Transpiration is the process where plants release water vapour into the atmosphere from their aerial parts, like leaves and stems, through tiny pores called stomata. Transpiration is essentially the evaporation of water from a plant's surface, similar to how humans sweat. 

When a plant absorbs more water than it can transpire (evaporate through stomata), the excess water, along with dissolved minerals and sugars, is pushed out through hydathodes. The process through which it does this is called guttation.