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Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher. the sequoia tree

admin2010-10-14  38
问题     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 plants acted as pumps. But many experiments demonstrated that the stem 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 tress, 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 form the evaporation of water at the top of the plant. As water is lost form 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 heights without being broken.
How do botanists know that root pressure is not the only force that moves water in plants?
选项 A、Some very tall trees have weak root pressure.
B、Root pressures decrease in winter.
C、Plants can live after their roots die.
D、Water in a plant’s roots is not connected to water in its stem.
答案A
解析 主旨题。植物学家如何得知根压不是植物内运输水分的惟一力量?A项一些很高的树，根压却很小。B项根压在冬季降低。C项植物在根死后仍能存活。D项植物根中的水与茎中的水互不相连。
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