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(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(duì)于植物中輸送水的其它解釋都基于根壓--植物 底端的根對(duì)水的推動(dòng)。 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(huì)問：它是怎樣

到達(dá)樹頂?shù)哪?根據(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(lái)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(dòng)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(dāng)水被限制在內(nèi)徑很小的管道中時(shí)，內(nèi)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(qiáng)度相當(dāng)于一根直徑相同的鋼絲的強(qiáng)度。

這種內(nèi)聚壓力使得水柱被拉到非常高的 地方而不會(huì)斷裂。

譯路通武漢漢口翻譯公司

2012.8.7