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In science, a theory is a reasonable explanation of observed evnets that are related. A theory often involves an imaginary model that helps scientists picture the way an obseved event could be produced. A good example of this is found in the kinetic molecular theory, in which gases are pictured as being made up of many small particles that are in constant motion. A useful theoty, in addition to explaining past observations, helps to predict events that have not as yet been observed. After a theory has been publicized, scientists design experiments to test the theory. If observations confirm the scientists' predictions, the theory is supported. If observations do not confirm the predictions, the scientists must search further. There may be a fault in the experiment, or the theory may have to be revised or rejected. Science involves imagination and creative thinking as well as collecting information and performing experiments. Facts by themselves are not science. As the mathematician Jules Henri Poincare said: "Science is built with facts just as a house is built with bricks, But a collection of facts cannot be called science any more than a pile of bricks can be called a house." Most scientists start an investigation by finding out what other scientists have learned about a particular problem. After known facts have been gathered, the scientist comes to the part of the investigation that requires considerable imagination. Possible solutions to the problem are formulated. these possible solutions are called hypotheses. In a way, any hypothesis is a leap into the unknown. It extents the scientist's thinking beyond the known facts. The scientist plans experiments, performs calculations and makes observations to test hypotheses. For without hypotheses, further investigation lacks purpose and direction. When hypotheses are confirmed, they are incorporated into theories. 重点单词： By the mid-nineteenth century, the term "icebox" had entered the American language, but ice was still only beginning to affect the diet of ordinary citizens in the United States. The ice trade grew with the growth of cities. Ice was used in hotels, taverns, and hospitals, and by some forward-looking city dealers in fresh meat, fresh fish, and butter. After the Civil War(1860-1865), as ice used to refrigerate freight cars, it also came into household use. Even before 1880, half the ice sold in New York, Philadelphia, and Baltimore, and one-third of that sold in Boston and Chicago, went to families for their own use. This had become possible because a new household convenience, the icebox, a precursor of the modern refrigerator, had been invented. Making an efficient icebox was not as easy as we might now suppose. In the early nineteenth century, the knowledge of the physics of heat, which was essential to a science of refrigeration, was rudimentary. The commonsense notion that the best icebox was one that prevented the ice from melting was of course mistaken, for it was the melting of the ice that performed the cooling. Nevertheless, early efforts to economize ice included wrapping the ice in blankets, which kept the ice from doing its job. Not until near the end of the nineteenth century did inventors achieve the delicate balance of insulation and circulation needed for an efficient icebox. But as early as 1803, an ingenious Maryland farmer. Thomas Moore, had been on the right track. He owned a farm about twenty miles outside the city of washington, for which the village of Georgetown was the market center. When he used an icebox of his own design to transport his butter to market, he found that customers would pass up the rapidly melting stuff in the tubs of his competitors to pay a premium price for his butter, still fresh and hard in neat, one-pound bricks. One advantage of his icebox, Moore explained, was that farmers would no longer have to travel to market at night in order to keep their produce cool. 重点单词： Aside from perpetuating itself, the sole purpose of the American Academy and Institute of Arts and Letters is to "foster, assist and sustain an interest" in literature, music, and art. This it does by enthusiastically handing out money. Annual cash awards are given to deserving artists in various categories of creativity: architecture, musical composition, theater, novels, serious poetry, light verse,painting,sculpture. One award subsidizes a promising American writer's visit to Rome. There is even an award for a very good work of fiction that falled commercially-once won by the young John Updike for The poorhouse Fair and, more recently, by Alice Walker for In Love and Trouble. The awards and prizes total about $750,000 a year, but most of them range in size from $5,000 to $12,500, a welcome sum to many young practitioners whose work may not bring in that much in a year. One of the advantages of the awards is that many go to the struggling artists, rather than to those who are already successful. Members of the Academy and Institute are not eligible for any cash prizes. Another advantage is that, unlike the National Endowment for the Arts or similar institutions throughout the world, there is no government money involved. Awards are made by committee. Each of the three departments--Literature (120 members), Art(83), Music(47)--has a committee dealing with its own field. Committee membership rotates every year, so that new voices and opinions