Why is a cactus a fleshy water filled body in the dry hot desert?

The insides of all plant tissues are fleshy. The cactus is particularly so, because of its adaptation to its native environment. Its native environments are the deserts and jungles of Latin and North America.

In terms of the desert, a cactus needs to hold onto every drop of moisture that its roots get hold of. Once inside, everything is done to keep water and its solutions and related products inside. So the inside is thick and fleshy, to keep water from evaporating through the stem. The breathing pores, or stomata, are controlled by guard cells that approve the entry of gases and disapprove the exit of water. And the entire internal contents are encased in the stem's thick outer covering that is equally hostile to the loss of water and watery products and solutions.