新東方優美背誦短文50篇 Unit40:Cohesion-tension Theory

40 Cohesion-tension Theory

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.

內聚壓力理論
大氣壓能夠支持10米高的水柱，但植物可將水送得更高。美洲紅杉就能把水泵到地面以上100多米高的樹頂。直到19
世紀末，水在樹木和其它高大植物中的輸送還是一個謎。一些植物學家假定植物中的活細胞充當了水泵的角色。但許多實驗表明細胞都已死亡的植
物莖干仍能將水輸送到相當可觀的高度。對于植物中輸送水的其它解釋都基于根壓--植物底端的根對水的推動。但根壓完全不足以將水推到樹頂。
況且，最高樹木中的松柏只有很低的根壓。如果水不是被泵到高樹的樹頂，也不是被推到樹頂，那么我們會問：它是怎樣
到達樹頂的呢?根據目前為人們所接受的內聚壓力的理論，水是被拉到上面去的。一株植物中作用于一個正在升高的水柱之上的拉力來自該植物頂部水的蒸發。
由于水從葉子表面喪失，一個負壓力，或張力就得以產生。蒸發出去的水被植物里流動的水代替。這些水形成
水柱從植物頂端一直延伸到根部。在任何水樣中造成表面張力的力支持著這些不斷的水柱。
當水被限制在內徑很小的管道中時，內聚壓力(水分子之間的相互吸引力)是如此之大以致一支水柱的強度相當于一根直徑相同的鋼絲的強度。
這種內聚壓力使得水柱被拉到非常高的地方而不會斷裂。