In dicotyledonous stems, the vascular tissues lie in between cortex and pith. It occurs in the form of a continuous or split cylinder. The phloem is situated on the peripheral or external side. In contrast to root, xylem and phloem of stem lie on the same radii. Each strand of the split vascular cylinder is designated as vascular bundle that is separated by interfascicular parenchyma.
A vascular bundle with external phloem only is termed as collateral bundle. In some plants belonging to families Solanaceae, Cucurbitaceae, Apocynaceae etc. phloem is also present on the inner side of xylem.
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.