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.
内聚重压理论
大方压可以支持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。但很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讲解都基于根压||植物底端的根对水的推进。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非常低的根压。假如水不是被泵到高树的树顶，更不是被推到树顶，那样大家会问：它是如何
到达树顶的呢?依据现在为大家所同意的内聚重压的理论，水是被拉到上面去的。一株植物中用途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。
因为水从叶子表面丧失，一个负重压，或张力就得以产生。蒸发出去的水被植物里流动的水代替。这类水形成
水柱从植物顶端一直延伸到根部。在任何水样中导致表面张力的力支持着这类持续的水柱。
当水被限制在内径非常小的管道中时，内聚重压(水分子之间的相互吸引力)是这样之大以致一支水柱的强度等于一根直径相同的钢丝的强度。
这种内聚重压致使水柱被拉到特别高的地方而不会断裂。
