Question

Which vascular tissue is responsible for the unidirectional movement of substances across the plant?

Answer 1

The plant absorbs water and minerals from the soil through the roots. These substances enter the xylem of the root and are conducted to different parts of the plant. Hence, the movement of substances in xylem is unidirectional from roots to stem and leaves.
However, the movement of substances in phloem is bidirectional. The photosynthates (food) produced by leaves are transported to various parts of the plant body. Also sometimes, the food that is stored in roots gets transported to growing regions of the plant, when required.