Generally, the term plane, together with point, line, and solid, is considered an undefined term. Every definition in mathematics attempts to use simpler and better understood terms to define more complex ones. As the terms to be defined become ever simpler, this eventually becomes impossible. The simplest terms are so well understood that there is little sense in attempting a formal definition, s…
The Plane family is a family of trees and large shrubs known to botanists as the Platanaceae. This family has a single genus, Platanus, and 7-10 different species. The two most familiar species are the American sycamore (Platanus occidentalis), which is native to eastern and central United States, and the London plane, a hybrid tree species which is commonly planted as an ornamental in the United …
A planet is a relatively cold body that orbits a star. Planets are thought to have formed from the same gas and dust that condensed to make the parent star. They can be seen by eye and telescope because of the light they reflect from their star. The planets themselves often have orbiting moons and dust rings. The nine planets in our solar system that are in elliptical orbits near the ecliptic plan…
Is there another planet beyond Pluto? Prior to 1781 that question could have been asked in regard to Saturn. In that year, Sir William Herschel discovered Uranus, after detecting what he believed to be a comet. Calculations to determine the orbit of Uranus were made, and the planet was found to conform to the "law" of planetary distances suggested by Johann Elert Bode (1747-1826). Ho…
The term planetary atmosphere refers to the envelope of gases that surrounds any of the planets in our solar system. A complete understanding of the properties of a planet's atmosphere involves a number of different areas including atmospheric temperatures, chemical composition of the atmosphere, atmospheric structure, and circulation patterns within the atmosphere. The study of planetary a…
Planetary geology is a branch of geology devoted to the study of structure, composition, processes, and origin of major and minor planetary bodies in our solar system and beyond, and to the effects of interaction between planetary bodies within our solar system. Planetary geology interfaces with many other fields including astronomy, biology, chemistry, and physics. Planetary geologists work in th…
High-density interstellar dust or clouds are referred to as nebulae. These nebulae, both dark and luminous, are equally important since the chemical analyses of these objects contribute significantly to the study of cosmic abundances. Bright or incandescent nebulae, just as dark nebulae, are not self-luminous. It is the star or stars imbedded in these nebulae which produce the luminous objects and…
Plant breeding began when early humans saved seeds and planted them. The cultural change from living as nomadic hunter-gatherers, to living in more settled communities, depended on the ability to cultivate plants for food. Present knowledge indicates that this transition occurred in several different parts of the world, about 10,000 years ago. Today, there are literally thousands of different cult…
Like human beings and other animals, plants are subject to diseases. In order to maintain a sufficient food supply for the world's population, it is necessary for those involved in plant growth and management to find ways to combat plant diseases that are capable of destroying crops on a large scale. There are many branches of science that participate in the control of plant diseases. Among…
Plasma is the liquid portion of blood which is about 90% water and transports nutrients, wastes, antibodies, ions, hormones, and other molecules throughout the body. Humans typically have about 1.3-1.5 gal (5-6 l) of blood, which is about 55% plasma and 45% cells-red blood cells, white blood cells, and platelets. The plasma of humans and other vertebrates is nearly colorless, since the red color o…
Plastic surgery is the specialized branch of surgery concerned with repairing deformities, correcting functional deficits, and enhancing appearance. Unlike most surgical specialties, plastic surgery is not confined to one specific anatomical or functional area of the body. Often, plastic surgery is classified as either reconstructive or aesthetic surgery. All plastic surgery procedures seek to res…
In the twentieth century, the term plastic has come to refer to a class of materials that, under suitable conditions, can be deformed by some kind of shaping or molding process to produce an end product that retains its shape. When used as an adjective, the term plastic (from Greek plastikos meaning to mold or form) describes a material that can be shaped or molded with or without the application …
The term platonic solids refers to regular polyhedra. In geometry, a polyhedron, (the word is a Greek neologism meaning many seats) is a solid bounded by plane surfaces, which are called the faces; the intersection of three or more edges is called a vertex (plural: vertices). What distinguishes regular polyhedra from all others is the fact that all of their faces are congruent with one another. (I…
A principle feature of Platonism, which refers to the doctrines or philosophies influenced by Plato, is the belief in the existence of a distinction between the world that appears to the senses and a real realm that can be grasped only by the intellect. This latter realm contains the transcendental Ideas or Forms which, although existing independently of both the empirical world and human consciou…
