In vascular plants, phloem is the tissue that carries organic nutrients, particularly glucose. In trees, the phloem is part of the bark, hence the name, derived from the Greek word for "bark". See also xylem.
Phloem sap moves from sugar sources to sugar sinks. A sugar source is any part of the plant that is producing or releasing sugar. During the plant's growth period, usually during the spring, storage organs such as the roots are sugar sources, and the plant's many growing areas are sugar sinks. After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks. Developing seed-bearing organs (such as fruit) are always sinks. Because of this multi-directional flow, coupled with the fact that sap cannot with ease move between adjacent sieve tubes, it is not unusual for sap in adjacent sieve tubes to be flowing in opposite directions.
Phloem tissue consists of sieve-tube memberss and companion cellss. The sieve-tube elements are large, cylindrical cells, with large pores in the cell wall at either end. They are almost entirely dead, and have no organelles. All of their functions of a sieve-tube element are carried out by its (much smaller but quite living) companion cell.
Phloem sap moves by little flow, using the sieve tubes as pipes. This flow is accomplished by a process called phloem loading and unloading. Cells in a sugar source "load" a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect.