Transport of water (and ions) up the plant. Also gives strength to the plant
The transpiration stream is the movement of water from the roots of the plant up the steam to the leaves. Water moves into the plant roots by osmosis and continues to move across the cells of the roots to the xylem in the centre. There is a continuous gradient of water potential from water in the soil to the cells in the centre. The water in the soil has a higher water potential then that of the water inside the cells so water is continuously moving into the cells. This is because water is continuously moving up the xylem. The body of water moving across the cells exerts a pressure on the water in front of it (root pressure).
The energy from the sunlight gives water molecules at the surface of the leaf enough energy to move out into the air which creates a tension (a negative pressure or a pull) on the water molecules further down the xylem.
The cohesion of water molecules causes the attraction of water at the top of the plant being evaporated to pull the water molecules further down up the plant. Xylem tubes are relatively narrow, which helps in the adhesion (attraction of water molecules of different types) of the water molecules with the cellulose and lignin surrounding the walls of the xylem. This adhesion helps to drag water up the narrow tubes and is called capillarity. This mechanism is also called the evaporation -tension -cohesion mechanism.
*Transpiration is the loss of water from the leaves of plants. -water vapour is lost though the stomates.