♪♪♪.

Living through a cold winter is not easy — for any of us living things. Our cells just don't do well in the freezing cold.

Animals that live in cold climates have all kinds of adaptations to keep themselves alive when it's freezing. Bears hibernate; seals have plenty of blubber; humans seek shelter and wear clothing and shoes. But plants in cold climates also need ways to get themselves through the winter.

And they don't really have the option of blubber — or shoes, for that matter. Many plants just die every winter and regrow in the spring. But trees are too big to regrow every year.

So, to get themselves to spring, they have other, stranger adaptations. Including, in some cases, turning themselves into glass. The problem begins when the temperature falls below the freezing point of the water in a tree's cells which normally happens at 0 degrees Celsius.

Water expands when it freezes, and if that's allowed to happen inside a cell, ice crystals can pierce through the cell's membranes. Which is not great! In fact, it kills the cell.

Luckily for trees, ice only forms inside their cells when the temperature drops below freezing very suddenly. When it falls more gradually — which is what usually happens as winter approaches ice tends to form in the spaces between the cells, in a process called extracellular freezing. As the temperature slowly falls, ice forms on the outside of the tree's cell walls before the inside of the cell gets cold enough to freeze.

Then, once there's some ice between the cells, the water inside the cells does something weird. It flows outward toward the ice. That movement is because of a property known as chemical potential.

As a general rule, substances move from areas with higher chemical potential to areas with lower chemical potential. And because of the way its molecules are arranged, ice has a lower chemical potential than the liquid inside the cell. So the water inside the cell moves toward the ice outside the cell and freezes there, instead of freezing inside the cell and destroying it.

Great! Problem solved! Except, there's more to surviving in the winter than just keeping your cells from rupturing.

I mean, pretty much all plants undergo extracellular freezing — whether they're cold-tolerant or not. But not all of them survive the process. See, the water moving out of the cells causes another problem: dehydration.

Dehydration is bad for all kinds of reasons, but for trees, the main danger is that as a cell shrinks from water loss, its cell membranes can move close enough together to react. That can tear the membranes apart and is generally just not a good idea. So, trees that are good at surviving cold temperatures have a whole bunch of different strategies to avoid this dehydration problem.

One of the most common techniques they use is called supercooling. That's what happens when water falls below the temperature where it would normally freeze, but stays liquid. There are a few reasons a liquid might not freeze at its normal temperature, but for trees' cells, the thickness of the liquid inside them, also known as viscosity, is one of the main ones.

This liquid gets thicker during extracellular freezing, as water is drawn from the cell and leaves behind a thicker concentration of dissolved substances. The thicker the liquid, the harder it is for ice crystals to begin forming, and the more it can be supercooled. As temperatures fall, trees that use supercooling also start producing more of certain molecules, like sugars, that make the liquid inside them even thicker.

You might be familiar with this liquid. We call it sap. With supercooling, trees can avoid getting too dehydrated by extracellular freezing and hold more liquid inside their cells without it freezing into ice.

The combination of extracellular freezing and supercooling can keep trees alive through temperatures as low as -40 degrees Celsius. But around -40 or -50 degrees, supercooling backfires. At a certain point, it doesn't matter what you do to try to keep ice crystals from forming — all of the liquid will just spontaneously freeze.

And if a tree's cells still have a bunch of liquid inside them, a sudden freeze is very much a death sentence. Luckily, for the vast majority of the world's surface, this limit isn't important. Protection down to -40 degrees is more than enough.

But that's not true everywhere. In some places — like in the Arctic — normal winter temperatures fall as low as -60 degrees Celsius. And there are still trees that live there!

In fact, there are plenty of trees, like the black locust, white pine, and northern white cedar, that can survive being submerged in liquid nitrogen — a temperature of -196 degrees Celsius. And some trees, like the Japanese white birch, can survive exposure to liquid helium, which is -269 degrees Celsius. Not that they'd ever encounter that in the wild.

But, you know. Sometimes, you've got a tree and a bunch of helium, and one thing leads to another…. Anyway.

These trees don't survive those temperatures through supercooling. Instead, the insides of their cells turn into glass, in a process known as vitrification. Researchers describe it as a kind of suspended animation, where molecules don't really move.

Normally, ice spreads as crystals of ice come in contact with other molecules. But if the molecules aren't moving, they're not reacting with each other, either. We don't yet know the exact mechanism that leads to this vitrification, but researchers think it's helped along by high concentrations of sugars in the tree's cells, along with proteins called dehydrins.

These proteins seem to bind themselves to the cell's membranes, keeping them apart from each other. Meanwhile, other parts of the proteins might tangle with the sugars in the cell, helping arrange them into a glassy state. Once the tree's cells turn into glass, it doesn't really matter how cold it gets.

They're essentially preserved in a way that keeps the cells from being damaged, so whenever it does warm up again, they can pretty much get back to normal. Which I kind of wish I could do during winters here in Montana, but as far as I know, humans still can't turn themselves into glass. So, a coat and boots it is, I guess.

But even if you're out there shivering through winter like me, you've got to admire trees and their ingenious tricks for making it through the cold. Thanks for watching this episode of SciShow! And if you want to find out about even more reasons why trees are amazing, you might like this video about how trees use underground networks to communicate with each other.

You can watch that one right after this. ♪♪♪.