Evidence to support cohesion-tension

• Evidence for cohesion-tension: Change in diameter of tree trunks

  • Daytime: Tree trunks reduce in diameter when transpiration is at its greatest (photosynthesis and temperature)
  • This is because adhesion of water molecules to walls of xylem results in a tension which pulls the xylem walls in
• Evidence for cohesion-tension: Change in diameter of tree trunks

  • At night, when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases
• Evidence for cohesion-tension: Xylem vessel broken

  • If a xylem vessel is broken and air enters it, the tree can no longer draw up water
  • This is because the continuous column of water is broken and so the water molecules can no longer stick together
• Evidence for cohesion-tension: Xylem vessel broken

  • When a xylem vessel is broken, water does not leak out, as would be the case if it were under pressure
  • Instead air is drawn in, which is consistent with it being under tension