the Miller-Urey experiment fundamentally established that Earth's primitive atmosphere was capable of p…
Millipedes are long, cylindrical, segmented, many-legged terrestrial arthropods in the class Diplopoda, in total comprising about 10,000 species. The common name of these animals is derived from the Latin word for "thousand legs," although most species actually have fewer than 200 legs, and some as few as about 60. Millipedes have an elongate, almost cylindrical body form, with two s…
Mimicry may broadly be defined as imitation or copying of an action or image. In biological systems, mimicry specifically refers to the fascinating resemblance of an organism, called the "mimic," to another somewhat distantly related organism, called the "model." The set of mimic and model species involved is often referred to as a mimicry complex. Usually through escap…
Mineralogy is the branch of geology concerned with the study of minerals. A mineral is a naturally occurring, homogeneous solid with a definite chemical composition and a highly ordered atomic structure. A homogeneous substance is one that can be divided into repeating units that are exactly the same. A mineral, by definition, cannot be a liquid or a gas. The chemical composition of a mineral is d…
In ordinary usage, minerals are the natural, non-living materials that compose rocks and are mined from Earth. Examples are metals, gemstones, clays, and ores. The scientific definition of a mineral is more limited. To be considered a mineral, a substance must be solid under ordinary conditions, thus excluding petroleum and water. Minerals must be single, homogeneous (uniform) substances. Therefor…
Mining is the process by which ores or related materials are extracted from the Earth. Ore is defined as a rock or mineral, generally metallic, which can be mined, processed, transported, and sold at a profit. Therefore, the classification of an Earth material as ore depends as much on economics and technology as geology. Nonmetallic substances that are commonly mined but not considered to be ores…
Minks are carnivores in the family Mustelidae, which also includes badgers, weasels, marten, and otters. Mink are closely related to the weasels and ermine, and are included in the same genus (Mustela spp.). Mink have a long, compact body, with relatively short legs, webbed toes, and a long, bushy tail. Minks are larger and more stout than other animals in the weasel group. Male minks are consider…
Minnows are a diverse group of about 1,600 species of small exclusively freshwater fish in the family Cyprinidae. The most familiar of these fish are carp, minnows, tenche, and barbs. Species in the minnow family occur in Africa, Asia, Europe, and North America. In addition, some cultivated and game species have also been introduced to South America, New Guinea, Australia, New Zealand, and some ot…
There are many thousands of minor planets, also termed miniplanets or asteroids, within our solar system. They vary in size from a foot or so in diameter to hundreds of miles in diameter. The majority of asteroids, the main-belt asteroids, circle the Sun in a donutshaped region between the orbits of the planets Jupiter and Mars. Several other asteroid families have been identified, including a lar…
The mint family (Labiatae or Lamiaceae) is a large group of dicotyledonous plants occurring worldwide in all types of climates except in extreme arctic and antarctic conditions. There are about 3,000 species in the mint family and 200 genera. The most diverse groups are the genus Salvia with 500 species, Hyptis with 350 species, and Scutellaria, Coleus, Plectranthus, and Stachys, each with 200 spe…
The longest continuous presence of man in space began in 1989, with the Russian launch of a 20.4 ton cylinder that formed the core of the space station called Mir. By 1996, a total of six modules had been linked to complete the sprawling station, which has been likened to a spaceborne assembly of tinkertoys. To build Mir, the Russians drew from lessons learned with the Salyut stations of the 1970s…
Any reflective material can act as a mirror, because it will throw back enough light to form an image of its surroundings. The surface may be a plane, concave or convex. Planar or flat mirrors present a virtual image that reverses the object being reflected. Curved surfaces act more like lenses, without the aberrations to which glass lenses are prone. Nowadays such surfaces can be made from glass,…
Miscibility means how completely two or more liquids dissolve in each other. It is a qualitative rather than quantitative observation—miscible, partially miscible, not miscible. (To state exactly how miscible two liquids were, a scientist would use the larger concept of solubility, usually in a specific weight or volume per liter of solution.) Two completely miscible liquids will form a hom…
Mistletoe belongs to the family Viscaceae and to the genus Viscum, Phoradendron, or Arceuthobium. Most commonly, mistletoe refers to either the Eurasian shrub Viscum album or one of the American species, such as Phoradendron flavescens. Mistletoe grows on the trunks and branches of a wide variety of trees. Mistletoe is an evergreen, and its stems have numerous branches. The plants have tough, oblo…
Mites are tiny arthropods in the order Acari (or Acarina), which also includes the somewhat larger ticks. The Acari is in the class Arachnida, which also encompasses the spiders and scorpions. Arachnids have four pairs of segmented legs, a body divided into a cephalothorax (that is, a united head and thorax), and abdomen, and they have a simple respiratory apparatus consisting of tracheae and/or b…
Mitosis is a process that sorts and evenly distributes a cell's genetic instructions to the nuclei of two daughter cells during cell division. Mitosis distributes identical DNA instructions to new cells when the old cell divides. Growth is based on cell division and mitosis. Some cells in the body—such as nerve and skeletal muscle cells—cannot divide, and they stay with us for…
A chemical mixture is a collection of molecules or atoms of different types. A mixture is distinguished from a pure substance, which has constant composition (is composed of a only one type of molecule or atom), and a unique set of physical properties (no matter how large or small a sample is observed). The properties of a mixture depend not only on the types of substances that compose it, but als…
A Möbius strip is a twisted surface in space that is made by starting with a rectangular piece of paper, twisting one side through 180° (relative to the opposite side), and then joining it to the opposite side. That is, Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. using the …
Mockingbirds, thrashers, and catbirds are 31 species of medium-sized birds that are sometimes known as mimid thrushes, in the family Mimidae. This is an American family of birds, occurring widely from southern Argentina and Chile, through to southern Canada. The usual habitat of mimids is brushlands, forest edges, shrubby riparian areas, and recently disturbed forests. Mimids range in body length …
The mode of a set of numbers is the number that occurs most frequently. There may be more than one mode. In the set (1,4,5,7), all four numbers are modes. But in the set (1,4,4,6), 4 is the only mode. The mode is one of the measures of central tendency, the others being the mean and the median. …
Modular arithmetic derives from the concept of congruence modulo m, written symbolically as where a and b are any integers and m is a positive integer greater than 1. This means that a - b is divisible by m. For example, since 36 - 16 = 20 is divisible by 5. Likewise 11 ≡38 (mod 9) because 11-38 = -27 is divisible by 9. The concept of congruence was first used by Carl Friedrich Ga…
Mohs' hardness scale provides an index and relative measure of mineral hardness (i.e., resistance to abrasion). In 1812, German geologist Frierich Mohs (1773–1839) devised a scale with specimen minerals that offered comparison of hardness qualities that allows the assignment of a Mohs hardness number to a mineral. Mohs' scale utilizes ten specific representative materials that…
Molds are fungi that are microscopic in size. Even though they can approximate bacteria in size, molds are eukaryotic organisms. That is, their genetic material is enclosed within a specialized membrane that lies in the interior of the organism. Molds are present in virtually every environment that has been examined. Molds grow indoors and outdoors and, depending on the species, can grow year-roun…
A standard unit for counting numbers of particles is needed in chemistry, because atoms and molecules react with one another particle by particle. The amount of a chemical reaction—how much of the chemicals are used up or produced—is determined by the numbers of particles that are reacting. Weighing the chemicals wouldn't tell us anything very meaningful unless we knew how to …
Mole-rats are small, fossorial rodents, which means they spend their entire lives underground in a sealed burrow system. Native to Africa, these little animals are found from the southernmost tip of the continent to about 10 degrees north of the equator. Mole-rats make up the family Bathyergidae, which includes 12 species in five genera (not to be confused with an unrelated family Spalacidae, cont…
Molecular biology is the study of biological molecules and the molecular basis of structure and function in living organisms. Molecular biology is an interdisciplinary approach to understanding biological functions and regulation at the level of molecules such as nucleic acids, proteins, and carbohydrates. Following the rapid advances in biological science brought about by the development and adva…
The molecular formula specifies the actual number of atoms of each element in a molecule. It is important to remember that the molecular formula—in contrast to the simpler empirical formula that specifies only the relative number of atoms or moles present in a compound—identifies the actual number of atoms present in a molecule. More information is required to construct a molecular f…
The arrangement of atoms within a molecule determines not only the shape of the molecule, but also many of its physical properties. In many cases, dealing with molecules of biological significance, the spatial arrangement of atoms determines whether or not the molecule—or the compound containing the molecule—is "biologically active" (e.g., whether a drug containing a pa…
To know more about "molecular weights," one must first become familiar with the concept of "atomic weights." Because an element (e.g., carbon, oxygen, sulfur, etc.) often exists as a mixture of two or more (stable and unstable forms) natural isotopes that have the same number of protons but differ in the number of neutrons, atomic masses of these isotopes are slightly d…
It was not until knowledge about atoms and elements was gained that the make-up of the millions of different substances around us was understood. All the scientific knowledge we have today indicates that these different substances are made from only 92 different kinds of atoms that make up the naturally occurring elements. These atoms are able to join together in millions of different combinations…
Moles are small burrowing animals of the order Insectivora, mammals with teeth designed for crushing the outer shells of insects. The true moles and desmans (water-living moles) make up the family Talpidae, which inhabit most of North America and northern Eurasia. The similar golden moles of Africa, south of the Sahara, make up the family Chrysochloridae. Moles vary from 1-3 in (24-75 mm) in lengt…
Mollusks (phylum Mollusca) are the second largest group of invertebrates (the arthropods being the largest), with over 100,000 species. They are characterized by a head with sense organs and mouth, a muscular foot, a visceral hump containing the digestive and reproductive organs, and an envelope of tissue (the mantle) that usually secretes a hard, protective shell. Practically all of the shells fo…
Momentum is a property of motion that in classical physics is a vector (directional) quantity that in closed systems is conserved during collisions. In Newtonian physics momentum is measured as the product of the mass and component velocity of a body. For massless particles (e.g., photons) moving at the speed of light (v = c) the momentum (p) is equal to Planck's constant divided by the wav…
