A scientific theory can never be disproven.

A basic principle in science is that any law, theory, or otherwise can be disproven if new facts or evidence are presented. If it cannot be somehow disproven by an experiment, then it is not scientific. Take, for example, the Universal Law of Gravitation

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem