Above: Gutation on grass, photographed soon after sunrise.
Most people who get out of bed, pull back the curtains
and look out of the window to see what the weather is like will have seen what they think
is dew on the grass of their lawn, knowing that dew is often formed overnight. Dew
forms on objects that get cold and water vapour in the air condenses out onto them.
However what looks like 'dew drops' on the blades of grass above is not dew but droplets
of water formed by a process called gutation. Gutation is the process that many
plants use to expel liquid water from within when the water pressure inside the
plant gets too high. This often happens overnight when transpiration is reduced to a
minimum. (Transpiration is the process by which plants lose water vapour mostly
during the daytime, helping them to control their temperature.) Only certain
structures within the leaves of plants will allow liquid water to emerge (gutation) and
these structures are different to those which allow transpiration to occur.
Gutation is driven by excess water pressure from the roots of plants and often occurs when
the soil around them is very wet, or when atmospheric humidity is high and effective
transpiration (loss of water vapour) is not possible. This can sometimes
happen overnight after a period of rain when the skies clear and the temperature drops.
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