How Plants Use Water

This week's experiment is one that popped into my mind as we were watering the plants. Why do plants need so much water? Some of Lisa's flowers drink more every day than I do.

To find out why they need so much water, you will need:

• a tree or bush
• a clear, plastic bag
• a wire twist-tie or piece of string

Find a branch that is easy to reach and out of the way. Put the bag over the end of the branch, being sure that several leaves are inside. Use the wire twist-tie to fasten the bag over the branch, sealing it as tight as you can to the branch. You do not want air to be able to get in or out of the bag easily. Leave the bag there for a while. Check on it every hour or so.

What did you notice? Soon, the bag had lots of tiny drops of water on it. As more and more of these drops form, they will start to slide down the side of the bag, collecting other drops and forming a small puddle in the bottom of the bag. By the end of the day, you probably had quite a bit of water. Where did it come from?

The water is coming from the leaves of the plant, through a process called transpiration. The underside of the leaf has lots of tiny holes called stomata. These holes let air get in and out of the leaf so the plant can take in carbon dioxide and give off oxygen. They also let water get out. Some plants lose more water than others, but a good sized tree can lose over 80 gallons of water a day.

Isn't that awfully wasteful? Well, it does use a lot of water, but it serves a purpose. To understand why plants need to lose all this water, think about your body. Your blood carries oxygen and nutrients to all the cells in your body. A plant's sap, which is mostly water, serves basically the same purpose. But, we have a heart to pump the blood through us. Plants don't have hearts. OK, so artichokes do, but an artichoke heart does not pump the sap through the plant. How can a tree get sap to move up from to roots to the highest branches?

The answer is transpiration. As water evaporates from the leaves, more water is pulled up from the roots. As this water moves up, it carries nutrients with it. The water evaporates, but the nutrients are left in the leaf, where they are needed. Only about 1% of the water is actually used in the process of photosynthesis. The rest of it goes out into the air.

All that water going into the air serves another purpose. It is a major part of the water cycle. Almost twice as much water enters the atmosphere through plants as evaporates from the surface of all the oceans.

If you want to take this experiment another step, try putting bags on different kinds of plants. Compare broad leafed trees with a tree that has needles, like a pine. Control the variables or differences. If one bag is in the sun and the other is in the shade, you will not know whether the difference is because of the type of plant or the amount of sun.

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