Many plants have thorns on their stems or leaves. what is the most likely explanation for the evolution of thorns?

Xerophytes have thorns to adapt themselves to the arid regions where there is scarcity of water (thorns reduce the loss of water through transpiration) whereas some of the plants like rose have thorns to protect themselves from predators

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere.

The atmosphere does not play an important role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are  solids at the symbolic ranges of temperature and pressure present on Earth.

Thus, the earths spheres like biosphere, Cryosphere  and lithosphere are included in the phosphorus cycle but atmosphere is your answer.

Explanation: