Cacti are some of the most resilient plants on the planet, and can survive the harshest of desert conditions on very little water and with constant exposure to extreme heat.
The reason these plants can survive so well in the desert is thanks to several adaptations of their roots, leaves and stems. All of these adaptations are aimed at minimizing water loss while also storing as much water as possible for use during times of drought.
In this article, we will discuss the properties of cacti that set them apart from other plants and give them the ability to survive in the desert. If you want to learn more about these fascinating features, then keep reading.
How do cacti survive in the desert?
There are thousands of cactus species, and they come in all shapes and sizes. Even so, they have a very particular look that sets them apart from other plants, both succulent- and non-succulent. They have been a feature of many cultures over thousands of years, to whom they are a source of food and even medicine.
In deserts all over the world, including in Australia, the Americas and Africa, cacti are visited by tourists who travel long distances just to be able to marvel at them. Certain species have been proven to be hundreds of years old, yet still they seem perfectly able to deal with the harsh elements of their surroundings.
These plants are able to survive in desert conditions thanks to specific properties and adaptations they have evolved over millennia. We will discuss these below.
Rain does occur in the desert on occasion, and cacti have developed particularly shallow roots that can extract water from the top layers of soil, in case the rainwater does not penetrate very deep.
Additionally, the plant actually grows temporary roots when the soil becomes moist, with the goal of absorbing as much water as possible during that short period of rain. It can grow these roots in the space of about two hours! When the rains cease, the roots wither.
As well as being close to the surface to maximize water absorption, the roots also extend to cover a large area. Once they have absorbed the water, the roots transport it to the plant’s stems for storage.
Cacti lack true branches and leaves that other plants have. Instead, their leaves have been modified to become spines. Their branches have small bumps, called areoles, from which the spines sprout from the plant.
This raises the question: How can the plant photosynthesize in the absence of leaves? Well, unlike other plants, in cacti it is the stems, not the leaves, that perform photosynthesis. As a result, cacti do not need leaves in the same way that other plants do. Additionally, because the desert gets plenty of sunlight, the stems have constant and easy access to sunlight.
The spines are critical in preventing water evaporation, as they are thicker and have less surface area than leaves. Summer causes the majority of desert shrubs to droop and cease photosynthesis. That is not the case with cacti: they can continue to produce food, which is why they stand tall even during the sweltering summer months.
The spines on a cactus also create a buffer around the plant, reducing air flow and thus water evaporation. During the hot desert summers, the moist air contained within this buffer plays an important role in preventing water from evaporating from the plant’s surface.
Another function of the spines is to collect water for the plant. The Atacama Desert in Chile is one of the driest deserts on the planet, but it is common for this desert to be blanketed in fog in the early hours of the morning. This heavy dew, referred to as “camanchaca” by the locals, settles on the spines, liquefies into water, and drips onto the ground beneath the plant. The water is then absorbed by the cactus’ roots, which helps hydrate the plant and keep it alive.
The spines have a grooved structure that enhances the efficiency of their water collection.
Spines also provide the cactus with some shade, even though they look a bit small for this job. There is a reason for the dense population of needles on a cactus, with a single plant having literally thousands of them. This density is designed to cover as much surface area as possible, providing sufficient shade to prevent a certain amount of evaporation.
The spines can also protect the plant from predators that might harm it. The desert is teeming with herbivorous animals who would relish the opportunity to munch on the juicy flesh of a cactus, but its spines are sharp enough to pierce through skin, making them very effective at keeping these animals away.
The cactus’ stem has the ability to store a significant amount of water, which is why the plant can thrive in even the hottest desert conditions.
Photosynthesis is the process by which plants manufacture food, using carbon dioxide in conjunction with sunlight and water. The carbon dioxide is absorbed through pores called stomata, and while carbon dioxide is inhaled, oxygen is exhaled. During this process a significant amount of moisture is also lost. And, while moisture loss may not be a major concern for plants that receive regular rainfall, it is a different story for those that grow in the desert.
Desert plants must conserve as much water as possible, and to accommodate this, they only perform photosynthesis at night. Their stomata only open at night when temperatures are cooler, resulting in reduced water loss during the day. The opening and closing of the stomata is as regular as clockwork: they close automatically at dawn and reopen automatically at night. This process of night-time photosynthesis is referred to as crassulacean acid metabolism in scientific circles.
The stomata on a cactus are also extremely small, and are located deep within the tissue rather than at the surface. Both of these factors help to reduce water loss.
The cactus’ stem has thicker skin than the leaves of a normal plant, making it an efficient water reservoir that can hold more water than regular leaves.
Another special feature of a cactus’ stem is that it can expand to store more water. If you were to cut off the stem and examine the cross-section, you would see that it is shaped like an accordion, which can easily expand and contract. As the plant uses up its water reserves, its stems will continue to shrink, and will expand again when they absorb water from the next rainfall.
The surface of a cactus is covered with a thin layer of waxy skin that aids in the prevention of water evaporation.
In addition, this surface helps keep the cactus cool in hot weather. Without it, the plant would dry out in the extreme heat.
Short growing season
In contrast to most plants that grow continuously, the cactus has periods during which it does not actively grow. A large amount of water is required for growth, and this is scarce in the plant’s natural environment.
For this reason, cacti only grow for a short season before taking a break until the following growing season begins. It is no surprise, then, that cacti grow more slowly and live longer than most other plants.
Cacti are some of the most resilient, drought-tolerant plants in the world. They have evolved over millennia to develop characteristics that aid their survival in harsh desert conditions.
These plants’ unique capabilities are what sets them apart from other plants that grow in more temperate climates.
The reason that cacti can survive in the desert is because of their shallow roots that can absorb water quickly, spines that provide shade and protection, stems that can photosynthesize economically and store a lot of water, skin that helps keep moisture locked in, and a short growing season.
All of these amazing traits have, for many thousands of years, ensured the survival of cacti in the world’s harshest climates.
Image: istockphoto.com / DGHayes