Guttation - or when Dew is not Dew

Oh, to the days when I would walk past grass in the morning, see droplets of water on the grass blades accept that it was 'dew' and carry on. Recently I heard of a phenomena that looks like dew and is what, at least some of the time I was confusing with dew, but is called guttation and is created by a very different process.

Dew is  water vapour condensating onto leaves, such as the blades of grass in our example. For this to occurr, the temperature of the air must reach what's called the dew point. It's at this point when water vapour held in the air condenses into droplets, changing from a gas to a liquid.

Guttation is the process whereby plants release excess water through cells called hydathodes. The water released in this way typically occur at the tip of the leaf and is caused by root pressure.

Root pressure is a cool process whereby water is drawn into the roots by osmosis, then when the root cells become fully inflated the water follows the route of least pressure, which in this case is into the xylem. The xylem is the 'passage way' that goes uninterrupted from the roots, up through the stem and into the leafs - which is part of what we see at the leaf veins. This pressure can be seen when you cut the stem of a herbaceous stem and see liquid coming out.

This is normally fine because the leaves transpire (breathe) through their stomata pores, completing the full circuit of root pressure and transpirational pull. This transpirational pull, pulls more water into the leaf when water is vaporised from the leaf and needs replacing. This is a rather amazing process and it's possible for a tree like a 48 foot silver mapble to transpire 2220 litres every hour! It's also said that the average humble tomato plant transpires 115 litres during its growing phase - anyway back to guttation.

At night plants close their stomata and don't transpire, but the roots still take in water, therefore the root pressure can build up to a point that water needs to be released from the plant. Hydathodes are special pores (originally for removing excess salt) that allow the water to be squeezed out of them during the night.

This can result in guttation droplets all the way down the edge of the leaves, but normally at the leaf tip, as in the guttation that's occurring in the leaf blade tips in this photo.