Transpiration and Translocation

• flowering plants have two separate types of vessel- xylem and phloem- for transporting stuff around the plant.
• Phloem tubes transport food

  1. Made of columns of elongated living cells with small pores in the end walls to allow cell sap to flow through
  2. They transport food substances (mainly dissolved sugars) made in the leaves to the rest of the plant for immediate use or for storage
  3. The transport goes in both directions
  4. This process is called translocation
• Xylem tubes take water up

  1. Made of dead cells joined end to end with no end walls between them and a hole down the middle. They're strengthened with a material called lignin
  2. They carry water and mineral ions from the roots to the stems and leaves
  3. The movement of water from the roots through the xylem is called the transpiration stream
• Transpiration is the loss of water from the plant

  1. Transpiration is caused by the evaporation and diffusion of water from a plant's surface. Most transpiration happens at the leaves.
  2. This evaporation creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it.
  3. This in turn means more water is drawn up from the roots, and so there's a constant transpiration stream of water through the plant.

  
• Transpiration is a side-effect of the way leaves are adapted for photosynthesis. They have to have stomata in them so that gases can be exchanged easily. Because there's more water inside the plant than in the air outside, the water escapes from the leaves through the stomata by diffusion.