The primary food source commonly called as sucrose is transported by the vascular tissue phloem from the source to the sink. The source is the plant part which can synthesize the food, like leaves. Sink is the plant part that requires the food and stores the food. The source and sink are reversed based on the season or the needs of the plant. The sugars stored in the roots would be made to move as the source of food in the spring while the tree buds act as sink receiving the food from the roots. The photosynthetic apparatus development and growth of the plant parts occurs by the energy generated from the food that is received by the sink. The positions of source and sink are variable and hence the movement of food through phloem can be bi-directional (upwards or downwards). The movement of water in the xylem is always in upward direction. The flow of water in transpiration is towards up and the movement of phloem sap occurs in both the directions, depending on the source of the sugar and sink that can make use, store and remove the sugars. The major constituents of phloem sap are water and sucrose while the hormones, amino acids and sugars are also translocated through the phloem.