Transpiration is basically evaporation with a
plant added in. It’s called evapotranspiration
once we add in the evaporation. As you know, when
liquid water turns to water vapor, that’s
evaporation. The molecules of water don’t change
when they become vapor, they’re just moving a lot
more. The energy it takes to move more comes from
heat. You probably know that things evaporate
faster when you add more heat. You don’t have to
boil water to make it evaporate, that just makes
evaporation go even faster.
Now let’s look at the plant. Plants have many
tiny tubes inside. Some of these go all the way
from the roots to tiny holes in the plant leaves.
These carry things around a plant, sort of like
our blood vessel carry things around our bodies.
One major difference is that the plant has no
heart to pump fluid around. When sap made in the
leaves is going down to the roots, gravity does
the job. But how does water get from the roots to
the leaves? The energy comes from the sun.
Water molecules are attracted to each other
because they have a semi-positive end and a
semi-negative end. You know what they say,
“opposites attract.” You know how water beads up
on a piece of glass, or how you can carefully add
drops of water to a container so that it actually
bulges over the top? That’s because water
molecule are attracted to each other. Imagine
that they like to hold hands.
Okay, water molecules don’t have hands, so
imagine the aliens from Toy Story holding hands.
The claw comes down and grabs one of them, and
lifts it up. All of the ones who are holding
hands get pulled up with it.
Now imagine that I pull on the water molecule
at the top of the tubes in the plant. Not only do
I pull on that molecule, I pull on all the ones
it’s “holding hands” with. The tiny tubes inside
the plant are also very “sticky” for water. As a
water molecule evaporates out through the tiny
leaf holes into the air, more water molecules get
pulled up the tiny tube behind it. The pull goes
all the way down to the roots. The water below is
also pushing into drier places. It’s like a big
solar pump taking water from the ground and
putting it into the air. The water also has
nutrients in it that the plant needs.
One thing that doesn’t come up from the roots
is carbon dioxide. Plants need carbon dioxide to
do photosynthesis. See if you can figure out
where that comes from.
If you are interested in plants, you may want
to study botany.
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