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The pressure required to prevent solvent from passing through a semipermeable membrane from a region of higher concentration of solute to a region of lower concentration of solute is called osmotic pressure. It is also a measure of the tendency of a solution to gain water by osmosis when separated by a membrane from pure water.

 

Root pressure is the pressure (osmotic) developed by living cells in the root that forces water upwards into the stem.  Thus sap rises upwards to the required regions of the plant.

Root pressure also causes exudation from cut stems and guttation of water from leaves. It occurs when the soil moisture level is high either at night or when transpiration is low during the day.