Is in a desert plant saves water by its stem for making food?

Cacti (plural) is a common name given to a group of xerophytic plants (adapted to dry regions).

They are able to survive to to a number of unique adaptations to their morphology.

They have long roots that spread out just under the surface in search of water, they grow more roots (feeder roots) when heavy rain falls these roots are then cut of by the plant so there is less plant to feed and less chance of precious water seeping out.

They also have spines which are modified leaves to keep animals from eating the stem; the needles also catch water then send it in towards the plant so it can slide of the vertical stem on to the floor to be absorbed by the roots. Reduction in leaves also means less water is lost through transpiration.

Cacti look swollen because they have adapted their stems and leaves (where they still have) to store as much water as possible and to lose as little water as possible by evaporation. They also have a very thick and waxy cuticle which cuts down water loss to the absolute minimum.

The whole of the stem is green and contains photosynthetic cells, so the plant does not need broad leaves to pick up light. There is plenty of light around anyway, so the plant can photosynthesize very well for most of the day.

Some cacti (particularly small ones) have rolled up leaves so there is less surface area exposed to the sun. It may have thorns to protect it from being eaten, and also be a grayish color so to reflect the heat of the sun and to prevent water loss.