Passive transport of fluid up the plant stem. The xylem cells form a continuous tube through which water with dissolved nutrients (sap?) is pulled up at a rate of mm/h by a mixture of capillary action (mainly for small stems) and the turgor pressure generated by transpiration. Considering that most trees can reach many meters in height, some sort of pumping might be thought necessary. However, as long as the column of water is unbroken (by bubbles, for example) the distance it travels is not too important. On the other hand, the pressure necessary to move water 100 m up a tree is 3 MPa. There are mechanisms to maintain such an unbroken column, such as removing bubbles.