Explain pressure flow hypothesis of translocation of sugars in plants.

Explanation

Translocation of sugars froom source to sink can be explained by pressure flow or mass flow hypothesis. As glucose is prepared at the source (by photosynthesis) it is converted to sucrpse.
The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport. This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis.
As osmotic pressure builds up, the phloem sap moves to areas of lower pressure . At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which wil use the sugar-converting it into the energy starch or cellulose. As sugars are removed, the osmotic pressure decreases and water moves out of the phloem and reaches xylem.
This can be shown by following illustration.