The source and sink relationship is variable in plants so, the direction of movement in the
phloem can be upwards or downwards, i.e., bidirectional. It is opposite to xylem, where the moment is always unidirectional. Unlike one-way flow of water in transpiration, food in phloem sap can he transported in any required direction as long as there is a source of sugar and a sink able to use, store or remove the sugar. Here, in case of unidirectional flow of xylem tissue, it is important to note that root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.