Categorize this entry under unsolved mysteries. I bike-commute past a few willow trees that inhabit the banks of lazy little Paradise Creek. As I pass under their canopy, cool air braces me and a spritz of moisture douses my face.
A while back, after enjoying this brief “rain,” my eyes widened with a sudden eureka moment. “I wonder,” said I, “if this moisture bath is why they call it weeping?” (It isn’t. Willows are from genus Salix with about 400 species, one of which is Salix babylonica, named weeping willow because of the gentle arcuate curves of its trailing branches. Overall, a weeping willow looks like melting candle wax, but fluffier. This drooping shape is fancifully called weeping.)
But I can be as lazy as Paradise Creek and for months I didn’t bother to check. I operated under the private delusion that “weeping” willows were the ones that dripped moisture. But that dripping began to bother me, because drops of water are typically a few millimeters across when they fall.
But the spray of moisture I experience under a willow tree is composed of droplets too small to be seen. My nose for science quivered. This paradox must be explained! Somehow, the willow tree is producing wee little droplets like an atomizer (spray bottle).
So I cracked open a textbook, finally. I shortly found out I was wrong about the term “weeping.” Shrugging that off, I learned about transpiration. Transpiration is evaporation in leaves, evaporation being the transformation of liquid water to gaseous water (steam, or water vapor). Transpiration happens inside leaves on the wet surfaces of cells called mesophyll cells. It’s crucial to the flow of water in the plant because the evaporation causes the layer of water on the mesophyll cell to thin out. That creates a negative water pressure that sucks water from the roots all the way to the leaves.
As regards my water droplet mystery, what is missing in that description of transpiration is water droplets. If the textbook is right, a healthy tree leaf drinks water from its stem and evaporates it into nearby air, and we should expect no droplets.
We should expect a cooling of the air, because if energy is conserved (It is, always. Trust a physicist.) then there is a cost to liberating a water molecule from its liquid phase to its vapor phase (called the latent heat of evaporation) and that cost is bourn by nearby air, which cools off. This is the principle of the so-called swamp cooler. The same thing, in reverse, drives thunderhead formation.
I observed, to my delight, a cooling of the air under a willow tree, so that’s all good. And willows love water, clearly, since they are generally found at the edges of ponds and streams. So we can expect active transpiration in willows.
So where’s that water coming from? I don’t know, but the internet has one suggestion: aphids.
The idea is that aphids infest the tree. The moisture is “honeydew” secreted by the li’l bugs. Honeydew is sweet and sticky and the aphids deposit droplets of it on leaves and stems. If the infestation is severe, the droplets can coalesce and it can “rain” under your infested plant.
Gee, thanks, internet. That’s really gross.
Also, internet, I don’t believe you. Firstly, this spritz of water is constant. If the willow is leafed out, I feel it. That would imply that the tree always has a case of aphids. Secondly, I looked. To the limits of my eyesight, I can’t spot any aphids on the willow leaves. Thirdly, it doesn’t solve the droplet-size problem.
And that’s where the mystery stands. I’m pretty sure there is a beyond-textbook explanation for feeling moist underneath a willow tree. Perhaps the leaves spit extra water out through their stomata (pores) somehow, although I can’t readily picture how this might occur. Or perhaps the cooler air under the willow encourages very-local fog formation; wee droplets condensing out of the humid air under the willow’s canopy.
If a herbologist would like to comment, by gum I’d sure like to hear from you. In the meantime, I’ll just enjoy that delicious cool moment when I pass under the willow trees on the way to work.