Answer:
Xylem cells transport water and as water is carried from the roots to all parts of the plant, hence the xylem sap will be unidirectional always.
On the other hand, the function of the phloem cells is to transport food to all parts of the plant. Th food is made in the leaves and has to be transported to all parts of the plant from the leaves. Hence, the phloem sap needs to move in any direction so that the food is provided to each part of the plant.
