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The word 'demonstrated' in line 6 is closest in meaning to (A) ignored (B) showed (C) disguised (D) distinguished 4. (D) Botanists have changed their theories of how water moves in plants.ģ. (C) Atmospheric pressure draws water to the tops of tall trees. (B) Botanists have proven that living cells act as pumps. Which of the following statements does the passage support? (A) Water is pushed to the tops of trees. The passage answers which of the following questions? (A) What is the effect of atmospheric pressure on foliage? (B) When do dead cells harm plant growth? (C) How does water get to the tops of trees? (D) Why is root pressure weak?Ħ.

(D) Water in a plant's roots is not connected to water in its stem.Ģ. (C) Plants can live after their roots die. (A) Some very tall trees have weak root pressure. How many theories does the author mention? (A) One (B) Two (C) Three (D) Four This cohesive strength permits columns of water to be pulled to great heights without being broken. 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. The same forces that create surface tension in any sample of water are responsible for the maintenance of these unbroken columns of water. 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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