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A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C15H20O4 (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various…

A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately…

Habituation to a specific practice, such as drinking alcoholic beverages or using drugs (for example, heroin, cocaine, or amphetamines), and disorders characterized by the chronic use of a drug, alcohol, or similar substance. Addiction and addictive…

A complex endocrine organ in proximity to the kidney. The adrenal gland (see illustration), often called simply as the adrenal, is an important endocrine organ located on top of the kidneys. Adrenal gland tissue is present in all vertebrates from…

Malfunctions of the paired adrenal glands. The paired adrenal glands are complex endocrine organs in proximity to the kidneys (see illustration), and disorders of the adrenal glands can have serious consequences. Under specific circumstances,…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab100An0 to Ab90An10, where Ab (= albite) is NaAlSi3O8 and An (= anorthite) is CaAl2Si2O8 (see illustration). Albite occurs in crustal igneous rocks as a major…

A plasma protein produced by the liver that maintains fluid balance in the blood and transports fatty acids in the plasma and interstitial fluid. Albumin is the most abundant protein in human serum (see illustration), as well as being one of the…

Any of a number of alternative forms of a gene. An allele is a variant form of a gene (see illustration). Allele is a contraction of allelomorph, which is a term that William Bateson used to designate one of the alternative forms of a unit showing…

A glycoprotein that is normally present in significant amounts only in the serum of the fetus. Alpha fetoprotein (AFP) is an important plasma glycoprotein found in a developing fetus. This protein is also detectable in the mother's blood by passing…

A disease of the nervous system characterized by a progressive dementia that leads to profound impairment in cognition and behavior. Dementia occurs in a number of brain diseases where the impairment in cognitive abilities represents a decline from…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. The time necessary to transform resin…

A lithium aluminum phosphate mineral of basic formula LiAl(PO4)(F). The structure of amblygonite consists of phosphate (PO4) groups of tetrahedra and AlO6 groups of octahedra. Each PO4 tetrahedron is connected to an AlO6 octahedron. Corner-sharing…

The transparent purple to violet variety of the mineral quartz. Although quartz is perhaps the commonest gem mineral known, amethyst (see illustration) is rare in the deep colors that characterize fine quality. Amethyst is usually colored unevenly…

A nesosilicate mineral, composition Al2SiO5, crystallizing in the orthorhombic system. Andalusite (Fig. 1) occurs commonly in large, nearly square prismatic crystals. The variety chiastolite has inclusions of dark-colored carbonaceous material…

A plagioclase feldspar with composition Ab70An30 to Ab50An50 (Ab = NaAlSi3O8; An = CaAl2Si2O8). Andesine (see illustration) occurs primarily in igneous rocks, often in a glassy matrix as small, chemically zoned, lathlike crystals known as…

Loss of sensation with or without loss of consciousness. Anesthesia is used to prevent pain during surgery and other medical procedures. There are several ways of producing anesthesia, with the choice dependent on the type of surgery and the medical…

A mineral with the chemical composition PbSO4. Anglesite occurs in white or gray, orthorhombic, tabular or prismatic crystals or compact masses (see illustration). It is a common secondary mineral, usually formed by the oxidation of galena. Fracture…

A mineral with the chemical composition CaSO4. Anhydrite (anhydrous calcium sulfate) occurs commonly in white and grayish granular masses, rarely in large, orthorhombic crystals (see illustration). Fracture is uneven and luster is pearly to…

The formation of new animal individuals, which may occur by asexual or sexual means. All animals must reproduce asexually or sexually to create new individuals of their species. In asexual reproduction, a single parent can produce offspring that are…

The calcium-rich plagioclase feldspar with composition Ab0An100 to Ab10An90 (Ab = NaAlSi3O8; An = CaAl2Si2O8). Anorthite (see illustration) occurs in olivine-rich igneous rocks and rare volcanic ejecta (for example, at Mount Vesuvius, Italy, and…

