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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 member s and companion cell s. 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 transportingIn cell biology, active transport is the mediated transport of biochemicals, and other atomic/ molecular substances, across membranes. Unlike passive transport, this process requires chemical energy. In this form of transport, molecules move against eithe solute molecules into it. This causes water to move into the sieve-tube element by osmosisOsmosis is the diffusion of a solvent through a semipermeable membrane from a region of low solute concentration to a region of high solute concentration. The semipermeable membrane is permeable to the solvent, but not to the solute, resulting in a chemic, 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.

Phloem is produced in phases. Primary phloem is laid down by the apical meristemThe apical meristem or the growing tip, is a completely undifferentiated meristematic tissue found in the buds of plants. Its main function is to begin growth of new cells in young seedlings at the tips of roots and shoots (forming buds, among other thing. Secondary phloem is laid down by the vascular cambiumVascular cambium is a tissue found in the stems of perennial dicots. Vascular cambium is a part of the plants meristem the series of tissues consisting of embryonic (incompletely differentiated) cells from which the plant grows. It is the innermost layer to the inside of the established layer(s) of phloem.