2.3.2 The Plant Organ System

• What is transpiration?

  Transpiration is the process where plants release water vapour into the atmosphere, usually through their leaves.
• What is transpiration caused by?

  Evaporation and diffusion.
• What are the three steps of transpiration?

  1. Evaporation occurs at the leaves.
  2. A slight shortage of water is caused at the leaves so more water is drawn up from the rest of the plant through the xylem vessels to replace it.
  3. This means more water is drawn up from the roots, and so there’s a constant transpiration stream of water through the plant.
• What is transpiration a side effect of?

  Photosynthesis.
• Where does the water escape from in transpiration?

  The stomata.
• What do phloem tubes transport?

  Sugars and other nutrients.
• What are phloem tubes made of?

  Columns of elongated living cells with small pores in the end walls to allow cell sap to flow through.
• Why does the phloem transport food?

  For immediate use or storage.
• What is cell sap?

  A liquid made up of the substances being transported and water.
• What is translocation?

  The movement of dissolved sugars around a plant.
• What do xylem tubes do?

  Take water up the plant.
• What are xylem cells made of?

  Dead cells joined end to end with no end walls between them and a hole down the middle. They are strengthened with a material called lignin.
• Where does water go from and to in a xylem tube?

  The roots to the stem and leaves.
• What is the transpiration stream?

  The movement of water from a plant’s roots, through the xylem and out of the leaves.
• What factors affect transpiration?

  • Humidity - drier air = faster transpiration.
  • Temperature - warmer = faster transpiration.
  • Air flow - better = faster transpiration.
  • Light intensity - brighter = faster transpiration.