Distribution of xylem and phloem in dicotyledonous plants

• what is a dicotyledonous plant?

  a flowering plant with two embryonic seed leaves (or cotyledons) that usually appear at germination
• label the diagram

  A) epidermis

  B) cortex

  C) medulla

  D) vascular bundle

  E) xylem

  F) vascular bundle

  G) phloem

  
• label the diagram

  A) root hair cells

  B) epidermis

  C) cortex

  D) endodermis

  E) xylem

  F) phloem

  
• what is the distribution of tissues in the roots?

  • xylem - cross shape in the centre of the root
  • phloem - found in between arms of cross (in centre of root)
  • gives the roots strength to withstand pulling forces they are exposed to
• what is the distribution of tissues in the stem?

  • xylem - inner side of vascular bundles which are arranged around outside of stem
  • phloem - outer-side of vascular bundles - which are arranged around outside of stem
  • for strength and support but retain some flexibility
• what is the distribution of tissues in the leaves?

  • xylem - upper side of central midrib and leaf veins
  • lower side of central midrib and leaf veins
  • support the leaf, preventing it collapsing under its own weight
• why are xylem vessels dead?

  this is so there is nothing to impede the flow of water
• what is the function of the bordered pits?

  pores to allow movement of water in/out of xylem and between adjacent vessels
• how do xylem cells line up?

  line up end to end forming a hollow tube to allow the passage of water and dissolved mineral ions
• what are the various patterns of lignin?

  • annular form
  • spiral form
  • reticulate form
  • xylem are not completely lignified to allow flexibility of the root/stem
• what is lignin?

  • strengthens cell wall
  • makes walls waterproof
  • for support to prevent xylem vessels collapsing due to the negative pressure inside
• what is xylem parenchyma?

  thick-walled, packs around the xylem vessels, storing food, and containing tannin deposits - bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores
• what is the function of xylem vessels?

  transport water and mineral ions up and around the plant

  e.g. to the leaves from the roots for photosynthesis
• what is the structure of xylem vessels?

  • hollow - with no cell contents
  • bordered pits - pores allowing movement of water between adjacent vessels
  • lignin - strengthens, supports and waterproofs walls - under negative pressure in vessel
• what were bordered pits once?

  plasmadesma
• why does lignification kill xylem cells?

  waterproofs them, depriving them of water and oxygen
• what are phloem cells comprised of?

  • sieve tube elements
  • companion cells
• what is transported in the sieve tube elements?

  sucrose solution (sap)
• why do the sieve tube elements have a thin layer of cytoplasm?

  prevents cell contents from impeding the flow of sucrose solution
• why do the companion cells have a dense cytoplasm?

  due to many mitochondria
• why do companion cells have lots of mitochondria?

  they need lots of energy for active transport and they also need to supply the energy needs of the sieve tube elements
• what is the function of phloem?

  transport nutrients and sucrose solution around the plant (transport of assimilates)
• what is the structure of phloem?

  • live cells
  • sieve tube elements
  • companion cells
  • plasmodesma linking
• why do sieve the elements only have a thin layer of cytoplasm?

  prevents cell contents from impeding flow of sucrose solution - maximises volume
• why are sieve tube elements not true cells?

  • no nucleus
  • have companion cells to support them and carry out all necessary functions