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

Explanation

As the water is absorbed by root from soil, it is to move only upwards through xylem making it unidirectional flow (upward). Food is formed in leaf and it is required both for root system and shoot system. It occurs through phloem. Phloem sap moves upwards and downwards making sugar transport bidirectional. Here sourcesink relationship works depending upon the season, need of plant. Moreover, phloem is well adapted for bidirectional flow.