Transpiration

• Transpiration is the process of water constantly evaporating from the surface of the leaves.
• Transpiration starts with the evaporation of water from cells inside the leaf. The water vapour diffuses through the air spaces in the spongy mesophyll and out of the leaf through the stomata. Water passes from the xylem into the leaf to replace water lost. Finally, water is drawn into the root hair cells and up the xylem vessels to the leaf.
• Transpiration brings water to the leaf, which is required for photosynthesis. The transpiration stream also transports dissolved mineral ions such as magnesium.
• The evaporation of water from the leaf cools the leaf down, especially in warm weather.
• Transpiration is affected by:

  • Temperature (evaporation occurs faster)
  • Dry Conditions (evaporation occurs faster)
  • Windy Conditions (removes water vapour, more water can evaporate)
  • Light Intensity (increases rate of photosynthesis)
• Stomata are surrounded by two guard cells. When light intensity is high, guard cells swell and change shape, causing stomata to open. Under hot conditions, the plant closes its stomata, however can no longer photosynthesise.