Why do plant cells that transport water against the force of gravity contain many more mitochondria than other plant cells do?
This specific process requires energy and ATP in mitochondria provide energy.
The process of transporting water against gravity is called active transport, so called because it requires energy to occur (as opposed to passive transport which occurs naturally).
Now, the molecule which provides cells with energy is called ATP (adenosine triphosphate), which is found in mitochondria. So cells which use active transport need more mitochondria so that they have the energy needed for the process.