Morning is metaphorical awakening, at times sparkling with spectral, diamond-like glistenings of water droplets on grass leaves and foliage of other plants, and even spider webs throughout the landscape. Across cultures, that early moisture connotes a common freshness to daily life. Hosts for Japanese tea ceremonies might moisten the stone entry so as to welcome guests along “the dewy path.”
A character in The Broken Heart, a play in Huntington collections by Shakespeare’s contemporary, John Ford, remarks: ‘I am, gay creature, with pardon of your deities, a mushroom on whom the dew of heaven drops now and then.“ Robert Lewis Stevenson pines from the Rare Book Stacks: And my heart springs up anew, bright and confident and true. And the old love comes to meet me, in the dawning and the dew. We are advised by Shakespeare to “enjoy the honey-heavy dew of slumber” and learn that Anne barely sleeps for fear: “For never yet one hour in his bed Have I enjoy ‘d the golden dew of sleep.”
Shakespeare and others held that dew falls, somehow, in the evening. And honestly, there are times when a light rain, a drizzle, creates the look of dew. But morning dew from light drizzle is still rain, by any measure. The more overwhelming origins of dew are atmospheric also, but revolve around humidity, the battle between temperature and water.
Humidity is a measure of the relative amount of water vapor present in the air around us, but the amount of vapor that can hang out in air depends on temperature. The higher the temp, the more water vapor air can abide. When humid air contacts cooler surfaces, water condenses, something people who wear glasses and photographers experience on exiting a cool, dry air conditioned space into another space that’s warm and humid. Their clouded glasses and camera lenses evidence a violation of the “dew point,” the point at which the air is so saturated with vapor that liquid water readily forms wherever the temperature drops – in this case, on surfaces that are relatively cool.
The Atlanta Journal-Constitution claimed to “cover Dixie like the Dew” – a regionally-intuitive reference. Air in the Southeast can be incredibly humid, to the point that condensation is ever-present; people there live in the most frigid air-conditioned spaces possible, and are accustomed to windows obscured by condensate, water dripping from the walls of pipes, and fogged glasses. At any time of the day, going outside can precipitate some moist encounters.
My sense is that most people do not connect that ordinary experience with “dew” – I guess that’s because dew happens in the early morning, whether or not someone lives in an air conditioned home.. But it’s much the same phenomenon. The heavily vapor-laden Southeastern atmosphere cools a little bit overnight, changing the dew point (the temperature at which water condenses on surfaces). Suddenly, any surface that is a tiny bit cooler than the air (especially thin materials that cool more easily) will be bathed in condensate. So that is the “dew” the Atlanta rag claims, dew that soaks anyone walking through grass and shrubbery first thing in the morning. It even forms droplets on spider webs.
BUT – there is a completely different phenomenon people regard as dew, and it is the most striking in many cases. We have to start with how water moves through plants, which are industrious, incredibly capable of moving water from the soil to stems, leaves, flowers, and fruit. Indeed, water goes further – plants move water from the soil to the atmosphere, and the end of that process (called evapotranspiration) is one of the world’s quiet forces. Rhett Butler notes that in the Amazon basin: Each canopy tree transpires some 200 gallons (760 liters) of water annually, translating to roughly 20,000 gallons (76,000 L) of water transpired into the atmosphere for every acre of canopy trees.
Transpiration is useful to a plant, aiding in movement of water and nutrients to upper leaves, as well as cooling leaves when water evaporates from the surfaces. Understanding evapotranspiration is important to farmers, and to Southern California gardeners who hope to manage their water budgets efficiently.
And, understanding how water moves through plants explains “guttation” – the process that generates morning water droplets on leaf tips people also call dew. This happens because even though plants rely on evaporation as a mechanism to “pull” water through stems, roots, independently, do some of the work. The internal anatomy of a root is different from that of a stem, having structure that actually zones off layers. This compartmentalization allows a plant to concentrate sugars in roots, forming a concentration gradient that creates pressure, forcing water up the columns of xylem tissue (cells through which water moves.)
This isn’t a huge force, there is no way root pressure could push water to the top of a redwood. But in smaller plants, especially when the plant is well hydrated and the xylem tissues are full of liquid, root pressure can force plant sap out the ends of veins. On early mornings, before sunlight warms the foliage, especially when humidity is high, it’s common to see droplets of water exuded at leaf tips where main veins terminate. Grasses are great at this. So are other plants, as seen in the accompanying photo pf Nasturtium leaves producing guttation droplets along their margins.
Dew, then, is a dual issue. On the right mornings, moisture condenses on leaves, as well as almost any cool surface. But the more pointedly bejeweled lenses on leaf tips are totally different, they are guttation droplets pushed through leaves rich in moisture.
Believe it or not, this can happen indoors, which may not be a good thing. Before the Huntington Gallery was restored in 2008, the original floor of the main gallery was surfaced with “fumed” oak. Fumed oak has integral stain infused through a smoking kind of process; when you sand a fumed oak floor, the color darkens, because the underlying tissues retain the shade imparted through fuming.
We used to decorate the gallery with cultivated cache pots of Diffenbachia, a tropical understory plant that has become an interior greenscaping staple because it can survive in very low light. I kept noticing light spots in the floor surface around the cache pots, spots that resembled stains from drops of water. After some investigation, we realized that when Diffenbachia is freshly watered, given the cool temperatures and reasonable humidity maintained in the galleries, root pressure forces plant sap through the leaf tips, i.e., the leaves guttate. Evidently droplets were large enough actually to peel off, falling to the floor. Just a drop of water, you’d think, potentially harmless. But plant sap isn’t pure water, it contains sugars and even dilute acids. The staining on the floor, we decided, was caused by plant sap bleaching the integral stain in the oak flooring. That was the end of Diffenbachia in the galleries.
I have to note that interior stains on wood are not the only evils ascribed to dew. In the Library Rare Book collections sits a 1485 volume, De proprietatibus rerum, credited to Bartholomew (the Cleric), warning that untimely dew could destroy wheat crops through disease: “When Wheat is gathered, some of the straw is burnt to help and amend the land ; and some is kept to fodder of beasts, for it is first meat that is laid to fore beasts. And the kind thereof is cold, that it suffereth not snow that falleth to shed, and is so hot that it compelleth apples for to ripen. Of corrupt dew, that cleaveth to the leaves, cometh corruption in corn, and maketh it as it were red or rusty. And among all manner corn Wheat beareth the price, and to mankind nothing is more friendly, nothing more nourishing.”
I’m thinking the same could be true for rain, or drizzle. Water has been the ruin of many a good crop. We need water, but the when and the where are, like humidity, relative.
*Rhett Butler (https://rainforests.mongabay.com/amazon/rainforest_ecology.html )
References:
Shakespeare’s King John and Richard III, by Francis Pierrepont Barnard, 1897. https://play.google.com/store/books/details?id=cn47AAAAYAAJ&rdid=book-cn47AAAAYAAJ&rdot=1