The exudation of liquid water onto a plant surface. It occurs under conditions of high humidity when the saturated atmosphere prevents *transpiration
. The increase in *root pressure
forces water out of special *hydathodes
. The secreted water may contain calcium salts, which dry as a white crust at the leaf margins. Morning 'dew' on grass is often the product of guttation, as the lower temperatures at night provide ideal conditions for the process to occur.