are constantly heard. The most financially rewarding of all the Academy-Institute awards are the Mildred and Harold Strauss Livings. Harold Strauss, a devoted editor at Alfred A. Knopf, the New York publishing house, and Mildred Strauss, his wife, were wealthy any childless. They left the Academy-Institute a unique bequest: for five consecutive years, two distinguished (and financially needy) writers would receive enough money so they could devote themselves entirely to "prose literature" (no plays, no poetry, and no paying job that might distract). In 1983, the first Strauss Livings of $35,000 a year went to short-story writer Raymond Carver and novelist-essayist Cynthia Ozick. By 1988, the fund had grown enough so that two winners, novelists Diane Johnson and Robert Stone, each got $50,000 a year for five years. 重点单词： Archaeological records-paintings, drawings and carvings of humans engaged in activities involving the use of hands-indicate that humans have been predominantly right-handed for more than 5,000 years. In ancient Egyptian artwork, for example, the right hand is depicted as the dominant one in about 90 percent of the examples. Fracture or wear patterns on tools also indicate that a majority of ancient people were right-handed. Cro-Magnon cave paintings some 27,000 years old commonly show outlines of human hands made by placing one hand against the cave wall and applying paint with the other. Children today make similar outlines of their hands with crayons on paper. With few exceptions, left hands of Cro-Magnans are displayed on cave walls, indicating that the paintings were usually done by right-handers. Anthropological evidence pushes the record of handedness in early human ancestors back to at least 1.4 million years ago. One important line of evidence comes from flaking patterns of stone cores used in toolmaking: implements flaked with a clockwise motion (indicating a right-handed toolmaker) can be distinguished from those flaked with a counter-clockwise rotation (indicating a left-handed toolmaker). Even scratches found on fossil human teeth offer clues. Ancient humans are thought to have cut meat into strips by holding it between their teeth and slicing it with stone knives, as do the present-day Inuit. Occasionally the knives slip and leave scratches on the users' teeth. Scratches made with a left-to-right stroke direction (by right-handers) are more common than scratches in the opposite direction (made by left-handers). Still other evidence comes from cranial morphology: scientists think that physical differences between the right and left sides of the interior of the skull indicate subtle physical differences between the two sides of the brain. The variation between the hemispheres corresponds to which side of the body is used to perform specific activities. Such studies, as well as studies of tool use, indicate that right- or left-sided dominance is not exclusive to modern Homo sapiens. Population of Neanderthals, such as Homo erectus and Homo habilis, seem to have been predominantly right-handed, as we are. 重点单词： Plants are subject to attack and infection by
a remarkable variety of symbiotic species and
have evolved a diverse array of mechanisms
designed to frustrate the potential colonists.
These can be divided into preformed or passive
defense mechanisms and inducible or active
systems. Passive plant defense comprises physical and
chemical barriers that prevent entry of
pathogens, such as bacteria, or render
tissues unpalatable or toxic to the invader.
The external surfaces of plants, in addition to being covered
by an epidermis and a waxy cuticle, often
carry spiky hairs known as trichomes, which either
prevent feeding by insects or may even puncture
and kill insect larvae. Other trichomes are sticky
and glandular and effectively trap and immobilize
insects. If the physical barriers of the plant are breached,
then preformed chemicals may inhibit or kill the intruder,
and plant tissues contain a diverse array of toxic or
potentially toxic substances, such as resins,
tannins, glycosides, and alkaloids, many of which
are highly effective deterrents to insects that feed
on plants. The success of the Colorado beetle in infesting
potatoes, for example, seems to be correlated
with its high tolerance to alkaloids that normally repel
potential pests. Other possible chemical defenses, while
not directly toxic to the parasite, may inhibit some
essential step in the establishment of a parasitic
relationship. For example, glycoproteins in plant cell walls
may inactivate enzymes that degrade cell
walls. These enzymes are often produced by bacteria and fungi.
Active plant defense mechanisms are comparable to the
immune system of vertebrate animals, although
the cellular and molecular bases are fundamentally different.
Both, however, are triggered in reaction to intrusion,
implying that the host has some means of recognizing
the presence of a foreign organism. The most dramatic
example of an inducible plant defense reaction is the hypersensitive
response. In the hypersensitive response, cells undergo
rapid necrosis--that is, they become diseased
and die--after being penetrated by a parasite; the parasite
itself subsequently ceases to grow and is therefore
restricted to one or a few cells around the entry site.
Several theories have been put forward to explain the basis
of hypersensitive resistance. 重点单词： |
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