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 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.
职场英语:怎样利用周末的时间提升自己(双语)
职场英语:如何让手中的钱越变越多?
职场英语:再也不在party上做哑巴
职场英语:微软元老离职信字字珠玑 回首职场生涯12年
职场英语:避免犯的8个职场错误
职场英语:十大名企掌门人告诉你 我需要什么样的员工
职场英语:求职碰壁也许是你被HR拉黑了
职场英语:对现在工作不满意 如何找更适合的职位
职场英语:十句职场黑话 你被忽悠过没(双语)
职场英语:如何把握内定的招聘职位
职场英语:春天到了,该如何着装?
职场英语:10件小事让你工作效率更高
职场英语:网络时代面会时间减少“面会时间”英语怎么说
职场英语:能提高生活品质的办公桌布置方案
职场英语:这样写商务邮件
职场英语:6种方式在职场赢得尊重
职场英语:员工薪资是保密还是公开?
职场英语:如何帮助朋友找工作
职场英语:办公室里的四类人(双语)
职场英语:警惕!你可能选错工作的7个信号
职场英语:开开心心去上班,如何在工作时还保持好心情
职场英语:2013年压力最大的工作vs最轻松的工作
职场英语:职场聪明人的口头禅(双语)
职场英语:小熊维尼告诉我们的人生哲学
职场英语:职场女性擦亮眼 别被男人的微笑给骗了
职场英语:工作中不要说的13句话(双语)
职场英语:职场女性擦亮眼,别被男人的微笑给骗了
职场英语:活着就是为了改变世界
2013年压力最大的10个职业
职场英语:教你怎样高效利用时间
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |