3.4:VASCULAR BUNDLE

• Xylem


  The water transporting tissue in vascular plants
• Phloem


  The tissue that transports sugar and other organic compounds like amino acids through the plant
• Xylem


  • Complex tissue in monocots and dicots
  • Composed of 5 different cell types: tracheids, vessels, parenchyma, sclereids, fibres
• Phloem


  • Composed of sieve tubes, companion cells, phloem parenchyma, sclereids and fibres
• Translocation

  Transport of organic materials either up or down the stem in phloem
• Sugar is transported from the leaves where it is made (source tissue)


  To any part of plant which is using or storing it (sink tissue)
• Sieve tubes


  Translocate materials in phloem
• Companion cells


  Support sieve tubes
• Phloem parenchyma


  Store starch
• Sclereids


  Provide support
• Fibres


  Provide support
• The conducting cells in the phloem remain alive and form the sieve tubes
• Sieve tubes


  • Formed from vertical columns of cells joined end to end
  • End walls are perforated allowing flow of substances through the sieve tube
  • Cell walls are not lignified so cell contents are alive although they lose their nuclei
• Phloem


  • Contains supporting cells - fibres and sclereids (reduced form of sclerenchyma) which are highly thickened and lignified
• Fibres


  Cells that are long and thin, often bundled together to provide maximum stability for plants
• Parenchyma and companion cells


  Located on either side of the sieve tube, manage the flow of nutrients through the sieve tube
• Companion cells


  Provide ATP, proteins and other nutrients to the sieve tube
• Vascular bundles


  • Have a supporting function as well as transport function, containing vessels, fibres and other thick-walled, lignified and elongated cells
• Main points


  • The vascular bundle or tissue contains the xylem and phloem
  • The xylem and phloem are the transport tissues in plants
  • Transport of substances is from source to sink tissues
  • Xylem is responsible for water and mineral transport, done by tracheids and vessel elements
  • The phloem translocates sugar and other manufactured substances, done by the sieve tubes
  • Xylem is composed of tracheids, vessel element, sclereids, xylem parenchyma and fibers
  • The phloem is composed of sieve tubes, companion cells, phloem parenchyma, sclereids and fibres
  • Lignin is a tough, water-proof substance that provides strength and support to plants