explain the pressure flow hypothesis of translocation of sugar in plants.?

This theory is also called Much's mass flow hypothesis.  

Munch explained how food is translocated from leaves to other parts of the plant body.

To explain this, he constructed a small experiment. He hs taken two osmometers A and B. The two osmometers are connected to each other by glass tube. The osmometer A is fiiled with concentrated solution and B osmometer is also filled with concentrated solution, but the osmometer A is more concentrated than osmometer B. Both the osmometers are dipped in water.

Due to osmotic differences the water from beaker enters into osmometer A by endosmosis and this creates a osmotic pressure on osmometer B. Due to this the water from osmometer B comes out by a process called exosmosis.

Along with the flow of water from A to B osmometer, The solutes are also travel from A to B.

Now, he compares osmometer A to the source i.e. leaves and osmometer B as sink i.e other than leaves.