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Water moves into the xylem down a water potential gradient.
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Pressure from water entering the xylem via osmosis gives the bottom of the xylem a high hydrostatic pressure.
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Transpiration at the leaves creates a low hydrostatic pressure at the top of the xylem.
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Water, under tension, is pulled up in a continious column.
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This is aided by: cohesion - the attraction of water molecules to each other, forming hydrogen bonds.
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adhesion - the attraction of water molecules to the xylem wall.
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Capillary action also aids the process - this is where the water rises because it is placed in a very narrow tube.
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All of the above bullet points lead to the water moving up the xylem by mass flow, from a higher hydrostatic pressure to a lower hydrostatic pressure, down a pressure gradient.
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