Water is mainly absorbed by roots which goes upwards so as to replace water loss in transpiration and to be used in photosynthesis. This upward movement of water from roots to leaves through stem against force of gravity is called ascent of sap. Different theories have been put forward in support of ascent of sap. They include root pressure theory, capillary theory and transpiration pull theory. Root pressure is developed when rate of absorption is more than rate of transpiration and so water is pushed up in the tracheary elements. Capillary force theory was given by Boehm according to which xylem vessels act as minute capillaries and water rises in these capillaries due to capillary action or surface tension. According to transpiration pull theory by Dixon and Jolly, due to water loss by transpiration suction pressure is increased and water is absorbed from adjacent xylem vessels of leaves and thus sap in under tension.