The Secret Life of Cacti

By: Geo Kim

When you picture a cactus, you probably imagine a spiny green column basking under a scorching desert sun. You’re not wrong. After all, if you are a Nevadan, chances are you have been a first-hand witness to such a sight.  What you may not know, conversely, are the various evolutionary and physical routes that it takes to get there. Allow this blog to shed light into that territory. 

At the heart of a cactus’s survival strategy is water conservation. Deserts are characterized (according to my AP Environmental Science Premium Prep textbook) by less than 25 centimeters of annual rainfall and coarse, sandy soil that are usually 30 degrees north and south of the equator. The major types of vegetation that inhabit the area are low-water adapted plants, such as, of course, cacti. 

Although these cacti are accustomed to such low-water environments, it doesn’t necessarily mean that they require no water. 

A saguaro cactus.

Stems 

Cactus' thick, fleshy stems are specialized to hold gallons of water after a single rainfall. The stems are usually ribbed or pleated, allowing them to expand and contract depending on how much water they’re storing. Some species, like the saguaro, can weigh several tons when fully hydrated. Since they lack traditional leaves, cactuses also perform most of their photosynthesis through their stems. The waxy outer layer, called the cuticle (and no, not like the cuticles on your fingers), minimizes water loss while still allowing sunlight in. 

To further combat dehydration, cactuses have evolved a unique form of photosynthesis known as CAM (a.k.a. Crassulacean Acid Metabolism). Unlike most plants that open their stomata (tiny pores on the leaf surface) during the day, cactuses open theirs only at night. This reduces evaporation from the sunlight and allows them to take in carbon dioxide when it is significantly cooler. During the day, they close their stomata and use the stored CO₂ to photosynthesize.

Spines 

Cactus spines are often called the leaves of cacti, and it’s true. They’re just highly modified, and serve multiple functions. Spines provide shade, reducing surface temperature and further preventing water loss. They also help collect dew and channel it toward the roots. In some species, spines even act as a form of insulation, reducing the amount of heat absorbed during the day and lost at night—a crucial adaptation in places with extreme temperature swings. 

Spines do also certainly help defend against some herbivores that try to eat them for the water that they store. However, as you may know, animals like camels have thick lips, a hard palate, and special chewing methods that help minimize pain while still allowing them to munch on those cacti. 

Roots 

Cactuses tend to have shallow but wide-reaching root systems that can quickly absorb water from light rains. Some roots lie just an inch or two beneath the surface, allowing the plant to take advantage of any moisture before it quickly evaporates. Precipitation can also trigger rapid root growth, enabling the cactus to gather water more efficiently. 

Cactus roots, for reference 

How they grow 

To clarify, not every cactus spends its life under full sun. While many of the towering species like the saguaro do stand tall in open areas, lots of smaller cacti prefer a bit of cover. Species such as the Echinopsis, the Hedgehog Cactus, often live under the protection of grasses or low shrubs.

And even the saguaro doesn’t start out in clearings. It typically germinates and grows in the shade of a nurse plant, like the Palo Verde. These trees shelter young saguaros from the harsh sun and extreme temperatures. Ironically, once the saguaro matures and outgrows its protector, it can end up killing the very plant that sheltered it by monopolizing water, space, and light. 

Flowers! 

Flowers of Mammillaria spinosissima (or the Spiny Pincushion Cactus)

Despite their quite dry lifestyle, cactuses produce some of the most spectacular flowers in the plantae kingdom. These flowers are often large, colorful, and short-lived, attracting pollinators like bats and birds; since the activities of such animals are based in the nighttime, the flowers also tend to open when there is less sun to match. 

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About the Author:

Geo Kim

Community Volunteer Blog Writer

Geo Kim is a high school student with a passion for environmental issues and writing. Her love for nature has been a constant since childhood, and her enthusiasm for words led her to work in the school newspaper as a staff writer. She hopes to combine her two passions through nature writing, creating fun and informative content that highlights the beauty and importance of the natural world.