Dear Emma,

When I’m thirsty, I pick up a glass of water with my paws and drink it—just like you do. But plants don’t have paws or mouths, so how (and why) do they drink it?

To find the answer, I talked with my friend Helmut Kirchhoff. He’s a scientist at Washington State University. He studies plants and biochemistry.

He told me plants need water inside their cells. Water makes plant cells strong and flexible. It also dissolves stuff. That makes it possible for chemical reactions to happen inside plant cells—like the reactions a plant uses to make energy during photosynthesis. Plants also need water to move around nutrients and other molecules required for life.

“Water is essential for life, but plants must move nutrients from the soil to the leaves,” Kirchhoff said. “So, they have this very nice transport system called xylem. Xylem is an ancient Greek word that means wood. It works like a straw to move water and nutrients from the roots to the leaves.”

You can see the xylem yourself. Place a rib of celery in a cup of water and food coloring. Be sure the leafy part is pointed up. Over time, the colored water will move up the celery stem. If you look at the cut end of the celery, you’ll see dark dots. That’s the xylem. Photo: Fir0002/Flagstaffotos

We usually think of water flowing down, so it might seem weird that water is moving up the plant. The way water moves from the roots to the stem and up to the leaves is called the transpiration stream. First, it moves from the soil into very fine hairs on the roots. Then it travels from cell to cell up the plant’s roots.

That’s when the pull of transpiration really kicks in. Transpiration is how plants release water into the air through their leaves. It works because there are super tiny openings on the underside of a plant’s leaves. They are so small you need a microscope to see them. They’re called stomata. They look like itty bitty mouths with lips. The “lips” are guard cells. They open and close the stomata to release water or keep it inside.

You can see stomata, too. Paint the underside of a leaf with a little clear nail polish. Then, peel it up and look at it under a light microscope.

When the stomata open, water evaporates into the air. That causes suction—sort of like sucking on a straw. The suction pulls water—and the nutrients dissolved in the water—from the roots up the plant stem and out to the leaves.

One big way that plants use water is photosynthesis. Plants use the sun’s light to change water and carbon dioxide into oxygen and sugar. Then, the plant moves some of the sugar back down the plant using another transport system called phloem. The plant stores the sugar partly in its roots. When the plant needs energy, it can break down the sugar and use the energy stored there.

Water is important for the phloem, too. It dissolves the sugar and other stuff the plant needs moved down to the roots. Or up to the flowers and small growing leaves that still need sugar from older leaves to thrive.

Plants might not have paws or mouths, but their bodies still need water. It’s just another way plants aren’t so different from us after all.


Dr. Universe