Both Xylem and Phloem are vascular tissue found in plants that is responsible for the transport of nutrients is solution. Xylem transports water from the root through out the plant, while phloem is responsible for transporting nutrients (food) from photosynthetic parts of the plant to non-photosynthetic areas of the plant. Although these two plant tissues have the same basic function, evolution has specialized them for their purpose in very different ways.
Xylem is dead at maturity with no organelles or other components. Transport is done without energy expense in the xylem. The opposite is true in the phloem. The phloem is a living tissue that uses active transport to disperse the plants food supply to the areas where it is needed.
The advantages phloem has by having an active transport allows it to be selective in what materials it is sending where. Phloem consists of sieve-tube members accompanied by companion cells. The companion cells allow the phloem to be selective in nature by participating in phloem loading and unloading. The loading and unloading occurs via active transport. If the companion cells load the vessel-elements it causes water to enter the element via osmosis which pushes sap downwards into sugar sinks (areas that us sap for fuel causing an energy loss). The opposite is true when the companion cells unload the sieve tube elements. In this case the companion cells actively transport sap out of the elements creating a pressure that pulls the sap up the phloem. These companion cells and their active transport allow phloem to function in a manner that is bidirectional.
The reason the bidirectionality of the phloem is an advantage is because it can send nutrients to different parts of the plant based on necessity. For example, during the spring growing season, the storage areas of the plants send nutrients out the regions of primary growth. When the growing season has completed the storage areas become the sinks as sugar is...