Push/pull Theory

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when soil is almost completely saturated with water and leaves are not losing much water through evaporation. When the xylem is full of water, the molecules will not move back down the structure, but rather need to be expelled from the plant. They will move, by the process of osmosis, to an areas where there is the least amount of pressure. These areas are usually openings located on plant leaves. Once the water molecules reach the leaves, the excess water will be forced out of the plant by root pressure.
This push theory presents two major inconsistencies 1)many plants exhibit no root pressure and 2)root pressure is not strong enough. In many plants and trees, there is a substantial amount of root pressure that could possibly explain the movement of so much water throughout a tree, defying gravity. However, in many plants, root pressure is not nearly enough to exert the amount of force needed to circulate such large quantities of water throughout the plant and against gravity. ...

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