It's difficult for the non scientifically employed gardener to measure a cactus plant's water levels. But there's a way of guesstimating. A cactus plant has ribbed sides that fill out with more water inside, and sink in with less. Filled out ribbing identifies a cactus plant as being 90%+ full of water. The sunken in look indicates 75% and less.
A cactus competes with other plants for water.
Cactus
A cactus has roots just as do other plants. These roots absorb water from the soil when it is available and store water in their tissues for ties of drought.
inside the chloroplast in the plants cells.
cactus
cactus, ipomea biloba
Water lilies will have less. Water plants have stomata on the sides exposed to air. Or if totally submerged, there are no stomata present. A cactus is covered with stomata but not as much as average plants.
That depends on what kind of plant it is. Some plants, such as cactus, do not need much water. Other kinds of plants need more. So you would have to ask about a specific plant in order to get a specific answer.It really depends on the plant. Some plants need more water than others, such as a cactus and a bean ,the bean needs more water than the cactus because the cactus stores water and beans don't.
The cactus" HAS water INSIDE of IT so IT drinks IT FROM inside
They are succulent plants, having thick fleshy leaves or stems to store water
As with all terrestrial plants, cacti absorb water through their roots.
Because that is pretty much the only thing availible.