Do plants need defense mechanisms?

In the evolutionary sense, plants most certainly need defense systems, either as a remnant of a formerly existing evolutionary necessity or a present-day need. In the broadest terms, cacti have defenses against dehydration that would offer disastrous results in environments with abundant moisture, and they developed these defenses in responses to a persistently drying environment. Just as optimized predatory qualities and procreation are important, defenses mechanisms are an integral aspect of evolutionary theory. While these quirky qualities of plants may seem dismissible, at one point or another they were most likely to have been the crucial survival response in the history of that particular species.

Plants need defense mechanisms of some sort because if they don't have a defense then they will be completely eaten by hungry animals or they will not be able to survive harsh weather. Imagine if all plants were made of jello, with that type of defense they would melt, fall apart or be eaten rather fast.