Transpiration and Translocation

• Phloem tubes are columns of elongated living cells with small pores in the end walls to allow cell sap to flow through. They transport food substances (mainly dissolved sugars) made in the leaves to the rest of the plant for immediate use or for storage.
• In phloem tubes, transport goes in both directions.
• Xylem tubes are 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.
• Xylem tubes carry water and mineral ions from the roots to the stem and leaves.
• The movement of water from the roots, through the xylem and out of the leaves is called the transpiration stream.
• Transpiration is caused by the evaporation and diffusion of water from a plant's surface. Most transpiration happens at the leaves. 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. 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.