When the plant is growing, the leaves act as the source of food carrying out photosynthesis. The vascular bundle, phloem is responsible for conducting the food from thesource to the sink. But the process is reversed during seasons like spring because during that term the food which is stored in sink is then supplied to the growing parts of the plant. The source and sink relation is hence variable so direction of movement of food via the phloem can be either upwards or downwards i.e. bidirectional.
But in the xylem, water transport only occurs when water is absorbed from the roots and is conducted via the xylem to the leaves. Thus this kind of absorption via the xylem occurs only in one direction which means that the movement of water and minerals is unidirectional via the xylem.
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