John Aitken was a rather serious Victorian who is remembered mainly for the rather clever way he found of counting tiny specks of matter in the atmosphere. But sometimes he applied his intellect to more accessible, familiar things - like the experience of walking on a bright, crisp, cloudless morning when you seem . . .
To scatter round you,
as you lightly pass,
a shower of diamonds
from each blade of grass.
For centuries these diamond-showers were known as dew-drops. And dew forms, as we noted yesterday, when still, humid air in contact with a cold surface is cooled to the point where it can no longer accommodate all the moisture it contains; the moisture condenses as tiny drops of water on the cold surface underneath.
What bothered Aitken, however, was that very often, in the case of grass and other plants, the leaves were not uniformly coated with the tiny water drops. Instead, a single large drop sometimes hung from the tip of each blade. He wondered why dew formed only there, and not along the entire surface of the leaf, large areas of which must be more or less uniform in temperature.
He gave his diagnosis in a paper called "On Dew" 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 from 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 Aitken had observed was subsequently called guttation - from gutta, the Latin word for drop - and it 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 time, when during the day roots must supply enough water to keep leaf temperatures from rising so high that plant cells might be damaged, and this water flow continues even after sundown.
Aitken concludes: "The difference between these exuded drops and true dew can be detected at a glance. The 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. The exuded liquid forms a large glistening diamond-like drop, whereas dew coats the blade with a fine pearly lustre."