If you were a jackrabbit hopping through the desert, you’d be relieved to find a cactus. The succulent flesh of these plants serves as a vital water source for many desert animals. Native to the Americas, cacti, known for their spines and succulent stems, come in all shapes and sizes. They have evolved to not just survive, but thrive, in some of the harshest desert climates on Earth.
So, how do cacti manage to survive in such harsh conditions? The answer lies in their spines. Contrary to popular belief, the spines of a cactus, such as the prickly pear, are highly modified leaves. A normal leaf’s large surface area would be ill-suited to the desert, transpiring massive amounts of water under the baking sun. The dramatically reduced surface area of the spines limits water loss. They also shade the cactus and reflect the sun’s rays, reducing the plant’s core temperature during the day. At night, when air temperatures plummet, the spines act as an insulating layer, keeping the cactus from cooling down too much. These functions are just as important, if not more, than defending against predators.
From Cuba to Mexico, and as far south as Brazil and Peru, Melon cacti grow on limestone soils in seasonally dry tropical forests, where they’re constantly exposed to the beating sun. They rely on another adaptation common to cacti: a thick skin, which is coated in a waxy substance called a cuticle that limits water loss. Meanwhile, the stomata—tiny holes that allow the exchange of gases that enable photosynthesis—remain firmly closed until night when they open. The lower temperatures at night mean the cactus loses less water from the stem when the stomata open. The bulk of the plant acts as a large barrel of water, storing it for times of need.
In North America’s Sonoran Desert, the towering Saguaro cactus can grow up to 20 meters tall and live for up to 200 years. Woody tissue, like the kind found in tree trunks, gives the Saguaro its height, but the Saguaro survives with way less water than most trees. Most of its roots are only a few inches deep. Just below the soil’s surface, they spread out laterally for meters and hold the plant in place. Even its single deepest root, the taproot, extends less than one meter into the ground. After a rain, the lateral roots respond in real time, rapidly growing and spreading. They produce ephemeral rain roots that quickly take up the available water. The water is then pulled up into the plant body and stored in cells that contain mucilage, a gluey substance that clings to water molecules and stops them from evaporating if the plant’s tissue is ever damaged and exposed. As the soil dries after the rain, the small rain roots also start to dry and wither away, and the cactus awaits the next time it can take advantage of a shower.
Taken together, these features make cacti well-equipped to survive their environments, from the driest desert to a tropical rainforest. The mistletoe cactus, for instance, can live on the branches of trees in the rainforest. Though there’s lots of water around, not much of it reaches the cactus here, and there’s nowhere for its roots to go. So even here, the cactus survives using adaptations that long ago helped its ancestors survive the desert.
1. How do cacti manage to survive in harsh desert conditions?
2. What is the purpose of the spines on a cactus?
3. How does the thick skin of melon cacti help them survive in tropical climates?
4. What role do stomata play in the survival of cacti?
5. How does the Saguaro cactus store and conserve water?
6. What unique features allow cacti to thrive in different environments?
7. Can cacti survive in a tropical rainforest? If so, how?
8. How have cacti adapted over time to survive in their respective habitats?
Imagine you are a cactus in the desert. Write a diary entry for a day in your life, describing how you use your adaptations to survive. Include details about how you use your spines, thick skin, and stomata to conserve water and protect yourself from the harsh desert conditions.
Draw a comic strip that illustrates the relationship between the jackrabbit and the cactus in the desert. Show how the jackrabbit relies on the cactus for water and how the cactus benefits from this relationship.
Choose one type of cactus mentioned in the article (prickly pear, melon cacti, or Saguaro) and research more about it. Create a poster that includes pictures, facts, and information about its adaptations and how it survives in its specific environment.
Conduct a simple experiment to understand how cacti store water. Fill a sponge with water and observe how it retains water. Compare this to how a cactus stores water in its cells. Write a short report on your observations and findings.
Write a short essay comparing how cacti survive in different environments, such as the desert and the rainforest. Use the mistletoe cactus as an example of a cactus that has adapted to survive in the rainforest. Discuss the different adaptations that cacti have developed to survive in these different environments.
jackrabbit – a large hare or rabbit with long legs and large ears, native to western North America. – The jackrabbit bounded across the field, its long legs propelling it forward with incredible speed.
cactus – a spiny succulent plant native to arid regions, typically having swollen stems or pads for storing water. – The desert was dotted with various species of cacti, including the iconic saguaro cactus.
desert – a barren or arid area of land, often sandy or rocky, with little or no precipitation and sparse vegetation. – The scorching heat of the desert made it a harsh environment for any living creature.
succulent – a plant that is able to store water in its leaves, stems, or roots, allowing it to thrive in arid conditions. – The juicy leaves of the aloe vera plant make it a popular succulent for both decorative and medicinal purposes.
spines – sharp, pointed structures that protrude from the surface of certain plants, such as cacti, providing protection against herbivores. – The spines of the prickly pear cactus served as a deterrent to animals attempting to eat its fleshy pads.
water – a transparent, odorless, tasteless liquid that is essential for the survival of all known forms of life. – The camel’s hump stores fat, not water, helping it survive in the water-scarce desert.
climates – the average or prevailing weather conditions of a region, including temperature, humidity, precipitation, wind patterns, and other variables. – The desert and the rainforest have drastically different climates, resulting in unique ecosystems in each.
survival – the state or fact of continuing to live or exist, often despite difficult or challenging circumstances. – The adaptations of desert animals are crucial for their survival in extreme heat and limited water availability.
adaptation – the process or state of adjusting to new conditions, often resulting in physical or behavioral changes to improve an organism’s chances of survival. – The long ears of the jackrabbit are an adaptation that helps it dissipate heat and detect predators more effectively.
roots – the part of a plant that typically lies below the surface of the ground, absorbing water and nutrients from the soil. – The deep and extensive root system of desert plants enables them to access water from deep underground sources.
