Explain why xylem transport is unidirectional, and phloem transport is bi-directional.

Explanation

Xylem Transport is Unidirectional : The force for xylem transport is generated by transpiration. Transpiration occurs only in the aerial parts, mostly those having leaves. It creates a negative pressure or tension in the xylem channels that cause the upward movement of sap. Xylem transport is, therefore, unidirectional or in the upward direction only.
Phloem Transport is Bidirectional : Phloem transport is based upon the developement of positive pressure in the region where nutrients are loaded. The movement is towards the sink region where there is less turgot pressure, and a mechanism to offload the nutrients. Positive or high turgor pressure develops in the area of food manufacture. Translocation occurs both towards roots (for storage, and consumption), and towards growing points of the shoot (for consumption). Phloem transport is, therefore, bidirectional, both towards the upper side as well as towards lower side.