If you were a jackrabbit hopping through the desert,
you'd be glad to stumble, well, maybe not stumble, across a cactus. The succulent
flesh of these plants is a water source 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 just survive, but thrive in some of the harshest desert climates on earth.
So, how do they do it? A cactus's spines are 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 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 the plant's core temperature during the
heat of the day. Then, 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
Stomata open. The bulk of the plant acts as a large barrel of water, storing it
for times of need. But to survive the desert, the cactus can't just limit water
loss. It has to be prepared to take full advantage of the rare situations
where water is readily available. In North America's Sonoran Desert, the towering
Saguaro Cactus can grow up to twenty 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. The spread out
laterally for meters, and hold the plant in place. Even its single deepest root,
the tap root, 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 begin 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 rain forest? The Mistletoe Cactus can live on the branches of trees
in the rain forest. 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. Other
plant adaptations more closely resemble booby traps, from Bedbug Harpoons to
Fly Swatters. Check out some of the amazing ways plants defend themselves with
this video.
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