新托福写作考试想拿到高分非常不容易，但是参加新托福写作考试的同学都想要拿到高分，那么研究一两篇优秀范文，可以让我们了解新托福写作的高分要素。
范文的题目：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.
>>点击查看托福写作备考全集合专题，阅读更多相关文章！
-
托福写作字数和实际得分关系深度分析 合理安排写作篇幅让文章内容更有料
托福考生对于写作部分的字数要求一直存有疑问。写得字数太少担心被扣分，写得太多时间又会不够用。如何才能把握好作文字数和实际得分之间的关系呢?本文将为大家深度分析托福写作字数和得分的关系，帮助大家了解托福写作的正确篇幅安排。
- 备考托福写作范文资料备考如何高效运用？这3个练习重点不容忽视
- 托福独立写作开头段写法基础4步骤讲解 循序渐进让高分作文开个好头
- 托福写作7种常用高分句型汇总分享 结合句式实例提升写作用句水平
- 托福写作机经正确使用方法技巧和注意事项一览 掌握用法才能发挥机经价值