The coldest, windiest, and driest continent. The lowest temperature ever measured on Earth, −89.2°C (−128.5°F), was recorded at the Russian Antarctic station of Vostok in July 1983. Katabatic (cold, gravitational) winds with velocities up to 50 km/h…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

One of three naturally occurring mineral forms of calcium carbonate (CaCO3). The other forms (or polymorphs) are the abundant mineral calcite and the relatively rare mineral vaterite. Still other forms of calcium carbonate are known, but only as…

Arenite (rock composed of sand-size fragments) that contains a high proportion of feldspar in addition to quartz and other detrital minerals. Arkose is also known as feldspathic sandstone. Although there is no universal agreement, many geologists…

Any treatment or procedure that includes the in-vitro handling of oocytes (immature ova or egg cells) and sperm or embryos for the purpose of establishing a pregnancy. Assisted reproductive technology (ART) is undertaken as a treatment of…

A group of monoclinic calcic pyroxenes which have the general chemical formula (Ca,Mg,Fe)(Mg,Fe)-Si2O6, in which calcium is the dominant cation in the first cation position. Monoclinic pyroxene with substantial iron or magnesium in place of calcium…

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as…

Any of a class of plant growth substances (phytohormones or plant hormones) that have an essential role in coordination of many growth and developmental processes during the life cycle of plants, especially the ability to increase the rate of shoot…

A basic carbonate of copper with the chemical formula Cu3(OH)2(CO3)2. Azurite is normally associated with copper ores and often occurs with malachite. Azurite is monoclinic. It may be massive or may occur in tabular, prismatic, or equant crystals…

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence.…

The production of visible light (usually blue-green) by bacteria. Luminous bacteria emit visible light by converting chemical energy to light energy. The phenomenon of bacterial luminescence is seen in many species of several genera, including…

A mineral with chemical composition BaSO4. It is isostructural with celestite, SrSO4, and anglesite, PbSO4. Barite is orthorhombic, space group Pnma, a = 0.887 nanometer, b = 0.545 nm, c = 0.714 nm. The illustration shows typical crystals. Barite…

The most common volcanic rock on Earth. Basalt is an igneous rock characterized by small grain size (less than about 5 mm) and approximately equal proportions of calcium-rich plagioclase feldspar and calcium-rich pyroxene, with less than about 20%…

The primary ore used to produce aluminum metal and alumina (Al2O3). Bauxite (see illustration) is a naturally occurring weathering product composed of one or more aluminum hydroxide minerals, such as gibbsite [Al(OH)3], boehmite [AlO(OH)], and…

Any of several leguminous plants, or their seeds, long utilized as food by humans or livestock. There are more than 40,000 varieties of beans, with only a small number of these being mass-produced for human or animal consumption. In particular,…

The most common beryllium mineral. Beryl, Al2[Be3Si6O18], crystallizes in the hexagonal system with space group P6/mcc. The crystal structure consists of six-membered rings of corner-sharing silicon-oxygen (SiO4) tetrahedra cross-linked by…

Minerals containing beryllium as an essential component. Over 50 beryllium minerals (Fig. 1) have been identified, even though beryllium is a scarce element in the Earth's crust. The unusual combination of low charge (+2) and small ionic radius…

The study of skeletal remains from archeological sites by biological (or physical) anthropologists. Bioarcheology differs in several ways from traditional skeletal research (see illustration). Previous work focused on individual case studies (for…

Fungi that exhibit the natural phenomenon of bioluminescence (the ability of an organism to emit visible light). Bioluminescence is the emission of photons of visible light by living organisms via the chemical reaction of molecular oxygen (O2) with…

A field that combines the disciplines of biology and engineering mechanics and utilizes the tools of physics, mathematics, and engineering to quantitatively describe the properties of biological materials. Biomechanics helps explain how the…

An integrated device consisting of a biological recognition element and a transducer capable of detecting the biological reaction and converting it into a signal that can be processed. Ideally, the sensor should be self-contained, so that it is not…

An iron-magnesium-rich layer silicate; it is also known as black mica. Biotite (see illustration) is the most abundant of the mica group of minerals. The name is derived from that of the French chemist J. Biot. The formula for the ideal end member,…

