Section: Understanding Transpiration

  • Transpiration is the process by which water moves up from the roots to the leaves of a plant and then evaporates into the air.
  • It is primarily an unavoidable consequence of the gas exchange needed for photosynthesis.
  • During this process, water vapour is lost from the stomata (small pores) located predominantly on the undersides of leaves.

Section: Factors Influencing Transpiration

  • Many factors affect the rate of transpiration, including temperature, humidity, light intensity, and air movement (wind).
  • Higher temperatures and wind speed increase the rate of transpiration by speeding up the evaporation of water.
  • Conversely, high humidity reduces the rate as the concentration gradient between the leaf and the environment decreases.

Section: The Role of Stomata

  • The stomata play a crucial role in transpiration. They open to let in carbon dioxide for photosynthesis and close to prevent excessive water loss.
  • These openings are controlled by two surrounding guard cells that swell and shrink based on the water potential inside them.

Section: The Transpiration Stream

  • Transpiration creates a suction, drawing up water and dissolved mineral nutrients from the roots, through the xylem vessels in a continuous transpiration stream.
  • Water molecules are cohesive (stick together) and adhesive (stick to the xylem walls), which assists in upward movement.

Section: Transpiration and Plant Adaptations

  • Plants have adaptations to either reduce or increase the rate of transpiration. For instance, plants in hot, dry climates often have small, thick leaves, or fewer stomata to reduce water loss.
  • It’s important to understand that whilst transpiration can lead to water loss, it is also necessary for photosynthesis and maintaining plant structure.