Why Don't Plants Freeze?

Cold man Those of us who live in places where the temperature can get as low as 40 below zero, see a large number of ways that living things survive the cold. Birds, insects, reptiles, amphibians, and mammals all have unique ways of avoiding death by freezing. Most of us have never thought about how plants survive the cold. It would seem that trees and bushes would be especially vulnerable to freezing as they have to sit in the wind and the elements, waiting for the thawing of spring. If you have ever seen a barrel or a flower pot left out with water in it in the winter, you know that they break as the water freezes and expands. There is a demonstration I do with my physics students in which we take a solid steel ball, fill it with water and freeze it, and the freezing water breaks the steel--just as it will the block of your car. Why does that not happen to the trunk of a tree or the bud of a bush?

Some plants produce chemicals that act like antifreeze. The chemicals lower the temperature at which the water will freeze in the same way that antifreeze keeps your radiator from freezing. Another method plants use has been discovered by botanists at Cornell University. When the air is warm, plants have on their surface a thin layer of water with various other molecules mixed in. As the temperature drops, this water freezes, but the other molecules are not included in the ice crystals. As the concentration of these other molecules increases, water inside the plant cells is drawn out by osmosis. This process continues until the cells in a plant bud are almost totally dehydrated. If there is no water in the bud, the bud can freeze without damage. In the spring when the temperature rises, water is drawn back into the bud and the cells revive. Where we live, in the early fall you see plants covered with what is called rime. This is the water drawn out of the plant. It falls away and the plant is winterized. In the spring, water can get back into the bud and start the plant working before the ground is even thawed, giving the plant a jump on spring.

Trees and snow One amazing thing about plants is that they seem to be able to grow anywhere, but that only happens because they have a design that makes them able to survive even hostile environments. As we learn more of those designs we have to be impressed with the wisdom and complexity of them--a testimony to the God who created them with purpose and function in mind.

Reference--Natural History, December, 1999/January2000 page 31.

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