The movement of water from the roots to the leaves is a critical function in a plant's life. The flow of water depends upon air pressure, humidity, adhesion, and cohesion. At sea level, normal air pressure can force water up the columns of xylem from the roots to a height of many feet. These columns of water continue to flow upward because water molecules stick to the walls of xylem by adhesion and stick to one another by cohesion. Water initially moves into the root hair cells by osmosis, because the mineral content of the cells is higher than that of the surrounding environment. Thus, a root pressure is established and extends into the microscopic tubes of the xylem.
At the far end of the system, water either moves out of the plant through the stomata or diffuses into the photosynthetic cells and the spaces around them. This loss of water from the xylem at the far end allows water to flow up the xylem tubes, creating continuous water movement.
In the phloem cells, fluids move in a somewhat similar fashion. In many cases, there is a flow of carbohydrate from the source of production to a depository known as the “sink.” For example, sucrose is produced in a photosynthesizing leaf and is transported into phloem tubes. It exits the phloem far away in the root. This is the “source‐sink” phenomenon. A constant flow of water is established in the phloem because water follows the carbohydrates by osmosis.