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. As water is lost from the surface of the leaves, a negative pressure, or tension, is created. The pull on a rising column of water in a plant results from the evaporation of water at the top of the plant. 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. Furthermore, the conifers, which are among the tallest trees, have unusually low root pressures. But root pressure is not nearly great enough to push water to the tops of tall trees. 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. Some botanists hypothesized that the living cells of plants in which all the cells are killed can still move water to appreciable heights. ![]() Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. But plants can move water much higher, the sequoia tree can pump water to its very top, more than 100 meters above the ground. ![]() 95年10月TOFEL阅读(-24) C Questions 1-13 Atmospheric pressure can support a column of water up to 10 meters high.
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