Final answer:
A cactus survives in a desert by having a waxy layer on its stem to prevent water loss. This adaptation, along with other features like modified spines and water storage in stems, enables cacti to live in arid environments.
Step-by-step explanation:
The adaptation that helps a cactus survive in a desert environment is a waxy layer of cells covering the stem to prevent water loss. This adaptation is essential for the plant to conserve water in the extremely dry desert conditions.
Desert plants like cacti (xerophytes) have evolved several unique features to thrive in harsh environments where water is scarce. One of the most vital adaptations is the development of a thick waxy cuticle on their stems. This layer significantly reduces water evaporation, allowing cacti to retain moisture for prolonged periods.
Moreover, photosynthesis in cacti has evolved to become more water-efficient, with some species conducting the process at night when temperatures are cooler and water loss is minimized. The leaves of many cacti are also modified into spines, reducing the surface area through which water can be lost while simultaneously providing protection from herbivores. Additionally, the stem of a cactus is adapted to store water, swelling after rain and slowly using this reserve during drier periods.