mass flow (pressure flow) A hypothesis to explain the movement of sugars in the phloem tissue of plants. At a source (site of production) sugars are actively secreted from phloem companion cells into the sieve elements, 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 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. 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.

mass flow A down-slope slide of sediment which moves under the force of gravity. Mass flows include rockfalls (accumulations of scree), slumps and slides (where masses of sediment move downslope along discrete shear planes), debris flows (in which ill-sorted masses of sediment move downslope due to the loss of internal strength of the sediment mass), liquefied sediment flows, grain flows, and turbidity flows.

mass flow A down-slope slide of sediment which moves under the force of gravity. Mass flows include rockfalls (accumulations of scree), slumps and slides (where masses of sediment move down-slope along discrete shear planes), debris flows (in which ill-sorted masses of sediment move down-slope following the loss of internal strength of the sediment mass), liquefied sediment flows, grain flows, and turbidity flows.