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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.
内聚压力理论
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。 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?根据目前为人们所接受的内聚压力的理论，水是被拉到上面去的。 一株植物 中作用于一个正在升高的水柱之上的拉力来自该植物顶部水的蒸发。 由于水从叶子表面丧 失，一个负压力，或张力就得以产生。 蒸发出去的水被植物里流动的水代替。 这些水形成 水柱从植物顶端一直延伸到根部。在任何水样中造成表面张力的力支持着这些不断的水柱。 当水被限制在内径很小的管道中时，内聚压力(水分子之间的相互吸引力)是如此之大以致一 支水柱的强度相当于一根直径相同的钢丝的强度。 这种内聚压力使得水柱被拉到非常高的 地方而不会断裂。