Guttation

The grapevine leaves in our garden, yesterday morning, had droplets of water around their margins, yet were perfectly dry in the middle.  I'm pretty sure this is an example of 'Guttation'. 

  Over night, most plants close their stomata (pores) which prevents transpiration; so, when there is high humidity or a lot of moisture in the soil, we get 'root pressure'.  Water is then forced out through 'hydathodes' (a secretory structure - either glandular hairs or water pores) which are usually found at vein endings.

  Sometimes the water droplets contain calcium salts, which leave a white crust behind (not evident here).  This is not to be confused with 'dew'.  Dew is moisture condensed from the atmosphere onto the plant surface - in guttation, the water comes from within the plant.