It's hot in the desert. It's awful dry too. Succulent plants such as cacti, aloes, and agaves, beat the dry heat by storing plenty of water in their roots, stems, or leaves.
How? For starters, when it does rain, succulents absorb a lot of water quickly. In the desert, water evaporates rapidly, never sinking deep into the soil. Thus, most succulents have extensive, but shallow root systems. Their roots absorb water just a half inch or so below the surface.
Succulents have evolved a number of strategies for holding onto this water. They tend to have a thick waxy coating, which helps seal in moisture.
All plants are covered by tiny pores called stomates, which allow plants to take in gasses for photosynthesis. However, these pores also allow water to be lost. Succulents have fewer stomates per cubic inch through which water can evaporate. In addition, succulents have a reduced surface area and, if they have leaves at all, they're thick and fleshy.
Many succulent plants also have a modified way of conducting photosynthesis. Other plants open their stomates during the day to take in carbon dioxide for photosynthesis. Many succulents, however, keep their stomates closed during the heat of the day and open them in the coolness of the night to take in carbon dioxide, which they store until the next day.
Finally, because water is a scarce commodity in the desert, succulents have to protect themselves against thirsty animals. These plants protect their water supplies by being prickly like many cacti or in other cases, by being toxic, by growing in inaccessible locations, or by camouflage.