Xylem

• Transpiration is the evaporation of water vapour from plants
• Xylem vessels are found on the inside of the vascular bundle
• Xylem cells are dead and resemble a series of tubes
• Xylem carries water and mineral ions from the roots to the leaves
• Xylem tubes are kept open and strengthened by rings of lignin
• Water vapour evaporates from the stomata of leaves as a result of transpiration
• Water molecules form hydrogen bonds between one another, known as cohesion
• Water forms a continuous unbroken column
• As water vapour evaporates in the leaves, water molecules are drawn up the xylem as a result of cohesion - known as the transpiration pull
• The transpiration pull places the xylem under negative pressure, so there is tension within the xylem hence the name cohesion-tension theory
• Diameter of tree trunks changes according to the rate of transpiration

  • when rate is highest during the day, more tension shrinks the diameter
• If a xylem vessel is broken, tree can no longer draw up water because the continuous column is disrupted
• Adaptations of a xylem

  • dead cells form a continuous column
  • narrow tubes
  • pits allows water movement in and out
  • lignin supports and allow xylem to stretch
  • no end walls
  • no cell contents