transportation in plants

The supply and demand or source and sink determines the direction of movement of substances. Phloem sap moves from sugar sources to sugar sinks by means of turgor pressure. 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 fruits and flowers) are always sinks. So in case of phloem, translocation can be up or down the stem, since leaves or roots can be sources.

But the only source of water for plants is the soil. From here, water is absorbed by the roots and transported upwards to all the other aerial parts of the plant body. Hence only unidirectional flow is seen in xylem.