Question

how was cactus modified itself to live in a desert

Answer 1

Like all plants and living things, cacti need water to survive. Given where they live, there is obviously very little access to water. To compensate for that, cacti have developed special proclivities that allow them to conserve the water they receive for a long time.



As an example, the prickly pines are actually modified leaves that protect them from animals and also to restrict the air flow near the cactus and thus reducec water loss.
Many species of cacti also have very shallow roots that allow them to soak up any rainfall that may come their way. They also possess specialized stems that enables them to store water for a very long time.

Answer 2

By adapting the conditions of desert and also, its leaves are modified in thorns or spikes to prevent water loss