内聚压力理论(Cohesion

1、内聚压力理论（Cohesion内 聚压力理论(Cohesion-tensionTheory)内聚压力理论(Cohesi on-te nsi onTheory)cohesi on-tensiontheoryatmospheric pressure can support a colu mn 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 tha n 100 meters above the groun

2、d,until the end of the nineteenthcen tury, the moveme nt of water in trees and other tall pla nts was a mystery. some botanists hypothesized that the living cells of pla nts acted as pumps.but many experime ntsdem on stratedthat the stems ofpla nts in which all the cells are killed can still move wa

3、ter to appreciable heights. other expla nati ons for the moveme nt of water in pla nts have bee n based on root pressure, a push on the water from the roots at the bottom of the pla nt. but root pressure is not n early great eno ugh to push water to the tops of tall trees. furthermore, the conifers,

4、 which are among the tallest trees, have unu sually 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? accord ing to the curre ntly accepted cohesi on-tension theory, water is pulled there. t

5、he pull on a risi ng colu mn of water in a pla nt results from the evaporati on of water at the top of the pla nt. as water is lost from the surface of the leaves, a n egative pressure, or tension, is created. the evaporated water is replaced by water movi ng from in side the pla nt in un broke n co

6、lu mns that exte nd from the top of a pla nt to its roots.the same forces that create surface tension in any sampleof water are responsiblefor the maintenance of theseun broke n colu mns of water. whe n water is confined in tubes of very small bore, the forces of cohesi on (the attractio n betwee n

7、water molecules) are so great that the stre ngth of a colu mn of water compares with the stre ngth of a steel wire of the same diameter. this cohesive stre ngth permits colu mns of water to be pulled to great heights without being broke n.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

8、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