During my visit to Tucson, Arizona during the winter break, I had many close encounters with cacti on the hiking trail, including getting pricked by a jumping cholla cactus. However, I decided to research possibly the most iconic cactus in the world: the massive saguaro cactus.
The saguaro cactus is not very common; it is only found in Arizona and parts of northern Mexico. The Sonoran Desert in Arizona is one of the few places with naturally growing saguaro cacti. Saguaro cacti are amazing plants. For one thing, large saguaro cacti are incredibly valuable. This is because it takes a saguaro cactus several hundred years to grow to that size. In fact, the signature “arms” of the saguaro actually don’t grow until the cactus is at least 60-years-old.
The saguaro cactus has a unique, accordion-like skin texture that can expand to gather more water in wet weather. Amazingly, some can expand up to 16 inches during a rainy season. Yet another adaptation that the saguaro and some other desert plants have developed is a thin web of roots just below the surface. This allows them to capture rainwater even if deeper soil is not very saturated. Weather significantly affects the growth of a cactus’s arms. If a winter is unusually cold, the cold could weaken an arm and make it sag. If the damage is not too severe, the arm will continue growing in its new direction.
The saguaro flower, the state flower of Arizona, is typically only open for one day. When it is open in the day, it is pollinated by various birds and insects, including bees and white-winged doves. At night, it is pollinated by lesser long-nosed or Mexican long-tongued bats.
The spines of a saguaro are very unique adaptations. While they resemble, say, a hedgehog’s spines, they are actually modified leaves. Their first purpose of the spines of a saguaro is fairly obvious—to protect them from predators. But this does not deter all predators. For example, javelinas (a type of wild pig), tortoises, and pack rats are unfazed by the painful spines. The main reason that the leaves of a saguaro have evolved into spines is that spines lower the transpiration rate, or the rate at which water is lost via water vapor. Stomata are minute pores on leaves, which allow water vapor to escape. Since saguaro spines have no stomata, the transpiration rate is reduced. The third purpose of a saguaro’s spines, surprisingly, is to provide shade for the cactus. While a single spine does not seem to provide much shade, multiply that spine by one hundred or one thousand, and you will realize how much help these spines provide. The shade these spines provide helps lower the surface temperature of a cactus, which lowers the amount of water lost to the atmosphere.
The way a cactus has evolved to life in the desert is quite amazing. I can never forget the sight of hundreds of towering saguaros standing in the Sabino Canyon near Tucson. Despite their daunting appearances, they provide shelters to little birds and reach their arms out as if to welcome people to the Sonoran Desert.
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