Guttation is the appearance of water droplets near the end of the leaves of vascular herbaceous plants at the end of the night. These droplets are not to be mistaken for dew.
This phenomenon is caused by root pressure: at night there is no transpiration to suck the water towards the leaves, but the roots continue to actively absorb minerals from the soil. These minerals are trapped in the plant and cause water to enter by osmosis.
The pressure causes the raw sap to rise leading to the formation of water droplets on specialised structures: the hydathodes.
Water droplets formed by the root force of the horsetail. © Luc Viatour CC by-sa 3.0