How does a cactus keep water in the desert?

Think about a puddle on a hot, sunny day. The sun shines down on it, and the puddle shrinks and shrinks until it is gone.

In the desert, the sun shines hot day after day, and rain hardly ever falls. Yet a cactus stays green and plump for months. So how does a cactus keep water in the desert?

When rain finally comes, the cactus drinks it from the wet sand with its roots and sends it into its thick green stem. That fat stem works like a water tank. Inside, it is soft and spongy, so it can hold a lot of water.

A tall cactus has long ridges running up and down its stem, like the folds of a paper fan. People call these folds pleats, which means folds that can open out and close up again. When the cactus drinks, its pleats spread open, so the stem can swell up and hold even more water.

Now the stem is full. But the desert air is hot and dry, and dry air pulls water out of anything it touches, just like the sun dried up that puddle. What do you think the cactus has to stop its water from escaping?

A smooth, shiny skin covers the whole cactus. This skin is its cuticle, which means a thin, waxy coat on the outside of a plant. Like the wax on a crayon, the cuticle does not let water slip through, so the water stays inside the stem.

Day after day, the cactus sips a little water from its tank to stay alive. As it uses up the water, the pleats fold back in and the stem grows thinner.

Pleats and cuticle. A cactus keeps water in the desert by storing it in its thick, spongy stem, where the pleats open to hold more, while its waxy cuticle stops the dry air from pulling the water out.

Next time you see a cactus standing tall in the sun, you will know it is quietly holding on to its own secret store of rain.