1.2 Transpiration (T) Transpiration is the loss of water under the form of vapor by the plants, predominantly by means of leaves, although in woody plants a tiny loss might also occur through the lenticels of the bark of branches. On the leaves evaporation befalls from the cell walls into the direction of air intercellular spaces, coming up the diffusion process through the stomata to atmosphere. The stomata acts as a fundamental regulator of transpiration rates along with the adjacent air layer to the leaf. An alternative path to the stomata is the foliar cuticle, although under good water supply conditions the preferential via is the stomachic. The maintenance of transpiration is achieved by the reposition of lost water, at the vapor phase, by the water from the transpiration current that takes place throughout the conductor system from the roots up to the leaves, as a function of a water potential gradient from the soil (ψsoil) to the air (ψair) as shown in Figure 1. The atmosphere with its water potential highly negative performs as a drain for water vapor. The drier the air is (low relative humidity), the higher (more negative) the suction force of such a drain will be.