Final answer:
Cacti, like the saguaro, have adapted to their desert environment through efficient water storage in their barrel-shaped stems, the opening of stomata at night to reduce water loss, and the modification of leaves into spines to decrease water evaporation.
Step-by-step explanation:
The adaptation that helps a cactus survive in a desert environment is primarily its efficient water storage and conservation mechanisms. During rare rainfalls, the saguaro cactus utilizes its widespread root system to absorb as much moisture as possible. Once absorbed, the water is retained in its large, barrel-shaped stem that functions as a storage container. To minimize water loss, cacti have evolved variations of the light-independent reactions of photosynthesis which allow them to open their stomata only at night, reducing evaporation due to cooler temperatures. Consequently, the cactus can perform photosynthesis with the stored carbon dioxide during the day without losing precious water.
Additionally, the leaves of a cactus are modified into spines, decreasing the surface-to-volume ratio and further reducing water loss. The stem, which conducts photosynthesis, is also adapted to retain water. These adaptations ensure that cacti are highly efficient in using every drop of water for survival in the harsh, dry heat of the desert.