Structure/Function of the Xylem and Phloem

• Xylem and Phloem in the root

  • The vascular bundle is found at the centre of the root. There is a central core of xylem, often in the shape of an X.
  • The phloem is found in between the arms of the X-shaped xylem tissue. This arrangement provides strength to withstand the pulling forces to which roots are exposed to during winds.
  • Around the vascular bundle is a special sheath of cells called the endodermis. The endodermis has a key role in getting water into the xylem vessels.
  • Just inside the endodermis is a layer of meristem cells (cells that remain able to divide) called the pericycle.

  
• Xylem and Phloem in the root (2)

  
• Xylem and Phloem in the stem

  • The vascular bundles are found near the outer edge of the stem.
  • In non-woody plants the bundles are separate and discrete.
  • In woody plants the bundles are separate in young stems, but become a continuous ring in older stems. This arrangement provides strength and stability to withstand the bending forces.
  • The xylem is found towards the inside of each vascular bundle and the phloem towards the outside.
  • In between the xylem and phloem is a layer of cambium. The cambium is a layer of meristem cells that divide to produce new xylem and phloem.

  
• Xylem and Phloem in the stem (2)

  
• Xylem and Phloem in the leaf

  • The vascular bundles form the midrib and veins of a leaf.
  • A dicotyledonous leaf has a branching network of veins that get smaller as they spread away from the midrib. Within each vein, the xylem is located on top of the phloem.
  • It also helps to support the structure of the leaf.

  
• Xylem and Phloem in the leaves (2)

  
• Structure and function of xylem cells

  • Lignified cell walls- Adds strength to withstand the hydrostatic pressure so the vessels do not collapse, they’re also impermeable to water. Keeps vessels open even at times when water may be in short supply. It can form rings, spirals or relatively solid tubes with lots of small unlignified areas called bordered pits.
  • Non-Lignified bordered pits- Allows for lateral movement of water where water leaves the xylem and moves into other cells of the plant. Also allow water to leave one vessel and pass into the next vessel.

  
• Structure and function of xylem cells (2)

  • No end plates- Allows the mass flow of water and dissolved solutes as cohesive (between water molecules) and adhesive (between water and the walls) forces are not impeded.
  • No protoplasm(cells are dead when mature)- Doesn't impede the mass flow of water and dissolved solutes (transpiration stream).
  • Small diameter of vessels (narrow)- Helps prevent the water column from breaking and assists with capillary action (adhesion).
• Structure and function of xylem cells (3)

  • Thick-walled xylem parenchyma packs around the xylem vessels, storing food, and containing tannin deposits.
  • Tannin is a bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores.
• The flow of water is not impeded, because:

  • There are no cross-walls
  • There are no cell contents, nucleus or cytoplasm
  • Lignin thickening prevents the walls from collapsing

  
• Structure and function of the sieve tube in the phloem

  • Sieve plates with sieve pores- allows for the continuous movement of the sap containing organic compounds such as sucrose and amino acids.
  • No nucleus, vacuole or ribosomes- maximises space for the translocation of assimilates.
  • Cellulose cell wall- strengthens the wall to withstand the hydrostatic pressures that move the assimilates.
  • Thin cytoplasm- reduces friction to facilitate the movement of the assimilates.

  
• Structure and function of the companion cell in the phloem cell (1)

  • Nucleus and other organelles present, e.g. RER- Provides metabolic support to sieve tube elements and helps with the loading and unloading of the assimilates.
  • Transport proteins in plasma membrane- Moves assimilates into and out the sieve tube elements.
  • Large numbers of mitochondria- to provide ATP for the active transport of assimilates into or out of the companion cells. (very important that the phloem has living cells).

  
• Structure and function of the companion cell in the phloem (2)

  • Plasmodesmata (channels in the cell wall)- They link to sieve tube elements which allows organic compounds to move from the companion cells into the sieve tube elements.