Xylem and Phloem

• Phloem tubes

  • Transport food substances
  • Phloem tubes are made of columns of living cells called sieve tube elements.
  • These have perforated end-plates to allow stuff to flow through.
• Sieve tube elements 

  • have no nucleus. 
  • This means that they can't survive on their own, so each sieve tube element has a companion cell. 
  • These cells carry out the living functions for both themselves and their sieve cells.
• Phloem vessels transport food substances (mainly sugars) both up and down the stem to growing and storage tissues.

  • This movement of food substances around the plant is known as translocation.

  Sugars are usually translocated from photosynthetic tissues, e.g. the leaves, to non-photosynthetic tissues, e.g. the roots.

  • The sugars enter the phloem by active transport.
  • They are then pushed around by water, which enters the phloem by osmosis.
• Xylem tubes 

  • Take water and ions up.
  • Xylem tubes are made of dead cells joined end to end with no connecting cell walls between them (to create a long tube) and a hole (lumen) down the middle.
  • The thick side walls are made of cellulose.
  • They're strong and stiff, which gives the plant support. The cell walls are also strengthened with a material called lignin
  • Xylem tubes carry water and mineral ions (e.g. nitrates) in aqueous solution from the roots up the stem to the leaves in the transpiration stream. 
  • Hollow block structure
• Aqueous solution - ions are dissolved in the water.
• Lignin - makes the stem waterproof.
• Transpiration

  • is the loss of water from the plant
  • Transpiration is caused by the evaporation and diffusion of water from a plant's surface.
  •  Most transpiration happens at the leaves.
• Evaporation and diffusion


  1. Creates a slight shortage of water in the leaf

  2. More water is drawn up from the rest of the plant through the xylem vessels to replace it
• More water is drawn up from the roots


  Constant transpiration stream of water through the plant
• Transpiration


  A side-effect of the way leaves are adapted for photosynthesis
• Leaves


  • Have stomata so that gases can be exchanged easily
• There's more water inside the plant than in the air outside


  Water escapes from the leaves through the stomata by diffusion