Transpiration is the loss of water vapour from the leaves of a plant. Water evaporates from the surface of cells inside the leaf into the air spaces, and the vapour then diffuses out through the stomata.
Stomata are tiny pores, found mostly on the underside of a leaf. Each stoma is surrounded by two guard cells. When the guard cells take in water they become turgid and the stoma opens. When they lose water they become flaccid and the stoma closes. Open stomata let carbon dioxide diffuse in for photosynthesis, but they also let water vapour diffuse out.
As water is lost from the leaf, more water is pulled up the xylem from the roots to replace it. This continuous movement of water through the plant is called the transpiration stream. It brings mineral ions from the soil to the leaves, supplies water for photosynthesis, keeps cells turgid and helps to cool the leaf.
