A hypothesis to explain the movement of sugars in the phloem tissue of plants. At a source (site of production) sugars are loaded into companion cells and thence into the sieve elements (see phloem loading), causing water to follow by osmosis. The pressure of water in the tubes (the hydrostatic pressure) causes it to move along the tubes to a sink (site of utilization), where the reverse process occurs. Here sugars diffuse or are actively transported from the sieve elements into the companion cells and then into the surrounding tissues, establishing a concentration gradient from source to sink (see phloem unloading). Although different solutes can be transported in the phloem in different directions at the same time, it is argued that the mass flow hypothesis can still apply provided transport occurs in different sieve elements.