Mass flow hypothesis is the best supported hypothesis to explain the movement of sap through the phloem. It was proposed by Munch. 

In it the higher concentration of organuc substances like sugar, inside cells of phloem at a source, such as leaf creates a diffusion gradient that draws water into  cells from adjacent xylem. This creates turgor pressure known as hydrostatic pressure in the phloem. Movement in phloem is bidirectional while in xylem it is unidirectional.