The monarch or old-world flycatchers are about 400 species of arboreal, insectivorous songbirds that make up the family Muscicapidae. There are three subfamilies in this group: the monarch and paradise flycatchers (Monarchinae), the fantails (Rhipidurinae), and the typical flycatchers (Muscicapinae). Some taxonomists consider these to be separate families, but they are treated as a single group he…
Mongooses are African and Asian carnivores of the family Herpestidae. They are small, long, slender mammals, some of which are most widely known for their willingness and skill in attacking and killing poisonous snakes. In the past, members of the mongoose family have been included with the weasels (family Mustelidae), and sometimes the mongooses are included with civets and genets (family Viverri…
Monitor lizards include 24 living species of large lizards in the genus Varanus, family Varanidae. Monitors inhabit tropical and sub-tropical regions of Africa, Asia, and Australia. Monitors are among the most advanced of the lizards, in terms of achieving an active, predaceous lifestyle. The largest species of monitor, and the world's largest lizard, is the extremely impressive Komodo drag…
Monkeys are tree-dwelling mammals that, along with prosimians, apes, and humans, make up the order Primata of the primates. The primate suborder Anthropoidea includes two different infraorders: the Platyrrhini, comprised of New World monkeys, marmosets and tamarins; and the Catarrhini, the Old World monkeys, apes, and humans. The major division between New World and Old World monkeys, in addition …
Monoculture refers to the practice of cultivating an agricultural species or tree under conditions where other species are absent or virtually absent. This is done in order to lessen the intensity of competition on growth of the desired crop species. …
A monomer is a molecule or compound, usually containing carbon, with a relatively low molecular weight and simple structure; monomers form the fundamental building blocks of polymers, synthetic resins, and elastomers. Thus, vinylidene chloride is the monomer from which polyvinylidene chloride is made, and styrene is the monomer from which polystyrene resins are produced. One of the simplest monome…
The identification of MSG began with the isolation of glutamic acid from a mass of wheat protein, called gluten, in 1886. The chemical structure of glutamic acid, a naturally occurring amino acid, was later identified in 1890. The flavor enhancing ability of MSG was discovered by the Japanese chemist Ikeda Kibunae (1864-1936). From a kelplike seaweed, which was traditionally used to add flavor to …
The order Monotremata (one-holed creatures) is comprised of two families, the Ornithorhynchidae, including the platypus, and the Tachyglossidae, including the long- and short-beaked spiny anteaters or echidnas. Monotremes are found only in Australia, Tasmania, and New Guinea. Monotremes are a derivative of an ancient mammal stock but there is no direct evidence of what it might have been. Monotrem…
A monsoon is a seasonal change in the direction of the prevailing wind. This wind shift typically brings about a marked change in local weather. Monsoons are often associated with rainy seasons in the tropics (the areas of Earth within 23.5° latitude of the equator) and the subtropics (areas between 23.5° and about 35° latitude, both north and south). In these areas, life is c…
Earth's moon is a roughly spherical, rocky body orbiting Earth at an average distance of 238,000 mi (382,942 km). Its diameter is about one-fourth Earth's diameter. Compared to moons of other planets, this is a The first footprint on the moon's surface, made by astronaut Neil A. Armstrong on July 20, 1969, shows the fineness of the lunar soil. The soil is produced when moon…
The mooneye is a freshwater fish with very large eyes. Usually measuring between 16-21 in (40-51 cm) long, it has a deep, laterally thin body. Because it generally resembles a shad or herring, the mooneye has acquired such common names as toothed herring, big-eyed shad, or white shad. Mooneyes can be distinguished from shads and herrings by the presence of well-developed teeth on their jaws and to…
The moose (Alces alces), also known as elk in Europe, is a horse-sized, northern species of deer that occurs in the boreal and north-temperate forests of both North America and Eurasia. At one time, the Eurasian and American moose were considered to be separate species, but these animals are fully interfertile and are now thought to be the same species. However, there are many geographically disti…
Morphine has similar painkilling properties to endorphins and enkephalins, a group of amino acid compounds produced in the pituitary gland. The molecular structure of morphine is so much like that of endorphins that it is able to bind to and occupy specialized receptor sites located in various pain centers in the central nervous system. Morphine also alters the release of neurotransmitters. The pe…
Mosquitoes belong to an order of insects called Diptera, which includes the common house fly. All together, the Diptera order, the flies, are responsible for carrying diseases to more than 50% of the world's population. Some 120,000 species of Dipteran flies have been catalogued, which includes more than 2,500 species of mosquitoes. More than 15,000 species of flies and 150 species of mosqu…
The mosses are the largest of the three classes in the plant phylum Bryophyta. They have been divided into three subclasses: the true mosses (Bryideae), the peat mosses (Sphagnideae), and the granite mosses (Andreaeideae). The familiar small, green, and "leafy" moss plant is known as the gametophyte. This haploid multicellular phase is dominant in the moss life cycle. The mosses are …
Moss animals, polyzoa, or ectoprocts (phylum Bryozoa) are some of the most abundant and widespread organisms in the animal kingdom. A considerable amount of confusion has surrounded the taxonomic arrangement of the moss animals. A similar, although not so diverse, group of animals known as entoprocts have now been distinguished in their own phylum, Entoprocta. The vast majority of moss animals are…
Mössbauer effect was first reported by Rudolph Mössbauer in 1958. Three years later, he won the Nobel Prize with his discovery. Since then, it is believed that nuclear γ-ray emission and absorption process can take place in recoil-free fashion. In reality, of course we have both recoil and recoil-free events. Mössbauer also utilized the Doppler (velocity) shift to modu…
Moths, along with butterflies, belong to the order Lepidoptera, the second largest order of insects. They possess two pairs of wings covered with microscopic, overlapping scales having distinctive colors and patterns. The body and legs are covered with scales, or with long, hairlike bristles. The adult lepidopteran lacks mandibles (found in most other insects); it feeds on liquids, mostly nectar, …
The process by which something moves from one position to another is referred to as motion; that is, a changing position involving time, velocity and acceleration. Motions can be classified as linear or translational (motion along a straight line), rotational (motion about some axis), or curvilinear (a combination of linear and rotational). A detailed description of all aspects of motion is called…
Motion pictures, also called film, cinema or movies, are a series of images, recorded on strips of film, that create the illusion of continuous motion. A series of rapidly shown images appear to move because of a phenomena called the persistence of vision. The eye does not react instantly to changes in its field of vision. In fact, the eye continues to "see" an image for 1/10-1/20 of…
The megapodes, or moundbuilders, are a fascinating group of birds found in Australia, New Guinea and its surrounding islands, eastern Indonesia, and the Philippines. Also known as "thermometer birds," scrubfowl, or brush turkeys, 22 species are recognized (seven genera) in the family Megapodiidae. A wide range of habitats are occupied by these species, ranging from semi-arid scrublan…
Mounds are artificially constructed heaps or banks of earth built to contain sacred objects. Their basic construction is the same all over the world: a pit is dug and lined, and the sacred contents are deposited and covered with earth. Sometimes these objects are sprinkled with red ocher, a pigment used to make paint, perhaps as a way to revive the spirits thought to dwell within them. If we were …
A mountain is a large-scale topographic feature that is set apart from the local landscape by being much higher in elevation (topographic means having to do with the shape of the land surface). When the edge of a plate of Earth's crust runs over another plate, forcing the lower plate deep into Earth's elastic interior, a long, curved mountain chain of volcanos usually forms on the fo…
Mousebirds, or colies, are six species of non-migratory birds of sub-Saharan Africa, making up the family Colidae. Their usual habitat is open brushlands, savannas, and forest edges. Mousebirds are about 11-14 in (29-36 cm) long, at least one-half of which is made up of their long tail. This structure is composed of ten feathers of variable, but graduated length, the longest being in the center of…
The mulberry family occurs primarily in tropical and semi-tropical regions, and includes a wide variety of herbs, shrubs, and trees, characterized by a milky sap and reduced, unisexual flowers. This family includes 40 genera and 1,000 species, of which 500 species are members of the fig genus, Ficus. The Moraceae is a member of the order Urticales, class Magnoliopsida (the dicotyledons), division …
Multiple personality disorder (MPD) is a chronic and recurrent emotional illness. A person with MPD plays host to two or more personalities. Each identity has its own unique style of viewing and understanding the world and may have its own name. These distinct personalities periodically control that person's behavior as if several people were alternately sharing the same body. Because those…
Multiplication is often described as repeated addition. For example, the product 3 × 4 is equal to the sum, 4 + 4 + 4, of three 4s. The law on which this is based is the distributive law: a(b + c) = ab + ac. In this instance, the law is applied to 4(1 + 1 + 1), which gives 4(1) + 4(1) + 4(1) or 4 + 4 + 4. When one or both of the multipliers are not natural numbers, the law still applies, 0.…
The Murchison meteorite was a meteorite that entered Earth's atmosphere in September 1969. The meteor fragmented before impact and remnants were recovered near Murchison, Australia (located about 60 mi [96.5 km] north of Melbourne). The fragments recovered dated to nearly five billion years ago—to the time greater than the estimated age of Earth. In addition to interest generated by …
Muscle relaxants are drugs that are administered to relax muscles. They are given to relieve the discomfort of muscle spasm or involuntary muscle contracture and also in cases of surgery to relax muscles and provide easier access for the surgeon. Some nonprescription drugs are available to combat painful contraction of the uterus during a woman's menstrual period. Muscles can be divided int…
The muscular system is the body's network of tissues for both conscious and unconscious movement. Movement is generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, are involved in voluntary movements such as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntary muscles…
Mushrooms are the fruiting bodies of certain species of higher fungi. The vegetative tissues of these fungi consists of immense lengths of microscopic, thread-like hyphae, and their aggregations known as mycelium, which grow in surface soils, organic debris, and in association with plant roots. Strictly speaking, a mushroom is the sporulating or fruiting body of a fungus in the division Basidiomyc…
Muskoxen or muskox (Ovibos moschatus) are a species of large mammal in the family Bovidae, which also includes cattle, buffalo, antelope, sheep, and goats. The muskox is anatomically intermediate between sheep and cattle, and there has been taxonomic debate over which of these two groups the muskox should be more closely aligned with. As a result, its genus, Ovibos, is a composite of the scientifi…
The muskrat or musquash (Ondatra zibethicus) is a relatively large, amphibious rodent that is native to North America. The northern range of the muskrat reaches as far as the limits of the boreal forest from Alaska to Labrador and Newfoundland. The southern range of the muskrat extends through much of the United States as far south as northern Baja California, although not in the coastal plains of…
