Xylem and phloem tissues are found in groups called vascular bundles. The position of these bundles varies in different parts of the plant. In a leaf, for example, the phloem is usually found closer to the lower surface.
Xylem vessels are tough and strong, so the vascular bundles are in the centre of the root to resist forces that could pull the plant out of the ground.
The stem has to resist compression (squashing) and bending forces caused by the plant’s weight and the wind. The vascular bundles are arranged near the edge of the stem, with the phloem on the outside and the xylem on the inside.