The leaves carry out photosynthesis and act as the source of the food. The phloem conducts the food from the source to the sink (the part of the plant requiring or storing food).  But, the source and sink may be reversed depending on the season, or the plant’s needs.  During spring, 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) as the realtion of source and sink is variable.

However, 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.