The driving force for water leaving the leaf is the difference between the water vapor pressure of the air compared to the water vapor pressure inside the leaf. This difference is called the vapor pressure deficit (VPD).
Fog systems and poly tents reduce VPD by minimizing the difference between the vapor pressure of the leaf and air.
Misting a cutting puts a film of water on the leaf. This water on the surface evaporates in response to VPD preventing water loss by the leaf.
