What is transpiration?

Transpiration is the process in plants by which water travels in a stream from the roots to the leaves where it evaporates. The water is pulled up from the roots through the xylem via the cohesion-tension mechanism. A force called cohesion holds the water molecules together and adhesion holds the water and the cells walls of the xylem together. Once the water arrives in the leaves it is lost from the plant through evaporation from the mesophyll cells. 

The transpiration stream is controlled by the opening and closing of the stomata on the leaf surface. It must be regulated in order to prevent the plant losing too much water.