The mustard family, or Brassicaceae, contains about 3,000 species of plants. These plants occur widely on all continents except Antarctica and in a wide range of habitats from tundra and desert to forests of all types. Most species in the mustard family occur in the temperate zones, and many occur in the alpine or arctic tundra. The flowers of members of the Brassicaceae have four petals arranged …
Mustard gas is a substance used in chemical warfare. It is the popular name for the compound with the chemical designation 1,1-thiobis(2-chloroethane) (chemical formula: Cl-CH2-CH2-S-CH2-CH2-Cl). Mustard gas has a number of other names by which it has been known over the years, including H, yprite, sulfur mustard and Kampstoff Lost. Because the impure substance is said to have an odor similar to t…
In the living cell, deoxyribonucleic acid (DNA) undergoes frequent chemical change, especially when it is being replicated. Most of these changes are quickly repaired. Those that are not, result in a mutation. Thus, mutation is a failure of DNA repair. Mutagens are chemicals or physical factors (such as radiation) that increase the rate of mutation in the cells of bacteria, plants, and animals (in…
Mutagenesis is the induction of genetic change in a cell by the alterations in the cell's genetic material (usually deoxyribonucleic acid [DNA]). This change or alteration can subsequently be inherited from one cell to the next. While many mutations are benign, some can be detrimental and cause human genetic disease. If the mutation occurs in a gamete (sex cell), the genetic alteration may …
A mutation is the alteration in the composition in deoxyribonucleic acid (DNA). Mutations that are inherited can change the character of a species. Living organisms rely upon change as a means of adapting to new environments or conditions. Change is a crucial survival mechanism. Evolution hinges on the appearance and inheritance of mutations. Mutations are the source of genetic variation in humans…
Mutualism is a biological interaction that is beneficial to both parties. Most mutualisms are facultative, meaning the partners can successfully live apart. However, some mutualisms are so intimate that the interacting species can no longer live without each other; they have a mutually obligate interdependence. Many mutualisms are fascinating in their intricacies and reciprocal usefulness as is ap…
A "fungus root" or mycorrhiza (plural: mycorrhizae) is a fungus living in a mutually beneficial symbiosis (or mutualism) with the roots of a vascular plant. In this intimate relationship, the fungus benefits from access to energy-containing carbohydrates, proteins, and other organic nutrients excreted by, or contained in, the roots while the host plant benefits from an enhanced suppl…
Mycotoxins are toxic substances produced by molds and fungi, such as mushrooms. These toxic substances, known as secondary metabolites, are by-products of metabolism that are inessential to fungal growth. Although some mycotoxins can be used for medicinal purposes, most are poisonous if eaten in sufficient quantity. Numerous mycotoxins have been studied and identified. Of particular interest are m…
Mynah or myna birds are species in the family Sturnidae, which also includes many species of starlings. The distinction between starlings and mynahs is not always clear, and these common names are sometimes used interchangeably. However, as considered here, the mynahs are tropical, Asian species, the most prominent of which are in the genus Acridotheres and Gracula. The word mynah is derived from …
In both the New and Old Worlds many genera of the myrtle family (Myrtaceae) unfurl their waxy, leathery leaves. Containing both trees and shrubs, this angiosperm family takes its name from the shrub Myrtus, which is found near the Mediterranean, in North Africa, and in South America. Other well known genera from the Myrtaceae include ornamentals such as Leptospermum (Australian tea tree), Eucalypt…
Nanotechnology describes technologies where the component parts can measure just a few atomic diameters (generally around a millionth of a millimeter). The general goal of nanotechnology research programs is to reduce complex and sophisticated machinery into very small operational units. Nanotechnology involves the development of techniques to build machines from atoms and molecules. The name come…
A narcotic is a substance that produces insensibility, or a stuporous state. The most notable characteristics of narcotics are their ability to decrease the perception of pain and alter the reaction to pain, and their extremely addictive properties. Narcotics often induce a state of euphoria or extreme well being. The word narcotic is derived from the Greek word narké (meaning stupor), and …
Natural fibers may be of animal, vegetable, or mineral origin. Although the annual production of vegetable fibers outweighs that of animal or mineral fibers, all have long been useful to humans. Animal hair fibers consist of a protein known as keratin. It has a composition similar to human hair. Keratin proteins are actually crystalline copolymers of nylon, where the repeating units are amino acid…
Natural gas is a mixture of hydrocarbons (molecules that contain only carbon and hydrogen) and gases (most notably methane, ethane, propane, and butane) that exist naturally in rocks beneath the surface of the earth. It is widely used as a heating source, and in some cases specific portions of the natural gas are used as starting materials in industrial processes. Natural gas is the product of the…
The natural numbers are the ordinary numbers, 1, 2, 3,... with which we count. Sometimes they are called "the counting numbers." They have been called "natural" because much of our experience from infancy deals with discrete objects such as fingers, balls, peanuts, etc. We quickly, if not naturally, learn to count them. The mathematician Kronecker is reported to have sa…
Nautical archaeology is the branch of archaeology concerned with the excavation, identification, and study of the remains of sunken ships. The techniques used in nautical archaeology can be applied to the study of submerged ports, lost cities, sacrificial wells, and other underwater sites. The ocean conceals a vast number of unexplored, and potentially valuable, archaeological sites. But the techn…
Advanced by Massachusetts Institute of Technology Professor Richard P. Binzel in 1995, the Torino scale is a revision of the Near-Earth Object Hazard Index. In 1999, the International Conference on Near-Earth objects adopted the scale at a meeting in Turino (Turin), Italy (from which the name of the scale is derived). The Torino scale is used to portray the threat to Earth of an impact with a part…
Nectar is a sweet, pleasant-tasting liquid secreted by nectaries. These are specialized organs located inside flowers, and also outside. For example, the "glands" on leaf stalks of cherry (Prunus) and wattle (Acacia) trees are nectaries. The nectar produced inside flowers attracts crawling or flying insects, birds, or mammals. As a result of feeding on nectar, these visitors may tran…
Negative is a term in mathematics that usually means "opposite." An electron's charge is called negative not because it is "below" but because it is opposite that of a proton. A surface with negative curvature bulges in from the point of view of someone on one side of the surface but bulges out from the point of view of someone on the other side. A line with nega…
Neptune is the eighth planet from the Sun and about four times the size of Earth. Astronomers consider Neptune to form with Uranus a subgroup of the Jovian planets (Jupiter, Saturn, Uranus, and Neptune). Neptune and Uranus are similar in size, mass, periods of their rotation, the overall features of their magnetic fields, and ring systems. However they differ in the structure of their atmospheres …
Nerve growth factor (NGF) is a polypeptide, a molecule composed of several amino acid units that has a protein-like behavior, but is not as complex as a protein in structure. NGF increases the growth of nerve cells, especially those in the peripheral nervous system, and directs the growth and orientation of nerve cell axons (processes which carry impulses away from the nerve cell body to adjoining…
In contrast to the endocrine system that achieves long-term control via chemical (hormonal) mechanisms, the nervous system relies on more rapid mechanisms of chemical and electrical transmission to propagate signals and commands. The rapid conduction of impulses is essential in allowing the nervous system to mediate short-term and near immediate communication and control between various body syste…
The nervous system coordinates behavior and helps to maintain the internal stability of animals. It may be as simple as the nerve net of Cnidarians or as complex as the centralized system of mammals. In all nervous systems the functional unit is the nerve cell or neuron, a cell specialized to transmit and receive a stimulus. …
When humans are in good health, the nervous system and musculature work together so smoothly there is little awareness of how efficiently this complicated biochemical machine functions. Neuromuscular diseases include a vast and bewildering array of related and unrelated disorders that have a certain similarity of symptoms in that both nerves and muscles are usually impaired. This term is usually a…
Neurons are nerve cells (neurocytes), which, together with neuroglial cells, comprise the nervous tissue making up the nervous system. The neuron is the integral element of our five senses and of countless other physical, regulatory, and mental faculties, including memory and consciousness. A neuron consists of a nerve cell body (or soma), an elongated projection (axon), and short branching fibers…
Neuroscience is the study of the nervous system and its components. Neuroscientists may examine the nervous systems of humans and higher animals as well as simple multicellular nervous systems, or investigate nervous phenomenon at the cellular, organelle, or molecular level. Neuroscience principally originated with three European scientists working at the end of the nineteenth and the beginning of…
Neurosurgery is a medical specialty concerned with the diagnosis and treatment of the central and peripheral nervous system. The nervous system is composed of the brain, spinal cord and spinal column, as well as the nerves that travel through all parts of the body (hands, legs, arms, face). Neurosurgeons treat degenerative and congenital diseases of the spine, pain from pinched nerves in the neck,…
Neurotransmitters are chemical agents secreted at the end of axons of nerve cells that diffuse across the synaptic gap and transmit information to adjoining cells such as neurons, muscle cells, and glands, by altering their electrical state or activity. There are many neurotransmitters with a variety of structures and functions; two of the principle ones are acetylcholine and norepinephrine. Since…
In chemistry, the process in which an acid and a basereact with each other to form a salt and water is known as neutralization. Just as a neutral color contains no vivid colors and a neutral person has no strong opinions, a neutralization reaction between an acidic and a basic (alkaline) solution lessens the acidic and basic properties of both solutions. Taking an antacid to settle a sour stomach,…
Neutrinos are elusive subatomic particles that result from certain nuclear reactions. Neutrinos have no electrical charge and only a tiny mass, travel at nearly the speed of light, come in three types—electron neutrinos, muon neutrinos, and tau neutrinos—and barely interact with normal matter. Because their interaction rate is so low, neutrinos produced in the core of the Sun fly dir…
The atom's structure has a nucleus at the center, surrounded by a cloud of electrons. Inside the nucleus are two types of particles, neutrons (electrically neutral) and protons (positively charged). Under special experimental conditions, neutrons can be released from the nucleus. They are useful for creating new radioactive materials or for producing large amounts of nuclear energy. The ato…
Neutron activation analysis is an analytical technique for determining the elements present in a material as well as the amount of each element in the sample. The technique is based on a well-known reaction from nuclear chemistry. When an element is bombarded with neutrons, some of the atoms of that element may capture neutrons and incorporate them into their nuclei. Those atoms that do so have th…
