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

During the growth of a plant, its leaves act as the source of food as they carry out photosynthesis. The phloem conducts the food from the source to the sink (the part of the plant requiring or storing food). During spring, this process is reversed as the food stored in the sink is mobilised toward the growing buds of the plant, through the phloem. Thus, the movement of food in the phloem is bidirectional (i.e., upward and downward).
The transport of water in the xylem takes place only from the roots to the leaves.
Therefore, the movement of water and nutrients in the xylem is unidirectional.

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