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If you were a jackrabbit hopping through 
the desert,

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you'd be glad to stumble, well, maybe not
stumble, across a cactus. The succulent

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flesh of these plants is a water source
for many desert animals. Native to the

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Americas and known for

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their spines and succulent stems, cacti 
of all shapes and sizes have evolved to

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not just survive, but thrive in some of 
the harshest desert climates on earth.

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So, how do they do it? A cactus's spines
are key to its survival, but not for the

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reason you might think. Take a look at
the Prickly Pear. Its spines are highly

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modified leaves. A normal leaf's large 
surface area would be ill-suited to the

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desert, transpiring massive amounts of 
water under the baking sun.

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The dramatically

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reduced surface area of the spines limits
water loss. They also shade the cactus,

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and reflect the sun's rays, reducing the 
the plant's core temperature during the

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heat of the day. Then, at night, when air 
temperatures plummet, the spines act as

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an insulating layer, keeping the cactus
from cooling down too much. These

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functions are just as important, if not
more, than defending against predators.

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From Cuba to Mexico, and as far south as 
Brazil and Peru, Melon Cacti grow on

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limestone soils in seasonally dry tropical
forests, where they're constantly exposed

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to the beating sun. They rely on another 
adaptation common to cacti: a thick skin,

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which is coated in a waxy substance called
a cuticle that limits water loss.

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Meanwhile, the Stomata, tiny holes that
allow the exchange of gases that enable

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photosynthesis, remain firmly closed until
night, when they open. The lower

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temperatures at night mean the cactus 
loses less water from the stem when

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Stomata open. The bulk of the plant acts
as a large barrel of water, storing it

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for times of need. But to survive the
desert, the cactus can't just limit water

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loss. It has to be prepared to take 
full advantage of the rare situations

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where water is readily available. In North
America's Sonoran Desert, the towering

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Saguaro Cactus can grow up to twenty 
meters tall and live for up to 200 years.

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Woody tissue, like the kind found in 
tree trunks, gives the Saguaro its

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height, but the Saguaro survives with way
less water than most trees. Most of its

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roots are only a few inches deep, just
below the soil's surface. The spread out

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laterally for meters, and hold the plant
in place. Even its single deepest root,

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the tap root, extends less than one meter
into the ground. After a rain, the lateral

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roots respond in real time, rapidly 
growing and spreading. They produce

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ephemeral rain roots that quickly take up
the available water. The water is then

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pulled up into the plant body and stored
in cells that contain Mucilage, a gluey

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substance that clings to water molecules
and stops them from evaporating if the

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plant's tissue is ever damaged and 
exposed. As the soil dries after the rain,

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the small rain roots also begin to dry
and wither away, and the cactus awaits

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the next time it can take advantage of a
shower. Taken together, these features

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make cacti well-equipped to survive their
environments, from the driest desert to...

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a tropical rain forest? The Mistletoe 
Cactus can live on the branches of trees

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in the rain forest. Though there's lots of
water around, not much of it reaches the

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cactus here, and there's nowhere for its
roots to go. So, even here, the cactus

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survives

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using adaptations that long ago helped
its ancestors survive the desert. Other

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plant adaptations more closely resemble 
booby traps, from Bedbug Harpoons to

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Fly Swatters. Check out some of the 
amazing ways plants defend themselves with

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this video.

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♪ (Music) ♪