The long-distance movement of organic substances from the source or storage end (region of manufacture or leaf) to the region of utilization or sink. The sink and source can be reversed depending on the need of the plant or the seasonal changes. Sugar mainly in form of sucrose that is stored in roots is mobilized to become a source of food in the early spring when the buds act as sink and require energy in form of $\mathrm{ATP}$ for their unloading. Since the source-sink relationship is highly variable, the direction of movement of organic solutes in phloem is bi-directional, i.e. can be upwards or downwards. It is in contrast to the unidirectional movement of water in the xylem.