CIE A Level Biology复习笔记7.1.2 Xylem & Phloem Distribution

Xylem

• The functions of xylem tissue in a plant are:
  • Vascular tissue that carries dissolved minerals and water up the plant
  • Structural support
  • Food storage
• 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 as the different organs are under different stresses:
  • In the roots the vascular bundle is found in the centre and the centre core of this is xylem tissue. This helps the roots withstand the pulling strains they are subjected to as the plant transports water upwards and grows
  • In the 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 the leaves the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The xylem tissue is found on the upper side of the bundles (closest to the upper epidermis)

Phloem

• The function of phloem tissue in a plant is to:
  • Transport organic compounds, particularly sucrose, from the source (eg. leaf) to the sink (eg. 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 the companion cells
• Other cell types of phloem tissue also include parenchyma for storage and strengthening fibres
• The location of the vascular bundles is dependent on which organ they are in as the different organs are under different stresses:
  • In the roots the vascular bundle is found in the centre and on the edges of the centre core is the phloem tissue
  • In the stems, the vascular bundles are located around the outside and the phloem tissue is found on the outside (closest to the epidermis)
  • In the leaves, the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line. The phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)

The distribution of xylem and phloem tissue in the roots, stem and leaves in a herbaceous dicotyledonous plant