Transpiration is the evaporation of water \/ loss of water from those parts of plants that are outside the earth , especially leaves but also stems , flowers and fruits . The rate of transpiration is directly related to the evaporation of water molecules from plant surface , especially from the surface openings , or stoma , on leaves . Stomatal transpiration accounts for most of the water loss by a plant , but some direct evaporation also takes place through the cuticle of the leaves and young stems .