Open your mouth and take a deep breath, then breathe back out onto your
hand. Does your hand feel moist? Plants have more stomata only if they
can afford to lose that much moisture , fewer stomata if they must
conserve water . Are you new to botany? In case you're a bit green,
let's take a step back.
What are stomata, exactly?
Stomata are like the mouths of plants, except that they can have many
hundreds of "mouths" per leaf where we only have one for our whole body.
Recall that:
- Plants use carbon dioxide (CO2) that they "breathe in" to make sugars (plant food).
- A byproduct of that process is oxygen , which the plants " breathe out".
- This "breathing" is actually passive flowing of gasses in and out of the stomata.
- There is water in the air, but more water in the plant's leaves.
- Every time a stoma opens, water vapor is one of the gasses that leaves... the leaves.
So
we now know that stomata are critical to the plant as they get
essential molecules from the air into the leaf parts that make plant
food. Stomata open their lips (guard cells) when the plant's chemical
process es signal that more CO2 is needed. You'll recognize the name of
the mechanism by which plants make sugar: Photosynthesis. As you likely
know, photosynthesis requires sunlight. Adding that piece to the puzzle,
we start to see why desert plants must really be special. If plants
make food when it's light out, in the heat of the day, that is when
they'll be opening their stomata for more CO2 - gulping in air and
spitting out water. But "the heat of the day" in the desert is so hot
and dry!
How do plants survive in the desert?
How do plants survive in the desert?
Although plants get plenty of sun light in the desert for
photosynthesis, it's actually too much. Extra heat and low humidity mean
that water will pass out of the plant that much faster. Also, there
isn't as much water in the soil, so the plant cannot reliably replace
water that is lost. Because of this, you'll find that anyone who' s
anyone in the community of desert plants will share many of the
following traits:
- Waxy coating to help keep water in the plant.
- Thick, small leaves that reduce the amount of surface area exposed to heat.
- Large pockets for absorbing water that can then be saved for a rainy… eh, a not-so-rainy day.
- Thorns to keep animals from eating the plant to drink that stored water.
- Limiting certain critical phases (growth, reproduction) to short periods of the year.
But
my favorite desert plant adaptation by far is that of hording. Many
desert plants use an alternative photosynthesis mechanism called "CAM"
photosynthesis (Crassulacean Acid Metabolism). Photosynthesis can only
occur when tons of CO2 flood the inside of the plants' cells. Rather
than leave the stomata gaping during the day to provide that CO2 , CAM
plants suck in CO2 at night and stockpile it until day. At that time, it
is released in saturating bursts to the right plant parts when the sun
is up, without the stomata having to open in the heat.
So, do desert plants have fewer stomata?
Yes. Although many of these other adaptations, including CAM
photosynthesis, make the stomata less burdensome, desert plants still
have fewer per square inch. One addition, more subtle desert plant
adaptation is that many desert plants grow much slower and are just…
well… lazy. To increase food production by increasing stomata, at the
risk of losing more water than can be regained, is not energetically
worth it. The plant would not live to reproduce and, thus, nature has
made its selection.
