<P> The Pressure Flow Hypothesis, also known as the Mass Flow Hypothesis, is the best - supported theory to explain the movement of sap through the phloem . It was proposed by Ernst Munch, a German plant physiologist in 1930 . A high concentration of organic substances, particularly sugar, inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient (osmotic gradient) that draws water into the cells from the adjacent xylem . This creates turgor pressure, also known as hydrostatic pressure, in the phloem . Movement of phloem sap occurs by bulk flow (mass flow) from sugar sources to sugar sinks . The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward). Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve - tubes, it is not unusual for sap in adjacent sieve - tubes to be flowing in opposite directions . </P> <P> A sugar source is any part of the plant that is producing or releasing sugar . </P>

How does phloem sap move from source to sink
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