From wriggling amoebas to roving rhinos, delicate dandelions to majestic redwoods, everything that lives on Earth needs one superstar element to survive: water. Luckily for us, water is abundant on the planet -- the catch is that it's not evenly distributed.
If life is anything, it's tenacious. Over time organisms have evolved to fill even the most barren ecological niches. The microbes commonly referred to as extremophiles, are a great example. Thermophiles like it nice and toasty, halophiles think super-salty is the way to go and nothing will do for psychrophiles but the big chill.
In the case of plants, there's a special set known as xerophytes, which have evolved to survive in climates where water is extremely scarce. And among the xerophytes, we find an assembly of plants called succulents. The term succulent (and xerophyte for that matter) doesn't denote a traditional taxonomic group, however. Rather, succulents are diverse plants share common adaptations for making the most of every bit of water they can get. It's also important to note that the line between succulent plants and ordinary plants is not cut and dry. On a short-term basis, all plants are generally able to do what succulents are so renowned for -- it's just that succulents take things to a whole other level.
The succulent that probably springs to mind most readily is the cactus, although there are succulents in a wide variety of plant families. You might imagine succulents only inhabit wind-swept arid desert regions where little else can grow, but that's not the case. Succulents are abundant in a variety of locales. You can find them growing in the upper reaches of tropical rain forests, rooted to high-rise patches of moss or bark where the competition for water is fierce. You can find them in lofty mountainous regions where cold weather, severe winds and rocky soil make finding moisture a challenge. You can even find them on the shores of salty bodies of water, where brackish conditions hinder normal botanical water routines.
It's worth noting, too, that succulents aren't superheroes -- there are some regions where the desert conditions are so extreme that only the most hardcore xerophytes can survive. While succulents are champions at water collection and conservation, most need at least a few inches of rain annually to get by. There are a few other exceptions as well. In some places, deserts spring up too quickly for local plants to evolve and in others, the plants just find other ways to adapt.
Now that we've got the basics down, continue on to the next pages for the dirt on how these plants are able to get by with such tiny amounts of water.
The Dirt on the Succulent
Over millions of years, as the planet's land and climate have altered, so too have the plants that pepper that landscape. For example, millions of years ago, continents drifted and mountain ranges slowly rose out of vast prehistoric jungles, blocking air currents and altering the climate. Seasons developed, so did climatic belts. In many parts of the world, water gradually became a scarce commodity.
Succulents eventually rose to take over the wastelands where little else could live. They're champs at quickly gathering any water that comes their way, storing it for future use and protecting it from thirsty visitors.
Although succulents can vary greatly in appearance, they share a number of basic fundamental characteristics. The most important of these is the succulent's ability to store water, from which its name is derived. Plants all have a certain degree of succulence. But succulents took this talent to new heights -- some can store years' worth of water in either their stems, roots or leaves for times of severe drought. Think of the wide stem of a saguaro cactus or the thick fleshy leaves of an aloe plant, and you're basically looking at an expandable botanical rain barrel.
But how are they going to collect any water in all those wild environments they inhabit? Many succulents have fantastic root systems completely specialized for their environments. In mountain regions, roots often dig down deep to harvest any subterranean water sources. But up on the plains, their roots are often broadly spread but incredibly shallow in order to catch any moisture that might lick the surface of the land, such as morning dew.
Plants wage a constant battle to get all the nutrients they need in order to function. Besides water, they also need sunlight and carbon dioxide for photosynthesis. The CO2 comes in through diffusion so most plants have a huge number of pores, called stomata, to coax it in. But at the same time the sun is providing them with the energy they need to perform photosynthesis, it's also evaporating water (a process that in plants is called transpiration) through the exposed stomata. It's a difficult tightrope to walk, so succulents have managed to get themselves a safety harness.
Most plant species are hardwired to open their stomata wide when they sense certain triggers like sunlight -- a bad idea for plants trained to carefully avoid any water loss. It does have the benefit of temporarily maximizing photosynthesis, but succulents just don't have that luxury, one of the reasons they're typically slow-growing plants. Succulents bypass this hang-up by opening their stomata at night, which decreases transpiration and retains precious water. This means they've had to develop a special type of metabolism called Crassulacean acid metabolism (or CAM). With the CAM method, plants can take in CO2 during the night and store it to use for photosynthesis the next day.
