If you were a rabbit hopping through the desert, you'd be happy to stumble—well, maybe not stumble—on a cactus: the succulent flesh of these plants is a source of water for many desert animals.
Native to the Americas and known for their spines and succulent stems, cacti of all shapes and sizes have evolved to not only survive but thrive in some of the harshest desert climates on Earth.
So how do they do it?
A cactus's spine is a key to its survival
But not for the reason you might think. Take a look at the prickly pear. Its spines are highly modified leaves.
A typical leaf's large surface area would not be suitable for the desert, which transfers large amounts of water to sunlight. A dramatic decrease in spinal cord surface area limits dehydration.
They also shade the cactus and reflect the sun's rays, lowering the plant's core temperature during the heat of the day. Then, at night, when the air temperature drops, the spines act as an insulating layer, preventing the cactus from getting too cold.
These functions are just as important
If not more so, 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 are exposed to constant sunlight.
They rely on another adaptation common to cacti: a thick skin, coated in a waxy substance called a cuticle that limits dehydration. Meanwhile, the stomata—small openings that allow the exchange of gases that enable photosynthesis—are tightly closed until night when they open.
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 when needed.
But to survive in the desert, cactus can't just limit water loss—it has to be prepared to take full advantage of the rare conditions where water is readily available.
In the Sonoran Desert of North America
the giant saguaro cactus can grow up to 20 meters tall and live up to 200 years. Woody tissue, such as that found in tree trunks, gives the saguaro its height, but the saguaro survives with less water than most trees.
Most of its roots are only a few inches deep. Just below the soil surface, they later extend for meters and hold the plant in place. Even its only deep root, the taproot, extends less than a meter into the ground.
After rain, lateral roots respond in real time, growing and spreading rapidly. They develop temporary rain roots that quickly take up available water.
The water is then drawn into the plant's body and stored in cells that contain mucilage, a sticky substance that clings to water molecules and if the plant tissue is ever damaged and If masked, it prevents it from evaporating.
As the soil dries out after a rain
the tiny roots also dry out and wither, and the cactus waits to take advantage of the next shower.
Together, these characteristics make cacti well-equipped to survive in their environments, from the driest desert to... a tropical rainforest? Mistletoe cactus can live on tree branches in the rainforest.
Although there is plenty of water around, not much of it reaches the cactus here, and its roots have nowhere to go. So here, too, the cactus survives using adaptations that helped its long-ago ancestors survive in the desert.
Comments
Post a Comment