Washington State University
Dr. Universe: How do plants that need very little water survive? – Alivia, 11, Maryland
My neighbor has a very prickly garden. It’s full of cactuses – including one thorny plant nearly as tall as my house. That’s not something you see every day in the Pacific Northwest. Cactuses usually live in dry places like deserts.
I talked about your question with my friend Linda Chalker-Scott. She’s a garden scientist at Washington State University.
She told me plants that need very little water have adapted to dry conditions. Adaptations are changes in the physical body or behavior that help a plant or other living thing survive. Then, it can pass on those changes to its babies.
“Plants are very adaptable,” Chalker-Scott said. “They will colonize almost every environment, and some are able to survive low water conditions. Over time, these plant populations evolve ways to conserve water.”
Another way to describe plants that live in dry places is drought tolerant. They can survive times with little water by holding on to the water inside them.
One way they do that is by adapting their leaves. The outside of a leaf is called the epidermis – just like your skin. One of the jobs of the plant’s epidermis is to make wax and ooze it out. The wax protects the leaf from damage and holds in water.
Many drought-tolerant plants, including cactuses and succulents like jade plants, have evolved to make more wax than other plants. A thick wax layer slows down how much water the plant can lose.
Plants lose water through small openings called stomata. If you zoomed in with a microscope, you’d see the stomata look like tiny mouths. The “lips” are called guard cells.
Plants use their roots to suck up water from the ground. It travels up the stem. Then, when the guard cells open the stomata, plants release some water through the openings. It evaporates into the air as water vapor.
Many drought-tolerant plants have evolved smaller leaves. Smaller leaves mean fewer stomata to let out water.
Some plants take it to an extreme. Their leaves adapted into spines. All those prickly spines you see on a cactus? Those are modified leaves. They don’t have stomata, so they don’t lose water through their leaves.
But plants still need stomata. That’s how they take in carbon dioxide and release oxygen when they make food using the sun’s light. So, cactuses have stomata on their stems and open them at night when it’s cooler.
Plants also adapt their roots to survive in dry places. Some cactuses have shallow roots that spread out to suck up lots of water when it rains. Others have a very long central root that reaches the water deep underground.
Chalker-Scott told me that even plants adapted to dry conditions love water.
“Most plants will use as much water as they can get,” she said. “It’s a misconception that you don’t need to water your cactuses and succulents very much because they’ve adapted. If you water them, they’ll grow very quickly.”
Maybe that’s why my neighbor has such happy cactuses. With all that water, they can grab life by the thorns.
Adults can help kids submit a question at askdruniverse.wsu.edu/ask.