Mass flow theory is also known as pressure flow hypothesis was given by Munch. It explains the movement of sap through the phloem. It is known as translocation. The glucose is produced by photosynthesis in the mesophyll cells of green leaves. It is actively transported to the companion cells and increases the sugar concentration which draws water from nearby cells xylem cells and creates hydrostatic pressure and pushes the sugar towards the sink. The low sugar content in the companion cells again releases the water to nearby xylem cells.