Xylem transports only inorganic nutrients like water and minerals. The source for water and minerals is soil; from the soil the inorganic nutrients move upward. Since this movement is in only one direction xylem transport is unidirectional. Phloem transports mainly organic substances like food. The source in this transport is understood to be that part of the plant which synthesises the food, i.e., the leaf and sink, the part that needs or stores the food. But, the source and sink may be reversed depending on the season, or the plant’s needs. Sugar stored in roots may be mobilised to become a source of food in the early spring when the buds of trees, act as sink; they need energy for growth and development of the photosynthetic apparatus. Since the source-sink relationship is variable, the direction of movement in the phloem can be upwards or downwards, i.e., bi-directional.