A distinctive, fine-grained mudrock that derives its dark color from an unusually high content of organic matter, ranging from around 2% to over 50% of the weight of the rock (typical shales contain less than 1% organic matter). Most black shales…

Regional metamorphism with the highest pressures and lowest temperatures, commonly above 500 megapascals (5 kilobars) and below 400°C (750°F). Metamorphic rocks of the relatively uncommon blueschist facies contain assemblages of minerals that record…

A clastic rock composed of angular gravel-size fragments; the consolidated equivalent of rubble. The designation gravel-size refers to a mean particle diameter greater than 2 mm, which means that 50% or more of the particles (by volume) are this…

Terrestrial and aquatic animals that are capable of excavating holes in the ground for protection from adverse environmental conditions, as well as for storing food. Burrowing animals are uniquely adapted species, having the capacity to dig burrows…

A mineral composed of calcium carbonate (CaCO3). Calcite is one of the most common minerals found in the Earth's crust. Calcite (Fig. 1) may be found in a great variety of sedimentary, metamorphic, and igneous rocks. It is also an important…

A hormone, the only known secretory product of the parenchymal or C cells of the mammalian thyroid and of the ultimobranchial glands of lower animal forms. The hormone calcitonin (also referred to as thyrocalcitonin) has been isolated and…

A member of the mammalian family Camelidae in the order Artiodactyla. Camels are even-toed, hoofed mammals (ungulates). There are three extant camel species: the Bactrian camel (Camelus bactrianus; Fig. 1), the wild Bactrian camel (C. ferus), and…

A type of carved gemstone in which the background is cut away to leave the subject in relief. Often cameos are cut from stones in which the coloring is layered, resulting in a figure of one color and a background of another. The term cameo (see…

The common name for a malignant neoplasm or tumor. Cancer is a serious medical condition that develops when the orderly relationship of cell division and cell differentiation becomes disordered, leading to the development of neoplasms. Neoplasms are…

Cells within a tumor that possess the capacity to self-renew and cause the heterogeneous lineages of cancerous cells that comprise the tumor. Cancer stem cells (CSCs) typically represent a small percentage of all cells in a tumor (rarely more than…

A condition caused by the thickening of ligaments and tendon sheaths at the wrist, with consequent compression of the median nerve at the palm. The carpal tunnel is a thin passageway among ligaments and tendon sheaths located in the wrist joint area…

A firm, resilient connective tissue of vertebrates and some invertebrates. Cartilage (see illustration) is an important structural element found in the bodies of vertebrates and some invertebrates. It is a resistant, elastic connective tissue that…

A mineral with the chemical composition SrSO4. Celestite occurs commonly in colorless to sky-blue, orthorhombic, tabular crystals (see illustration). Fracture is uneven and luster is vitreous. Hardness is 3–3.5 on Mohs scale and specific gravity is…

The mechanism by which cells in a multicellular organism become specialized to perform specific functions in a variety of tissues and organs. Cell differentiation is the process through which cells are transformed into divergent types of cells…

The movement of cells, the movement of materials within cells, and changes in cell shape (including cell division). Cell motility is an important facet of cell biology, encompassing the movement of individual cells, the coordinated movement of many…

The organizing and compartmentation of eukaryotic cells. Eukaryotic cells are divided into several compartments, and each compartment has a characteristic structure, biochemical composition, and function (Fig. 1). These compartments are called…

The limited capacity of all normal human and other animal cells to reproduce and function. The gradual decline in normal physiological function of cells is referred to as aging or senescence (Fig. 1). The aging process ends with the death of…

The field concerning the interactions among cells and molecules of the immune system, and how such interactions contribute to the recognition and elimination of pathogens. Humans (and vertebrates in general) possess a range of nonspecific mechanical…

A common mineral in the upper oxidized zone of ore deposits containing galena (PbS). Cerussite (see illustration) is usually associated with anglesite, smithsonite, malachite, and other secondary minerals. Normally cerussite is close in composition…

