Xylem and Phloem A Plant’s Dynamic Duo

Xylem and Phloem: A Plant’s Dynamic Duo Sap is transported through a plant’s circulatory system by the actions of the xylem and the phloem. The xylem carries mineral rich water from the roots to the rest of the plant, while the phloem carries sap, that has organic nutrients, to parts of the plant on an as needed basis. Though they have similar functions, the xylem and phloem have different processes by which they work and the plant puts them to different uses.
In the process of moving water and minerals from the roots to the outer shoots, the sap is moved through the xylem by the action of a few different forces and physical properties. At times of minimum transpiration, minerals continue to flow into the xylem. The stele prevents these minerals from flowing back out and results in a negative water potential. Water flows in and raises the water pressure in the root causing an upward flow in the xylem. The pressure is only enough to move the sap a few meters, and may cause gluttation at night. However, the xylem relies on other forces to move water to the tips of larger plants.
A major contributing force moving sap in the xylem is known as transpirational pull. During the day, water is lost through the leaves by transpiration. As this water is lost, tension on the surface of the water film becomes curved and concave. This curvature has an increased tension and results in a negative water pressure. This causes water to be pulled from areas, such as the roots, that have a higher water pressure. Hydrogen bonding results in the entire tube feeling the pull, due the action of the water molecules pulling on the adjacent molecules.
Contributing to the action of the water pull, and the hydrogen bonding, is adhesion. The hydrogen bonds in the water cause it to tightly adhere to the hydrophilic walls of the xylem. The extremely small diameter of the xylem vessels and tracheids enhances the adhesion and in effect causes the water in the xylem to overcome the effects of gravity.
The plant transports organic sap throughout the plant in a system of sieve tubes known as phloem. In the phloem, the sap flows from sugar sources to sugar sinks. These sites will vary over the course of seasons as the plant stores and uses starch and sugar. This necessitates that the phloem is able to move sap from the roots upward as well as from the leaves and into other parts of the plant.
Phloem "loading" is initiated when sugar flows from a sugar source into the phloem and displaces the water. The resulting loss of pressure in the sieve tube causes water to be absorbed from the surrounding tissues. This absorption results in an increase of hydrostatic pressure and forces the sap to flow. As the sap reaches the sugar "sink", sugar is "unloaded" and causes a further decrease in water pressure at the sugar sink. In the case of leave to root translocation, excess water in the phloem will be cycled back to the xylem where it will be moved back to the leaves by transpirational pull.
References

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Pearson Education, 2002.