Question

Xylem transports sap upwards only. Where as Phloem transports food in both the directions. Why do you think there is a difference?

Answer 1

as the water is collected form the roots and due to suction pressure it is ment to move in upward (therefore xylem transports water unidirectionally)

WHEREAS

the food is  produced by the leaves (by photosynthesis) so in order to transfer it to all parts of the plant phloem works bidirectionally .