Original: 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.
Simplified: Transpiration is the evaporation of water \/O/ loss of water from those parts of plants that are outside the earth, especially leaves but also stems, flowers and fruits.