40 Cohesion1-tension Theory
Atmospheric2 pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia3 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 botanists4 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 appreciable5 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 evaporation6 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 cohesive7 strength permits columns of water to be pulled to great heights without being broken.
內(nèi)聚壓力理論
大氣壓能夠支持10米高的水柱，但植物可將水送得更高。美洲紅杉就能把水泵到地面以上100多米高的樹(shù)頂。直到19
世紀(jì)末，水在樹(shù)木和其它高大植物中的輸送還是一個(gè)謎。一些植物學(xué)家假定植物中的活細(xì)胞充當(dāng)了水泵的角色。但許多實(shí)驗(yàn)表明細(xì)胞都已死亡的植
物莖干仍能將水輸送到相當(dāng)可觀的高度。對(duì)于植物中輸送水的其它解釋都基于根壓--植物底端的根對(duì)水的推動(dòng)。但根壓完全不足以將水推到樹(shù)頂。
況且，樹(shù)木中的松柏只有很低的根壓。如果水不是被泵到高樹(shù)的樹(shù)頂，也不是被推到樹(shù)頂，那么我們會(huì)問(wèn)：它是怎樣
到達(dá)樹(shù)頂?shù)哪?根據(jù)目前為人們所接受的內(nèi)聚壓力的理論，水是被拉到上面去的。一株植物中作用于一個(gè)正在升高的水柱之上的拉力來(lái)自該植物頂部水的蒸發(fā)。
由于水從葉子表面喪失，一個(gè)負(fù)壓力，或張力就得以產(chǎn)生。蒸發(fā)出去的水被植物里流動(dòng)的水代替。這些水形成
水柱從植物頂端一直延伸到根部。在任何水樣中造成表面張力的力支持著這些不斷的水柱。
當(dāng)水被限制在內(nèi)徑很小的管道中時(shí)，內(nèi)聚壓力(水分子之間的相互吸引力)是如此之大以致一支水柱的強(qiáng)度相當(dāng)于一根直徑相同的鋼絲的強(qiáng)度。
這種內(nèi)聚壓力使得水柱被拉到非常高的地方而不會(huì)斷裂。
1 cohesion
n.團(tuán)結(jié)，凝結(jié)力
參考例句：
I had to bring some cohesion into the company.我得使整個(gè)公司恢復(fù)凝聚力。
The power of culture is deeply rooted in the vitality,creativity and cohesion of a nation. 文化的力量，深深熔鑄在民族的生命力、創(chuàng)造力和凝聚力之中。
2 atmospheric
adj.大氣的，空氣的；大氣層的；大氣所引起的
參考例句：
Sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面溫度與大氣環(huán)流是密切相關(guān)的。
Clouds return radiant energy to the surface primarily via the atmospheric window.云主要通過(guò)大氣窗區(qū)向地表輻射能量。
3 sequoia
n.紅杉
參考例句：
The sequoia national forest is at the southern end of the sierra nevada range.紅杉國(guó)家公園位于內(nèi)華達(dá)山脈南端盡頭處。
The photo shows the enormous general Sherman tree in California's sequoia national park.照片顯示的是加利福尼亞州紅杉國(guó)家公園內(nèi)巨大的謝爾曼將軍樹(shù)。
4 botanists
n.植物學(xué)家，研究植物的人( botanist的名詞復(fù)數(shù) )
參考例句：
Botanists had some difficulty categorizing the newly found plant. 植物學(xué)家們不大容易確定這種新發(fā)現(xiàn)的植物的種類(lèi)。 來(lái)自辭典例句
Botanists refer this flower to the rose family. 植物學(xué)家將這花歸入薔薇科。 來(lái)自辭典例句
5 appreciable
adj.明顯的，可見(jiàn)的，可估量的，可覺(jué)察的
參考例句：
There is no appreciable distinction between the twins.在這對(duì)孿生子之間看不出有什么明顯的差別。
We bought an appreciable piece of property.我們買(mǎi)下的資產(chǎn)有增值的潛力。
6 evaporation
n.蒸發(fā)，消失
參考例句：
Be careful not to lose too much liquid by evaporation.小心不要因蒸發(fā)失去太多水分。
Our bodies can sweat,thereby losing heat by evaporation.我們的身體能出汗，由此可以蒸發(fā)散熱。
7 cohesive
adj.有粘著力的；有結(jié)合力的；凝聚性的
參考例句：
She sealed the parcel with cohesive tape.她用粘膠帶把包裹封起來(lái)。
The author skillfully fuses these fragments into a cohesive whole.作者將這些片斷巧妙地結(jié)合成一個(gè)連貫的整體。
Atmospheric2 pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia3 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 botanists4 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 appreciable5 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 evaporation6 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 cohesive7 strength permits columns of water to be pulled to great heights without being broken.