A mineral belonging to the zeolite family of silicates. Chabazite (see illustration) commonly occurs in well-crystallized groupings of crystals with external rhombohedral symmetry (point group 3 2/m) with nearly cubic angles. This well-developed…

A mineral with the chemical composition CuSO4· 5H2O. Chalcanthite commonly occurs in blue to greenish-blue triclinic crystals or in massive fibrous veins or stalactites (see illustration). Fracture is conchoidal and luster is vitreous. Hardness is…

A fine-grained fibrous variety of quartz, silicon dioxide. The individual fibers that compose the mineral aggregate usually are visible only under the microscope. Subvarieties of chalcedony recognized on the basis of color differences (induced by…

The term “chalk” is sometimes used in a broad sense for any soft, friable, or weathered fine-grained limestone; however the term is mostly restricted to pelagic (biogenic) limestones. Chalk is a uniformly fine-grained, typically light-colored marine…

Neural systems used for physiological reception of and response to chemical stimulation by excitation of specialized receptors. In vertebrates, the senses of smell (olfaction) and taste (gustation), along with the so-called common chemical sense,…

The administration of chemical substances for the treatment of disease, especially cancer, or for the prevention of aberrant growth of cells or tissues (neoplasms). In general, chemotherapy is the use of chemicals to treat any disease, but the term…

A hard, dense sedimentary rock composed of fine-grained silica (SiO2), most commonly quartz. Chert is characterized by a semivitreous to dull luster and a splintery to conchoidal fracture, and is most commonly gray, black, reddish brown, or green…

One of a group of layer silicate minerals, usually green in color, characterized by a perfect cleavage parallel to (001). Chlorite cleavage flakes are flexible but inelastic, with a luster varying from pearly or vitreous to dull and earthy. The…

A hydrous iron aluminum silicate mineral commonly found in low-grade metamorphic rocks such as slate and phyllite. Chloritoid is found as platy, black or dark green crystals, rarely more than a few millimeters in size (Fig. 1). Its density ranges…

An illness of unknown cause and uncertain prevalence, often resulting in debilitating fatigue accompanied by diverse symptoms. Chronic fatigue syndrome (CFS) is a complicated and disabling illness. Characterized by extreme degrees of fatigue (see…

A mineral having composition BeAl2O4 and crystallizing in the orthorhombic system. Chrysoberyl crystals (see illustration) are usually tabular parallel to the front pinacoid and frequently in pseudohexagonal twins. There is good prismatic cleavage.…

A silicate mineral with composition essentially (Cu,Al)2H2Si2O5(OH)4·nH2O; iron, and more rarely manganese, may substitute for copper. Although acicular monoclinic crystals have been reported, the wide variation in the composition of analyzed…

A fibrous asbestos mineral with a tubular morphology for each fibril. Chrysotile (Fig. 1) is a member of the serpentine mineral group, as are antigorite and lizardite. Chrysotile aggregates make up serpentine asbestos, which is the most common type…

The collective processes by which metabolic materials are transported from one region of an organism to another region. The circulatory system, often termed the cardiovascular system, is responsible for the functions of circulation, allowing…

Pathologies and abnormal conditions that involve the circulatory system. Circulation disorders include both localized disturbances of circulation and diseases of the circulatory system (Fig. 1). Circulation 1 Illustration of the thickening and…

Fine-grained, hydrous, layer silicates that belong to the larger class of sheet silicates known as phyllosilicates. Clay minerals form on Earth in many different environments, including the weathering environment, the sedimentary environment, and…

The long-term records of precipitation, temperature, wind, and all other aspects of the Earth's climate. Climate, like the Earth itself, has a history extending over several billion years. Climatic changes have occurred at time scales ranging from…

The adaptation of microbiological techniques to the study of the etiological agents of infectious disease. Clinical microbiology, also termed medical microbiology, is a branch of science concerned with the investigation of pathogenic microorganisms…

The major fibrous protein in animals. Collagen is present in all types of multicellular animals and is probably the most abundant animal protein in nature. It is estimated that collagen accounts for about 30% of the total human body protein.…

