陳蕙雅思教材 - 雅思閱讀精選 (Cohesion-tension Theory)

Cohesion-tension Theory

Atmospheric pressure cansupport a column of water up to 10 meters high.

But plants can move watermuch higher; the sequoia tree can pump water to

its very top more than100 meters above the ground. Until the end of the

nineteenth     century, the movement of water in trees andother tall plants

was a mystery. Somebotanists hypothesized that the   livingcells of plants

acted as pumps. But manyexperiments demonstrated that the stems of plants

in which all the        cells are killed can still move water toappreciable

heights. Otherexplanations for the movement of water in      plantshave been

based on root pressure, apush 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, haveunusually low root pressures.

If water is not pumped tothe top of a tall tree, and if it is not pushed

to the top of a talltree, then we may        ask: how does itget there?

According to thecurrently accepted cohesion-tension theory, water  is

pulled there. The pull ona rising  column of water in a  plant results

from the evaporation ofwater at the top  of the  plant. As water is lost

from  the surface of the leaves, a negativepressure, or tension, is

created. The evaporatedwater is   replaced by water moving frominside the

plant in unbroken columnsthat extend from the top of a plant to its  roots.

The same forces thatcreate surface tension in any sample of water are

responsible for themaintenance of these unbroken columns of water. When

water is confined intubes of very small bore, the forces of cohesion (the    

attraction between watermolecules) are so great that the strength of a

column of water compareswith the strength      of a steel wire ofthe same

diameter. This cohesivestrength permits columns of water to be pulled to

great heights withoutbeing broken.

內聚壓力理論

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

世紀末，水在樹木和其它高大植物中的輸送還是一個謎。 一些植物學家假定植物中的活細胞充當了水泵的角色。  但許多實驗表明細胞都已死亡的植

物莖幹仍能將水輸送到相當可觀的高度。  對於植物中輸送水的其它解釋都基於根壓--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，那麼我們會問：它是怎樣

到達樹頂的呢?根據目前為人們所接受的內聚壓力的理論，水是被拉到上面去的。 一株植物 中作用于一個正在升高的水柱之上的拉力來自該植物頂部水的蒸發。

由於水從葉子表面喪 失，一個負壓力，或張力就得以產生。  蒸發出去的水被植物裡流動的水代替。  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。

當水被限制在內徑很小的管道中時，內聚壓力(水分子之間的相互吸引力)是如此之大以致一 支水柱的強度相當於一根直徑相同的鋼絲的強度。  

這種內聚壓力使得水柱被拉到非常高的 地方而不會斷裂。