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Why Are Cactuses Spiky?

Sunset in the Sonoran Desert with Saguaro Cacti, Phoenix, Arizona.

What makes a cactus a cactus? And what are you supposed to call a group of these plants--cacti, cactuses, or cactus?! We'll find out in today's episode, as we learn more about the cactus family with Kimberlie McCue of the Desert Botanical Garden in Phoenix, Arizona. She'll answer kid questions about why cactuses are spiky and how they got those spikes, as well as why teddy bear cactuses aren't actually cuddly!

Download our learning guides: PDF| Google Slide | Transcript


Those prickly spines that are so characteristic of the cactus family are actually modified leaves! Cactuses don't have the kind of leaves like a maple or oak tree. But they might have had leaves that were at least a little more like that way way back in the past. Over time, those leaves evolved into the spiky spines we see on cactuses today because they help the plants survive in hot, dry environments.

Why are cactuses spiky? -Noah, Iowa

"They can be a defense mechanism to discourage herbivores - animals that eat plants - from eating the cactus. But, also, spines create shade!" explains Kimberlie McCue.

"When you're covered in spines, as the sun moves across the sky, those spines are casting shadows on the body of the cactus. They're little shade umbrellas!"

All cactuses are native to desert environments, and some live in places where it never rains at all. So how do they get water to survive? Well, Kimberlie tells us that these plants grow not too far from the ocean.

"Early in the morning, there will be fog that comes off the water. Those spines provide a place for the water to condense, form little droplets of water that run down the spine, to the body of the plant, down to the ground and to the roots."

Cactuses are also extremely important parts of their desert environments, as they hold soil in place and provide shelter for birds and other animals. Those insects and birds in turn help pollinate the cactus flowers. Cactuses are also an important local food source for humans.

Unfortunately, cactuses are in danger from people who poach (illegally take) wild plants from their environment. Kimberlie McCue says one way to help make sure cacti stay healthy and plentiful is to be careful when you buy cactus plants. Check to see where the plant seller got the cactus and make sure they're taking care to be ethical stewards of these plants before you buy.


Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.
Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
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