explain wht xylem transport is unidirectional and pholem transport is bidirectional?

xylem transports water nd minerals so it has to be from roots to other parts. whereas phloem  transports organic and mineral nutrients. organic compounds synthesised in the photosynthetic leaves and are exported to all other parts including storage organs from where they are then re exported. here storage organs may be even roots(potato etc).hence it has to be multidirectional