model answers plants

• Describe the mass flow hypothesis?

  In the leaf substances are actively transported into the phloem,

  By companion cells,

  lowering the water potential of the sieve tube so water enters by osmosis,

  The increase in pressure causes mass movement towards root,

  Sugars converted in root for respiration.
• Describe the cohesion-tension theory of water transport in the xylem?

  Evaporation through leaves,

  lowers the water potential of leaf cells,

  Water is pulled up the xylem (creating tension),

  Water molecules cohere by hydrogen bonds creating a continous column,

  adhesion of water to the walls of the xylem.
• Support for mass flow hypothesis?


  Phloem is involved,

  Respiration is involved in movement,

  Cooling reduces movement,

  Agar block is the source,

  Roots are the sink
• Against mass flow hypothesis


  No buldge above ringing,

  No role for osmosis,

  Movement could be due to gravity,

  Roots still grow without phloem,

  No leaves to act a source.
• Why is phloem pressure reduced at the hottest part of the day?


  High rate of transpiration,

  Water lost through stomata,

  Causing less movement of water from xylem to phloem.