Atmopheric preure can upport a column of water up to 10 meter high. But plant can move water much higher; the equoia tree can pump water to it very top more than 100 meter above the ground. Until the end of the nineteenth century, the movement of water in tree and other tall plant wa a mytery. Some botanit hypotheized that the living cell of plant acted a pump. But many experiment demontrated that the tem of plant in which all the cell are killed can till move water to appreciable height. Other explanation for the movement of water in plant have been baed on root preure, a puh on the water from the root at the bottom of the plant. But root preure i not nearly great enough to puh water to the top of tall tree. Furthermore, the conifer, which are among the tallet tree, have unuually low root preure.
If water i not pumped to the top of a tall tree, and if it i not puhed to the top of a tall tree, then we may ak: how doe it get there? According to the currently accepted coheion-tenion theory, water i pulled there. The pull on a riing column of water in a plant reult from the evaporation of water at the top of the plant. A water i lot from the urface of the leave, a negative preure, or tenion, i created. The evaporated water i replaced by water moving from inide the plant in unbroken column that extend from the top of a plant to it root. The ame force that create urface tenion in any ample of water are reponible for the maintenance of thee unbroken column of water. When water i confined in tube of very mall bore, the force of coheion (the attraction between water molecule) are o great that the trength of a column of water compare with the trength of a teel wire of the ame diameter. Thi coheive trength permit column of water to be pulled to great height without being broken.[由www.haoZuowen.Com整理]