A neutron star is the dead remnant of a massive star. A massive star ends its life as a supernova, a catastrophic explosion that flings the star's outer layers into space, leaving only the core behind. If the mass of the core is between 1.4 and 2.5 times the mass of the Sun, it will become a neutron star, a solid mass of neutrons a hundred trillion times more dense than water. Neutron stars…
The New World monkeys of Central and South America belong to the family Cebidae and to the family Callitrichidae (the marmosets and tamarins). The Cebidae, or capuchin-like monkeys, are distinguished from the marmosets and tamarins by their possession of nails instead of claws on most fingers and toes, and three molars instead of two on either side of each jaw. Finally, cebids tend to give birth t…
Earthly and heavenly motions were of great interest to Newton. Applying an acute sense for asking the right questions with reasoning, Newton formulated three laws which allowed a complete analysis (mathematical) of dynamics, relating all aspects of motion to basic causes, force and mass. So influential was Newton's work that it is referred to as the first revolution in physics. …
Newts are lizard-shaped animals with a tail in the amphibian order Caudata (or Urodela), in the superfamily Salamandroidea, which also includes the salamanders. The distinction between newts and salamanders is not always obvious since both have a tail in the larval stage and the adult stage. However, newts do not have costal grooves on the sides of their body, they are less slippery than salamande…
The niche is an important ecological notion that considers the role that an organism or species plays in its community. Important aspects of the niche are environmental tolerance, use of resources, activities, and interactions with other organisms. Niche is an integrating concept because it considers organisms within the context of their biological tolerance of environmental extremes but is signif…
Nicotine is so highly addictive that the American Psychiatric Association includes it in their diagnostic manual under substance dependence. Nicotine addiction is also very difficult to break—only 5% of those who attempt to quit smoking are successful on their first try, and only 3% can kick the habit for a whole year. Only 10% of smokers are not addicted. To relieve the physical and psycho…
Night vision enhancement scopes or devices enable machines or people to form images when illumination in the visible band of the electromagnetic spectrum is inadequate. Although it is not possible to form images in absolute darkness, that is, in the absence of any electromagnetic radiation whatsoever, it is possible to form images from radiation wavelengths to which the human eye is insensitive, o…
The family of plants known as nightshades is also known as the Solanacene. It is a large group of plants composed of more than 2,000 species and 75 different genera. Most nightshades are herbs, but some species are shrubs, vines, or trees. Most of the members of the nightshade family are native to parts of Central and South America, but about 100 nightshades can be found in North America. Some spe…
Nitric acid is also formed from the reaction of nitrogen oxides produced during the combustion (burning) of fossil fuels in automobile engines. These nitrogen oxides react with water in the atmosphere and form nitric acid, one cause of acid rain. High levels of nitrates in drinking water can contribute to the formation of nitrosamines, a group of carcinogenic (cancer causing) compounds. …
Nitrification is an aerobic microbial process by which specialized bacteria oxidize ammonium to nitrite and then to nitrate. Nitrification is a very important part of the nitrogen cycle, because for most plants nitrate is the preferred chemical form of nitrogen uptake from soil or water. …
Nitrogen is the non-metallic chemical element of atomic number 7, with a symbol N, atomic weight 14.0067, specific gravity 0.96737 (compared to air), melting point -345.74°F (-209.86°C), boiling point -320.44°F (-195.8°C). Nitrogen is a non-metallic element located in group 15 of the periodic table. It has two stable isotopes: nitrogen-14, with an abundance of 99.634%, …
Nitrogen is a critically important nutrient for organisms, being one of the most abundant elements in their tissues, and an integral component of many biochemicals, including amino acids, proteins, and nucleic acids. The availability of biologically useful forms of nitrogen is a common limiting factor in the productivity of plants. This is especially true of plants growing in terrestrial and marin…
As late as the 1950s, most of the sounds on this planet were probably still of natural origin rather than derived from technological sources. Today, however, the opposite appears to be true. Cars, trucks, lawn mowers, leaf blowers, chain saws, power drills, television, radio, video games, computers... the list of noise makers in our modern life is almost endless, and our world keeps getting noisie…
Non-Euclidean geometry refers to certain types of geometry which differ from plane and solid geometry which dominated the realm of mathematics for several centuries. There are other types of geometry which do not assume all of Euclid's postulates such as hyperbolic geometry, elliptic geometry, spherical geometry, descriptive geometry, differential geometry, geometric algebra, and multidimen…
Non-point sources refer to situations in which there are numerous, relatively small sources of emission of gases, metals, pesticides, nutrients, or other substances into the environment. Collectively, these many sources can comprise a regional emission of large quantities of Non-point sources. Illustration by Hans & Cassidy. Courtesy of Gale Group. pollutants into the environment. Ho…
A nonmetal is a chemical element that generally does not conduct heat or electricity very well, is usually a solid or a gas at normal temperatures, and (for solids) is difficult to reshape by pounding or beating. Nonmetals include elements such as carbon, sulfur, oxygen, and nitrogen. They are normally defined in contrast to metals, which are bright, shiny, solid elements (with one exception) that…
The landmass occupied by the present-day countries of Canada, the United States, and the Republic of Mexico make up North America. Greenland (Kalaallit Nunaat), an island landmass to the northeast of Canada, is also included in North America, for it has been attached to Canada for almost two billion years. Plate tectonics is the main force of nature responsible for the geologic history of North Am…
Nova is a Latin word meaning new, and it describes the appearance of a seemingly "new star" in the sky, a brilliant object in a place where there was previously only a very faint star, or perhaps nothing at all. A nova is a phenomenon that happens in a binary star system containing a white dwarf and a stable companion star. A white dwarf is the dead, collapsed core of a star that for…
Procaine hydrochloride was first synthesized by Einhorn in 1905 as a substitute for cocaine, the first local anesthetic. Cocaine, an alkaloid obtained from the leaves of the coca plant, Erythroxylon coca, is highly addictive and toxic. Procaine hydrochloride replaced cocaine as a local anesthetic because it is much less toxic, cheaper and easier to produce, and it is easier to sterilize. In contra…
Nuclear fission is a process in which the nucleus of an atom splits, usually into two pieces. This reaction was discovered when a target of uranium was bombarded by neutrons. Fission fragments were shown to fly apart with a large release of energy. The fission reaction was the basis of the atomic bomb, which was developed by the United States during World War II. After the war, controlled energy r…
Nuclear fusion is the process by which two light atomic nuclei combine to form one heavier atomic nucleus. As an example, a proton (the nucleus of a hydrogen atom) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of "heavy hydrogen"). In general, the mass of the heavier product nucleus is less than the total mass of the two lighte…
Nuclear magnetic resonance, NMR, is a process in which the nuclei of certain atoms absorb energy from a magnetic field that gyrates, or has a direction which rotates about some fixed axis. NMR provides a means of measuring nuclear properties using ordinary electromagnetic fields rather than high-energy particles as in a particle accelerator. Its applications range from nuclear measurements to medi…
Nuclear medicine is a medical specialty that uses radioactive materials, called radionuclides, to help diagnose and treat a wide variety of diseases, and for biomedical research. The development of nuclear medicine reflects the advances in the fields of nuclear physics, nuclear chemistry, and later, molecular biology. While there was considerable research in the nuclear sciences during the first p…
Nuclear power is any method of doing work that makes use of nuclear fission or nuclear fusion reactions. In its broadest sense, the term refers to both the uncontrolled release of nuclear energy, as in fission or fusion weapons, and to the controlled release of energy, as in nuclear power plants. Most commonly, however, the expression "nuclear power" is reserved for the latter. Appro…
A nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion. At the present time, all commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium (the fuel) are broken apart into smaller nuclei, with the release of energy. It is theoretically possible to construct reactors that…
Nuclear weapons are explosive devices that release nuclear energy. An individual nuclear device may have an explosive force equivalent to millions of tons (megatons) of trinitrotoluene (TNT, the chemical explosive traditionally used for such comparisons), and is more than enough to inflict devastating physical damage to a city. The destructive power of nuclear weapons derives from the core of the …
Nuclear winter is a theory estimating the global climatic consequences of a nuclear war: prolonged and worldwide cooling and darkening caused by sunlight-blocking smoke and soot entering the atmosphere. The term nuclear winter was first defined and used by American astronomer Carl Sagan (1934–1996) and his group of colleagues in their 1983 article (later referred to as the TTAPS-article, fr…
Two nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are found in living things which serve to store, translate, and pass on the genetic information of an organism to the next generation. Nucleic acids are universal to all life, in eukaryotic and prokaryotic cells, as well as in viruses. The mitochondria of eukaryotic cells also contain some DNA, known as mitochondrial DNA. N…
Nucleon is a generic word for the heavy particles that make up the atomic nucleus: the protons and neutrons. It is like the generic word fruit, used to include apples, oranges, and many others, except that the class of nucleons contains only two members. The use of the ending -on for the names of subatomic particles began with electron, a word that was coined in 1891 by the Irish physicist George …
The nucleus is a large membrane-bound cell organelle which houses the chromosomes and which occupies roughly 10% of the volume of all eukaryotic cells. The nucleus is separated from the rest of the cell and the cytoplasm by a double membrane known as the nuclear envelope. The outer layer of the nuclear membrane is studded with small openings called nuclear pores, which allow for the controlled mov…
Numbats are the sole members of the family Myrmecobiidae. "Mumbat" is the aborigine name for these small marsupial mammals, otherwise known as banded anteaters. They are slightly larger than rats and weigh about 1 lb (0.5 kg). Considered one of the most attractive marsupials, their general color varies from grayish-brown to reddish-brown, broken by several prominent white bars across…
Number theory is the study of natural, or counting numbers, including prime numbers. Number theory is important because the simple sequence of counting numbers from one to infinity conceals many relationships beneath its surface. Number theory is full of famous formulas that illustrate the relationships between whole numbers from 1 to infinity. Some of these formulas are very complicated, but the …
