Transpiration

• Water is constantly evaporating from the surfaces of leaves. This process is called transpiration.
• Transpiration starts with the evaporation of water from the cells inside the leaf.
• The water vapour then diffuses through the air spaces in the spongy mesophyll and out of the leaf through the stomata.

  
• Finally, water is drawn into the root hair cells and up the xylem vessels to the leaf. (to replace the water that has been lost)
• The whole process is called the transpiration stream.
• The transpiration stream transports dissolved mineral ions such as magnesium which play important roles in the plant.
• The evaporation of water from the leaf cools the leaf down, especially in warm weather.
• The rate of transpiration is greater at higher temperatures. This is because evaporation is faster when temperatures are higher.
• Transpiration is faster under dry conditions, when the air is not humid. This is because evaporation takes place more quickly under dry conditions.
• The rate of transpiration increases in windy conditions. This is because wind removes any water vapour, allowing more water to evaporate.
• The rate of transpiration when the light intensity increases. This is because high light intensity increases the rate of photosynthesis.
• Once the stomata have opened to let in carbon dioxide, water vapour can now pass out of the leaf.
• Stomata are surrounded by guard cells.

  
• When the light intensity is high from example during the day, the guard cells swell and they change shape.

  This causes the stomata to open.

  Now carbon dioxide diffuses into the leaf and be used for photosynthesis.
• Under hot conditions, that plant closes its stomata to reduce water loss by transpirations. This means that the plant cannot photosynthesise.