What do the stems on a cactus do?

The stem is the cactus body part in which water is moved, processed and stored. It also is the body part where breathing pores called stomata allow for the gas exchange necessary for keeping proper temperatures and stopping water loss. It takes on various forms. For example, it may look like a thick column, such as in the case of the giant saguaro cactus [Carnegie gigantea]. Or it may look roundly compact, such as in the case of the birthday cake cactus [Mammillaria hahniana].