Numeration systems are structured methods or procedures for counting in order to determine the total units in a collection. Numeration systems consist of counting bases (base 2, base 5, base 10, base 20, etc.) and some form of representation. This representation might be as primitive as the hand signals used in aborigine cultures and in the trading pits of stock exchanges, or it could be written o…
A nut is a type of fruit. Like all fruits, a nut develops from the ovary of a mature, fertilized flower. A nut is thick, dry, hard, and partly or entirely enclosed by a husk. A nut is indehiscent, in that it does not open along a naturally occurring seam, and remains closed even when fully mature. A nut is a simple fruit, in that it is derived from the pistil of a single flower. Although a nut con…
Nuthatches are small, short-tailed, large-headed birds in the family Sittidae in the order Passeriformes, the perching birds. There are 25 species of nuthatches, occurring on all continents except South America, Africa, or Antarctica. Most species of nuthatches are forest birds which clamber over the bark of trees seeking insects, spiders, and arthropod eggs. Nuthatches can climb in any direction-…
The nutmeg family, Myristicaceae, order Magnoliales, consists of evergreen trees of the tropical rain forests. The genus Myristica includes about 120 species, the best known of which is the nutmeg tree (Myristica fragrans). The nutmeg tree is native to the Moluccas, a group of islands in eastern Indonesia, also known as the Spice Islands. However, nutmeg is now cultivated in much of southern Asia,…
Nutrient deficiency diseases occur when there is an absence of nutrients which are essential for growth and health. Lack of food leading to either malnutrition or starvation gives rise to these diseases. Another cause for a deficiency disease may be due to a structural or biological imbalance in the individual's metabolic system. There are more than 50 known nutrients in food. Nutrients ena…
Nutrients are any chemicals that are required for life. Nutrients can be of two basic types: (1) inorganic substances absorbed by autotrophic organisms such as plants and certain microorganisms for use in their synthetic reactions and metabolism; or (2) biomass ingested as nourishment by animals and heterotrophic microorganisms. Plants absorb a wide range of mineral nutrients, which they utilize i…
Nutrition is the means by which organisms obtain and use nutrients. Nutrition is also the determination of the kinds and quantities of substances (nutrients) needed by organisms to sustain life. Some organisms such as plants require only a supply of light, water, and a few other molecules and ions in order to thrive, and are known as autotrophs, or self nourishers, for they literally build their o…
The nux vomica tree (Strychnos nux-vomica) is a species in the tropical family Loganiaceae. The range of the nux vomica in cultivation extends from Sri Lanka, India, southern China, southeast Asia, and northern Australia. The nux vomica grows as tall as 49.2 ft (15 m). The nux vomica has roundish, opposite leaves and attractive white flowers. The roughly spherical fruits of the nux vomica are larg…
Oaks (Quercus spp.), members of the Beech family (Fagacea), are trees and shrubs having simple, alternate leaves found throughout the world. Characterized by their strong, complex wood, wind-pollinated flowers, fruits called acorns, and their ability to live for centuries, oaks have played an important role in temperate landscapes. Of the 500 species in the genus Quercus, approximately 90 are foun…
Obesity can be defined as a body weight that is greater than 30% above a favorable target weight. In order to fit the definition of obesity, the excess weight must be due to adipose, or fat, tissue. Muscle mass does not account for the weight attributed to obesity. Therefore, a body-builder with tremendous muscle mass for example is not, by definition, obese. A deleterious condition, obesity is ha…
The main concern of psychiatrists and therapists who treat people with obsessions is the role they play in a mental illness called obsessive-compulsive disorder (OCD). Obsessions need to be distinguished from compulsions in order to understand how they interconnect with compulsive behavior and reinforce this debilitating illness. In psychiatric literature, obsessions are defined as disruptive thou…
Oceans are large bodies of saltwater that surround Earth's continents and occupy the basins between them. Ocean basins are the part of the seafloor that lies beyond the margins of the continents, generally in water deeper than 600 ft (183 m). Therefore, an ocean is both larger in area and deeper than a sea. It may be that most of the water on Earth today has been cycling between the oceans,…
Ocean basins are that part of Earth's surface that extends seaward from the continental margins, ranging from an average water depth of about 6,500 ft (2,000 m) down into the deepest trenches. The ocean basins constitute one of the two major topographic features of Earth's surface, the other being the continents. Ocean basins cover about 70% of the total sea area and about half of th…
The ocean sunfish (Mola mola), also called the headfish, is so named because of its unique shape: it looks as if it is all head and no body. The ocean sunfish is a very large species that lives in tropical and temperate waters. The ocean sunfish has a flattened, oval body that may measure 11 ft (3.5 m) in length and weigh as much as 2,000 lb (1,000 kg). In contrast to its huge size, it has a verte…
Ocean zones are layers within the seas that contain distinctive plant and animal life. They are sometimes referred to as ocean layers or environmental zones. A system of zonation frequently used by oceanographers grew out of suggestions made by Joel Hedgpeth in 1957. According to that system, the ocean environment is first divided into two broad categories, known as realms, the benthic realm, cons…
Oceanography, the study of the oceans, is a combination of the sciences of biology, chemistry, geology, physics, and meteorology. Ancient explorers of the ocean were sailors and fishermen who learned about marine biology by observing the sea life and discovering when it was most plentiful. They observed the effects of wind, currents, and tides, and learned how to use them to their advantage, or to…
The octet rule allows chemists to predict the placement of electrons around the nucleus (electron orbitals), the identification of electrons added or lost during chemical reactions, and the chemical reactivity of atoms based upon their particular electron configuration. The octet rule is used when drawing Lewis dot structures and disgraming electron configurations. The octet rule is used to descri…
The octopus is an invertebrate in the class Mollusca (the molluscs), which also includes snails, clams, and squid. Octopi are cephalopod molluscs which are generally considered to be the most advanced members of the class. There are about 220 species of octopus. Octopi are found in every ocean of the world, ranging in size from a tiny Philippine species barely an inch across to giant specimens tha…
Ohm's law is a relationship between the voltage across an electric circuit, the electrical resistance in the circuit, and the current in the circuit. This law is named after its discoverer, Georg Simon Ohm. Ohm found that for most electric circuits, the voltage across the circuit was equal to the current flowing through the circuit times the electrical resistance of the circuit. For the sam…
Petroleum is a critically important natural resource. However, petroleum is often mined in places that are far away from the regions where most of its consumption occurs. Accordingly, petroleum must therefore be transported in large quantities, mostly by oceanic tankers, barges on inland waters, and both subsea and overland pipelines. Any of these transportation systems can release pollution throu…
Oil and natural gas has been found in geologic strata of Earth from the surface to depths exceeding 30,000 ft (9,144 m). Bogs and seeps in the ancient world were the initial source of oil and gas. As advancing economic systems and industries emerged with the development of nations and expanding populations, the need for plentiful and more efficient sources of energy were required. Hydrocarbon base…
Old-growth forests are natural ecosystems dominated by large, old trees, usually of a mixed species composition, and with all ages present in the community. Old-growth forests also contain many scattered, dead trees, both standing and lying on the forest floor. In the tropics, these forests are threatened by conversion to agriculture and by other disturbances, while old-growth forests in the tempe…
The olive family is a family of flowering plants known to botanists as the Oleaceae. The Oleaceae have about 25 genera and over 500 species. Most species are native to temperate and tropical regions of the Northern Hemisphere. The best known trees of this family are olive and ash, while the most familiar shrubs are privet, lilac, and golden bell, all popular ornamental plants. …
An omnivore is any animal that is a generalist feeder, consuming a wide variety of foods that can include both animal and plant matter. Because they have attributes of both carnivores and herbivores, omnivores have relatively diverse linkages within ecological food webs. Some examples of omnivorous animals are pig and bear, both of which will eat a remarkably wide range of plant and animal product…
In mathematics, one-to-one correspondence refers to a situation in which the members of one set (call it A) can be evenly matched with the members of a second set (call it B). Evenly matched means that each member of A is paired with one and only one member of B, each member of B is paired with one and only one member of A, and none of the members from either set are left unpaired. The result is t…
Opahs (Lampris guttatus) are a bony ray-fin oceanic fish, with a world-wide distribution, in the Family Lamprididae of the Order Lampridiformes. Opahs are known only from specimens found stranded on the beach or captured accidentally by commercial travelers because these fish live at depths of 325-1,300 ft (100-400 m). Little is known of their life habits, but they are very fast swimmers and feed …
Many software programs must be purchased from a vendor. The purchase price entitles the buyer to the benefits of the functional software. The program code that makes these performance features possible (the source code), however, usually remains known only to the company that designed the software. Open source software runs counter to this philosophy. As its name implies, in open source software, …
The American or common opossum (Didelphis marsupialis) is the only member of the order Marsupialia to occur naturally in North America. The American opossum occurs from southern Ontario through to most of South America. The only other member of its genus is D. azarae, which occurs through much of South America. However, other genera and species of the family Didelphidae, the New World opossums, oc…
Opportunistic species of animals or plants are adapted to exploit newly available habitats or resources and are typically found in unpredictable, transient, and variable environments. For example, clear-cut forests create well-lit open areas which are colonized rapidly by the windbone seeds of opportunistic species of plants, many of which are regarded as weeds by farmers and gardeners. Besides pr…
Almost from the invention of the laser, researchers were considering the possibilities of optical data storage. Throughout the 1960s and 1970s, a number of companies were at work on optical data storage systems, held back in large part by the cost and performance level of available lasers. In 1982, Sony Corp. revolutionized the music industry with the introduction of the compact disc (CD). CD-ROM …
Optics is the branch of physics that is concerned with visible light and its properties. Physicists who focus on optics study the properties of light. They also apply these properties to phenomena such as color, mirrors, and lenses. Geometrical optics treats light phenomena (e.g., the determination of focal points, image characteristics, etc.) through calculations derived from the geometry of rays…
Orang-utans (Pongo pygmaeus) are large, long-haired, red apes that inhabit lowland primary forests of Borneo and a small area in the mountains of northwest Sumatra. Their distribution was once much more widespread, extending throughout the tropical forests of Southeast Asia. However, these apes are now endangered, mostly because of the clearing of their forest habitat to develop agricultural land,…
