內(nèi)聚壓力理論(Cohesion

1、內(nèi)聚壓力理論（Cohesion內(nèi) 聚壓力理論(Cohesion-tensionTheory)內(nèi)聚壓力理論(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.內(nèi)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

8、紀(jì)末，水在樹木和其它高大植物中的輸送還是一個(gè)謎。一些植物學(xué)家假定植物中的活細(xì)胞充當(dāng)了水泵的角色。但許多實(shí)驗(yàn)表明細(xì)胞都已死亡的植物莖干仍能將水輸送到相當(dāng)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(dá)樹頂?shù)哪?？根?jù)目前為人們所接受的內(nèi)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(gè)正在升高的水柱之上的拉力 來自該植物頂部水的蒸發(fā)。由于水從葉子表面喪失，一個(gè)負(fù)壓力，或張力就得以產(chǎn)生。蒸 發(fā)出去的水被植物里流動的水代替。這些水形成水柱從植