The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the best-supported theory to explain the movement of food through the phloem. It was proposed by Ernst Munch, a German plant physiologist in 1930. A high concentration of organic substance inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells. Movement occurs by bulk flow (mass flow); phloem sap moves from sugar sources to sugar sinks by means of turgor pressure, also known as hydrostatic pressure. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, usually during the spring, storage organs such as the roots are sugar sources, and the plant's many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward).
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