Xylem and phloem

• Xylem structure:

  • Plants possess two specialist transport vessels called the xylem and phloem.
  • The xylem and phloem are arranged throughout the root, stem and leaves in groups called vascular bundles.
• Vascular tissue in a dicotyledonous plant (part 1):

  
• Vascular tissue in a dicotyledonous plant (part 2):

  
• Structure and function of the xylem:

  • Xylem vessels transport water and minerals from the roots to the stem and leaves in the transpiration stream.
  • The xylem has some key structural features that makes it adapted for the transport of water.
  • It is composed of dead cells joined end to end with no end walls between them.
  • This creates a hole (lumen) down the middle which forms a hollow tube.
  • Xylem cells are strengthened by a woody material called lignin which makes the cell walls strong, waterproof and stiff.
• Xylem cells lose their top and bottom walls to form a continuous tube through which water moves through from the roots to the leaves:

  
• Phloem structure (part 1):

  • Phloem tubes transport nutrients like dissolved sugars (e.g. sucrose) and amino acids both up and down the stem.
  • Nutrients are transported to new growing and storage tissue in a process known as translocation.
  • Phloem tubes are made of columns of living cells called sieve-tube elements which are joined end to end.
• Phloem structure (part 2):

  • Sieve tube elements contain end-plates with holes (sieve plates) which allow sugars and amino acids to flow easily through.
  • Sieve tube elements are supported by companion cells that carry out the living functions for the sieve tube elements.
  • This is because sieve tube elements contain no nucleus so can't survive on their own.
• Phloem cells form tubes similar to xylem vessels, except the cells still retain some subcellular structures and are therefore living:

  
• Comparison between Xylem and Phloem Tissue Table:

  
• Xylem are found in the plant stem. They form long tubes. These tubes carry eater and dissolved minerals from the roots to the leaves. Like this...

  
• Xylem cells have very thick walls containing lignin. This provides support to the plant. Because the cell walls are sealed with lignin, this causes the xylem calls to die. Also the end walls between the cells have broken down. This means that the cells now form a long tube so water and dissolved minerals can flow easily. Like this...

  
• Xylem cells have got no nucleus. Cytoplasm, vacuole, or chloroplast. That makes it easier for water and minerals to flow.
• Phloem tubes carry dissolved sugars up and down the plant. Phloem vessel cells have no nucleus and only limited cytoplasm. The end walls of the vessel cells have pores called sieve plates. Both these features allow dissolved sugars to move through the cell interior. Like this...

  
• Each phloem vessel cell has a companion cell connected by pores. Mitochondria in the companion cell provided energy to the phloem vessel cell. Like this...

  
• Root hair cells:

  These hairs increase the surface area of the root, so it can absorb water and dissolved minerals more effectively. Like this...

  
• The root hair increases the surface area of the root. Also root hair cells do not contain chloroplasts, because they are underground. Like this...