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 …
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…
The issues summoned up by the term pacifism are complex and varied because different concepts, traditions, and definitions exist throughout the world, often creating misunderstanding and confusion—sometimes intentionally so. For example, the term may be used pejoratively in political debates by individuals seeking to portray opponents who refuse to support a specific military action, or by …
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 …
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…
Because it refers neither to a single political ideology nor a clearly discernible philosophical tradition, Pan-Africanism is difficult to define. Many scholars avoid defining it, noting that black internationalism has varied drastically according to time and place. Indeed, various conceptions of Pan-Africanism have been aligned with disparate political and theoretical positions, from largely reli…
Pan-Arabism is the concept that all Arabs form one nation and should be politically united in one Arab state. The intellectual foundations of pan-Arabism were laid down in the early decades of the twentieth century, in the context first of Arab alienation from Ottoman rule and later in response to the imperialist partition of the Arab provinces of the Ottoman Empire after World War I. The doctrine…
A term of European origin, pan-Islamism denotes the intellectual and institutional trends toward Islamic unity that emerged among Muslim peoples, starting in the mid–nineteenth century and continuing throughout the twentieth century. The need for a unified Islamic identity was a product of the challenges posed by Western intervention in and domination of Muslim societies during the colonial…
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 …
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…
Paradigm is the key term in Thomas Kuhn's (1922–1996) very influential book, The Structure of Scientific Revolutions (1962). As is frequently the case when new ideas are presented, Kuhn took an existing term and gave it a specialized meaning. The term paradigm now occurs frequently in every kind of discourse, usually to mean something like "way of thinking" or "a…
The word paradise develops in Western languages from the Greek word paradeisos, the old Persian word pairidaeza, and the modern Arabic and Persian firdaus, all of which originally denoted a walled garden. In the arid environment of the Near East, a garden must be carefully and laboriously constructed with watercourses for irrigation, and its precious flowers and fruits protected from theft by a su…
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…
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.…
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…
Patriotism is one of a large class of words that are linked to the virtues of membership. To participate in relations of, for example, friendship, community, nationhood, citizenship, or marriage implies normative conventions. In other words, there are value expectations built into such membership. One important dimension of any membership relation is an expectation of loyalty. Fidelity or loyalty …
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 …
Currently concentrated in Africa, Asia, and Latin America, the peasantry has been defined differently by various scholars, depending on the degree of emphasis placed on any one of several characteristics. Definitions of the peasantry embrace some of the following characteristics: ownership and use of land, production methods, subordination to other social sectors, and the degree of integration int…