Write a brief account about the source and sink concept of the plants.

Food, primarily sucrose, is transported by the vascular tissue phloem from a source to a sink. Usually the source is the part of the plant which synthesizes 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 mobilized 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 but in the case of the xylem, the movement is always unidirectional, i.e., upwards. Hence, food in phloem sap can be transported in any required direction so long as there is a source of sugar and a sink able to use, store or remove the sugar.