文档介绍：新概念英语——新概念英语文章精选
美联英语提供：新概念英语——新概念英语文章精选
两分钟做个小测试，看看你的英语水平
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100 meters above the ground.
大气压能够支持10米高的水柱，但植物可将水送得更高。美洲红杉就能把水泵到地面以上100多米高的树顶。
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 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?
直到19世纪末，水在树木和其它高大植物中的输送还是一个谜。一些植物学家假定植物中的活细胞充当了水泵的角色。但许多实验表明细胞都已死亡的植物茎干仍能将水输送到相当可观的高度。对于植物中输送水的其它解释都基于根压——植物底端的根对水的推动。但根压完全不足以将水推到树顶。况且，最高树木中的松柏只有很低的根压。如果水不是被泵到高树的树顶，也不是被推到树顶，那么我们会问：它是怎样到达树顶的呢?
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 o