Natures 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 one
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 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 the 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,
a 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 20 meters tall
and live for up to 200 years.
Woody tissue, like the kind found
in tree trunks,
give 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,
they spread out laterally for meters
and hold the plant in place.
Even its single deepest root,
the taproot,
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 start 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 rainforest?
The mistletoe cactus can live on the
branches of trees in the rainforest.
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.