Dr. Universe: Why do trees have sap? -Aliyah, 8, Kirkland, WA
Dear Aliyah,
Just as blood moves important stuff around the human body, sugary sap moves important things around a tree.
My friend Nadia Valverdi told me all about it. She’s a researcher at Washington State University who studies how apple and cherry trees survive in different environments.
When we eat food, like a delicious apple or a handful of cherries, we get important nutrients.
Along with some help from the digestive system, our blood helps carry nutrients to different parts of the body to keep us strong.
Trees need nutrients, too. They use their roots to suck up nutrients and water from soil. They also have the ability to make their own food: sugars.
Trees absorb sunlight through their leaves and can use this energy from the sun to make sugars from water, carbon dioxide gas from the air and a few other ingredients.
A lot of sugars are made in the leaves, but they don’t do the tree much good if they just stay in one spot. The sugars have to get to other parts of the tree to help it survive. That’s where the sap comes in.
“Its main task is to make sure that every organ is well-fed and growing,” Valverdi said.
While our blood moves through tube-like veins and arteries, sap flows through two different tube-like parts of the tree.
One part, called xylem, moves important stuff like water and nutrients from the bottom of the tree to the top—from its roots to its leaves.
The other part, called phloem, moves important stuff from the leaves to other parts of the tree, such as the branches, roots and fruit.
I asked Valverdi how a sticky, gooey liquid like sap could move through these tubes. After all, sap doesn’t seem to move on its own.
It turns out that some liquids, such as sap, can move through a narrow space without any help from gravity or other outside forces.
This can happen in plants or trees when sap escapes through tiny, microscopic holes in the leaves. When sap molecules escape the leaf, more sap molecules move in to fill the empty space and keep the sap flowing upwards through the tree.
It’s a phenomenon we find happening everywhere from house plants to big apple trees to celery stalks.
“All trees and plants have sap,” Valverdi said. “The difference is that sometimes in big trees, we can see it with our eyes because it is more gooey.”
One really gooey kind of sap you might have seen before comes from sugar maple trees. You may even put it on your pancakes or waffles. You guessed it, maple syrup is a kind of sap.
Just like us, trees have systems that help them move important stuff around. These systems help the plants survive. When trees do well, that’s good for us, too. They do so many things for us from making the oxygen we all breathe to giving us delicious fruit to eat.
Sincerely,
Dr. Universe
You can learn more about the xylem and phloem in this simple activity using food coloring, water and some celery. https://www.youtube.com/watch?v=KIug9Foou3s
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