Transpiration and Guttation

A plant absorbs water constantly and continuously, but it puts hardly 5% of everything it takes in to actual use, for cell expansion and growth. The remaining 95% becomes surplus, and this surplus water is lost to the atmosphere through the plant's aerial parts. Of that surplus, only about 1% is lost in liquid form, while the remaining 99% is lost as vapour.

The loss of water in liquid form is called guttation, and it takes place through specialised structures called water stomata, or hydathodes, typically found along the margins or tips of leaves. Guttation is most noticeable in low-transpiration conditions — such as humid nights or early mornings — when root pressure continues pushing water up into the leaf even though the stomata that normally allow vapour loss are largely closed; with nowhere else for the excess pressure to go, water is forced out as visible liquid droplets through the hydathodes instead. Guttation fluid is not the same as dew: dew condenses onto the leaf surface from atmospheric moisture, whereas guttation fluid is pushed out from inside the plant's own vascular tissue by active root pressure. …