It is a big challenge for cacti to get enough water. There are three main strategies. One is to have very long roots that can tap into the water deep in the ground, where water may collect. Another is to have very long roots that spread out just below the surface so that when rain does fall, they catch a lot of it. Another potential source of water is the moisture in the air. When the water vapor in air hits a cooler surface, the vapor condenses into liquid water. That’s what we notice as dew. When it happens with cacti, the water falls to the ground, where small surface roots can absorb it. During the dry season, when they’re not needed, cacti cut off the water to the roots and let them die. This may seem odd, but picture two sponges next to each other. One is the root, one is the ground. Water will move from the wetter sponge to the drier sponge. If the soil is wet, roots absorb water. If the soil is dry, the soil will absorb water from the roots.

Recent research has shown that some cacti can actually absorb water right from the hair-like needles. The water condenses on the needles, then goes right through the plant’s surface at the base of the needles. In the desert, water tends to come all at once. Most cacti have pleats that allow them to expand and hold that water for later. The water inside the cactus can be stored in plant pulp or in glue-like substance.

Cacti don’t need a lot of water because they have several adaptations for conserving water. They are covered with a waxy waterproof coating to reduce evaporation. Instead of having leaves that lose moisture and collect heat, they do photosynthesis on their “trunks.” They may have furry spines that provide shade. All of these adaptations are good examples of the relationship between structure and function.

In photosynthesis, the cacti need to take in water and carbon dioxide. They give off oxygen. In most plants, the carbon dioxide enters—and oxygen leaves—through tiny holes in the leaves (stomata). Cacti have these holes on their surface. Unfortunately, water evaporates through these holes too. Cacti can’t completely escape this, but they only open those holes at night, when the air is cooler and moister. You probably know that plants need light to power photosynthesis. Since they can only store so much carbon dioxide at night, they can only do so much photosynthesis during the day. Photosynthesis is what provides sugars, starches, and other materials, so cacti grow very slowly Their sharp spines discourage animals from eating the tissue that takes so long to build.

Why do you think cacti are usually spaced so far apart in the desert?

Cacti obtain water in the same way that any other plant does: by using their roots and capillary action to draw it up out of the ground. What makes cacti unusual is their ways of storing water inside of their tissues and waiting for extended periods without rain.

Cacti take the water from the air [the atmosphere] and the earth. Cacti are very good at keeping the water they have.

A look at how cacti are able to survive and grow in the harsh, dry desert environment. The spines on a cactus help to protect it from humans and animals. Its roots are spread out to collect water when it does rain and it stores water in its body for future use.