2017公共英语一级阅读文章与技巧
对于参加公共英语一级考试的考生来说，想要通过公共英语一级不难，但是要拿到比较理想的成绩就得认真备考了。下面，小编为大家送上一篇公共英语一级阅读文章与技巧，希望对大家有所帮助。
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.
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。
附：公共英语一级阅读技巧
阅读理解考察考生获取信息的能力，这部分的内容在PETS的各级考试中都占相当的部分。要提高阅读理解部分的得分，考生必须提高英语的阅读能力。许多考生对于如何有效提高阅读水平和应试能力感到茫然和困惑，主要的原因在于大部分的考生将阅读理解简单的理解为词汇加语法，认为只要掌握了这两者，也就解决了阅读问题。那么，该如何提高自己的阅读水平呢?
首先，考生在平时要注意运用正确的阅读方法，要培养浏览、查找、预测、研读和宏观把握文章内容结构的能力。其次，要注意解题技巧的培养。
再次，考生应根据考试的'级别采取有针对性的练习，例如，在PETS三级考试中，阅读理解部分由A、B两节组成，考查考生理解书面英语的能力。A节3篇文章15题，选项为4选1，考查考生理解总体和特定信息的能力。B节1篇文章5题，考生须从7个选项中排除两个干扰项，将正确的概括与5段文字逐一搭配成对。这是考查考生理解文章的主旨要义的能力。在复习阅读理解部分时要有针对性，首先要复习理解文中的具体信息，并能根据上下文推测文中的生词。
平时训练时，要选择几本适合自己的阅读教材，在学习过程中要培养好的阅读习惯，不仅用眼，更要用脑去读，做到积极主动地阅读。用头脑去分析、整理所读到的内容，这是有效阅读的根本所在，也是阅读的最高境界。
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