树木如何度过冬天 How do Trees Survive Winter?

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天寒地冻的冬天,人类穿上了厚厚的衣服,小动物们纷纷打开了冬眠模式,而植物们都怎么保护自己度过这样严寒的天气呢?

Fossil records tell us that about 250 million years ago, there wasn't a tree on Earth designed to survive in temperatures below freezing.

They were all built for life in the tropics, where it's always warm, water is always liquid, and leaves are safe year round without fear of frost.

If you took a tropical tree, whether ancient or modern, and transported it to Siberia or Patagonia in winter, its water would freeze into sharp-edged ice crystals, fatally puncturing the living cells in all of its leaves.

The same thing that happens to lettuce and spinach when it freezes.

Cold weather also means that water in the tree's plumbing system freezes, and dangerous bubbles form in the ice from gases that were previously dissolved in the liquid water.

The ice itself doesn't cause much harm, but when it thaws, the bubbles remain, which is a problem because the whole plumbing system relies on the inter-molecular attraction of water molecules pulling each other upward against gravity.

Air bubbles break the chain of molecules, essentially shutting off the flow of water.

So to survive in cold weather, trees need to avoid two things: bubbles in their pipes and direct damage to living cells due to ice crystals.

Tree solved the first problem before ever leaving hot climates, because air bubbles are also a problem during drought.

When plants work harder to get water from the soil, their water-conducting pipes can accidentally suck in tiny pockets of air from the surrounding tissue.

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