Great question! The answer is that plants need to
move things up from the ground and they don't have
a pump like our heart.
Leaves have small holes in them that can be
open or closed. This allows them to take in
CO2, which you probably know is needed
for photosynthesis. It also allows them to get
rid of oxygen made in cellular respiration. Water
evaporates through these holes. As the water
evaporates, more water is drawn up to replace it.
Capillary action is the name of the process that
causes water to be drawn up into small tubes (like
the ones in a plant). It happens because water
molecules are attracted to each other.
So this flow of water up the plant happens
because of the evaporation. The water can then go
wherever it's needed and it has nutrients the
plant cells need dissolved in it. A lot of water
can be lost to evaporation, too, which is why they
need so much.
Not all plants need a lot of water. Plants
that evolved in the desert or climates with dry
seasons, have adaptations to help conserve water.
Can you think of some?
You may want to explore a career in plant
ecology or plant physiology.
Thanks for asking,