Skills: Identifying Xylem & Phloem (DP IB Biology: HL)

Xylem

• The functions of xylem tissue in a plant are:
  • To transport dissolved minerals and water up the plant via vascular tissues
  • Structural support
•  Xylem tissue is found, along with phloem tissue and other tissues, in vascular bundles
•  The location of the vascular bundles is dependent on which organ they are in:
  • In roots, the vascular bundle is found in the centre and in the middle of the centre core is the xylem tissue. This helps the roots withstand the pulling strains they are subjected to as the plant transports water upwards and grows
  • In stems, the vascular bundles are located around the outside and the xylem tissue is found on the inside (closest to the centre of the stem) to help support the plant
  • In leaves, the vascular bundles form the midrib and veins and therefore usually spread from the centre of the leaf in parallel lines. The xylem tissue is found on the upper side of the vascular bundles (closest to the upper epidermis)

The distribution of tissue types differs between roots, stems, and leaves.

Phloem

• The function of phloem tissue in a plant is to:
  • Transport organic compounds, particularly sucrose, from sources (e.g. leaves) to sinks (e.g. roots). The transport of these compounds can occur up and down the plant
•  Phloem is a complex tissue also made up of various cell types; its bulk is made up of sieve tube elements (which are the main conducting cells) and companion cells
• Other cell types within phloem tissue include parenchyma cells (for storage) and strengthening fibres
• The location of the vascular bundles is dependent on which organ they are in:
  • In roots, the vascular bundle is found in the centre and on the edges of the centre core is the phloem tissue
  • In stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis on the outer surface of the stem)
  • In leaves, the vascular bundles form the midrib and veins and therefore usually spread from the centre of the leaf in parallel lines. The phloem tissue is found on the lower side of the vascular bundles (closest to the lower epidermis)