內(nèi)聚壓力理論
大氣壓能夠支持10米高的水柱，但植物可將水送得更高。美洲紅杉就能把水泵到地面以上100多米高的樹(shù)頂。直到19
世紀(jì)末，水在樹(shù)木和其它高大植物中的輸送還是一個(gè)謎。一些植物學(xué)家假定植物中的活細(xì)胞充當(dāng)了水泵的角色。但許多實(shí)驗(yàn)表明細(xì)胞都已死亡的植
物莖干仍能將水輸送到相當(dāng)可觀的高度。對(duì)于植物中輸送水的其它解釋都基于根壓--植物底端的根對(duì)水的推動(dòng)。但根壓完全不足以將水推到樹(shù)頂。
況且，樹(shù)木中的松柏只有很低的根壓。如果水不是被泵到高樹(shù)的樹(shù)頂，也不是被推到樹(shù)頂，那么我們會(huì)問(wèn)：它是怎樣
到達(dá)樹(shù)頂?shù)哪?根據(jù)目前為人們所接受的內(nèi)聚壓力的理論，水是被拉到上面去的。一株植物中作用于一個(gè)正在升高的水柱之上的拉力來(lái)自該植物頂部水的蒸發(fā)。
由于水從葉子表面喪失，一個(gè)負(fù)壓力，或張力就得以產(chǎn)生。蒸發(fā)出去的水被植物里流動(dòng)的水代替。這些水形成
水柱從植物頂端一直延伸到根部。在任何水樣中造成表面張力的力支持著這些不斷的水柱。
當(dāng)水被限制在內(nèi)徑很小的管道中時(shí)，內(nèi)聚壓力(水分子之間的相互吸引力)是如此之大以致一支水柱的強(qiáng)度相當(dāng)于一根直徑相同的鋼絲的強(qiáng)度。
這種內(nèi)聚壓力使得水柱被拉到非常高的地方而不會(huì)斷裂。
1 cohesion
n.團(tuán)結(jié)，凝結(jié)力
參考例句：
I had to bring some cohesion into the company.我得使整個(gè)公司恢復(fù)凝聚力。
The power of culture is deeply rooted in the vitality,creativity and cohesion of a nation. 文化的力量，深深熔鑄在民族的生命力、創(chuàng)造力和凝聚力之中。
2 atmospheric
adj.大氣的，空氣的；大氣層的；大氣所引起的
參考例句：
Sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面溫度與大氣環(huán)流是密切相關(guān)的。
Clouds return radiant energy to the surface primarily via the atmospheric window.云主要通過(guò)大氣窗區(qū)向地表輻射能量。
3 sequoia
n.紅杉
參考例句：
The sequoia national forest is at the southern end of the sierra nevada range.紅杉國(guó)家公園位于內(nèi)華達(dá)山脈南端盡頭處。
The photo shows the enormous general Sherman tree in California's sequoia national park.照片顯示的是加利福尼亞州紅杉國(guó)家公園內(nèi)巨大的謝爾曼將軍樹(shù)。
4 botanists
n.植物學(xué)家，研究植物的人( botanist的名詞復(fù)數(shù) )
參考例句：
Botanists had some difficulty categorizing the newly found plant. 植物學(xué)家們不大容易確定這種新發(fā)現(xiàn)的植物的種類(lèi)。 來(lái)自辭典例句
Botanists refer this flower to the rose family. 植物學(xué)家將這花歸入薔薇科。 來(lái)自辭典例句
5 appreciable
adj.明顯的，可見(jiàn)的，可估量的，可覺(jué)察的
參考例句：
There is no appreciable distinction between the twins.在這對(duì)孿生子之間看不出有什么明顯的差別。
We bought an appreciable piece of property.我們買(mǎi)下的資產(chǎn)有增值的潛力。
6 evaporation
n.蒸發(fā)，消失
參考例句：
Be careful not to lose too much liquid by evaporation.小心不要因蒸發(fā)失去太多水分。
Our bodies can sweat,thereby losing heat by evaporation.我們的身體能出汗，由此可以蒸發(fā)散熱。
7 cohesive
adj.有粘著力的；有結(jié)合力的；凝聚性的
參考例句：
She sealed the parcel with cohesive tape.她用粘膠帶把包裹封起來(lái)。
The author skillfully fuses these fragments into a cohesive whole.作者將這些片斷巧妙地結(jié)合成一個(gè)連貫的整體。