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  • Living through a cold winter is not easyfor 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 blubberor 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 graduallywhich 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 freezingwhether 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 placeslike in the Arcticnormal 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.

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木が冬を乗り切る3つの極端な方法 (3 Extreme Ways Trees Survive the Winter)

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    林宜悉 に公開 2021 年 01 月 14 日
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