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.

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作文地带知识拓展:

• 英语（English）属于印欧语系日耳曼语族西日耳曼语支，最早被中世纪的英国使用，并因其广阔的殖民地而成为世界使用面积最广的语言。英国人的祖先盎格鲁部落是后来迁移到大不列颠岛地区的日耳曼部落之一，称为英格兰。这两个名字都来自波罗地海半岛的Anglia。该语言与弗里斯兰语和下撒克森语密切相关，其词汇受到其他日耳曼语族语言的影响，尤其是北欧语（北日耳曼语），并在很大程度上由拉丁文和法文撰写。

  英语的英文释义：

  English 名词 uk  /ˈɪŋ.ɡlɪʃ/ us  /ˈɪŋ.ɡlɪʃ/

  the language that is spoken in the UK, the US, and in many other countries

  英语

  American/British English

  美式／英式英语

  Do you speak English?

  你会说英语吗？

  English   形容词 adjective   uk  /ˈɪŋ.ɡlɪʃ/ us  /ˈɪŋ.ɡlɪʃ/

  in or relating to the English language

  英语的

  an English teacher

  英语老师

  relating to or from England

  英格兰的;英国的

  English films/food/people

  英国电影／食品／人

  English law

  英格兰法律