7.1.2—Xylem and phloem distribution

Xylem and phloem occur together in vascular bundles, but their relative positions change with the organ. Read the organ layout first, then identify the xylem and phloem positions rather than assuming one universal bundle pattern.

• Root: The vascular tissue is concentrated in the centre. Xylem forms the central transport region; phloem lies around the edge of the central core. This arrangement places conducting tissue where water and mineral ions entering from the roots can join the upward pathway.
• Stem: Vascular bundles are arranged around the outside of the stem. Within each bundle, xylem is on the inner side and phloem is on the outer side, nearer the epidermis. The distribution fits a stem that must conduct while also resisting bending and supporting the plant.
• Leaf: Vascular bundles form the midrib and veins and spread through the leaf. Xylem is above phloem within the bundle, closer to the upper epidermis; phloem is on the lower side, closer to the lower epidermis. The vein network supplies the leaf and carries substances away from source tissue.

For a transverse section, use a fixed read-out: (1) name the organ from the overall pattern, (2) locate the vascular bundle or central vascular region, (3) compare inner/outer or upper/lower position, then (4) label xylem and phloem only where the evidence supports the identification. Xylem carries water and dissolved mineral ions upward and also contributes structural support; phloem transports substances from source to sink.

Do not draw roots, stems and leaves with the same bundle layout. “Xylem is inside” is useful for roots and stems, but in leaves the key comparison is that xylem is above phloem. These are relative positions in herbaceous dicot transport tissues, not a complete map of every plant organ or an invitation to infer anatomy beyond the section.