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Cohesion-tensionTheory Atmospheric pressure can support a column of water up to 10 metershigh. But plants can move water much higher; the sequoia tree can pump water toits very top more than 100 meters above the ground. Until the end of thenineteenth century, the movement of water in trees and other tall plants was amystery. Some botanists hypothesized that the living cells of plants actedas pumps. But many experiments demonstrated that the stems of plants in whichall the cells are killed can still move water to appreciable heights.Other explanations for the movement of water in plants have been based onroot 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 thetops of tall trees. Furthermore, the conifers, which are among thetallest trees, have unusually low root pressures. If water is not pumped to the top of a tall tree, andif it is not pushed to the top of a tall tree, then we may ask: how doesit get there? According to the currently accepted cohesion-tension theory,water is pulled there. The pull on a rising column of water ina plant results from the evaporation of water at the top ofthe plant. As water is lost from the surface of the leaves, anegative pressure, or tension, is created. The evaporated wateris replaced by water moving from inside the plant in unbroken columns thatextend from the top of a plant to its roots. | 
