1995年10月托福阅读全真试题(doc13).doc

Questions 1-13

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 columns of
water to be pulled to great heights wIThout being broken.

1. How many theorIEs does the author mention?

(A) One
(B) Two
(C) THRee
(D) Four

2. 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?