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Why Do Cacti Have Spines? For Several Reasons

When thinking about a cactus, the first thing that comes to mind is often their sharp spines. Those spines are a great warning to not touch, but do they have any other purpose?

(Photo via Shutterstock)

Cactus spines are actually modified leaves! As plants adapt to a particular environment, how a leaf looks and functions can change over time. Plants lose a lot of water through their leaves. Most plants replace lost water with new rainfall. However, to survive in such a dry environment, desert plants adapted over time.

One adaptation was to get rid of their leaves altogether. Cacti originally had leaves, but since they were not beneficial, they evolved to become spines instead. Cactus spines help reduce water loss.

Cactus spines also help direct moisture down to the base of the plant. Any drops of rain that fall, or morning dew that condenses, can move down those spines like a waterslide to the bottom of the plant, where they are absorbed into the soil.

Cacti have adapted to have very shallow roots that grow outward from the plant just under the surface of the soil. This allows even the smallest amount of moisture to be captured by its roots. The roots draw the moisture into the stem of the cactus, where it can be stored for the plant to use.


Words to know

Adapt: To modify or change.

Beneficial: Favorable or resulting in good.

Condense: To change from a gas or vapor to a liquid.

Evaporate: To turn from liquid into vapor.

Modify: To make changes to.

Sprawling: Spreading out over a large area.


Cactus spines also provide the plant with shade. Thin spines don’t seem like they would do much, but as the sun moves across the sky, all those spines make shadows across the cactus. Shade is important because it keeps moisture from evaporating.

And, of course, cactus spines do protect them from hungry desert predators looking for a meal. The thick, waxy stems and pads of cacti hold the moisture the plant needs to survive and can make a juicy and nutritious meal, but one bite of the sharp spines will probably send even the hungriest critter looking elsewhere for food!

Cacti in Illinois?

But cacti are only found in the desert, right? Nope! There are native species growing right here in Illinois! Opuntia humifusa, or eastern prickly pear, can be found in about half of Illinois counties, including Will County. Unlike many towering cactus species found in desert habitats, eastern prickly pears are low-growing, sprawling plants. They have short, reddish-brown bristles, along with long gray spines that direct moisture and provide shade and protection.

A prickly pear. (Photo by Glenn P. Knoblock)

Prickly pears are important pollinator plants. They produce flowers on the top of their pads. Those flowers are pollinated by native bees that use them as a source of nectar and pollen. Pollinated flowers will produce fruits that are edible. The pads are edible too, but make sure to carefully remove the spines first!

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