Ernst Much proposed the Mass Flow Hypothesis, also called as the Pressure Flow Hypothesis in the year 1930. It is a hypothesis or a theory to explain the movement of sap through the phloem. 

Inside the cells of phloem, there is a high concentration of organic substances, particularly sugars. This creates a diffusion gradient or osmotic gradient between the environments inside and outside the phloem cells. This helps to draw water into the cells from the adjacent xylem, thereby creating turgor pressure in the phloem. Movement of phloem sap occurs by bulk flow or mass flow from inside the cells to outside of the cells in phloem. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional in upward direction. This was Munch Mass Flow hypothesis.