英语美文背诵 Cohesion-tension Theory

Cohesion¹-tension Theory

Atmospheric² pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia³ 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 botanists⁴ 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 appreciable⁵ 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 evaporation⁶ 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 cohesive⁷ strength permits columns of water to be pulled to great heights without being broken.

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

水柱从植物顶端一直延伸到根部。在任何水样中造成表面张力的力支持着这些不断的水柱。

当水被限制在内径很小的管道中时，内聚压力(水分子之间的相互吸引力)是如此之大以致一支水柱的强度相当于一根直径相同的钢丝的强度。

这种内聚压力使得水柱被拉到非常高的地方而不会断裂。

1 cohesion   

n.团结，凝结力

参考例句：

• I had to bring some cohesion into the company.我得使整个公司恢复凝聚力。
• The power of culture is deeply rooted in the vitality,creativity and cohesion of a nation. 文化的力量，深深熔铸在民族的生命力、创造力和凝聚力之中。

2 atmospheric   

adj.大气的，空气的；大气层的；大气所引起的

参考例句：

• Sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面温度与大气环流是密切相关的。
• Clouds return radiant energy to the surface primarily via the atmospheric window.云主要通过大气窗区向地表辐射能量。

3 sequoia   

n.红杉

参考例句：

• The sequoia national forest is at the southern end of the sierra nevada range.红杉国家公园位于内华达山脉南端尽头处。
• The photo shows the enormous general Sherman tree in California's sequoia national park.照片显示的是加利福尼亚州红杉国家公园内巨大的谢尔曼将军树。

4 botanists   

n.植物学家，研究植物的人( botanist的名词复数 )

参考例句：

• Botanists had some difficulty categorizing the newly found plant. 植物学家们不大容易确定这种新发现的植物的种类。 来自辞典例句
• Botanists refer this flower to the rose family. 植物学家将这花归入蔷薇科。 来自辞典例句

5 appreciable   

adj.明显的，可见的，可估量的，可觉察的

参考例句：

• There is no appreciable distinction between the twins.在这对孪生子之间看不出有什么明显的差别。
• We bought an appreciable piece of property.我们买下的资产有增值的潜力。

6 evaporation   

n.蒸发，消失

参考例句：

• Be careful not to lose too much liquid by evaporation.小心不要因蒸发失去太多水分。
• Our bodies can sweat,thereby losing heat by evaporation.我们的身体能出汗，由此可以蒸发散热。

7 cohesive   

adj.有粘着力的；有结合力的；凝聚性的

参考例句：

• She sealed the parcel with cohesive tape.她用粘胶带把包裹封起来。
• The author skillfully fuses these fragments into a cohesive whole.作者将这些片断巧妙地结合成一个连贯的整体。