新东方背诵文选 L40 Cohesion-tension Theory

Atmospheric¹ pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia² 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 botanists³ 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 appreciable⁴ 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 cohesion⁵-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.

1 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。

2 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。

3 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

4 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。

5 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。

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。