The cactus is a member of the taxonomic family Cactaceae, a group of succulent plants that perform photosynthesis in their stems instead of in their leaves. Many species are able to survive in extremely dry environments by storing water in their stems.
Cacti have widespread and relatively shallow roots in order to quickly capture any moisture that falls around them. In fact, a cactus can be as much as 90 percent water after a rain. To compensate, the body of the cactus has grooves that allow it to expand or shrink depending on its water content. The sharp spines slow airflow around the plant to reduce evaporation and retain moisture.