A loosely defined term used for a sedimentary mineral segregation that may range in size from millimeters to meters. Concretions (see illustration) are usually distinguished from the sedimentary matrix enclosing them by a difference in mineralogy,…

Structural or functional abnormalities of the body that develop during embryogenesis and the fetal period; also called birth defects. Congenital anomalies, or birth defects, are conditions present at or from birth that deviate from the body's normal…

The consolidated equivalent of gravel. Conglomerates are aggregates of more or less rounded particles greater than 2 mm in diameter. Frequently they are divided on the basis of size of particles into pebble (fine), cobble (medium), and boulder…

Any living or fossil member of the plant division Pinophyta, consisting chiefly of woody trees or shrubs that produce cones and evergreen needles. Conifers are predominantly cone-bearing, evergreen trees and shrubs belonging to the order Pinales, as…

One of the four primary tissues of the body. Connective tissue (Fig. 1) provides the solid foundation required for underpinning, binding, and protecting tissues and organs of the body. Connective tissue differs from the other three tissues…

Any of a group of diseases involving connective tissue as the pathological target. Connective tissue comprises one of the primary tissues of the body and is distinguished by an abundance of fibrillar and nonfibrillar extracellular components, which…

The response of corals to environmental stress in which they eject the single-cell algae that live within the transparent coral tissue, making the white coral skeleton visible. Corals (Fig. 1) are animals that live in a partnership (symbiosis) with…

An orthorhombic magnesium aluminosilicate mineral of composition Mg2[Al4Si5O18], a = 1.710 nanometers, b = 0.973 nm, c = 0.936 nm, Z = 4, space group Cccm. The crystal structure is related to beryl; magnesium is octahedrally coordinated by oxygen…

Any of a group of viruses that cause upper and lower respiratory tract diseases, gastroenteritis, hepatitis, and central nervous system infections in birds and mammals, including humans. Coronaviruses (Fig. 1) comprise a major group of common animal…

A genome-engineering technique that allows precise targeting of specific stretches of genetic code and editing of DNA at designated locations. The CRISPR/Cas9 system has been used as a general tool to engineer the genomes of many different…

A mineral with chemical composition Na3AlF6. Although it crystallizes in the monoclinic system, cryolite has three directions of parting at nearly 90°, giving it a pseudocubic aspect. Hardness is 2½ on Mohs scale and the specific gravity is 2.95.…

A large and heterogeneous group of photosynthetic microorganisms, formerly referred to as blue-green algae. Cyanobacteria (see illustration) are microscopic bacteria capable of photosynthesis. Originally and inaccurately termed blue-green algae,…

A member of a class of proteins containing hemes as prosthetic groups that catalyze oxidation–reduction reactions. Cytochromes are heme-containing proteins (see illustration) that play vital roles in electron transport within cells. It should be…

A system of protein filaments found in the cytoplasm of cells that is responsible for cell shape, support, movement, and organization. The protein fibers that compose the structural framework of a cell are collectively known as the cytoskeleton. The…

Marine animal life inhabiting the portion of the oceans below the upper limit of the continental slopes. The marine animals that make up the deep-sea fauna live below the photic zone of the oceans. In general, the deep sea may be regarded as that…

Inflammation of the skin; also known as eczema. Dermatitis, or eczema, is a general medical term that describes irritation or inflammation of the skin. Typically, the inflammation of the skin involves redness and scaling (see illustration). In acute…

An extensive field of investigation that includes the study of all changes associated with an organism as it progresses through its life cycle. Developmental biology investigates all of the life-cycle mechanisms, processes, and stages that determine…

The branch of genetics that studies how genes control embryonic development. Advances in the field of developmental genetics have emphasized the degree of conservation throughout evolution of the genes (basic units in inheritance consisting of DNA)…

The passage of loose or watery stools, usually at more frequent than normal intervals. Diarrhea is characterized by more-frequent and abnormally loose or watery stools (greater than 227 g or 8 oz per 24 h). The two principal mechanisms responsible…