When Old Capulet said "the air doth drizzle dew", it was his Shakespearean way of explaining the tears of his daughter Juliet, weeping, he assumed, over the untimely death of her cousin Tybalt. But Capulet had missed the point; Juliet's thoughts were on her lover, Romeo, and had nothing to do with Tybalt whatsoever.
Neither, of course, was he right about "the air doth drizzle dew".
Dew forms, as we know, when calm, humid air in contact with a cold surface is cooled to the point where it can no longer accommodate all its moisture; the moisture does not "drizzle" but condenses as tiny drops of water directly onto the cold surface underneath.
And to compound his errors, it may have been that from time to time what Capulet thought was dew on the grass just after sunset, was not really dew at all; it could be the product of a process called guttation. This was first identified by a Victorian scientist called John Aitken, who was troubled that very often in the case of apparently dewy grass and other plants, the leaves are not uniformly coated with the tiny drops.
Instead, a single large drop sometimes hangs from the tip of each blade. Why should the dew form only there, and not along the entire surface of the leaf, large areas of which must be more or less uniform in temperature? He published his diagnosis in 1885: "These large drops seen on plants at night are not dew at all, but are watery juices exuded by the leaves of plants. It is well known that plants transpire an immense amount of moisture, which passes off in an invisible form.
"The root, acting as a kind of force-pump, sends into the stem a regular supply of water - but what will be the result if transpiration is checked, while the root continues to send forward its supplies?" What subsequently came to be called guttation - from the Latin gutta , meaning "drop" - occurs when the humidity of the surrounding air is so high that the water exuded by the plant is unable to evaporate.
It is most likely to occur in summer when, during the day, roots must supply sufficient water to keep leaf temperatures from rising so high that plant cells might be damaged; this water flow continues even after sundown.
Aitken concludes: "The difference between these exuded drops and true dew can be detected at a glance. Moisture exuded by grass is always concentrated near the tip, and forms a drop of some size, but true dew collects evenly all over the blade."