题目内容
Plants and Mankind
Botany, the study of plants, occupies a peculiar position in the history of human knowledge. We don't know what our Stone Age ancestors knew about plants, but from what we can observe of preindustrial societies that still exist, a detailed learning of plants and their properties must be extremely ancient. They have always been enormously important to the welfare of people, not only for food, but also for clothing, weapons, tools, dyes, medicines, shelter, and many other purposes. Tribes living today in the jungle of the Amazon recognize hundreds of plants and know many properties of each. To them botany has no name and is probably not even recognized as a special branch of "knowledge" at all.
Unfortunately, the more industrialized we become the farther away we move from direct contact with plants. And the less distinct our knowledge of botany grows. Yet everyone comes unconsciously on an amazing amount of botanical knowledge, and few people will fail to recognize a rose, an apple, or an orchid. When our Neolithic ancestors, living in the Middle East about 10,000 years ago, discovered that certain grasses could be harvested and their seeds planted for richer yields the next season, the first great step in a new association of plants and humans was taken. Grains were discovered and from them flowed the marvel of agriculture: cultivated crops. From then on, humans would increasingly take their living from the controlled production of a few plants, rather than getting a little here and a little there from many varieties that grew wild and the accumulated knowledge of tens of thousands of years of experience and intimacy with plants in the wild would begin to fade away.
botany植物学
marvel令人惊奇的事(人)，奇迹
It is logical that a detailed learning of plants and their properties must be extremely ancient.
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If the salinity(含盐量) of ocean waters is analyzed, it is round to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation—conversion of liquid water to water vapor. In this manner, the salinity is increased, since the sales stay behind. If this is carried to the extreme, of course, white crystals of salt would be left behind; this, by the way, is how much of the table salt we use is actually obtained.
The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.
Normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be altered is associated with the formation and melting of sea ice. When seawater is frozen, the dissolved materials are left behind. In this manner, seawater directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water.
In the Weddell Sea, off Antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.
What does the passage mainly discuss？
Taking a cell, practically any cell, from your body, the theory goes, and through appropriate biological tinkering(摆弄) you can cause it to grow into a duplicate of yourself—identical from eyelashes to toenails. No need for procreational(生育的) sex any more;(51) this system, you can neatly reproduce yourself without a(52). Human cloning, it is called. Science fact or science fiction？ What would happen if human cloning became a(53)？ One favorite scenario is the(54)of a new(55)of Hitlers—or Einsteins. Scientists quickly disclaim the possibility. "It is more than genetic make-up that makes an individual." Says Markert of Yale University, "We are all products of a particular(56)era and of a special environment, with so many minute things(57)the way we develop each and every day, even in the womb, that a duplicate background--and therefore a duplicate(58)could never be created."
Already biologists studying the cell's(59)workings and the various methods of cloning have made discoveries that may ultimately lead to breakthroughs(60)the fight against cancer, control of the aging process, and the conquest of more than 100 presently incurable human genetic diseases. To(61)cloning-related research would mean closing the door(62)and important area of knowledge. To continue to probe the secrets of the cell, however, is perhaps to(63)the secret of human cloning. And, given the nature of man, if it can be done it will be done. What then is the(64)？
Says Congressman Rogers: "It is clear that human cloning is not yet possible. The day when it will be—if ever—is far(65). For now, at least, the benefits of cell-biology research outweigh the risks."
The author resolves the question of whether heat pumps run counter to the principle of energy conservation by ______.
Which of the following statements about the salinity of a body of water can best be inferred from the passage？
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