Water is transported  through the cells in the xylem vessel
the movement of water molecules is called osmosis and it is due to osmotic pressure.
osmotic pressure is developed due to transpiration
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Water in the soil has a weak solution of salts, the cell sap in the root cells has a stronger solution of salts. Therefore the water passes from the soil to the root, and as it reaches the first cell sap it dilutes the cell sap, and thus the water passes to another sap which has a stronger solution of salts than the first sap, and the process goes on and the water eventually reaches xylem vessels.
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