Video Transcript
Which of the following is not a way
plants control water loss? (A) Plants will wilt, collapsing
their leaves to reduce surface area. (B) Stomata will close if
temperatures are too high. (C) Plants have guard cells that
leave stomata permanently open to regulate transpiration. Or (D) leaves are covered in a waxy
cuticle to prevent excess evaporation of water.
Plants lose water through
transpiration, so let’s review this process. Transpiration is the process
through which plants return water from their leaves to the atmosphere. The water is first taken up via the
roots. There are more water molecules
outside of the roots than inside them. So water will move into the roots
by osmosis. The water will travel up the plant
to the leaves. Two processes help it to travel up
the plant. The first is a pushing effect
caused by the water entering the root. This is called root pressure. The second is a pulling effect from
the leaves as they lose water to the atmosphere. This is called transpiration
pull. The dynamic between the two is
illustrated here.
Once the water molecules finally
reach the leaves, they will evaporate out via small pores called stomata. We can see one magnified here. The two kidney-bean-shaped cells
are called guard cells and control whether the stomata are open or closed. The inner walls of them are thick,
whereas the outer walls are thinner. As water enters these cells, they
swell and internal pressure increases. As more pressure acts on the outer
walls, they’re pushed outwards and the stomatal pore opens. Once water leaves the cells, they
shrink and the stomatal pore closes.
Transpiration has multiple benefits
for a plant. It helps provide water to the
leaves for photosynthesis and helps regulate temperature because evaporation has a
cooling effect. Transpiration has four main
limiting factors: light, humidity, wind speed, and temperature. Let’s focus our attention on how
temperature limits transpiration. If the temperature is too high,
then the increased kinetic energy of the water molecules will cause them to
evaporate faster and will result in excess water loss.
There are several mechanisms by
which plants try to avoid this happening. First, if it’s too hot, stomata
will simply close. Plants can also wilt, which reduces
the surface area on the leaves and therefore water loss. Some plants in hot environments,
like cacti, will also have highly adapted leaves, such as spines covered in waxy
cuticles.
Now that we understand a little bit
more about transpiration, let’s look back at our question. We now understand that water loss
is caused by open stomata and that plants can close their stomata as a way to
prevent it. Therefore, we know that stomata do
not stay permanently open. So the correct answer is (C). The option that is not a way plants
control water loss is “plants have guard cells that leave stomata permanently open
to regulate transpiration.”
