Why do cacti have spines instead of leaves?

How it works

Leaves have tiny holes called stomata, and water escapes through them. In a desert, big flat leaves would be like a leaky bucket. A cactus's leaves have evolved into spines, which lose very little water. Spines also shade the stem a little and warn hungry animals to keep away.

Without big leaves, the green stem does the job of catching sunlight and making food. It has a waxy skin, like a raincoat that keeps water in. Many cacti keep their stomata shut during the hot day and open them in the cool night, which saves even more water.

Cactus roots are usually shallow and spread out wide like a net, ready to soak up water quickly whenever it rains. Many cacti have pleats or ribs like an accordion, so the stem can swell up when it is full of water and shrink as the water is used.

Where to see it in New Zealand

Almost all wild cacti are native to the Americas. In the Sonoran Desert of the United States and Mexico, giant saguaro cacti stand tall, and their pleated stems swell after rain. New Zealand has no native cacti, though many people grow them in pots and gardens.

Try it together

Wet two paper towels by the same amount. Lay one flat and roll the other into a tight tube, then leave both in the same spot. After a few hours, which is drier? The flat one is like a wide leaf and the roll is like a cactus: less surface means less water lost.