An orbit is the path followed by a celestial body moving in a gravitational field. When a single object, such as a planet, is moving freely in a gravitational field of a massive body, such as a star, the orbit is in the shape of a conic section, that is, elliptical, parabolic, or hyperbolic. Most orbits are elliptical. The exact path and position of an object in space can be determined by taking i…
The many species of orchids comprise one of the largest families of flowering plants, the Orchidaceae, which contains about 1,000 genera and about 20,000 species. Orchids have a worldwide distribution, and they occur in a wide variety of habitats, although their greatest diversity of species is in tropical rain forest. The most species-rich genera of orchids are Dendrobium and Bulbophyllus, each w…
The number 8 can be used in three ways: to tell "how many," to tell "where" in a ranking, and to name someone or something. The girl with the number 8 on her baseball uniform, who is 8th in the batting order, playing on a team that scores 8 runs, is using the same number in each of these ways. When she is 8th in the batting order, she is using the number as an ordinal n…
Ore is metalliferous rock that can be mined and processed at a profit. Although a broader definition includes nonmetallic rocks like rock salt and gypsum, most geologists classify these materials as industrial rocks and minerals. Unlike products from the forest and farm, ores are a nonrenewable resource. The economic survival of industrial societies is linked to the discovery of new supplies of me…
An organ is a functional structure of multicellular organisms which consists of a group of several different tissues. Many multicellular organisms have individual cells grouped together into tissues, a group of many associated cells with similar function; tissues grouped together into organs, a group of tissues interacting so as to form a functional unit; and organs grouped together into organ sys…
Organelles are internal cellular structures that perform dedicated functions. Oraganelles includes tructures such as ribosomes, mitochondria, chloroplasts (the site of photosynthesis in plants and other photosynthesizing organisms), endoplasmic reticulum (ER), Golgi apparatus, and lysosomes. The mitochondrion of all eukaryotes and the chloroplasts of plant cells are the only organelles that have t…
Organic agriculture refers to systems in which crops are grown using natural methods of maintaining fertility of the soil, and methods of pest control other than the use of synthetic pesticides. Compared with conventional agriculture systems that intensively use manufactured fertilizers and pesticides, much smaller environmental costs and damages are associated with organic systems. However, yield…
An organism is any individual living entity. Organisms range in size and complexity from microorganisms to multicellular plants and animals. Modern biologists classify Earth's organisms into five kingdoms on the basis of common patterns of the design of life, that is, in their cellular and sub-cellular organization, metabolism, reproduction, and behavior. Listed in order of their earliest a…
Organogensis refers to that period of time during development when the organs are being formed. After an egg has been fertilized, and has been implanted in the uterus, the developing form is known as the embryo. Organogenesis takes place during this embryonic phase. In fact, most organogenesis has begun as early as week five in humans (remember that a normal human pregnancy lasts an average of 40 …
In the human body an organ is composed of two or more different histological types of tissue that work together to carry out a complex function. An organ system consists of a group of organs that perform intricate functions necessary for the survival of an organism. Sometimes an organism can survive with an impaired or nonfunctioning organ. However, when a whole system of organs shuts down, the li…
There is not direct fossil-like evidence of how life originated on Earth, the molecular processes that preceded the appearance of cells do not leave such tangible evidence. However, fossils of single-celled microorganisms are present in rocks 3.0–3.5 billion years old (with some scientific controversy over which rocks contain the oldest true bacterial fossils), and chemical traces in Greenl…
The true or forest orioles include 28 species of medium-sized birds that make up the family Oriolidae. These birds occur in Africa, Europe, Asia, Southeast Asia, the Philippines, New Guinea, and Australia. Their usual habitats are forests, open woodlands, and savannas. Most species are tropical, but some migratory species occur in temperate regions. Orioles are jay-sized birds with long, pointed w…
Ornithology is the branch of zoology that deals with the study of birds. Birds are any organisms in the class Aves. They are warm-blooded (or homoiothermic) vertebrates that have feathers covering their body; forelimbs modified into wings; stouter hindlimbs used for walking, swimming, or perching; scaly legs and feet; jaws reduced to a toothless beak; and a four-chambered heart. Birds lay hard-she…
Orthopedics is the branch of medicine that specializes in diseases and injuries of bones. It is a specialty devoted to the diagnosis, treatment, rehabilitation and prevention of injuries and diseases of your body's musculoskeletal system. This complex system includes your bones, joints, ligaments, tendons, muscles, and nerves. Once devoted to the care of children with spine and limb deformi…
The oryx (Oryx gazella) is a species of antelope in the family Bovidae. Oryx are native to a rather wide range, extending from the Middle East through much of Africa. There are eight recognized subspecies of oryx, which vary greatly in body and horn shape, and in habitat A herd of scimitar-horned oryx. Photograph by Yav Levy. Phototake. Reproduced by permission. requirements. In some taxo…
In an oscillating chemical reaction, the concentrations of the reactants and products change with time in a periodic or quasi-periodic manner (i.e., they do not move directly or evenly toward their final concentrations). Chemical oscillators exhibit chaotic behavior, in which concentrations of products and the course of a reaction depend on the initial conditions of the reaction. Oscillating react…
An oscillation is a particular kind of motion in which an object repeats the same movement over and over. It is easy to see that a child on a swing and the pendulum on a grandfather clock both oscillate when they move back and forth along an arc. A small weight hanging from a rubber band or a spring can also oscillate if pulled slightly to start its motion, but this repeated motion is now linear (…
An oscilloscope is an instrument that provides a graphical display of electrical signals. It presents a considerable amount of information about the operation of a circuit almost instantly, and the visual nature of the display provides insights that tables of numbers do not offer. Oscilloscopes are extremely useful for monitoring and diagnosing electrical circuits or devices. Though it can plot an…
Osmosis is the movement of solvent, such as water, through a barrier from a less concentrated solution into a more concentrated solution. It occurs when two solutions are separated by a semipermeable membrane which allows only the solvent to pass through. Osmosis plays a major role in the chemistry of living things and also has applications in medicine and technology. Osmosis was first described i…
Osmosis is the movement of water across a membrane which is selectively permeable. In osmosis, water moves across a membrane from a region with low solute concentration to a region with high solute concentration. Thus, osmosis tends to equalize the solute concentrations in two separate membrane-enclosed regions. In living cells, water moves by osmosis across membranes between cells or between memb…
Ossification is the process of the synthesis of bone from cartilage. There are two types of ossification—intramembranous and endochondral ossification. Bone may be synthesized by intramembranous ossification, endochondral ossification, or a combination of the two. Intramembranous ossification is the transformation of the mesenchyme, cells of an embryo into bone. During early development of …
Osteoporosis is a condition in which bone mass, and therefore bone strength, is decreased. This results in a greatly increased risk of fracture. Primary osteoporosis is osteoporosis which occurs due to normal, predictable changes within the body during the aging process. Secondary osteoporosis occurs as a result of some other specific disease process which produces osteoporosis as one of its sympt…
Otter shrews are small otter-like aquatic mammals in the family Tenrecidae, with silvery fur. The three species of otter shrews belong to the order Insectivora, and all live in west and central Africa near the equator. The Potamogalidae family includes two genera—Micropotamogale and Potamogale. These genera contain a total of three distinctive species of otter shrew—the giant African…
Otters are small to medium-sized mammals with a long body, flattened head, broad muzzle, and long stiff whiskers. Their tail is strong, long, flattened, and somewhat tapered. Otters have short legs and webbed toes; they are well adapted to a semi-aquatic existence and are skilled swimmers. The outer fur of otters is short, very dense, and highly water resistant. They also have a layer of soft unde…
When weathering and erosion expose part of a rock layer or formation, an outcrop appears. An outcrop is the exposed rock, so named because the exposed rock "crops out." Outcrops provide opportunities for field geologists to sample the local geology—photograph it, hold, touch, climb, hammer, map, sniff, lick, chew, and carry it home. Classes often visit outcrops to see illustra…
The ovarian cycle is a series of events in the ovaries that occur during and after the maturation of the oocyte (egg or ovum). During their reproductive years, nonpregnant females usually experience a cyclical sequence of changes in their ovaries and uterus. Each cycle takes about one month and involves both oogenesis, the process of formation and development of oocyte, and preparation of the uter…
Ovenbirds are 200 species of birds that compose the rather large family Furnariidae, occurring from southern Mexico through Central and all of South America. Ovenbirds occur in a wide variety of habitats, ranging from mature tropical forests to semi-desert, and from coastal lowlands to alpine tundra. In other words, the ovenbird family is very rich in species, and these birds successfully exploit …
Oviparous is a zoological term that refers to animals that lay eggs which then hatch externally. Oviparous animals may fertilize their eggs either externally or internally. External fertilization involves the passage of the sperm to the ova through an ambient medium, usually water. For example, frogs achieve external fertilization of their eggs during amplexus, when the male deposits sperm over th…
Ovoviviparous is a zoological term that refers to animals that produce eggs but retain them inside the female body until hatching occurs, so that "live" offspring are born. The egg-hatching strategy of ovoviviparity occurs in a rather wide diversity of animals, including certain insects, fish, lizards, and snakes. However, ovoviviparity is much less common than the external developme…
Owls comprise two closely related families in the avian order Strigiformes: the barn owls (Tytonidae) and the typical owls (Strigidae). Owls are relatively large birds, with a big head and short neck, a hooked beak, talons adapted to seizing prey, and soft, dense plumage adapted for swift yet almost silent flight. Owls have large eyes located on the front of their face but almost fixed in their so…
Oxalic acid is the more common name of ethanedioic acid. The name ethanedioic acid communicates that the molecule has two carbon atoms (as in ethane) and two acid groups (COOH). It is a white solid used in removal of certain kinds of stains, in removing calcium ions from solutions, and in tanning leather. It occurs naturally and is toxic. The potassium and calcium salts of oxalic acid are found na…
Oxidation-reduction reactions, also known as redox reactions, are chemical processes in which electrons are transferred from one atom, ion, or molecule to another. Explosions, fires, batteries, and even our own bodies are powered by oxidation-reduction reactions. When iron rusts or colored paper bleaches in the sun, oxidation-reduction has taken place. Oxidation-reduction reactions can be thought …
