Final answer:
Pressure increases in the phloem due to osmotic water movement from the xylem caused by active phloem loading of sugars at source regions. This process, combined with sugar unloading at sink regions, establishes a pressure gradient that drives the flow of phloem sap throughout the plant.
Step-by-step explanation:
Pressure in the phloem increases due to the movement of water into this tissue as a consequence of phloem loading. Sugar, primarily sucrose, is actively transported into the sieve-tube elements of the phloem in regions of the plant known as 'source' areas, where it is manufactured or released from storage. This active transport decreases water potential inside the phloem, prompting water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure.
As the concentration of sucrose increases in the phloem, the osmotic pressure draws water into the sieve tubes, exacerbating the pressure increase. This change in pressure enables the bulk flow of phloem sap from source to sink. At the 'sink' areas, sucrose is removed, either being consumed by other parts of the plant or stored, thereby maintaining a continual pressure gradient.
Lastly, the process of transpiration also plays a role by causing water to return to the leaves through the xylem vessels, thus completing the cycle of water movement and maintaining the pressure necessary for the phloem's function.