Another important difference? In regular plants the stomata are sized like dense freeways; in succulents, they're not more than sparse two-lane country roads. So not only do they open at more opportune times, they're smaller when they do. Thirdly, succulent stomata are less numerous and are sunk down deep in leaves and stems, usually protected by a thick outer skin and various other features such as waxes, resins, hairs and needles to further decrease transpiration and retain water.
We're almost there, but we still have a bit of evolving to do before we're full-fledged succulents. On the next page we'll find out other reasons why succulents grow so slowly and how they prevent themselves from becoming the desert's version of a drinking fountain.
Succulent Shape: The Desert's Drinking Fountain
Another piece of the equation to keep in mind is how succulents receive sunlight, because there's a catch-22 here. They're often reducing their surface area to minimize water loss, especially in the stem succulents, but they're also reducing how much sunlight they can catch to motor their photosynthesis. This is why many succulents have such fun shapes -- rippled and ridged surfaces, crazy knobs and lumpy protrusions. These extensions, as well as adaptations to the stems, increase their ability to process sunlight, helping some types of succulents hang on. The shapes of succulents also typically vary depending on how much water the plants currently contain -- they expand when water is plentiful and contract when it's not. In some, ties with roots and other growths are severed when times get tough. Succulents can't afford to get sentimental.
When discussing succulents, it's important to understand how all the extensive adaptations they underwent to survive where water is scarce impacted them across the board. Their special metabolism and distinctive shapes are two aspects. Another example is how they grow. Many grow low to the ground -- practically burying themselves in order to avoid the harsh sunlight. Others are all about the shade; they enjoy living in someone else's shadow, especially if it's a well-placed rock or other form of shelter. Their rate of growth is also affected in several ways. For instance, plants get nutrients from the soil. (Ever buy a bag of fertilizer? Then you know what we're talking about.) But beyond that, plants need moisture to soak up all those yummy nutrients. This means that in terms of growth, succulents are the turtle in the race, not the hare. But hey, whatever works, right?
Another important factor with succulents is protection. Succulents are basically botanical water bottles, they thrive where others thirst. So how do they defend themselves against the dehydrated masses? If you've ever had the misfortune of bumping into a cactus, you've had a preview of how they pull this off. Spines and spikes, thick armored skin, yucky and sometimes poisonous juices are all among the defenses succulents have developed to keep meddlers away. Some of these protective measures even double up as rainwater collection devices -- like hollow spines that can slide water right inside.
In a rough-and-tumble environment, with the necessities for life few and far between, getting together for a date can be a bit of a challenge. Another way succulents are set up to survive is that many are self-propagating, whether through seeds or actual little plants all ready to go. Also, if you knock a chunk off a succulent, that piece can typically take root and start growing on its own fairly easily -- handy if the plant is in a place that suffers strong weather.
Whichever way you look at it, succulents are well armed to live days, months, and sometimes even years without a single fresh sip of water. They might not look like some of their non-succulent relatives, but they've got the right stuff to live where water is scarce. On the next page, you'll find lots more links about water, weather and all sorts of plants.
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More Great Links
- Binns, Corey. "How Cacti Survive: Surprising Strategies Quench Thirst." LiveScience.com. 7/24/2006. (10/30/2008) http://www.livescience.com/strangenews/060724_mm_cacti_survive.html
- British Cactus and Succulent Society Web site. (10/30/2008) http://www.bcss.org.uk/
- Cactus and Succulent Society of America Web site. (10/30/2008) http://www.cssainc.org
- Cactus and Succulent Society of New Zealand Web site. (10/30/2008) http://www.cssnz.org/what-is-a-cactus.phpvc
- Chidamian, Claude. "The Book of Cacti and Other Succulents." Timber Press. 1984. (10/30/2008)
- "Meet the Microbes: Archaea and Other Extremists. American Society for Microbiology. (10/30/2008) http://www.microbeworld.org/microbes/archaea/
- Raven, Peter et al. "Biology of Plants." Worth Publishers. 1992. (10/20/2008)
- "Succulent." Encyclopedia Britannica. (10/30/2008) http://www.britannica.com/EBchecked/topic/571271/succulent
- "Succulent Savvy." HGTV. (10/30/2008) http://www.hgtv.com/gl-plants-other/succulent-savvy/index.html