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.
上一篇: 最新GRE考试写作经典句式
下一篇: 如何理解GRE作文高频试题相关问题
【三维设计】2017届高考英语二轮复习(通用版)练习:第1板块 语法填空+短文改错组合练(6)(含解析)
中英阿富汗问题声明
2014届高考英语一轮复习话题阅读素材31
【三维设计】2017届高考英语二轮复习(通用版)练习:第1板块 语法填空+短文改错组合练(1)(含解析)
想来美国旅游?先交出社交账号
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解主旨大意题专练(2)(含解析)
2014届高考英语一轮复习精选阅读理解文章六十篇(5)
【三维设计】2017届高考英语二轮复习(通用版)练习:第1板块 语法填空+短文改错组合练(7)(含解析)
如何进行快速阅读?
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型4 阅读七选五专练(1)(含解析)
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解细节理解题专练(1)(含解析)
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解推理判断题专练(1)(含解析)
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解高考题型仿真练(1)(含解析)
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型4 阅读理解+阅读七选五板块提速练(2)(含解析)
【三维设计】2017届高考英语二轮复习(通用版)练习:第1板块 语法填空+短文改错组合练(4)(含解析)
2014届高考英语一轮复习精选阅读理解文章六十篇(11)
2014届高考英语一轮复习精选阅读理解文章六十篇(8)
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解词义猜测题专练(2)(含解析)
2014届高考英语一轮复习精选阅读理解文章六十篇(7)
【三维设计】2017届高考英语二轮复习(通用版)练习:第1板块 语法填空 补上一课(含解析)
外媒看中国:女员工该不该放姨妈假?
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解 第2讲 主旨大意题
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型3 阅读理解主旨大意题专练(1)(含解析)
2014届高考英语一轮复习精选阅读理解文章六十篇(12)
【三维设计】2017届高考英语二轮复习(通用版)练习:第3板块题型5 完形填空 教师用书(含解析)
如何加快自我成长的速度?
【三维设计】2017届高考英语二轮复习(通用版)练习:第2板块题型4 阅读七选五专练(3)(含解析)
2014届高考英语一轮复习精选阅读理解文章六十篇(10)
2017最新妆容流行趋势:大胆唇色、晒伤妆、雀斑妆
5大技巧教你高效阅读