How does the roots of a cactus adapt to the desert?

Cactus roots have adapted by burrowing deep to reach water that is not quickly evaporated by surface heat.

The stems grow very thick to act as a sort of insulation against evaporative loss.

And the leaves have evolved as needles which have a low surface area to reduce evaporation and to deter animals and insects that would pierce the stem to suck moisture from the plant.