The platypus is an egg laying mammal that is well adapted to the water. Physically, it looks like a mole or otter, with a beaver's flattened tail and a duck's bill. It also has short, powerful legs and webbed feet. While the fur on its back is dense, bristly, and reddish or blackish brown, the fur on its underbelly is soft and gray. Its eyes are very small, and it does not have exter…
Plovers are shore birds in the family Charadriidae, order Charadriiformes. Plovers have short, straight bills, with a small swelling towards the tip. Their wings are pointed at the tips, usually with a white wing-stripe on the underside, and the flight of these birds is fast and direct. Plovers and the closely related sandpipers (family Scolopacidae) are affectionately known as "peeps…
The ninth planet from the Sun, Pluto is one of the least well understood objects in the solar system. It is the smallest of the major planets, and has a most unusual orbit. Pluto's companion moon, Charon, is so large that the pair essentially form a binary system. How the Pluto-Charon system formed and how the system acquired its special 2-to-3 orbital resonance with Neptune are unanswered …
Pneumonia is an infection of the lung, and can be caused by nearly any class of organism known to cause human infections, including bacteria, viruses, fungi, and parasites. In the United States, pneumonia is the sixth most common disease leading to death, and the most common fatal infection acquired by already hospitalized patients. In developing countries, pneumonia ties with diarrhea as the most…
Podiatry is a medical specialty that focuses on the diagnosis and treatment of foot disease and deformity. The term is from the Greek word for foot (podos) and means "to heal the foot." Until recent years this specialty was called chiropody, literally meaning "to heal the hand and foot." References to physicians who treated abnormalities or injuries in the foot are foun…
As a science of interpretation, poetics has consistently been concerned with delineating its proper field of study ("What is a poetic text?") and then subdividing it into genres ("What kind of poetic text is under consideration and what interpretive approaches does it select?"). Although classical writers on the subject of literary types failed to formulate a comprehens…
A point source is a situation where large quantities of pollutants are emitted from a single, discrete source, such as a smokestack, a sewage or thermal outfall into a waterbody, or a volcano. If the emissions from a point source are large, the environment will be characterized by strong but continuous gradients of ecological stress, distributed more-or-less concentrically around the source, and d…
A chemical is said to be a poison if it causes some degree of metabolic disfunction in organisms. Strictly speaking, a toxin is a poisonous chemical of biological origin, being produced by a microorganism, plant, or animal. In common usage, however, the words poison and toxin are often used interchangeably, and in this essay they are also treated as synonyms. It is important to understand that pot…
One of the several systems for addressing points in the plane is the polar-coordinate system. In this system a point P is identified with an ordered pair (r,θ) where r is a distance and θ an angle. The angle is measured counter-clockwise from a fixed ray OA called the "polar axis." The distance to P is measured from the end point O of the ray. This point is called the &…
The polar ice caps cover the north and south poles and their surrounding territory, including the entire continent of Antarctica in the south, the Arctic Ocean, the northern part of Greenland, parts of northern Canada, and bits of Siberia and Scandinavia also in the north. Polar ice caps are dome-shaped sheets of ice that feed ice to other glacial formations, such as ice sheets, ice fields, and ic…
There are three viruses responsible for the infectious disease now called poliomyelitis. It has been called infantile paralysis and is now commonly referred to as polio. While the disease usually afflicts young children, adults can succumb to it also. A notable example of polio in an adult was the case of President Franklin Delano Roosevelt, the thirty-second president of the United States. He con…
Protests disrupt the peace, stop business as usual, and cause disquiet. Protesters in effect withdraw their consent to the way things are. Even if people are poor, unemployed, or seemingly without power, when organized together they can become a force that troubles the powerful. When the powerful are inconvenienced (or worse) by sit-ins, strikes, marches, and occupations, they will often negotiate…
Although in the early twenty-first century representative government is synonymous with democracy, the concept of political representation arose separately from the idea of the rule of the people. Broadly political representation refers to an arrangement whereby one is enabled to speak and act with authority in the behalf of some other. There are two issues that must be addressed in any theory of …
Political science, as currently conceived, is a relatively new concept that dates to the nineteenth-century United States. Prior to this time, the study of politics in the West remained a part of natural philosophy, and it tended to focus on philosophical, historical, and institutional approaches. The ancient Greek philosopher Aristotle is often named as the first "political scientist,�…
The term pollution is derived from the Latin pollutus, which means to be made foul, unclean, or dirty. Anything that corrupts, degrades, or makes something less valuable or desirable can be considered pollution. There is, however, a good deal of ambiguity and contention about what constitutes a pollutant. Many reserve the term for harmful physical changes in our environment caused by human actions…
