Nature's fortress: How cacti keep water in and predators out - Lucas C. Majure

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.

♪ (Music) ♪