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1995年10月托福阅读全真试题

时间:2021-09-06 17:00:39 托福英语 我要投稿

1995年10月托福阅读全真试题

Questions 1-13

1995年10月托福阅读全真试题

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 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