40 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 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
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修一Module 4《A Social Survey—My Neighbourhood》
山西省2017高考英语二轮复习专题训练:阅读理解(61)
山东济宁一中2017届高考高三英语二轮复习课件:专题四 动词和动词词组
山东济宁一中2017届高考高三英语二轮复习课件:专题十 特殊句式
山西省2017高考英语二轮复习专题训练:阅读理解(43)
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修一Module 2《My New Teachers》
山西省2017高考英语二轮复习专题训练:阅读理解(78)
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修三Module 6《Old and New》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修五Module 4《Carnival》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修五Module 5《The Great Sports Personality》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修一Module 1《My First Day at Senior High》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修四Module 4《Great Scientists》
2017届高考英语一轮单元总复习教案精品荟萃:外研版选修八Module 5《The Conquest of the Universe》
山西省2017高考英语二轮复习专题训练:阅读理解(45)
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修五Module 3《Adventure in Literature and the Cinema》
2017届高考英语一轮单元总复习教案精品荟萃:外研版选修六Module 3《Interpersonal Relationships—Friendship》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修五Module 2《A Job Worth Doing》
山东济宁一中2017届高考高三英语二轮复习课件:专题七 情态动词和虚拟语气
山西省2017高考英语二轮复习专题训练:阅读理解(63)
2017届高考英语一轮单元总复习教案精品荟萃:外研版选修六Module 2《Fantasy Literature—Philip Pullman》
2017届高考英语一轮单元总复习教案精品荟萃:外研版选修八Module 4《Which English》
山东济宁一中2017届高考高三英语二轮复习课件:专题六 非谓语动词
山西省2017高考英语二轮复习专题训练:阅读理解(60)
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修一Module 5《A Lesson in a Lab》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修四Module 5《 A Trip Along the Three Gorges》
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修四Module 1《Life in the Future》
山东济宁一中2017届高考高三英语二轮复习课件:专题三 形容词和副词
2017届高考英语一轮单元总复习教案精品荟萃:外研版必修一Module 3《My First Ride on a Train》
2017届高考英语一轮单元总复习教案精品荟萃:外研版选修六Module 6《War and Peace》
山东济宁一中2017届高考高三英语二轮复习课件:专题五 动词的时态和语态
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |