Loss of excess water absorbed by the roots, by some plants in the form of liquid droplets of water is called guttation. Burgerstein first observed it in 1887.
Every plant does not show guttation. So far, only 345 genera of mainly herbaceous plants have been known to explain this phenomenon. Specifically, this occurs when the rate of transpiration is lower than usual, and the rate of absorption is higher. Due to continuous absorption by roots, a positive root pressure develops which pushes the excess water through a specialized pore. Such pores are called hydathode or water stomata. They are present at the vein endings of only leaves. Each hydathode has a group of loosely arranged cells, which are colourless and parenchymatous. These are called as epithem. The water that goes out through guttation is impure, as it is in liquid form, and contains dissolved organic and inorganic salts. It is seen early in the morning in plants like balsam, tomato etc., which grow in a warmer, humid day which is followed by a cooler night.