The oxidation state of an atom is a description of how many electrons it has lost or gained from its original state. Each type of atom has a certain number of electrons (which varies from atom to atom) in its elemental form. When an atom forms a bond or otherwise interacts with another atom, it is possible that it will lose or gain an electron. If an atom is electronegative, it is more likely to t…
Oxygen is a non-metallic element of atomic number 8. Its symbol is O, the atomic weight is 15.9994, the specific gravity is 1.10535 (compared to air), the melting point is -361°F (-218.4°C), and the boiling point is -297.4°F (-183°C). Oxygen is a non-metal in group 16 of the periodic table. Its three stable isotopes have atomic weights of 16, 17, and 18. The first is b…
Oystercatchers are six rather similar-looking species of oceanic shorebirds that comprise the family Haematopodidae. Oystercatchers occur widely on sub-arctic, temperate, and tropical seacoasts, on all of the continents except Antarctica. Oystercatchers are relatively large shorebirds, with a body length of 15-21 in (40-53 cm). They have pointed wings, a short tail, short but heavy legs, and three…
In the vicinity of Los Angeles the maximum one-hour concentration of ozone can exceed 500 ppb, and it is typically greater than 100 ppb for at least 15 days per year. In other cities in North America, the annual maximum one-hour concentration is typically 150-250 ppb, and it is typically 90-180 ppb in London, England. Humans and other animals are sensitive to ozone. This gas irritates and damages …
Ozone is a gas that occurs naturally in relatively large concentrations in the upper-atmospheric layer known as the stratosphere. The stratosphere is between 5–10.6 mi (8–17 km) to about 31 mi (50 km) above the earth's surface. Stratospheric ozone is very important to life on the surface of Earth because it absorbs much of the incoming solar ultraviolet radiation, and thereby …
The heart is a unique organ that must function continuously to pump blood supplying oxygen to the body. It speeds up during special times of need, as when an individual is running or doing stressful work. It slows at night or during sleep when the demand for blood decreases. This tiny pump, about the size of a fist, squeezes approximately 2.5 fl oz (75 ml) of blood out into the body with each beat…
Pain, in its most basic form, can be described as an unpleasant physical sensation resulting from a variety of outside stimuli, from a pin prick to a serious burn. However, pain is a complex experience that eludes simple definitions. Not only does the central nervous system play a crucial role in the experience of pain, but psychological factors can also affect how an individual perceives pain. Mo…
Paleobotany endeavors to reconstruct past climates and regional vegetation systems by studying the fossilized remains of plants or preserved pollen samples. Such studies have yielded information regarding global climate change, both natural and man-made, and its effects on specific environments. Paleobotanists aid in the identification of various climatic episodes. By collaborating geological evid…
Paleoclimate studies analyze the variation of the climate in past geologic times, prior to instrumental measurements. Paleoclimate is expressed by its parameters—paleotemperature, precipitation in the past, circulation, sea surface temperature (SST) and sea level. The general state of Earth's climate is dependent upon the amount of energy the Earth receives from the solar radiation, …
Paleoecology is the branch of paleontology that studies ancient organisms and their environments. Paleoecologists study the physical structure and biological functions of organisms, their interactions with each other, and their role in ancient ecosystems. In addition to the basic principles of paleontology, paleoecology research depends on concepts from biology, sedimentology, and geochemistry. Th…
Paleomagnetism is the study of ancient magnetism in rocks. The phenomenon was first discovered by the French physicist Achilles Delesse in 1849. Delesse observed that certain magnetic minerals in rocks were lined up along the earth's magnetic field, just as if they were tiny compasses that had been set in place in the rocks. A related discovery that was even more startling was made by the F…
Paleontology is the study of ancient animal life and how it developed. It is divided into two subdisciplines, invertebrate paleontology and vertebrate paleontology. Paleontologists use two lines of evidence to learn about ancient animals. One is to examine animals that live today, and the other is to study fossils. The study of modern animals includes looking at the earliest stages of development …
Paleopathology is the study of the evidence of trauma, disease, and congenital defects in human remains. Archaeologists, geneticists, and physical anthropologists, conduct paleopathology studies in order to evaluate the effects of disease upon ancient populations. Often, such research is conducted to garner more information about the biological and genetic characteristics of prehistoric or ancient…
In genetics, the term palindrome refers to a sequence of nucleotides along a DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) strand that contains the same series of nitrogenous bases regardless from which direction the strand is analyzed. Akin to a language palindrome—wherein a word or phrase is spelled the same left-to-right as right-to-left (e.g., the word RADAR or the phrase …
The palm family is an ancient group of plants dating from at least the late Mesozoic Era, about 85 million years ago. Palms are flowering plants. Flowering plants have been subdivided into two major groups: the monocotyledons which bear only one seed leaf (cotyledon) and the dicotyledons which bear two seed leaves. Palms are among the most diverse of the families of monocotyledons, surpassed in nu…
Palynology is the study of fossil pollen (and sometimes plant spores) extracted from lake sediment, peat bog, or other matrices. The most common goal of palynological research is to reconstruct the probable character of historical plant communities, inferred from the abundance of species in dated portions of the fossil pollen record. Pollen analysis is an extremely useful tool for understanding th…
The name "panda" applies to two different species: the familiar and well-loved giant panda (Ailuropoda melanoleuca), and the lesser-known red panda (Ailurus fulgens). Scientists originally thought the two species were closely related because of similarities in their behavior, diet, anatomy, and distribution. However, despite these shared characters of the pandas, researchers have now…
Pangolins, also called scaly anteaters, are subsaharan African and Asian mammals that have horny scales covering the upper parts of their bodies. All seven species belong to one genus, Manis, making up the order Pholidota. A pangolin. Photograph by Nigel Dennis. The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. The overlapping brown, gold, olive, or p…
The papaya or pawpaw (Carica papaya) is a tropical tree originally native to the Americas, probably Mexico. This species is easily cultivated, produces large, edible fruits, and now is distributed worldwide to suitable climates where it is grown for subsistence and commercial agriculture. The papaya has large deeply incised, sometimes compound leaves that sprout near the top of the plant. This pla…
Paper was probably first produced from bamboo and rag fibers about 2,000 years ago. By the eighth century, papermaking technology had spread to the Middle East. By the middle of the twelfth century, the Moors had transplanted the technology to Spain, from where it spread throughout Europe. Rags continued to be the chief source of paper fibers until the introduction of papermaking machinery in the …
A parabola is the open curve formed by the intersection of a plane and a right circular cone. It occurs when the plane is parallel to one of the generatrices of the cone (Figure 1). A parabola can also be defined as the set of points which are equidistant from a fixed point (the "focus") and a fixed line (the "directrix") (Figure 2). The "axis" of a parabo…
Astronomers must use very indirect methods to measure the distances to stars and other astronomical objects. Measuring a star's parallax is a way to find its distance. This method takes advantage of the apparent shift in position of a nearby star as it is observed from different positions as the earth orbits the Sun. Because the parallax effect depends upon the earth's motion about t…
Two or more lines (or planes) are said to be parallel if they lie in the same plane (or space) and have no point in common, no matter how far they are extended. …
A parallelogram is a plane figure of four sides whose opposite sides are parallel. A rhombus is a parallelogram with all four sides of equal length; a rectangle is a parallelogram whose adjacent sides are perpendicular; and a square is a parallelogram whose adjacent sides are both perpendicular and equal in length. The area of a parallelogram is equal to the length of its base times the length of …
A parasite is an organism that depends on another organism, known as a host, for food and shelter. The parasite usually gains all the benefits of this relationship, while the host may suffer from various diseases and discomforts, or show no signs of the infection. The life cycle of a typical parasite usually includes several developmental stages and morphological changes as the parasite lives and …
Parity is both an operation and an intrinsic property used to describe particles and their wavefunctions (mathematical representations of one or more particles) in quantum mechanics (a branch of physics focusing on particles smaller than an atomic nucleus). The parity operation is a combination of a left-right trade (mirror reflection) with a top-bottom switch. This combination is also called a sp…
Parkinson disease (PD) is a disease in which cells in regions of the brain involved with muscular coordination and control suffer in impaired ability to synthesize the neurotransmitter dopamine. Parkinson disease or Parkinsonism is named for the English physician James Parkinson. His description of the various manifestations of the disease was published in 1817, in a work entitled "An Essay…
Parrots, macaws, lories, parakeets, and related birds, known collectively as psittacids, are 328 living species of birds that make up the family Psittacidae. The psittacids and the cockatoos (family Cacatuidae) are the only families in the order Psittaciformes. Species of psittacids occur in Central and South America, Africa, Madagascar, South and Southeast Asia, New Guinea, Australia, and New Zea…
Parthenogenesis in animals refers to reproduction in which a new individual genetically identical to the parent develops from an unfertilized egg. The analogous event in plants, which results in seed formation without fertilization, is called agamospermy. Parthenogenesis is viewed as an aberration of sexual reproduction because animals that reproduce by parthenogenesis evolved from organisms that …
Particle detectors are instruments designed for the detection and measurement of sub-atomic particles such as those emitted by radioactive materials, produced by particle accelerators or observed in cosmic rays. They include electrons, protons, neutrons, alpha particles, gamma rays and numerous mesons and baryons. Most detectors utilize in some way the ionization produced when these particles inte…
Partridges are species of fowl in the family Phasianidae, which also includes the pheasants, chickens, turkeys, grouse, peafowl, francolins, and quail. Partridges occur naturally in Eurasia, but they have been introduced as game birds to other places as well, including North America. Partridges are medium-sized, stocky birds with short, rounded wings, a short tail, and a short, stout bill, in whic…
Species of passion vines (Passiflora spp.; family Passifloraceae) that twine upward in tropical, subtropical, and some temperate regions of the world. They occur most often in wet forests, though some species may occur in drier, more open places. These vines have glossy leaves shaped like rounded human hands, and their flowers are often sweetly scented and packed with a ring of colorful filaments.…