Pollution control is the process of reducing or eliminating the release of pollutants into the enviroment. It is regulated by various environmental agencies which establish pollutant discharge limits for air, water, and land. Air pollution control strategies can be divided into two categories, the control of particulate emission and the control of gaseous emissions. There are many kinds of equipme…
Polybrominated biphenyls (or PBBs) are chemicals used to make plastics flame retardant. In Michigan in the early 1970s one type of PBB was accidentally mixed into livestock feed and fed to farm animals, resulting in the sickening and/or death of tens of thousands of animals. A large portion of Michigan's nine million residents became ill as a result of eating contaminated meat or poultry. P…
Polycyclic aromatic hydrocarbons, or polynuclear aromatic hydrocarbons, are a family of hydrocarbons containing two or more closed aromatic ring structures, each based on the structure of benzene. The simplest of these chemicals is naphthalene, consisting of two fused benzene rings. Sometimes there is limited substitution of halogens for the hydrogen of polycyclic aromatic hydrocarbons, in which c…
A polyhedron is a three-dimensional closed surface or solid, bounded by plane figures called polygons. The word polyhedron comes from the Greek prefix poly- , which means "many," and the root word hedron which refers to "surface." A polyhedron is a solid whose boundaries consist of planes. Many common objects in the world around us are in the shape of polyhedrons. The c…
Polymers are made up of extremely large, chainlike molecules consisting of numerous, smaller, repeating units called monomers. Polymer chains, which could be compared to paper clips linked together to make a long strand, appear in varying lengths. They can have branches, become intertwined, and can have cross-links. In addition, polymers can be composed of one or more types of monomer units, they …
There are various words that are used in conjunction with polynomials. The degree of a polynomial is the exponent of the highest power of x. Thus the degree of The most general form for a polynomial in one variable is Similar definitions apply to polynomials in 3, 4, 5 ellipsevariables but the term "polynomial" without qualification usually refers to a polynomial in one var…
The concept of polytheism was a creation of the Enlightenment. Before then, Europeans had characterized the religious universe in terms of Christianity, Judaism, paganism, and (eventually) Islam. As late as the sixteenth century, the Franciscan friar Bernardino de Sahagun, in his remarkably detailed and sensitive account of pre-Columbian Mexican religion and culture, equated various Aztec diviniti…
Poppies belong to a small family of flowering plants called the Papaveraceae. Poppies are annual, biennial, or perennial herbs, although three New World genera (Bocconia, Dendromecon, and Romneya) are woody shrubs or small trees. The leaves are alternate, lack stipules, and are often lobed or deeply dissected. The flowers are usually solitary, bisexual, showy, and crumpled in the bud. The fruit is…
Perceptions of the meaning, implications, and control of reproduction and sexuality, at both the individual and community levels, are among the most important, but also the most challenging, in the history of ideas. Changes in human population helped shape the twentieth century and are creating new tensions in the twenty-first, but, despite the profound significance of childbearing to individuals …
The numbers of humans on Earth have increased enormously during the past several millennia, but especially during the past two centuries. By the end of the twentieth century, the global population of humans was 6.0 billion. That figure is twice the population of 1960, a mere 30 years earlier. Moreover, the human population is growing at about 1.5% annually, equivalent to an additional 89 million p…
The number of human beings on Earth has increased enormously during the past several millennia, but especially during the last two centuries: from 1850 to 1950 the human population doubled, from 1.265 billion to 2.516 billion, and has more than doubled from 1950 to the present. Moreover, it is likely that the human population—presently at over 6.215 billion—will continue to increase.…
If the term populism was initially borrowed from radical farmers' movements in the United States in the late nineteenth century to describe early-twentieth-century political developments in Latin America, since then it has certainly acquired a special relevance for understanding this region's politics. The golden era for Latin American populism is usually cited as the 1930s to the 19…
Populism (from the Latin term populus, usually translated as "the people") is the name of a group of ideologies that stresses the need for a more equitable distribution of economic, political, and cultural power. Populists argue that an elite of some form or another holds an unfair concentration of political and economic power and typically that government intervention is required to…
Two families of rodents are called porcupines. They all have at least some hair modified into quills. The Old World porcupines belong to family Hystricidae of Europe, Asia, and Africa. The New World porcupines are 10 species of forest dwellers of the family Erethizontidae. The most common of these is the North American porcupine (Erthizon dorsatum). The name porcupine means "quill pig,�…