It is a big challenge for cacti to get enough
water. There are three main strategies. One
is to have very long roots that can tap into
the water deep in the ground, where water may
collect. Another is to have very long roots
that spread out just below the surface so that
when rain does fall, they catch a lot of it.
Another potential source of water is the
moisture in the air. When the water vapor in
air hits a cooler surface, the vapor condenses
into liquid water. That’s what we notice as dew.
When it happens with cacti, the water falls to the
ground, where small surface roots can absorb it.
During the dry season, when they’re not needed,
cacti cut off the water to the roots and let them
die. This may seem odd, but picture two sponges
next to each other. One is the root, one is the
ground. Water will move from the wetter sponge to
the drier sponge. If the soil is wet, roots absorb
water. If the soil is dry, the soil will absorb
water from the roots.
Recent research has shown that some cacti can
actually absorb water right from the hair-like
needles. The water condenses on the needles,
then goes right through the plant’s surface at the
base of the needles. In the desert, water tends to
come all at once. Most cacti have pleats
that allow them to expand and hold that water for
later. The water inside the cactus can be stored
in plant pulp or in glue-like substance.
Cacti don’t need a lot of water because they
have several adaptations for conserving water.
They are covered with a waxy waterproof coating to
reduce evaporation. Instead of having leaves that
lose moisture and collect heat, they do
photosynthesis on their “trunks.” They may
have furry spines that provide shade. All of
these adaptations are good examples of the
relationship between structure and
In photosynthesis, the cacti need to take in water
and carbon dioxide. They give off oxygen. In most
plants, the carbon dioxide enters—and oxygen
leaves—through tiny holes in the leaves (stomata).
Cacti have these holes on their surface.
Unfortunately, water evaporates through these
holes too. Cacti can’t completely escape this,
but they only open those holes at night, when
the air is cooler and moister. You probably
know that plants need light to power
photosynthesis. Since they can only store so much
carbon dioxide at night, they can only do so much
photosynthesis during the day. Photosynthesis
is what provides sugars, starches, and other
materials, so cacti grow very slowly Their
sharp spines discourage animals from eating the
tissue that takes so long to build.
Why do you think cacti are usually spaced so
far apart in the desert?
Thanks for asking,