Final answer:
Desert plants have thorns/spines to protect themselves from herbivores and ensure their survival in the harsh desert environment.
Step-by-step explanation:
Desert plants have thorns/spines to protect themselves from herbivores who want to eat them and obtain water.
Thorns and spines are modified branches and leaves respectively that serve as physical defenses against herbivores by causing damage and inducing allergic reactions.
These adaptations help deter animals from consuming the plants and ensure their survival in the harsh desert environment.