GRE写作范文下载(10篇)

满分网GRE 资料：作文篇

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目 录
GRE 写作范文1：Cohesion-tensionTheory ...................................................................................1
GRE 写作范文2：Americanblackbears..........................................................................................2
GRE 写作范文3：Coal-firedpowerplants.......................................................................................2
GRE 写作范文4：Statistics ............................................................................................................3
GRE 写作范文5：ObtainingFreshwaterfromicebergs ....................................................................4
GRE 写作范文6：ThesourceofEnergy ...........................................................................................4
GRE 写作范文7：Museums ...........................................................................................................5
GRE 写作范文8：SkyscrapersandEnvironment .............................................................................5
GRE 写作范文9：ArareFossilRecord.............................................................................................6
GRE 写作范文10：TheNobelAcademy..........................................................................................7
GRE 写作范文1：Cohesion-tensionTheory
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.