Work Step by Step
When sugars are made in a leaf's mesophyll, the sugar follows a gradient from the mesophyll into the companion cell. The sugars then diffuse from the companion cells into the attached sieve elements. Sugar accumulating in the sieve elements causes water to enter these cells by way of osmosis. The water causes pressure within the sieve elements. This pressure pushes the sieve element's cytoplasm and the sugars within it into the next sieve element. The sugar is then transported into the companion cells in the sink region. From the companion cells, the sugars diffuse into the sink cells, causing water to follow into the sink cells as well. The movement of the sugary fluid in a plant from a source to a sink as a result of osmotic pressure is called the pressure flow theory.