Paternity testing and the identification of genetic parents via DNA analysis has, within the last decade, become a highly reliable test upon which courts and medical staff increasingly rely. Paternity or parentage identification is based on the ability to establish the genetic relationship between the parent(s) and their biological offspring. …
Pathogens are organisms, frequently microorganisms, or components of these organisms, that cause disease. Microbial pathogens include various species of bacteria, viruses, and protozoa. Many diseases caused by microbial pathogens. A disease is any condition caused by the presence of an invading organism, or a toxic component, that damages the host. In humans, diseases can be caused by the growth o…
Pathology is the scientific study of disease processes that affect normal anatomy and physiology. Anatomical and physiological changes are pathological changes when they result from an underlying disease process or abnormality. Pathologists play an increasingly important role in diagnosis, research, and in the development of clinical treatments for disease. A specialized branch of pathology, foren…
PCR (polymerase chain reaction) is a technique in which cycles of denaturation, annealing with primer, and extension with DNA polymerase, are used to amplify the number of copies of a target DNA sequence by more than 100 times in a few hours. American molecular biologist Kary Mullis developed the techniques of PCR in the 1970s. For his ingenious invention, he was awarded the 1993 Nobel Prize in ph…
The peafowl are three large, and extraordinarily beautiful species of fowl in the family Phasianidae, which also includes the pheasants, chickens, partridges, francolins, turkeys, guinea fowl, and quail. Species of peafowl are native to Asia and Africa. However, these gorgeous creatures have been kept in captivity as ornamental birds for several thousand years, and they are now found in zoos and a…
Peanut worms are a group of over 300 species of worms, classified as a separate Phylum Sipunculida (Phylum Sipuncula, by some authors), called Sipunculids or Sipunculans in English. They have a simple tubular shape, with "a tube within a tube" body plan and the internal organs inside a body cavity (coelom), that is lined by a fine epithelium called peritoneum. They are non-segmented …
Peccaries are wild pigs (order Artiodactyla, family Tayassuidae) of the New World which are relatives of the wild pigs of the Old World. Peccaries are the only pigs native to the New World pigs, all other pigs in North and South America are formerly domestic animals that have escaped and become feral. Taxonomists recognize three species of peccaries: the collared peccary (Tayassu tajacu) of the so…
In genetics, a pedigree is a diagram of a family tree showing the relationships between individuals together with relevant facts about their medical histories. A pedigree analysis is the interpretation of these data that allows a better understanding of the transmission of genes within the family. Usually, at least one member of the family has a genetic disease, and by examining the pedigree, clue…
Pelicans are large coastal birds belonging to order Pelecaniformes, along with gannets, tropicbirds, darters, frigatebirds, and cormorants. All of them have throat pouches to one extent or another, but only the pelicans' are so big. They eat nothing but fish, and the pouch is a handy device for catching their food. The pelican has perhaps the most unusual bill in the bird world. It is quite…
Penguins are primitive, flightless birds that are highly specialized for marine life. Most species look A gentoo penguin with its chick. JLM Visuals. Reproduced by permission. rather similar, being generally dark-blue or dark-gray on top with a white belly. Some species, however, have a crest on their head and/or patches of color on their head and throat. The legs are set wide apart and t…
A peninsula is a piece of land surrounded by water on three sides and joined to a larger body of land by an isthmus, or neck. A peninsula is a topographic high spot; a dry land range of hills or mountains created during the formation of the earth's crust. It is left visible when the low areas on both sides either subside and become submerged, or when the water level rises and floods the val…
Pentyl or five carbon atom alkyl groups are also referred to as amyl groups. The term amyl is derived from the Latin word for starch, amylum, and is used because the five carbon atom amyl alcohols were first isolated from fermentation products. Amyl or pentyl alcohols consist of a chain of five carbon atoms with a hydroxyl group (-OH) connected to one of the carbon atoms. Each carbon atom is also …
The peony is an attractive flower, much beloved of gardeners. It is in the family Paeonaceae, though in the past it was in the family Ranunculaceae with other flowers such as the buttercup. The generic name is Paeonia and there are some 50 species in this group. The name peony comes from Paeon, a physician in Greek mythology. His teacher was jealous of his skills as a healer and intended to murder…
Pepper, one of the world's most important spices, comes from the fruit (peppercorns) of a flowering shrub, in the genus Piper, family Piperceaea. The pepper plant originates from India, which is still the world's largest producer of pepper. The plant grows in hot, humid regions such as India, Indonesia, Malaysia, and Brazil. The United States is the world's largest importer of…
Proteins are made up of amino acids, which are joined by peptide linkages. Although there are only 20 different naturally occurring amino acids, various combinations of these form the thousands of proteins used in metabolism. All amino acids have a similar structure. There is a central carbon atom, called the alpha-carbon and this is bonded to an amino group on one side and a carboxyl group on the…
Percent is a device for expressing hundredths. Thus P% stands for P × 0.01. For example, 6% is equal to six hundredths (6 × 0.01, or 0.06) and 7.2% is equal to 7.2 × 0.01, or 0.072. As shown by these examples, to change from a percent to a number, divide the percent figure by 100. Conversely, to convert hundredths to percent, multiply the number by 100. …
Human perception is the active reception and coordination of information received through our sensory systems in order to make sense of the environment and to behave effectively within it. In contrast with the direct and immediate sensations actually received and transmitted, perception is the transformation of that information into nerve cell activity that is transferred to the brain where furthe…
Perch belong to the class Osteicthyes, whose members have a skeleton of bone rather than cartilage. Bony fish comprise the largest group of vertebrates living today, both in the number of individuals (millions) and in the number of species (about 30,000). Perch occur in both fresh water and sea water throughout the entire world. Perch live at depths in the oceans as great as 7 mi (11.5 km) and in …
The peregrine falcon (Falco peregrinus) is a bird of prey in the family Falconidae. It is one of the most wide-ranging birds in the world with populations in both the Eastern and Western Hemispheres, and occurring on all continents except Antarctica. It is also the world's fastest-flying bird. Unfortunately, beginning in the 1940s, many populations of the peregrine falcon were decimated by …
A perfect number is a whole number which is equal to the sum of its divisors including 1 by excluding the number itself. Thus 6 is a perfect number becauses 1 + 2 + 3 = 6. Likewise 28 is a perfect number because 1 + 2 + 3 + 4 + 7 + 14 = 28. The search for even perfect numbers, then, is the same as the search for Mersenne primes. At the present time 32 are known corresponding to p = 2, 3, 7, 13,...…
A periodic function is a function whose values repeat at regular intervals. Given an interval of length t, and a function f, if the value of the function at x + t is equal to the value of the function at x then f is a periodic function. In standard function notation this is written f(x + t) = f(x) (read "f of x plus t equals f of x"). The shortest length t for which the function repe…
The arrangement of the chemical elements into periods (horizontal rows) and groups (vertical columns) is called the periodic table. The elements in the table are represented by symbols (one, two, or three letters) in individual squares. Above each chemical symbol appears the atomic number of the element. These whole numbers are the number of protons present in the nucleus of that element. Below th…
Geologists define permafrost as soil layers or rock and soil combinations that remain frozen (i.e., remain below the specific freezing temperature unique to the exact constituents of the formation) for a time period in excess of two years. The exact composition of permafrost can vary widely depending on the unique geology and morphology of the area in which the perafrost forms. Although ice is usu…
The term perpendicular describes a pair of lines or planes that intersect each other at a 90 degree angle. Perpendicularity is an important concept in mathematics, science, and engineering. A line l1 is perpendicular to a line l2 if the two intersect with congruent adjacent angles, which means that the angles are both equal to 90 degrees. Of course, a purely analytical definition of the term exist…
A pesticide is a chemical that is used to kill insects, weeds, and other organisms to protect humans, crops, and livestock. A broad-spectrum pesticide that kills all living organisms is called a biocide. Fumigants, such as ethylene dibromide or dibromochloropropane, used to protect stored grain or sterilize soil fall into this category. Generally, however, we prefer narrower spectrum agents that a…
Pests are any organisms that are considered, from the perspective of humans, to be undesirable in some ecological context. For example, pests could be insects that compete with humans for some common resource, such as agricultural production or timber. Other pests might be associated with diseases of humans, livestock, or agricultural plants. Pests could also be unwanted weeds that compete with ag…
Petrels and shearwaters are wide-ranging oceanic birds with a characteristic tubenose and other specialized features that equip them well for a life spent mostly at sea. Found throughout the world, these long-lived colonial nesting seabirds include some 79 species in four families, all in the Order Procelliformes. These seabirds show a great range in body size, from the giant petrel with a 6 ft (2…
In archaeology, petroglyphs and pictographs are terms used to describe forms of "rock art." Petroglyph refers to a rock carving or etching, while the term pictograph is commonly applied to a rock painting. Typically, these features are found on the vertical or overhanging surfaces of large boulders and are sometimes associated with nearby settlements. However, they are often found is…
Petroleum is a term that includes a wide variety of liquid hydrocarbons. Many scientists also include natural gas in their definition of petroleum. The most familiar types of petroleum are tar, oil, and natural gas. Petroleum forms through the accumulation, burial, and transformation of organic material, such as the remains of plants and animals, by chemical reactions over long periods of time. Af…
pH is a measure of the acidity or alkalinity of a solution based upon the dissociation of water. The variability of pH can have a dramatic effect on biological or physical chemical reactions (e.g., geochemical weathering processes). The pH scale was developed by Danish chemist Søren Peter Lauritz Sørensen (1868–1939) in 1909 and is generally presented as ranging from 0&#x…
Phalangers are a small group of arboreal mammals belonging to the family Phalangeridae, of which 20 species are recognized in six genera. Phalangers, more commonly known as possums and cuscuses, are marsupials but with a vague resemblance to some monkeys. Indeed many early European explorers thought that they were monkeys. These species occur in Australia, New Guinea and adjacent islands west to S…
Pharmacogenetics is the exploration of the relationship between inherited genes and the ability of the body to metabolize drugs. Although research interests are rapidly expanding, by early 2003 pharmacogenetics research concentrated on trying establish connections between the genes carried by an individual (genotype) and specific reactions to drugs (e.g., side effects, toxicities, etc.). Modern me…
