Xylem

Xylem is plant tissue that forms pathways for transporting water and dissolved mineral ions. These substances enter the xylem in the roots and travel up through the stem to other parts of the plant, including the leaves.

Xylem also helps to support the plant. Its walls are strengthened with a rigid material called lignin. These stiff walls help the stem resist bending and hold the plant upright.

Phloem

Phloem is plant tissue that transports sucrose and amino acids between parts of a plant. It carries these substances from places where they are available to places where they are needed.

For example, sucrose made in leaves can be transported through the phloem to growing roots. Amino acids can also be carried to growing parts of the plant. Transport is not limited to one direction: it depends on where the substances are available and where they are needed.

Xylem and phloem positions in dicotyledonous plant sections

In a cross-section of a non-woody dicotyledonous plant, xylem and phloem can be distinguished by their positions relative to each other and to the rest of the organ. Their arrangement differs in roots, stems and leaves.

Root cross-section

Xylem lies at the centre of the root, often in a star shape. Phloem lies between the arms of the xylem star.

Stem cross-section

Xylem and phloem occur together in vascular bundles. These bundles form a ring around the stem. Within each bundle, xylem is on the side nearer the centre of the stem, while phloem is nearer the outside.

Leaf cross-section

Vascular bundles lie within the leaf veins. In each bundle, xylem is towards the upper surface of the leaf and phloem is towards the lower surface.

When identifying these tissues in a diagram or image, first establish whether the section is from a root, stem or leaf. Then use the relevant positions—centre and arms, inner and outer sides of a bundle, or upper and lower surfaces—to distinguish xylem from phloem.

Xylem vessel structure and function

Xylem vessels are long tubes that carry water and dissolved mineral ions through a plant. Their structure provides a clear passage for water and helps support the plant.

Each vessel is formed from cells joined end to end. The cross walls between the cells are absent, creating a continuous passage. The cells also have no contents, so there is nothing inside them to obstruct the flow of water.

Xylem vessels have thick walls containing lignin. Lignin strengthens the walls, helping the vessels retain their shape as water passes through them. These strong walls also provide support to the plant.