编辑点评: 研习GRE作文考试的优秀范文不仅可以学到有用的词汇和句子,更能从中学习逻辑的构建方式和作文框架的结构等内容,这里为大家提供GRE的范文,希望对同学们的备考有所帮助。
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.
美多家银行同意支付抵押贷款索赔
广东催生智能手机新品牌
微尘&栋梁
为何达成财政悬崖协议如此艰难?
2012年英国大公司高管基薪仅涨3%
一对冲基金押注希腊债券获益5亿美元
美国枪支管制的最大障碍是选民
摩苏女人的大智慧
圣诞老人运动会 香港选手放异彩
西方主导地位为何下降?
喜欢你是静静的
世界末日不在眼前
朝鲜发射远程火箭,声称卫星进入轨道
美联储官员对QE3期限产生分歧
玫瑰花语
太原家乐福店被指价格欺诈
巴塞尔委员会放松银行流动性要求
海外现金难解美企国内之渴
在亚洲欢度圣诞的首选之地
Lex专栏:瑞银已没有犯错空间
投资东南亚股市须谨慎
谷歌地图重登苹果应用商店
安倍晋三当选日本首相
美国商学院遭遇募资难
印度强奸案在场男子指责警方应对不力
浪漫语丝
透过科幻片看未来经济
Love, love me do
希拉克回忆录——法国右翼政治入门
日本将挑起“全球货币战争”?
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |