Buds in Glass Houses
Just imagine you and I went walking one morning when the trees had dropped their leaves and the air was truly chilly. You pointed out dew glistening on the tight buds of willows and alders, all waxen and ready for winter. Then you asked me, the well-prepared science writer, why the buds seemed to gather water. I’d have nattered on about how cold air can’t hold as much water vapor as warm air can; I’d have pointed out that the small, exposed buds would have chilled down during the night just past, so their cold surfaces would be a natural place for water to condense. Probably, I’d have continued, we could even find some buds glazed with frost.
I’d have been smug about answering the question. And I’d have been wrong.
Oh, the physics in my explanation is accurate enough. Cold air can’t hold much water. That’s why the air in our houses gets so dry during the winter—we take outside air at fifty below that’s holding all the water it can, then bring it in and heat it up. If we don’t add moisture, we end up living in an atmosphere that makes the Sahara’s air seem soggy. And once you’ve seen frost on the windows but not the walls, you appreciate that cold surfaces are the first places to look for condensed moisture.
Fine. But I forgot that at this time of year, many plants are getting rid of as much moisture as they can. That glossy dew on the alder buds is likely provided—at least in part--by the alder itself.
It’s a protective mechanism. Water expands as it freezes. The molecules shove apart when they form the rigid arrangement we recognize as ice, setting up sharp-edged crystals. For a fragile, living cell, letting its contained water freeze would be like swallowing a passel of knives.
Leave the temperature set too low in the veggie drawer of your refrigerator and see what I mean. That greenish-black mush is what remains of frozen lettuce after its water-rich cells have been pulverized from the inside out by the forces of freezing.
Don’t wrap the lettuce, let it dry out a bit first, and the ensuing head of cold but paper-like leaves will stay green (if inedible) past Christmas.
To produce some healthy desiccation, northern plants take advantage of water’s ability to exclude the molecules of other substances as it freezes. (Thus, for example, travelers on the Arctic Ocean can melt sea ice for drinking water—and can find pockets of unfrozen briny water in that salt-free ice.) They also set up an osmotic pump.
Aha—you don’t remember that lesson from science class. Osmosis refers to that process by which substances in solution seek equilibrium; that is, put very salty water on one side of the proper sort of membrane, clean water on the other, and pretty soon you have evenly, moderately salty water on both sides.
As water on the outside of the buds begins to freeze, it leaves behind unfrozen water steadily more packed with foreign substances, molecules exuded by the plants or fallen from the air. Eventually the water inside the bud’s cells is cleaner than the substance-laden liquid remaining on its freezing surface. By osmosis, the interior water then makes its way out through the cellular membranes, the bud’s surface, and finally into the air. There it freezes—almost all of it. What’s left behind as liquid contains the freezing-rejected impurities. That keeps the osmotic pump operating and the water flowing out into the cold air, away from the vulnerable cells within the bud.
So—ask any alder--autumn’s dew is really a defense against winter.
