Final answer:
Mesquite trees have deep taproots for accessing deep moisture and providing stability, while the saguaro cactus has widespread shallow roots to quickly capture water from rare rainfalls.
Step-by-step explanation:
The root systems of the mesquite tree and saguaro cactus have adapted to their arid desert environments in different ways. The mesquite tree predominantly possesses a deep taproot system that is capable of reaching deep soil moisture that exists far beneath the surface. This deep taproot can extend down up to 20-30 feet, giving the mesquite tree access to water sources that are unavailable to other plants. Furthermore, this taproot provides structural support to anchor the tree in the desert environment.
On the other hand, the saguaro cactus has a more shallow fibrous root system that spreads widely and just below the surface of the soil. This enables the saguaro to capture as much moisture as possible from infrequent rainfall over a large area. Additionally, saguaros have adapted to rapidly produce new roots to take advantage of rain and then shed these roots once the soil dries, conserving energy and resources.
Both types of root systems are advantageous for desert plants but in different ways: taproots reach for deep water reserves and offer stability, while fibrous roots maximize surface area to collect sparse rainfall and help the plant respond quickly to variable conditions.