According to Munch, food materials are translocated from the source (leaves) to sink (roots) enmass, through turgor pressure gradient that occurs between leaves and roots. The mechanism of translocation can be explained as follows:
1. Sugars are formed in the mesophyll cells during photosynthesis, and are loaded into the phloem sieve elements of veins. This is called vein loading.
2. The osmotic potential in the phloem becomes more negative, and as a result water is drawn into phloem elements from xylem cells.
3. Turgor pressure increases in the sieve tubes of leaves, and at the same time the turgor. Pressure of roots becomes less. So a turgor pressures gradient occurs between mesophyll cells and root cells.
4. As a result, food molecules in solution form, are translocated from leaves to roots.
5. In the roots the food is consumed, and in the fruits it is converted into insoluble starch. So osmotic potential and turgor pressure in the roots and storage organs decreases.
6. Thus turgor pressure gradient continues between leaves, and roots, and translocation of food also continues.
Munch conducted a physical experiment to demonstrate the mass flow. He took two osmometers A and B, with concentrated sugar solution in A and B filled a with dilute solution. These are connected by a 'U' tube C. At B are immersed in a rough containing water.
Now, water enters into 'A', due to high osmotic pressure and this creates turgor pressure. This results in mass flow of sugar solution from A to B through 'C' until equilibrium is attained and maintained. He compared osmometer A to leaves, B to roots, C to phloem, and water in vessels to xylem vessels. This clearly explains the mass flow hypothesis.
