背诵篇章 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 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 whic salinity may be altered is associated with the formation and melting of sea ice. When sea water is frozen, the dissolved materials are left behind. In this manne sea water directly materials are left behind. In this manner, sea water directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. of course, when ths ice melts, it will tend to decrease the salinity of the surrounding water. In the Weddell Sea Antarctica, the densest water in the oceans is formed as a result of this freeing 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 salinity/s3 ' insti; s3 linati/ n[U] the high salinity of sea water海水的高含盐量 >>>saline /'seilain; US -li: n; selin 1adj[arb作定语](fm文) containing sa;saty含盐的;威的: a saline lake盐湖 saline springs盐泉 * saline solution, eg as used for gargling, storing contact lenses,εtc盐溶液(如用于漱喉、存放隐形眼镜等) 2.n[U](meda医) solution of salt and water盐水 40 Cohesion-tension Theory Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher; the oia tree can pump water to its very top more than 100 meters above the ground. Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. Some botanists hypothesized that the living cells of plan ts acted as pumps. But many experiments demonstrated that the stems of plants in which all the cells are killed can still move water to appreciable heights. Other explanations for the movement of wa ter in plants have been based on root pr on the water from the roots at the bottom of the plant. But root pressure is not nearly great enough to push water to the tops of tall trees. Furthermore, the conifers, which are among the tallest trees, have unusually low root pressures If water is not pumped to the top of a tall tree, and if it is not pushed to the top of a tall tree, then we may ask: how does it get there? According to the currently accepted cohesion-tension theory, water is pulled there. The pull on a rising column of water in a plant results from the evaporation of water at the top of the plant. As water is lost from the surface of the leaves, a negative pressure, or tension, is created. The evaporated water is replaced by water moving from inside the plant in unbroken columns that extend from the top of a plant to its roots. The same forces that create surface tension in any sample of water are esponsible for the maintenance of these unbroken columns of water. When water is confined in tubes of very small bore, the forces of cohesion(the attraction between water molecules)are so great that the strength of a column of water compares with the strength of a steel wire of the same diameter. This cohesive strength permits columns of water to be pulled to great heig ht without being broken 41. American black bears American black bears appear in a variety of colors despite their name. In the eastern part of their range, most of these brown, red, or even yellow coats. To the north, the black bear s actually gray or white in color. Even in the same litter, both brown and Black bears are the smallest of all American bears, ranging n length from five to six feet, weighing from three hundred to five hundred pounds Their eyes and ears are small and their eyesight and hearing are not as good as their sense of smell.背 诵 篇 章 17 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 sea water is frozen, the dissolved materials are left behind. In this manner, sea water directly materials are left behind. In this manner, sea water 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 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. NOTE： salinity / sэ'linэti; sэ`linэti/ n [U] the high salinity of sea water 海水的高含盐量. -→>>saline / 'seilain; US -li:n; `selin/ 1.adj [attrib 作定语] (fml 文) containing salt; salty 含盐的; 咸的: * a saline lake 盐湖 * saline springs 盐泉 * saline solution, eg as used for gargling, storing contact lenses, etc 盐溶液（如用于漱喉、存放隐形眼镜等）. 2. n [U] (medical 医) solution of salt and water 盐水. 40 Cohesion-tension Theory Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia tree can pump water to its very top more than 100 meters above the ground. Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. Some botanists hypothesized that the living cells of plan ts acted as pumps. But many experiments demonstrated that the stems of plants in which all the cells are killed can still move water to appreciable heights. Other explanations for the movement of water in plants have been based on root pressure, a push on the water from the roots at the bottom of the plant. But root pressure is not nearly great enough to push water to the tops of tall trees. Furthermore, the conifers, which are among the tallest trees, have unusually low root pressures. If water is not pumped to the top of a tall tree, and if it is not pushed to the top of a tall tree, then we may ask: how does it get there? According to the currently accepted cohesion-tension theory, water is pulled there. The pull on a rising column of water in a plant results from the evaporation of water at the top of the plant. As water is lost from the surface of the leaves, a negative pressure, or tension, is created. The evaporated water is replaced by water moving from inside the plant in unbroken columns that extend from the top of a plant to its roots. The same forces that create surface tension in any sample of water are responsible for the maintenance of these unbroken columns of water. When water is confined in tubes of very small bore, the forces of cohesion (the attraction between water molecules) are so great that the strength of a column of water compares with the strength of a steel wire of the same diameter. This cohesive strength permits columns of water to be pulled to great heig hts without being broken. 41.American black bears American black bears appear in a variety of colors despite their name. In the eastern part of their range, most of these brown, red, or even yellow coats. To the north, the black bear is actually gray or white in color. Even in the s ame litter, both brown and black furred bears may be born. Black bears are the smallest of all American bears, ranging in length from five to six feet, weighing from three hundred to five hundred pounds Their eyes and ears are small and their eyesight and hearing are not as good as their sense of smell